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In hyperalkaline ((Formula presented.)) fluids that have participated in low-temperature (

High Mg# andesites and dacites (Mg# = molar Mg/Mg + Fe) from western Aleutian seafloor volcanoes carry high concentrations of Sr (>1000 ppm) that is unradiogenic (87Sr/86Sr < 0.7029) compared to lavas from emergent volcanoes throughout the arc (200–800 ppm Sr, 87Sr/86Sr >0.7030). Data patterns in plots of 87Sr/86Sr vs Y/Sr and Nd/Sr imply the existence of an eclogite-melt source component – formed by partial melting of MORB eclogite in the subducting Pacific Plate – which is most clearly expressed in the compositions of western Aleutian andesites and dacites (Nd/Sr and Y/Sr < 0.02) and which dominates the source budget for Sr in volcanic rocks throughout the arc. When viewed in combination with inversely correlated εNd and 87Sr/86Sr, these patterns rule out aqueous fluids as an important source of Sr because mixtures of fluids from altered oceanic crust with depleted mantle and sediment produce compositions with 87Sr/86Sr higher than in common Aleutian rocks. The unradiogenic nature of Sr in the western Aleutian andesite–dacite end-member may be understood if H2O required to drive melting of the subducting oceanic crust is transported in fluids containing little Sr. Mass balance demonstrates that such fluids may be produced by dewatering of serpentinite in the mantle section of the subducting plate. If the eclogite-melt source component is present throughout the Aleutian arc, melting of the subducting plate must extend into minimally altered parts of the sheeted dike section or upper gabbros, at depths >2 km below the paleo-seafloor. Oxygen isotopes in western Aleutian seafloor lavas, which fall within a narrow range of MORB-like values (δ18O=5.1–5.7), are also consistent with this model. These results indicate that the subducting Pacific lithosphere beneath the Aleutian arc is significantly hotter than indicated my most thermal models.

The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH > 10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition.The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2-10 μm magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H2. Dissolved CH4 in the fluids exhibits the most isotopically heavy carbon in CH4 reported in the literature thus far. The CH4 may have formed through abiotic reduction of dissolved CO2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the Oman ophiolite.

The origin of the more oxidized nature of arc magmas as compared to that of mid-ocean ridge basalts (MORB) is debated, considered to be either a feature of their mantle source, or produced during crustal transit and eruption. Fe.sup.3+/Fe.sup.T ratios (Fe.sup.3+/[Fe.sup.3+ + Fe.sup.2+]) in arc volcanic rocks and glasses and thermodynamic oxybarometry on mantle xenoliths from arc lavas indicate elevated magmatic oxygen fugacity ( [Formula omitted]), whereas, redox-sensitive trace elements ratios and abundances in arc volcanic rocks have been used to suggest that arcs have source regions with [Formula omitted] similar to the MORB source. Here, we take an alternative approach by calculating the [Formula omitted] of the uppermost mantle and lowermost ultramafic cumulates from the accreted Jurassic Talkeetna arc (Alaska). This approach allows us to quantify the [Formula omitted] of the sub-arc mantle and of primary arc magmas crystallizing at the base of an island arc, which have not been affected by processes during crustal transit and eruption which could affect their [Formula omitted]. Implementing olivine-spinel oxybarometry, we find that the upper mantle (harzburgites and lherzolites) and ultramafic cumulates (clinopyroxenites and dunites) crystallized between + 0.4 and + 2.3 log units above the fayalite-magnetite-quartz buffer, consistent with previous studies suggesting that the sub-arc mantle is oxidized relative to that of MORB. In addition, the Talkeetna paleo-arc allows us to examine coeval lavas and their redox-sensitive trace element ratios (e.g., V/Sc). The average V/Sc ratios of high MgO (> 6 wt%) lavas are 6.7 ± 1.6 (2[sigma]), similar to that of MORB. However, V/Sc ratios must be interpretted in terms the degree of partial melting, as well as, the initial V/Sc ratio of the mantle source in order to derive information about [Formula omitted] of their mantle source. The V/Sc ratios of Talkeetna lavas are consistent with the elevated [Formula omitted] recorded in the sub-arc mantle and primitive cumulates (olivine Mg# [Mg/(Mg + Fe)] x 100 > 82) if a depleted mantle source underwent 15-20% melting. Our results suggest that the arc mantle is, on average, more oxidized than the MORB source and that V/Sc ratios must be interpreted in the context of a partial melting model where all model parameters are appropriate for arc magma genesis. This study reconciles V/Sc ratios in arc volcanic rocks with [Formula omitted] of primary arc basalts and the sub-arc mantle from the same locality., Author(s): Claire E. Bucholz [sup.1] , Peter B. Kelemen [sup.2] Author Affiliations: (Aff1) 0000000107068890, grid.20861.3d, Division of Geological and Planetary Sciences, California Institute of Technology, , 1200 E California Boulevard, [...]

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-Si[O.sub.2]-[H.sub.2]O-C[O.sub.2] 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 δ[sup.18]O 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. Keywords Serpentinization * Carbonation * Alteration * Peridotite * Oman * Ophiolite * Clumped isotopes, Introduction Mantle peridotite is far from thermodynamic equilibrium with the atmosphere and surface waters. Peridotite exposed at Earth's surface reacts readily with aqueous fluids to form products including serpentine and [...]

We investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the Oman Ophiolite. Our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase-rich lithology typical of the lower crust and an olivine-dominated harzburgite. We constrain deformation conditions using field relationships, geothermometry, grain size piezometry, and electron backscatter diffraction. The viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite, consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws. These flow laws predict that a significant viscosity contrast exists at the crust-mantle boundary where the crustal lithology is dominated by plagioclase, or where deformation localizes within plagioclase-rich layers. doi: 10.1130/G30880.1

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intraoceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to subaerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of [sup.143]Nd/[sup.144]Nd and [sup.87]Sr/[sup.86]Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the are, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [[epsilon].sub. Nd](t) = 6.0-7.8 and [sup.87]Sr/[sup.86][Sr.sub.int] = 0.703379-0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5-168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [[[epsilon].sub.Nd] = 5.6-7.2 and [sup.87]Sr/[sup.86][Sr.sub.int]= 0.703383-0.7013624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [[[epsilon].sub.Nd](t) = 4.0-5.5 and [sup.87]Sr/[sup.86][Sr.sub.int] = 0.703656-0.706252] and contain inherited xenucrystic Carboniferous-Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust. Keywords: island arc, arc plutonism, Talkeetna, Chugach, Alaska, zircon.

Shallow (garnet-free), depleted cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has a high Mg# (100 x Mg/(Mg + Fe) > 92) and is poor in Al and Ca compared to off-cratonic mantle. Here we compile data for many suites of shallow cratonic mantle xenoliths worldwide, and demonstrate a remarkably small range in their olivine Mg#, with an average of ~92.8. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine residue. Keywords: cratonic mantle, dunite xenoliths, residues, Archean, spinel peridotite.

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 x Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low [Al.sub.2][O.sub.3] and CaO contents (0.024-1.639 and 0.062-0.275 wt%, respectively). Spinel has 100 x Cr/ (Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of [sup.187]Os/[sup.188]Os values. The most unradiogenic [sup.187]Os/[sup.188]Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.

The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma. Keywords: Alaska, terrane accretion, subduction, tectonic erosion, collision.

The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tufts, and volcaniclastic debris-flow and turbidite deposits, showing significant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals ~7 km in thickness, similar to modern western Pacific arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calc-alkaline and shows no clear trend to increasing Si[O.sub.2] up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in melt-incompatible versus compatible high field strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy. REE-depleted mafic lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a north-dipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America. Keywords: sedimentology, arc volcanism, subduction, geochemistry, isotope geology.

Most people know that oceans cover about 70 percent of Earth's surface. Fewer people realize that the crust beneath oceans and continents is fundamentally different. Why this is so remains [...]

Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable [H.sub.2]O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing [H.sub.2]O concentration in the liquid changes the crystallization sequence. Low [H.sub.2]O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher [H.sub.2]O contents (> 3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with A1 in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/[cm.sup.3]. The combination of high pressure and high [H.sub.2]O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., Si[O.sub.2], [Al.sub.2][O.sub.3]) of igneous crust in arcs.

As the oceanic plates move apart at mid-ocean ridges, rocks from Earth's mantle, far below, rise to fill the void, mostly via slow plastic flow. As the rock approaches the [...]

The continental crust contains Mg/(Mg+Fe) and Ni in andesites which did not originate from differentiation of basaltic magmas. The High Mg# andesite (HMA) in the crust might have formed when peridotite was depressed to mantle, melted there and metasomatised. It might have also formed by reaction of ascending melts and peridotite. HMA is not found in island arcs at present but may be common in the past, or perhaps abundant in the lower and middle crust, being formed in the specific environment.

Seismic profiles of several modern arcs have identified thick, low-velocity midcrustal layers ([V.sub.p] = 6.06.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U-Pb zircon dates, radiogenic isotope data, and whole-rock geochemical analyses that provide the first high-precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion--thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 [+ or -] 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from [[epsilong].sub.Nd](t) = 5.2 to 9.0 and [sup.87]Sr/[sup.86]Srint = 0.703515 to 0.703947 and are similar to age-corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole-rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low-velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs. Citation: Rioux, M., J. Mattinson, B. Hacker, P. Kelemen, J. Blusztajn, K. Hanghoj, and G. Gehrels (2010), Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low-velocity middle crust in modern arcs, Tectonics, 29, TC3001, doi: 10.1029/2009TC002541.

The Gasherbrum Peaks, in the Himalaya of Pakistan, reach elevations of >8000 m. The relief between the peaks and the adjacent valley (Baltoro Glacier) is in excess of 3000 m. Eight samples of the Early Cretaceous Gasherbrum Diorite at elevations between 4880 and 7165 m on Gasherbrum IV were collected for fission-track dating. Zircon fission-track ages from the Gasherbrum Diorite vary from Early Cretaceous to middle Tertiary in age. There is no consistent pattern between age and elevation. The Cretaceous ages indicate that these rocks were never deeply buried, i.e., heated to temperatures in excess of 175 [degrees] C, to reset the zircons during Cenozoic time. These results also indicate that the uplift of this part of the Himalaya has been either very rapid and recent, or very slow since Early Cretaceous time. This latter possibility is not consistent with the high relief at Gasherbrum and what is known about regional tectonics. Gasherbrum IV zircons, currently at 4880 m, have never been at depths greater than 6 km, and less than 3 km of material has been removed from the top of the range by erosion since the Early Cretaceous. Rapid uplift has occurred very recently, and erosion rates have not been able to keep pace with this uplift.

Abstract: Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M-indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models. [Copyright &y& Elsevier]