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A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO2-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates. Effective viscosities of magnesian carbonates are higher than those for antigorite serpentine and they are markedly lower than those for H2O-saturated olivine. However, magnesian carbonates may extend to greater mantle depths than hydrous silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab dehydration. A simple model of shear heating and temperature-sensitive creep of carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by dehydration embrittlement, beyond the stability of antigorite serpentine in subduction zones. [ABSTRACT FROM AUTHOR]

With this Preface, we provide background information, a scientific overview of topics, and an annotated bibliography of the 63 papers published in the Journal of Geophysical Research (JGR) Special Issue on "Ophiolites and Oceanic Lithosphere, with a Focus on the Samail Ophiolite," collected online at https://agupubs.onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)2169-9356.SAMAIL1. Topics covered in the Special Issue include formation and alteration of igneous ocean crust at submarine spreading centers, subduction zone mass transfer at the leading edge of the mantle wedge, the tectonic emplacement and evolution of ophiolites, processes of alteration and weathering of tectonically exhumed mantle peridotite, and the subsurface biosphere in the peridotite weathering environment. The bibliography, in the form of a supplementary Excel spreadsheet with links to each paper, is organized by topic, but can be sorted by first author, order of publication, specific Oman Drilling Project boreholes, and other factors. Plain Language Summary: This is a Preface for a Special Issue of papers on "ophiolites"—fragments of oceanic crust and upper mantle, thrust onto continental margins—and related studies of oceanic "lithosphere"—data on submarine, oceanic crust and mantle in situ. The Issue includes papers on the formation and alteration of igneous crust at tectonic spreading centers, the thrusting of ophiolites over and onto continental margins, accompanied by an influx of fluids derived from underthrust materials, weathering of tectonically exhumed rocks from the Earth's mantle (both in ophiolites and on the seafloor), and the subsurface microbial biosphere associated with this weathering. Key Points: In understanding processes that form and modify oceanic lithosphere, the Samail ophiolite is an essential complement for seagoing studiesStudy of the base of the Samail ophiolite provides key insights on mass transfer at the leading edge of the mantle wedgeOngoing alteration of mantle peridotite generates highly reduced, alkaline fluids, and hosts an important subsurface microbial ecosystem [ABSTRACT FROM AUTHOR]

Serpentinization and carbonation of mantle rocks (peridotite alteration) are fundamentally important processes for a spectrum of geoscience topics, including arc volcanism, earthquake processes, chemosynthetic biological communities, and carbon sequestration. Data from a hydrophone array deployed in the Multi‐Borehole Observatory (MBO) of the Oman Drilling Project demonstrates that free gas generated by peridotite alteration and/or microbial activity migrates through the formation in discrete bursts of activity. We detected several, minutes‐long, swarms of gas discharge into Hole BA1B of the MBO over the course of a 9 month observation interval. The episodic nature of the migration events indicates that free gas accumulates in the permeable flow network, is pressurized, and discharges rapidly into the borehole when a critical pressure, likely associated with a capillary barrier at a flow constriction, is reached. Our observations reveal a dynamic mode of fluid migration during serpentinization, and highlight the important role that free gas can play in modulating pore pressure, fluid flow, and alteration kinetics during peridotite weathering. Plain Language Summary: Serpentinization, the reaction between water and upper mantle rocks, is a widespread process on Earth that has important implications for a variety of topics, including the origin of life and carbon sequestration. We monitored the acoustic pressure inside a well established in a serpentinizing region for 9 months, and found that intense swarms of gas bubbles were discharged into the well in several discrete episodes. The nature of these episodes indicates that free gas accumulates in the formation and then rapidly migrates through fractures when the gas pressure is high enough. This is the first time this type of gas pressure and migration cycle has been recognized during serpentinization. An important consequence is that gas is expelled out of the peridotite during its alteration, maintaining the weathering reactions far from equilibrium, a process which may be added in existing kinetics models of serpentinization. Key Points: This paper presents hydrophone array data from a borehole in an actively serpentinizing terrain contain acoustic signals from bubble swarms discharging into the holeFree gas accumulates in the formation and migrates rapidly in discrete swarm episodes when a pressure threshold is reachedResults reveal a gas migration mechanism associated with peridotite alteration, a process which enhances the kinetics of peridotite weathering [ABSTRACT FROM AUTHOR]

Carbonated serpentinites (listvenites) in the Samail Ophiolite, Oman, record mineralization of 1–2 Gt of CO2, but the mechanisms providing permeability for continued reactive fluid flow are unclear. Based on samples of the Oman Drilling Project, here we show that listvenites with a penetrative foliation have abundant microstructures indicating that the carbonation reaction occurred during deformation. Folded magnesite veins mark the onset of carbonation, followed by deformation during carbonate growth. Undeformed magnesite and quartz overgrowths indicate that deformation stopped before the reaction was completed. We propose deformation by dilatant granular flow and dissolution-precipitation assisted the reaction, while deformation in turn was localized in the weak reacting mass. Lithostatic pore pressures promoted this process, creating dilatant porosity for CO2 transport and solid volume increase. This feedback mechanism may be common in serpentinite-bearing fault zones and the mantle wedge overlying subduction zones, allowing massive carbonation of mantle rocks. Mantle rocks can efficiently bind carbon by reaction with CO2 if fluid pathways remain open. This study of samples from Oman demonstrates that coupling of synchronous reaction and deformation facilitates fluid flow and massive carbon sequestration. [ABSTRACT FROM AUTHOR]

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic "stratigraphy" of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO2‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front. Plain Language Summary: This paper reports initial results from study of core from Oman Drilling Project Hole BT1B and the surrounding area. It provides insights into subduction zone processes, including large fluxes of recycled CO2 from subducting sediments into the leading edge of the mantle wedge, and surprisingly low temperature ductile deformation at less than 200°C. Recycling of CO2 via carbon mineralization in the hanging wall of subduction zones may produce an important, lithospheric reservoir in the global carbon cycle. Ductile deformation of serpentinite, and during or after transformation of peridotite to listvenites (mixtures of carbonates and opal or quartz) could explain aseismic subduction atop some subduction zones. Key Points: The area at and around OmanDP Hole BT1B preserves nearly flat‐lying tectonic "stratigraphy" of the basal thrust of the Samail ophioliteRecycling of CO2 from subducting sediment into the mantle wedge produced listvenites (carbonated peridotite) at <200°CSubduction beneath the ophiolite was accommodated, in part, by ductile deformation of serpentinite and listvenite at <200°C [ABSTRACT FROM AUTHOR]

Completely carbonated peridotites represent a window to study reactions of carbon‐rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87Sr/86Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87Sr/86Sr values of up to 0.7241. δ13C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ13C as low as −27‰. Based on these results, we propose that during subduction at temperatures above >400°C, carbon‐rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87Sr/86Sr signature and the fractionated δ13C values of the serpentinites and listvenites in core BT1B. Plain Language Summary: Samples from Oman Drilling Project Hole BT1B provide a record of interactions of fluids rich in carbon dioxide with mantle rocks. These interactions lead to the formation of listvenites, rocks composed mainly of magnesite and quartz. Here, we describe the formation of listvenites in the Oman ophiolite using Strontium and Carbon isotopes to characterize the source and nature of the fluid that pervasively transform the mantle rocks that now store vast amounts of carbon dioxide. Key Points: Strontium and Carbon were added to the peridotites during alterationStrontium isotopes point to Hawasina calcite‐bearing clastic sediments C as source of fluidsAlteration fluids are derived from decarbonation reactions of subducted sediments [ABSTRACT FROM AUTHOR]

We investigate the cooling histories of peridotites and gabbros from localities that expose oceanic lithosphere formed beneath two fast seafloor spreading centers: Hess Deep as recovered from IODP Expedition 345 and ODP Leg 147, and the Oman Ophiolite as sampled by the Oman Drilling Project, ICDP Expedition 5057 (OmanDP). At these locations, relict crust‐mantle transition zones are directly sampled, enabling characterization of the thermal history of the crust‐mantle transition, and by inference, the depth extent of hydrothermal circulation beneath spreading centers. We measured major and trace element abundances in crustal gabbros and mantle peridotites from Hess Deep and OmanDP, and applied major and trace element‐based thermometers. Geospeedometric interpretation of the temperatures suggests similar cooling histories at both locations; cooling rates ranged from 0.02 to 2.6 °C/y from peak temperatures up to 1,350°C. The rates are consistent on either side of the paleo‐Moho (i.e., in the crust and mantle). Models for conductive cooling of the lower oceanic crust predict rates more than two orders of magnitude slower at the crust‐mantle transition zone, while thermal models that invoke deep and efficient hydrothermal circulation throughout the entire crustal section predict rates consistent with our observations. We infer that hydrothermal cooling extended to or near the petrologic Moho beneath the East Pacific Rise and the OmanDP paleo‐spreading center, consistent with the Sheeted Sills model for crustal accretion. Comparison with previously published rates recalculated using the methods we employed suggests the oceanic lower crust is cooled hydrothermally in some places and by conduction at others. Plain Language Summary: Most new crust on Earth is formed at mid‐ocean ridges, deep below the surface of the oceans. Though technological advances have enabled drilling to deeper depths than ever before, reaching the Earth's mantle remains elusive. With the help of faulting and tectonics, some relict sections of the rocks formed at the base of the crust and the top of the mantle have become more accessible. We examined the chemistry of rocks from two locations in order to determine how they cooled and how they may have formed at depth. According to the cooling rates determined from the rocks at both locations, we see evidence for deep circulation of cold seawater, and crustal accumulation by a network of small magma storage chambers (sills) at depth, rather than one large, hot, mass of magma‐mineral mush in the lower crust, as had been historically invoked in models of mid‐ocean ridges. Key Points: Speedometers applied to gabbros and peridotites from Hess Deep and OmanDP recover cooling rates of 0.02–2.6 °C/y across the paleo‐MohoThe new results are consistent with rapid, hydrothermal cooling of the lithosphere to the Moho from peak or near peak temperaturesPublished rates were recalculated for direct comparison, suggesting different cooling mechanisms operate in different places [ABSTRACT FROM AUTHOR]

Patients with relapsed/refractory chronic lymphocytic leukemia (R/R CLL) whose treatment failed with a Bruton's tyrosine kinase inhibitor have poor outcomes. We investigated tafasitamab plus idelalisib (cohort A) or venetoclax (cohort B) in this patient population in a phase II study (NCT02639910). In total, 24 patients were enrolled (cohort A: n = 11, median time on study, 7.4 months; cohort B: n = 13, median time on study, 15.6 months). The most common treatment-emergent adverse event (TEAE) in cohort A was anemia (63.6%) and in cohort B was infusion-related reaction (53.8%). The most common severe TEAE was neutropenia (cohort A: 45.5%; cohort B: 46.2%). The best overall response rate was 90.9% (cohort A) and 76.9% (cohort B). Undetectable minimal residual disease in peripheral blood was achieved in 2/8 patients (cohort A) and 6/7 patients (cohort B). Overall, these results suggest that anti-CD19 antibody-based combinations may be important in the treatment of patients with CLL. [ABSTRACT FROM AUTHOR]

The Oman Drilling Project "Multi‐Borehole Observatory" (MBO) samples an area of active weathering of tectonically exposed peridotite. This article reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites, and replacive. Dunites show evidence for "reactive fractionation," in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65%–100% hydrated. Ferric to total iron ratios vary from 50% to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30–150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14, and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO2 deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H2O‐H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores versus oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth. Plain Language Summary: The Oman Drilling Project developed a "Multi‐Borehole Observatory" (MBO) in an area of active weathering of tectonically exposed peridotite, to study the geochemistry, mechanics, and hydrology of peridotite alteration, which modifies the mineralogy, composition, density, and rheology of mantle lithologies, creates and sustains plate boundaries, and forms dramatic redox gradients. In turn, these redox conditions support a unique subsurface microbial ecosystem, form free H2 gas, facilitate methane generation, and potentially play a role in the origin of life on this and other planets. This paper provides an overview of the geology of the area within and surrounding the MBO, a summary of recent research on core and fluids from three new cored boreholes and four new rotary boreholes in the MBO, together with older Omani water monitoring well NSHQ14, and new data and calculations constraining ongoing peridotite alteration in this area. We constrain the igneous history of the mantle rocks, the extent to which they have been hydrated, carbonated, and oxidized. Highlights include new hypotheses on mechanisms of shallow sulfur depletion and deeper enrichment, and new insights into the interaction of water and minerals that controls the partial pressures of oxygen and hydrogen in this important geochemical environment. Key Points: Ongoing alteration decreases in intensity with increasing depth below the present erosional surfaceSulfur is accumulating in a supergene enrichment zone from 30 to 150 m below the present erosional surfaceDeep oxygen fugacity is fixed by H2 gas saturation = plus ongoing oxidation of Fe2+ in three holes and serpentine‐brucite equilibria in four holes [ABSTRACT FROM AUTHOR]

We quantified mineral proportions in listvenite (completely carbonated peridotite) in Hole BT1B drilled across the basal thrust of the Samail ophiolite by the International Continental Scientific Drilling Project Oman Drilling Project using 3D X‐ray Computed Tomography (XCT). We analyzed >250,000 XCT images from a continuous ∼200 m listvenite core. Histograms of the intensity of X‐ray attenuation of each XCT core‐slice image were fitted assuming that the listvenites are composed of magnesite, quartz, and dolomite. The XCT mineral peaks were confirmed by comparison with chemical mapping data obtained using an X‐ray fluorescence (XRF) core scanner. Listvenite matrix is composed almost entirely of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting the basal thrust acted as a source of Ca‐ and CO2‐rich fluids. The SiO2:MgO:CaO molar ratio in the entire core from BT1B is 42:52:6, similar to that of onboard XRF data for discrete samples (41:54:5), whereas average Oman peridotites have ratios of (39:60:1), indicating Ca addition perhaps during carbonation. P‐ and S‐wave velocities and density of listvenite are close to those of peridotite and are higher than those of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O. Listvenites formed in the mantle wedge above subduction zones may be an overlooked reservoir for carbon in the Earth's interior. Plain Language Summary: To understand planetary‐scale fluid circulation from the surface to deep regions of Earth, it is important to constrain how fluids are captured/released by hydration/dehydration and carbonation/decarbonation of minerals and the associated volumes of these fluids. Listvenite, completely carbonated mantle material, occurs in the Samail ophiolite. Continuous ∼200 m drill core samples of listvenite overlying the basal thrust of the Samail ophiolite were obtained by the International Continental Scientific Drilling Project Oman Drilling Project. We evaluated the mineral fractions of listvenite from 3D X‐ray Computed Tomography (XCT) images of these core samples with confirmation using chemical mapping data from an X‐ray fluorescence core scanner. More than 125,000 XCT images were analyzed using this method. We found that dolomite (CaMg(CO3)2) abundance is highest within 15 m of the base of listvenite layer, which is the paleo‐plate boundary. These results suggest that the mantle carbonation involved CO2‐, H2O‐, and Ca‐rich fluids and that the paleo‐plate boundary acted as a pathway for these fluids. Because listvenite contains 36 mol% CO2 and has seismic properties different from serpentinites and similar to volatile‐free peridotites, listvenite in the mantle wedge overlying subduction zone may be an overlooked reservoir for carbon in the Earth's interior. Key Points: A method was developed to determine the downhole mineral fractions of drill cores from continuous X‐ray Computed Tomography (XCT) imagesQuartz‐magnesite‐dolomite fractions were estimated from XCT images of listvenite cores from International Continental Scientific Drilling Project Oman Drilling Project Hole BT1BResults indicate limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O [ABSTRACT FROM AUTHOR]

Although ocean crust covers over 60% of Earth's surface, the processes that form, cool, and alter the ocean crust are not completely understood. We utilize shortwave infrared micro‐imaging spectroscopy of ∼1.2 km of rock cored by the International Continental Scientific Drilling Program's Oman Drilling Project to quantify hydration of basaltic dikes and gabbros from the Samail ophiolite as a function of depth, mineralogy and deformation. We develop a regression (R2 = 0.66) between area of the ∼1,350–1,650 nm OH/H2O absorption and measurements of loss on ignition of samples and apply this relationship to generate quantitative ∼250 μm/pixel hydration maps for all cores. The lowest mean hydration is observed in the most pervasively altered dike‐gabbro boundary (GT3A, H2Omean = 2.1 ± 1.6 wt%), consistent with the low H2O content of the dominant alteration minerals, amphibole and epidote. The highest H2O content occurs in deeper foliated and layered gabbros (GT2A, H2Omean = 3.2 ± 3.0 wt%) and layered gabbros (GT1A, H2Omean = 2.8 ± 3.1 wt%). The greater prevalence with depth of zeolite alteration as opposed to lower wt% H2O amphibole at shallow stratigraphic depths, as well as the occurrence of zones of intensive hydration associated with fault zones (H2Omean = 5.7 ± 4.0 wt%) lead to greater hydration of the lower ocean crust. This new approach provides an objective quantification of hydration in these cores, enabling an improved understanding of quantities and characteristics of ocean crust hydration. It highlights the importance of specific phases and faulting in controlling hydration, which has implications for ocean crust cooling, rheological properties, and the role of alteration in global biogeochemical cycling. Plain Language Summary: Ocean crust makes up much of Earth's crust. When it forms and cools, it reacts with seawater and other fluids to trap water in rock. The goal of this study was to determine how much water is stored within the ocean crust. We measured rock cores drilled from an ophiolite, a place where ocean crust has been thrust onto land. We used an infrared imaging spectrometer, commonly used on spacecraft to survey other planets, to measure reflected light as a function of wavelength to make maps of the amount of water in the core to determine what controls where the water is held in different minerals. We developed a method to determine the amount of water in the rocks using wavelength‐dependent patterns in absorptions in reflected light. Most of the ocean crust contains 1.5%–3.2% water by weight. Surprisingly, the mid ocean crust sampled by our cores has the least amount of water, whereas the lower segments have more water. This is due to the presence in the cores of specific minerals called zeolites that form at low temperatures and the high extent of hydration in the rocks surrounding deep fault zones. Key Points: An infrared spectral index using the area of OH/H2O absorptions at ∼1,400 nm spatially maps and quantifies H2O in rocks with high accuracyThe hydration ranges from 0 to 18 wt.% H2O with mean and medians for sheeted dike and gabbroic cores ranging from 1.5 to 3.2 wt.% H2ODeeper gabbroic oceanic crust is more hydrated due to relative prevalence of zeolite minerals and fault zones that facilitate hydration [ABSTRACT FROM AUTHOR]

Visible‐shortwave infrared (VSWIR) imaging spectrometers map composition remotely with spatial context, typically at many meters‐scale from orbital and airborne data. Here, we evaluate VSWIR imaging spectroscopy capabilities at centimeters to sub‐millimeter scale at the Samail Ophiolite, Oman, where mafic and ultramafic lithologies and their alteration products, including serpentine and carbonates, are exposed in a semi‐arid environment, analogous to similar mineral associations observed from Mars orbit that will be explored by the Mars‐2020 rover. At outcrop and hand specimen scales, VSWIR spectroscopy (a) identifies cross‐cutting veins of calcite, dolomite, magnesite, serpentine, and chlorite that record pathways and time‐order of multiple alteration events of changing fluid composition; (b) detects small‐scale, partially altered remnant pyroxenes and localized epidote and prehnite that indicate protolith composition and temperatures and pressures of multiple generations of faulting and alteration, respectively; and (c) discriminates between spectrally similar carbonate and serpentine phases and carbonate solid solutions. In natural magnesite veins, minor amounts of ferrous iron can appear similar to olivine's strong 1‐μm absorption, though no olivine is present. We also find that mineral identification for carbonate and serpentine in mixtures with each other is strongly scale‐ and texture‐dependent; ∼40 area% dolomite in mm‐scale veins at one serpentinite outcrop and ∼18 area% serpentine in a calcite‐rich travertine outcrop are not discriminated until spatial scales of <∼1–2 cm/pixel. We found biological materials, for example bacterial mats versus vascular plants, are differentiated using wavelengths <1 μm while shortwave infrared wavelengths >1 μm are required to identify most organic materials and distinguish most mineral phases. Plain Language Summary: Imaging spectroscopy from airplanes or satellites helps us to map different rock types on Earth and other planets, using reflected visible to shortwave infrared light. With this technique, each pixel in an image has a corresponding spectrum in hundreds of wavelengths with absorptions that are fingerprints, diagnostic of specific material compositions. Here, we demonstrate how imaging spectroscopy can be used at smaller scales, in the field, in the lab, and on future rover and lander missions. We took an imaging spectrometer to the Samail Ophiolite (Oman), which has many of the same water‐formed minerals and rock types as detected from orbit in some locales on Mars. We determined that imaging spectroscopy can differentiate between minerals that look similar from distance and provide the spatial context of mineral detections to understand the time order of events affecting a rock. Using an imaging spectrometer to find small pieces of unaltered rock left intact after fluids have transformed most of the rock reveals the pressure, temperature, and chemical environment during rock formation and later alteration. Rare minerals and unaltered relicts can then be sampled for intensive study. Plants and bacteria can also be mapped. Such capabilities are useful for environmental investigations on Earth and other planets. Key Points: Meso‐to micro‐scale imaging spectroscopy identifies and maps carbonates, serpentine, and other hydrothermal phases in the Oman ophiolitePigmentation from multiple types of biological materials including mats in travertine springs are distinguished even in shallow watersDetection thresholds of carbonate and serpentine when mixed can be >20area%, which has implications for interpretation of planetary data [ABSTRACT FROM AUTHOR]

The Oman Drilling Project established an "Active Alteration" multi‐borehole observatory in peridotites undergoing low‐temperature serpentinization in the Samail Ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to −750 mV) and pH (pH range 8.5–11.2) gradients within the 300–400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 101 to 2.9 × 107 cells/g. Low levels of biological sulfate reduction (2–1,000 fmol/cm3/day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH < 10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite‐vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh‐cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late‐stage carbonate veins are suggested as targets for future spatially resolved life‐detection investigations. The high‐quality whole‐round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by low‐temperature water/rock interaction, and how biosignatures of in situ microbial activity are generated. Plain Language Summary: Ultramafic rocks undergoing water/rock interaction, and storing fluids that are far from chemical equilibrium, may be one of the most common habitats in our solar system. Through the Oman Drilling Project we collected >1 km of intact serpentinite in contact with groundwaters. These cores capture parts of the rock‐hosted biosphere and show how cells are distributed within serpentinites that vary in their mineralogical, physical and chemical properties. The cores are also biologically active, enabling us to detect specific metabolisms, such as when microorganisms combine hydrogen as reductant and sulfate as an oxidant to fuel their metabolism. Although the distribution of microbial cells in the rock cores is very heterogeneous, there are many intervals where the abundance of cells constitutes robust biomass. In the deeper cores, slow, albeit detectable, microbial sulfate reduction proceeds. We suggest that this pervasive biological activity releases byproducts such as sulfide that can react with the serpentinite and change the optical and chemical properties of the rocks. The feedbacks between the rock alteration and microbial activity produce markers that enable us to focus our search for rock‐hosted life and any specific biosignatures it may produce on Earth and perhaps on other planetary bodies. Key Points: Highly serpentinized subsurface rocks exhibit steep redox gradients and host microbial cell abundances that vary >6 orders of magnitudeLow rates of microbial sulfate reduction in rock cores are inferred to result in optical darkening and sulfide overprinting of the mineralogyWidespread andradite garnet, abundant ferroan brucite, and rare carbonate are targets for future spatially resolved life‐detection efforts [ABSTRACT FROM AUTHOR]

In hyperalkaline (pH>10) fluids that have participated in low‐temperature (<150°C) serpentinization reactions, the dominant form of C is often methane (CH4), but the origin of this CH4 is uncertain. To assess CH4 origin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid‐hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (CH313D and CH212D2) relative abundances less than equilibrium, consistent with substantial microbial CH4 production. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations and δ13CCH4 across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source of CH4 is evidenced by the presence of CH4‐bearing fluid inclusions in the Samail Ophiolite and our measurement of high δ13C values of ethane and propane, which are similar to those reported in studies of CH4‐rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption of CH4 in the ophiolite may further enrich CH4 in 13C. We conclude that substantial microbial CH4 is produced under varying degrees of C limitation and mixes with abiotic CH4 released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions. Plain Language Summary: Mantle rocks from beneath Earth's crust can be thrust to the surface, where they are exposed to rain and air containing carbon dioxide (CO2). The groundwaters that become stored in these rocks often contain methane (CH4, a major component of "natural gas"), which can be formed from carbon dioxide in the subsurface. To investigate these methane‐forming processes, we sampled water, gas, and suspended particles from groundwaters using wells previously drilled into the rocks. The particles contained microbes with the genetic ability to produce methane. We also precisely measured the amounts of combinations of C and H atoms of different masses (isotopes) in the natural gas to determine how it was formed. The results of these measurements suggest that microbes could actively produce a considerable amount of the methane, which mixes with methane from another source that was formed by non‐biological processes, possibly long ago under different conditions than today's. Rocks like those studied here are widespread in the Solar System, so our finding that microbes live and produce methane in these rocks could help guide the search for life beyond Earth. Key Points: 16S rRNA gene sequences affiliated with methanogens and CH4 clumped isotopologue compositions suggest substantial microbial CH4 productionA second CH4 source, release of CH4 from fluid inclusions, is indicated by 13C‐enriched ethane and propaneC availability may influence the apparent C isotope effect of microbial methanogenesis [ABSTRACT FROM AUTHOR]

The formation and preservation of cratons—the oldest parts of the continents, comprising over 60 per cent of the continental landmass—remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses—the cratons.Cratons are the oldest parts of the Earth’s continents; this Review concludes that the production of widespread, thick and strong lithosphere via the process of orogenic thickening was fundamental to the eventual emergence of extensive continental landmasses. [ABSTRACT FROM AUTHOR]

Processes for formation, cooling, and altering Earth's ocean crust are not yet completely understood due to challenges in access and sampling. Here, we use contiguous micro‐imaging infrared spectroscopy to develop complete‐core maps of mineral occurrence and investigate spatial patterns in the hydrothermal alteration of 1.2 km of oceanic crust recovered from Oman Drilling Project Holes GT1A, GT2A, and GT3A drilled in the Samail Ophiolite, Oman. The imaging spectrometer shortwave infrared sensor measured reflectance of light at wavelengths 1.0–2.6 μm at 250–260 μm/pixel, resulting in >1 billion independent measurements. We map distributions of nine key primary and secondary minerals/mineral groups—clinopyroxene, amphibole, calcite, chlorite, epidote, gypsum, kaolinite/montmorillonite, prehnite, and zeolite—and find differences in their spatial occurrences and pervasiveness. Accuracy of spectral mapping of occurrence is 68%–100%, established using X‐ray diffraction measurements from the core description. The sheeted dikes and gabbros of upper oceanic crust Hole GT3A show more pervasive alteration and alteration dominated by chlorite, amphibole, and epidote. The foliated/layered gabbros of GT2A from intermediate crustal depths have similarly widespread chlorite but more zeolite and little amphibole and epidote. The layered gabbros of the lower oceanic crust (GT1A) have remnant pyroxene and 2X less chlorite, but alteration is extensive within and surrounding major fault zones with widespread occurrences of amphibole. The results indicate greater distribution of higher temperature alteration minerals in the upper oceanic crust relative to deeper gabbros and highlight the importance of fault zones in hydrothermal convection in the lower ocean crust. Plain Language Summary: The oceanic crust, the rock from the ocean floor to the mantle, forms much of Earth's solid crust, yet it is difficult to access, drill into, or collect samples from. This kilometers‐thick crust forms from cooling of molten rock, but we do not entirely understand how it forms, cools, and changes by chemical reactions with water. Ophiolites are places where rock from the ocean crust and uppermost portion of the mantle have been pushed upward and exposed on continents. One such location is in Oman, where the Oman Drilling Project drilled into continuous sections of ocean crust. We measured this drill core with imaging spectroscopy, a technique where we measure how infrared light at hundreds of wavelengths reflects off the rock. We use the characteristic infrared fingerprints of minerals to map them at sub‐millimeter scale, producing over one billion measurements. We find that much of the rocks closer to the ocean reacted with water at high temperatures to form new minerals. Some rocks deep within the ocean crust also interacted with large volumes of water, but intense fluid flow was concentrated in fractures and smaller areas, leaving less reacted rock. Key Points: Imaging spectroscopy efficiently and effectively mapped spatial patterns in hydrothermal alteration mineral occurrence in ocean crust coreSamail ophiolite upper ocean crust cores are dominated by chlorite, amphibole, and epidote, while deeper cores have more zeolite/prehniteHydrothermal alteration largely decreases with depth in the ocean crust but is locally intense in major fault zones, even in lower crust [ABSTRACT FROM AUTHOR]

Serpentinized peridotite is reacting with groundwater in the subsurface of the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they interact with a groundwater aquifer containing hyperalkaline fluids rich in H2 and CH4. Since the mechanisms by which H2 production may continue at low temperatures (<50°C) are not fully understood, core recovered during the Oman Drilling Project provides an excellent opportunity to study the mineralogy and Fe speciation in highly serpentinized harzburgite recording multiple stages of water/rock interaction. In Hole BA3A, early hydration of olivine and pyroxene, which likely occurred at temperatures of ∼100°C–200°C, produced serpentine, Fe‐rich brucite, awaruite, and little magnetite. Notably, Fe‐rich brucite is only preserved at >∼100 m depth in the core. Fe‐rich brucite is nearly absent within two near‐surface reaction zones where later stages of reaction are recorded, which include replacement of Fe‐rich brucite by Fe(III)‐bearing serpentine, and increases in the proportion of other Fe(III)‐bearing phases such as magnetite and hydroandradite. Thus, Fe‐rich brucite at depth represents substantial stored capacity for H2 production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life. Plain Language Summary: When ultramafic rocks from Earth's mantle come into contact with water, they undergo a set of hydration reactions leading to the formation of secondary minerals such as serpentine and brucite. Such "serpentinization reactions" also often involve the oxidation of iron, which can result in the production of hydrogen gas. Many models predict abundant hydrogen production at high temperatures >250°C, but the potential reactions that may produce hydrogen are more enigmatic at lower temperatures. Using rock cores drilled from the Samail ophiolite in Oman, we show that in highly serpentinized rocks in contact with hyperalkaline, low‐temperature, hydrogen‐rich fluids, multiple stages of reactions can be observed. By analyzing changes in mineralogy with depth, it is possible to identify reaction fronts where rocks that initially formed abundant Fe(II)‐bearing minerals such as brucite are now interacting with modern groundwaters at low‐temperature to form Fe(III)‐bearing minerals including magnetite, hydroandradite, and Fe(III)‐bearing serpentine. These reactions are likely responsible for the production of hydrogen gas that is able to support a rock‐hosted microbial community, and show that habitable conditions can be produced even during late, non‐hydrothermal stages of ultramafic rock alteration. Key Points: Subsurface serpentinites from the Samail ophiolite contain distinct reaction fronts with implications for H2 production and habitabilityWhere Fe(II)‐rich brucite is present, it has the potential to fuel future H2 productionFe(II,III)‐rich serpentine, hydroandradite, and magnetite all record past H2 production at low temperatures <200°C [ABSTRACT FROM AUTHOR]

Understanding the tectonic setting in which ophiolites form is necessary to determine how they can be used to study ocean spreading and subduction zone processes. Here, we present high‐precision U‐Pb zircon dates and Sm‐Nd isotopic data from two late magmatic series in the Samail ophiolite in Oman and the United Arab Emirates (UAE), which constrain its tectonic development. Volcanic rocks in the ophiolite record a progression from MORB‐like V1 lavas to subduction‐related V2 lavas. Plutonic rocks related to V2 magmatism yielded 206Pb/238U dates of 95.557 ± 0.063–95.289 ± 0.067 Ma. A second late magmatic series consists of felsic dikes and sills that intermittently intrude the upper mantle and are attributed to melting of a subducting slab. These dikes in Oman range from 95.201 ± 0.032 to 94.95 ± 0.10 Ma, with εNd(t) = −5.05 to −1.63. A single dike, attributed to V2 magmatism, has a higher εNd(t) = 7.35 and a date of 95.478 ± 0.032 Ma. Similar intrusions in the UAE are younger, ranging from 94.119 ± 0.057 to 90.998 ± 0.052 Ma. Our new and existing data indicate the following timeline of ophiolite formation during subduction initiation: (1) Initial sole metamorphism at ≥96.2 Ma; (2) Formation of the crust through primarily decompression‐related V1 magmatism from 96.1 to 95.6 Ma; (3) V2 magmatism related to H2O‐fluxed mantle melting from 95.6 to 95.2 Ma; and (4) Intrusion of slab‐derived felsic dikes from 95.2 to 95.0 Ma. The temporal progression of magmatism is similar to the timescales of subduction initiation predicted by geodynamic models and observed in the Izu‐Bonin‐Mariana forearc. Key Points: High‐precision U‐Pb zircon dating constrains the timing of late magmatism within the Samail ophioliteThe temporal and chemical evolution of the ophiolite is consistent with formation during subduction initiationThe ophiolite chronology is similar to timescales of subduction initiation in geodynamic models and the Izu‐Bonin‐Mariana forearc [ABSTRACT FROM AUTHOR]

Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg‐rich carbonates. Magnesium accounts for ∼25 – 30% of peridotite mass and its mobility can be used to trace this alteration. We report the first set of Mg isotope measurements from peridotites and their alteration products in Oman. Partially serpentinized peridotites have Mg isotope ratios that are indistinguishable from estimates for the average mantle and bulk silicate earth (δ26Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of δ26Mg values for our suite of alteration products from the mantle section is ∼4.5‰ (from −3.39‰ to 1.19‰), or >60% of the total range of terrestrial variability in δ26Mg values. Serpentine veins are typically enriched in 26Mg (max δ26Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low 26Mg/24Mg ratios (magnesite δ26Mg = −3.3‰, dolomite δ26Mg = −1.91‰). Our preferred explanation for the range in δ26Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >103. The coincidence of alteration products characterized by δ26Mg values that are both lower and higher than bulk silicate Earth and the finite 14C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ26Mgcal‐fl suggest that travertine formation in Oman sequesters a total of 106–107 kg CO2/yr, consistent with previous estimates. Key Points: The range of δ26Mg from samples of the mantle section in the Oman ophiolite is ∼4.5‰, or >60% of the total range of terrestrial variabilityThe range in δ26Mg values involves coprecipitation of serpentine and carbonates at high water‐to‐rock ratiosSerpentinization and carbonation are ongoing in the mantle section of the Oman ophiolite [ABSTRACT FROM AUTHOR]

This paper explores the relationship between individual mindfulness and organizational mindfulness. Many authors consider these concepts to be related and interlocked, however, the exact nature of this connection is still unclear. Therefore, we investigate the link between these two concepts using both literature review approach and theoretical reflection. The results of the literature review are classified and synthesized in a constructive way into a theoretical framework that sheds light on the main aspects of the relationship between individual and collective mindfulness. Our research shows that the existence of organizational mindfulness implies the existence of mindfulness on the individual level. Individual mindfulness is a necessary, but not sufficient condition to developing organizational mindfulness. Additional factors other than individual mindfulness must be included in order to increase organizational mindfulness. The reason for this is the difference between individual mindfulness, an intrapsychic process of individuals, and organizational mindfulness, which is a function of social procedures in an organization. [ABSTRACT FROM AUTHOR]

Hole BT1B of the Oman Drilling Project provides a continuous sampling from listvenite into the metamorphic sole that preserves the deformation, hydration, and carbonation processes of oceanic mantle peridotite at the base of the Samail ophiolite, Oman. We present evidence of multistage brittle deformation in listvenites and serpentinites based on field observations, visual core logging and petrography. About 10 vol% of listvenite and serpentinite in Hole BT1B is composed of cataclasite bands. Cataclasites contain lithic clasts of listvenite with spheroidal, zoned magnesite and quartz, and fragments of chalcedony‐carbonate veins that elsewhere crosscut listvenite—showing that cataclasis postdates listvenite formation. Locally the cataclasites are reworked and cut by thin, sharp faults, pointing to repeated reactivation of brittle structures. SEM‐EDS mapping shows that cataclasis was related to dissolution of carbonate and/or silica cementation. Dolomite veins crosscut cataclasites and breccias, suggesting that part of the Ca gain in BT1B is related to late fluids after listvenite formation. These results indicate a multistage tectonic overprint after peridotite carbonation and listvenite formation, which may be related to the tectonic history of the deformed continental margin under the ophiolite. These relatively late brittle structures should be excluded when trying to understand the carbonation of peridotite to listvenite. Key Points: Abundant cataclasites overprint previously formed listvenite in the Oman Drilling Hole BT1BCataclasis was related to fluid flow, which caused Mg loss and/or Si enrichment, and local redistribution of CrThe multistage tectonic overprint after peridotite carbonation in the Oman ophiolite should be excluded when analyzing early structures [ABSTRACT FROM AUTHOR]

Permeability profiles in the crust‐mantle sequences of the Samail ophiolite were constructed based on onboard measurements of the electrical resistivity of cores recovered during the Oman Drilling Project. For each sample, we measured dry and brine‐saturated resistivity during the description campaign on the drilling vessel Chikyu. Owing to the conductive brine in the pore space, wet resistivity is systematically lower than dry resistivity. The difference between dry and wet resistivity is attributed to the movement of dissolved ions in brine that occupies the pore space. We applied effective medium theory to calculate the volume fraction of pores that contribute to electrical transport. Using an empirical cubic law between transport porosity and permeability, we constructed permeability profiles for the crust‐mantle transition zone and the serpentinized mantle sections in the Samail ophiolite. The results indicate that (1) the gabbro sequence has a markedly lower permeability than the underlying mantle sequence; (2) serpentinized dunites have higher permeability than serpentinized harzburgites; and (3) discrete sample permeability is correlated with ultrasonic velocity, suggesting that the permeability variations predominately reflect crack density and geometry. Plain Language Summary: Aqueous fluids that circulated beneath the seafloor play an important role in heat transfer, chemical exchange, and microbial activity in the oceanic lithosphere. The Oman Drilling Project was successful in obtaining continuous drill cores through the crust‐mantle sequences in the Samail ophiolite, where the paleo‐oceanic basement was thrust onto the continental crust in the Late Cretaceous. In this study, we constructed profiles of permeability across the crust‐mantle sections using the effective medium theory and resistivity data, which provide insights into fluid circulation in the oceanic lithosphere. The results indicate higher permeability in the uppermost mantle sequence between the crust and the underlying mantle, suggesting that present‐day fluid transfer is predominant at the crust‐mantle boundary. Although the fluid flow and chemical reactions are likely coupled, the high permeability could promote the transformation of crust‐mantle materials and hence the recycling of water into the mantle. Application of this technique to forthcoming deep drilling projects through the Mohorovičić discontinuity (Moho) and into the upper mantle may provide insights into the permeability structure and fluid circulation system in the oceanic lithosphere. Key Points: Brine‐saturated resistivity is systematically lower than dry resistivity, with the differences attributed to the volume fraction of poresPermeability through the crust‐mantle sequences was modeled using the effective medium theory and resistivity dataThe dunite sequence is characterized by higher permeability than the overlying gabbro and underlying harzburgite sequences [ABSTRACT FROM AUTHOR]

To avoid dangerous climate change, new technologies must remove billions of tonnes of CO2 from the atmosphere every year by mid-century. Here we detail a land-based enhanced weathering cycle utilizing magnesite (MgCO3) feedstock to repeatedly capture CO2 from the atmosphere. In this process, MgCO3 is calcined, producing caustic magnesia (MgO) and high-purity CO2. This MgO is spread over land to carbonate for a year by reacting with atmospheric CO2. The carbonate minerals are then recollected and re-calcined. The reproduced MgO is spread over land to carbonate again. We show this process could cost approximately $46–159 tCO2−1 net removed from the atmosphere, considering grid and solar electricity without post-processing costs. This technology may achieve lower costs than projections for more extensively engineered Direct Air Capture methods. It has the scalable potential to remove at least 2–3 GtCO2 year−1, and may make a meaningful contribution to mitigating climate change. To remove CO2 from the atmosphere every year by mid-century will need new technologies. Here the authors proposed the use of magnesia (MgO) in ambient looping processes to remove CO2 from the air and they found that the proposed approach will cost $46–195 tCO2−1 net removed from the atmosphere considering both grid and solar electricity resources without including post-processing costs. [ABSTRACT FROM AUTHOR]

We report a reanalysis of active‐source wide‐angle seismic data collected along the Greenland‐Iceland Ridge during the 1996 Seismic Investigation of the Greenland Margin (SIGMA) experiment. Interpreting the crustal structure of volcanic rifted margins has suffered from nonuniqueness because thick crust at the continent‐ocean transition may not be totally of igneous origin. In this regard, the Greenland‐Iceland Ridge presents a unique opportunity because, together with the Faroe‐Iceland Ridge, it is generally considered to constitute the Iceland hotspot track, and as such, the bulk of its crust may be considered to be of igneous origin. From 15 ocean‐bottom instruments deployed along a 290‐km‐long refraction transect, we collect 5,383 Pg and 1,118 PmP travel times, and we use joint refraction and reflection seismic tomography with adaptive importance sampling to invert them to construct a two‐dimensional compressional wave speed (Vp) model across the Greenland‐Iceland Ridge. Based on the covariation of crustal thickness and Vp, the western part of the transect may be almost entirely continental. Even the eastern part could include a significant fraction of preexisting continental crust. When considered together with the seismic structure of the Icelandic crust and its geochemical characteristics, our results suggest that the putative Iceland mantle plume could be considerably weaker than commonly assumed. Key Points: A compressional‐wave speed model was built for the Greenland‐Iceland Ridge crustThe ridge may contain a substantial amount of preexisting continental crustThe putative Iceland mantle plume can be considerably weaker than commonly thought [ABSTRACT FROM AUTHOR]

Serpentinization plays a key role on the evolution of the physicochemical properties of the mantle lithosphere. The rate of serpentinization reactions idiscontinuities in the platelet separating ?1‐?m‐long segments of clinopyroxene. The presence of traces of magnetite in the discontinuities and their orientation in parallel to other magnetite grains suggest that the discontinuities were formerly filled with magnetite, which reacted during serpentinizations controlled by the transport of fluid, which itself depends on volume change during reaction. Element transfer can strongly modify the magnitude and sign of volume change. Here, we measure solid volume change and element transport perpendicular to a serpentine vein in a serpentinized dunite collected at depth during the Oman Drilling Project. The sample is extensively replaced (extent of reaction > 80 %) by a serpentine/brucite mixture parallel to a main serpentine vein network. The Mg content of serpentine and brucite indicates reaction with a small amount of fluid at temperatures below 100 °C. Concentrations of fluid‐mobile trace elements (Na, Ca, Sr, Rb, and Ba) decrease perpendicular to the main vein. Primary olivine contains parallel platelets of a clinopyroxene/magnetite symplectite. Tomography at the nanoscale reveals that these inclusions do not react during serpentinization but are cracked and displaced. We use these inert markers to measure a 59 % to 74 % positive volume change that is close to the 52 % expected for reaction in a closed system. Chemical data indicate no change in major element composition during reaction except for the addition of water. The initial olivine zoning in Al, Ti, V, Sc, and Cr is still preserved in serpentine and brucite. Serpentinization can thus be a local replacement process during which the solid volume homogeneously increases at the micrometer scale and the transport of aqueous species is limited. Key Points: A dunite collected during the Oman Drilling Project records serpentinization at low temperatureThe solid volume increases by more than 50 % during reaction as predicted for a closed systemThe primary olivine trace element zoning is preserved during serpentinization [ABSTRACT FROM AUTHOR]

Mantle peridotite in Wadi Fins in eastern Oman exhibits three concentric alteration zones with oxide and sulfide mineralogy recording gradients in fO2 and fS2 (fugacity) of more than 20 orders of magnitude over 15-20 cm. The black cores of samples (approx. 5 cm in diameter) exhibit incomplete, nearly isochemical serpentinization, with relict primary mantle minerals (olivine, orthopyroxene and clinopyroxene) along with sulfide assemblages (pentlandite/heazlewoodite/bornite) recording low fO2 and moderate fS2. In addition to the black cores, two alteration zones are evident from their colouration in outcrop and hand samples: green and red. These zones exhibit non-isochemical alteration characterized by intergrowths of stevensite/lizardite. All three reaction zones are cut by calcite±serpentine veins, which are most abundant in the outer, red zones, sometimes are flanked by narrow red and/or green zones where they cut the black zones, and thus may be approximately coeval with all three alteration zones. The alteration zones record progressively higher fO2 recorded by Ni-rich sulfides and iron oxides/hydroxides. These alteration zones lost 20- 30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron is reprecipitated in goethite intermixed with silicates due to higher fO2. Thermodynamic modelling at 60°C and 50MPa (estimated alteration conditions) reproduces sulfide assemblages, fO2 changes and Mg and Fe mobility. [ABSTRACT FROM AUTHOR]

De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average = 13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site's carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites. [ABSTRACT FROM AUTHOR]

The interactions between reactive fluids and solids are critical in Earth dynamics. Implications of such processes are wide ranging: from earthquake physics to geologic carbon sequestration and the cycling of fluids and volatiles through subduction zones. Peridotite alteration is a common feature in many of these processes, which—despite its obvious importance—is relatively poorly understood from a geodynamical perspective. In particular, although frequently observed in nature, it is still unknown how rocks can undergo 100% hydration/carbonation. One potential explanation of this observation is the mechanism of reaction‐driven cracking: that volume changes associated with these reactions are large enough to fracture the surrounding rock, leading to a positive feedback where new reactive surfaces are exposed and fluid pathways are created. The purpose of this study is to investigate the relative roles of reaction, elastic stresses, and surface tension in alteration reactions. In this regard, we derive a system of equations describing reactive fluid flow in elastically deformable porous media, which we apply to a model of serpentinization in a subseafloor environment. The stoichiometry of the serpentinization reaction predicts net volume reduction of the multiphase system, which, according to our numerical simulations, can lead to failure in tension. We also explore a parameterization of surface energy in the model, which allows fluid to infiltrate regions of dry peridotite, significantly changing the stresses and potentially leading to growing crack fronts. Key Points: We use poroelasticity and chemical mass balance to derive a model of fluid flow, elastic deformation, and mass transfer in porous mediaWe derive a parametrization of surface energy effects and demonstrate its ability to drive fluid flow into low-permeability rockUsing a model of serpentinization in a subseafloor environment we predict that reaction-driven volume changes can lead to tensile failure [ABSTRACT FROM AUTHOR]

Abstract: Deformation caused by reaction‐driven volume increases is an important process in many geological settings. The interaction of rocks with reactive fluids can change permeability and reactive surface area, leading to a large variety of feedbacks. Gypsum (CaSO4·2H2O) is an ideal material to study these processes. It forms rapidly at room temperature via bassanite (CaSO4·[1/2]H2O) hydration and is commonly used as an analog for rocks in high‐temperature, high‐pressure conditions. We conducted uniaxial deformation experiments on porous bassanite aggregates to study the effects of applied axial load σa on deformation during the formation of gypsum. While hydration of bassanite involves a solid volume increase, gypsum exhibits significant creep compaction when in contact with water. These two processes occur simultaneously. Samples exhibited an initial phase of deformation followed by a slower secondary phase. A particular value of σa separates expansion from compaction during each of the deformation phases. At σa≤150 kPa, samples expanded initially; for σa≥230 kPa, samples compacted initially. Up to σa≈3.2 MPa, samples expanded after compacting initially, while for σa≥3.6 MPa, no further deformation or continued compaction occurred. This behavior implies that crystallization‐induced stresses depend on porosity and reaction extent such that larger stresses cannot be generated by the reaction. We explain aspects of the observed behavior with a model that predicts strain evolution using kinetic relationships for the reaction and creep rates and consider the implications of our results for reaction‐induced fracturing during serpentinization. [ABSTRACT FROM AUTHOR]

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite ("gabbro," "alkaline peridotite," "hyperalkaline peridotite," and "gabbro/peridotite contact") that vary strongly in pH and the concentrations of H2, CH4, Ca2+, Mg2+, NO3-, SO42-, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation, as well as the oxidation of methane, hydrogen and small molecular weight organic acids utilizing nitrate and sulfate as electron acceptors. Dynamic nitrogen cycling may be especially prevalent in gabbro and alkaline peridotite fluids. These data suggest water-rock reaction, as controlled by lithology and hydrogeology, constrains the distribution of life in terrestrial ophiolites. [ABSTRACT FROM AUTHOR]

Tabular dunite bodies are thought to represent remnants of high-porosity pathways for efficient melt extraction from the mantle. They form by melt-rock reaction, an important physical process that affects the compositions of dunite-hosted basaltic melts and the mantle they originate from. To better understand melt-rock interactions in dunite channels, we analyzed clinopyroxene and orthopyroxene in samples collected across an ~20 m wide dunite-harzburgite-lherzolite-plagioclase lherzolite sequence in the previously well-studied Trinity ophiolite. We found spatial variation and fractionation in minor and trace elements in the constituent minerals. Rare earth element (REE) and high field strength element concentrations increase in unison about 9 m from the dunite-harzburgite contact. Minor elements in clinopyroxene also increase ~9 m from the dunite-harzburgite contact, and NiO contents in olivine increase ~3 m from the dunite-harzburgite contact. Clinopyroxene grains in plagioclase lherzolite samples farthest from the duniteharzburgite contact exhibit core-to-rim variations in minor and trace elements that mimic the outcrop-scale chemical trends. Collectively, the lithological sequence and major and trace element concentration gradients suggest that a two-stage history of evolution is preserved at Trinity. In the first stage, a cooling melt infiltrated a harzburgitic residue of partial melting, precipitating plagioclase and pyroxene and forming plagioclase lherzolite. In the second stage, a trace element depleted, pyroxeneand plagioclase-undersaturated melt migrated from the dunite channel into the plagioclase lherzolite, forming a hybridized composition by reaction with the plagioclase lherzolite. Because Ni is relatively fast diffusing and compatible in olivine, it was chromatographically fractionated from other trace elements during the infiltration event. Orthopyroxene-saturated melt precipitated new clinopyroxene with depleted major and trace element compositions as it cooled in the dunite, harzburgite, and lherzolite. The REE abundances of the melts in equilibrium with dunite, harzburgite, and lherzolite are similar to those of boninitic dikes that cut crustal units at Trinity, and the infiltrating melts may be genetically related to the dikes. Dunite-harzburgite-lherzolite-plagioclase lherzolite sequences from Trinity and other peridotites probably formed by similar processes. The infiltration of dunite-hosted melts into peridotitic host-rock may be common, providing an explanation for the wide array of melt-peridotite interactions observed in abyssal peridotites and some ophiolites. This outcrop demonstrates that dunite channels can be sources of melt infiltration as well as melt extraction pathways. [ABSTRACT FROM AUTHOR]

Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5-10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. [ABSTRACT FROM AUTHOR]

We review data and recent research on arc composition, focusing on the relatively complete arc crustal sections in the Jurassic Talkeetna arc (south central Alaska) and the Cretaceous Kohistan arc (northwest Pakistan), together with seismic data on the lower crust and uppermost mantle. Whereas primitive arc lavas are dominantly basaltic, the Kohistan crust is clearly andesitic and the Talkeetna crust could be andesitic. The andesitic compositions of the two arc sections are within the range of estimates for the major element composition of continental crust. Calculated seismic sections for Kohistan and Talkeetna provide a close match for the thicker parts of the active Izu arc, suggesting that it, too, could have an andesitic bulk composition. Because andesitic crust is buoyant with respect to the underlying mantle, much of this material represents a net addition to continental crust. Production of bulk crust from a parental melt in equilibrium with mantle olivine or pyroxene requires processing of igneous crust, probably via density instabilities. Delamination of dense cumulates from the base of arc crust, foundering into less dense, underlying mantle peridotite, is likely, as supported by geochemical evidence from Talkeetna and Kohistan. Relamination of buoyant, subducting material-during sediment subduction, subduction erosion, arc-arc collision, and continental collision-is also likely. [ABSTRACT FROM AUTHOR]

The composition of much of Earth's lower continental crust is enigmatic. Wavespeeds require that 10-20% of the lower third is mafic, but the available heat-flow and wavespeed constraints can be satisfied if lower continental crust elsewhere contains anywhere from 49 to 62 wt% SiO2. Thus, contrary to common belief, the lower crust in many regions could be relatively felsic, with SiO2 contents similar to andesites and dacites. Most lower crust is less dense than the underlying mantle, but mafic lowermost crust could be unstable and likely delaminates beneath rifts and arcs. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogites and may continue to descend into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than continental upper crust. These more felsic rocks may rise buoyantly, undergo decompression melting and melt extraction, and be relaminated to the base of the crust. As a result of this refining and differentiation process, such relatively felsic rocks could form much of Earth's lower crust. [ABSTRACT FROM AUTHOR]

Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago-coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone-a process probably more common in the Archaean-can produce juvenile continental crust. [ABSTRACT FROM AUTHOR]

Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50-70% SiO2. Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y (<15 ppm) or Yb (<1·5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (>1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50-350, La/Yb = 8-35, Dy/Yb = 2·0-3·5) with low relative abundances of Nb and Ta (La/Ta > 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with εNd = 9·1 ± 0·3 (n = 31) and εHf = 14·5 ± 0·6 (n = 27). Lead isotopes are variable and decrease with increasing SiO2 from basalts with 206Pb/204Pb = 18·51 ± 0·05 (n = 11) to dacites and rhyodacites with 206Pb/204Pb = 18·43 ± 0·04 (n = 18). Western seafloor lavas form a steep trend in 207Pb/204Pb-206Pb/204Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206Pb/204Pb > 18·6 and 207Pb/204Pb > 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36-50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc. [ABSTRACT FROM AUTHOR]

The increasing concentrations of CO2 in the atmosphere are attributed to the rising consumption of fossil fuels for energy generation around the world. One of the most stable and environmentally benign methods of reducing atmospheric CO2 is by storing it as thermodynamically stable carbonate minerals. Olivine ((Mg,Fe)2SiO4) is an abundant mineral that reacts with CO2 to form Mg-carbonate. The carbonation of olivine can be enhanced by injecting solutions containing CO2 at high partial pressure into olivine-rich formations at high temperatures, or by performing ex situ mineral carbonation in a reactor system with temperature and pressure control. In this study, the effects of NaHCO3 and NaCl, whose roles in enhanced mineral carbonation have been debated, were investigated in detail along with the effects of temperature, CO2 partial pressure and reaction time for determining the extent of olivine carbonation and its associated chemical and morphological changes. At high temperature and high CO2 pressure conditions, more than 70% olivine carbonation was achieved in 3 hours in the presence of 0.64 M NaHCO3. In contrast, NaCl did not significantly affect olivine carbonation. As olivine was dissolved and carbonated, its pore volume, surface area and particle size were significantly changed and these changes influenced subsequent reactivity of olivine. Thus, for both long-term simulation of olivine carbonation in geologic formations and the ex situ reactor design, the morphological changes of olivine during its reaction with CO2 should be carefully considered in order to accurately estimate the CO2 storage capacity and understand the mechanisms for CO2 trapping by olivine. [ABSTRACT FROM AUTHOR]

This workshop report describes plans for scientific drilling in the Samail ophiolite in Oman in the context of past, current, and future research. Long-standing plans to study formation and evolution of the Samail crust and upper mantle, involving igneous and metamorphic processes at an oceanic spreading center, have been augmented by recent interest in ongoing, low temperature processes. These include alteration and weathering, and the associated sub-surface biosphere supported by chemical potential energy due to disequilibrium between mantle peridotite and water near the surface. This interest is motivated in part by the possibility of geological carbon capture and storage via engineered, accelerated mineral carbonation in Oman. [ABSTRACT FROM AUTHOR]

Information about a workshop on scientific drilling held in Palisades, New York on September 13-17, 2012 on the plans for scientific drilling in the Samail ophiolite in Oman is presented. Topics include the characterizing of the rates and mechanisms of ongoing mineral hydration and carbonation and understanding the subsurface biosphere. In the workshop participants shared their views on planned drilling.

Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites. This assemblage is unstable with respect to serpentine + talc or talc + quartz under most conditions. Serpentine in the carbonated serpentinites in this study is more iron rich than in most serpentinites reported in previous studies, and samples with co-existing quartz contain the most iron-rich serpentines. Calculations of thermodynamic equilibria in the MgO-SiO-HO-CO system suggest that serpentine + quartz may be a stable assemblage at low temperatures (e.g., <~15-50 °C) and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Based on these calculations, serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater for all samples analyzed, while the δO of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater for the majority of samples analyzed. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment. [ABSTRACT FROM AUTHOR]

Near-surface reaction of CO2-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO2 storage, and perhaps for direct capture of CO2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes (''at the outcrop'') rather than ex situ processes (''at the smokestack''). [ABSTRACT FROM AUTHOR]

Information about several topics discussed at a workshop held September 9-11, 2010 sponsored by the Alfred P. Sloan Foundation on the use of technological innovations for deep sea drilling is presented. Topics include the use of drilling for studying the mantle dynamics, technological capabilities of drilling, and studying Earth's interior using self-sinking capsules. The workshop featured several scientists and engineers including Robert Hazen, Shuichi Kodaira and Greg Myers.

Information on the Reaching the Mantle Frontier: Moho and Beyond workshop held at the Carnegie Institution of Washington in Washington D.C. on September 9-11, 2010 is presented. Topics include the carbon cycle of the Earth's surface, Apollo lunar rock sampling project, and seismic profiles of oceanic Moho. The workshop featured several speakers including Robert Hazen, Donald Beattie, and Shuichi Kodaira.

As one of two well-exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu-Hf and Sm-Nd garnet ages, 40Ar/39Ar hornblende, mica and whole-rock ages, and U-Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U-Pb ages establish the main period of arc magmatism as 202-181 Ma in the Chugach Mountains and 183-153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu-Hf and ∼182 Ma Sm-Nd garnet ages indicate that 25-35 km deep sections of the arc remained above ∼700°C for as much as 15 Myr. The 40Ar/39Ar hornblende ages are chiefly 194-170 Ma in the Chugach Mountains and 175-150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U-Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from ∼180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are ∼137 to 129 Ma, indicating no Cenozoic regional heating above ∼180°C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One-dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories. [ABSTRACT FROM AUTHOR]

Evidence from abyssal peridotites suggests that significant chemical reaction with peridotite can occur during the early stages of cooling and crystallization of mid-ocean ridge basalt (MORB) magmas. We evaluate the hypothesis that reactive crystallization (crystallization influenced by such melt-rock reaction) could cause magma compositions to evolve along a different chemical trajectory than expected for fractional crystallization, and that reactive crystallization might be common in MORB petrogenesis. If correct, this hypothesis implies that a component of major element variability in fractionation-corrected MORB, commonly interpreted solely in terms of mantle source composition and potential temperature, could reflect reactive crystallization. The compositional evolution of MORB magmas undergoing reactive crystallization is predicted using thermodynamic calculations. We find that the decreasing melt MgO content during reactive crystallization is accompanied by nearly constant Mg-number [defined as molar MgO/ (MgO+FeO)], whereas melt SiO2 and Na2O contents evolve to higher values than in fractional crystallization. However, the extent of crystallization as a function of temperature is essentially identical during the initial ∼30-40% of both fractional and reactive crystallization. Comparison of melt transport and reaction timescales in a 1-D, steady-state, porous flow column shows that melt migration via grain-scale porous flow at the transition from melting to crystallization beneath ocean ridges will most probably give rise to reactive crystallization, whereas melt transport through the thermal boundary layer in larger conduits (dikes) will lead to fractional crystallization. Scatter in fractionation-corrected major element compositions could therefore reflect sample-to-sample variations in melt transport dynamics. Using a global compilation of MORB glass compositions, we show that 40-70% of the variability in fractionation-corrected MgO contents observed worldwide is also typically present in groups of samples collected from within 30 km of each other. Such short length-scale variability in MgO, and hence in the temperature of primitive magmas, cannot be due to variability in mantle potential temperature. There is a negative correlation in the variability of fractionation-corrected MgO (and most other compositional variables) with spreading rate. We infer that this negative correlation reflects a greater role for reactive crystallization in the thicker thermal boundary layers present beneath slow-spreading ridges. We demonstrate the ability of combined reactive and fractional crystallization to account for major element variability at several case-study locations, and argue that reactive crystallization can explain many observations of 30 km scale variability. Interpreted in terms of reactive crystallization, fractionation-corrected MgO variability could potentially bound geodynamic parameters such as the depth of onset of diking and the fraction of gabbro emplaced into residual mantle peridotite beneath the igneous crust. [ABSTRACT FROM AUTHOR]