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The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest. ▪ Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine. ▪ Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep. ▪ Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions. ▪ Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition.

Child abuse and neglect in young children can lead to trauma-related stress symptoms that can be challenging to manage. Parent-Child Interaction Therapy (PCIT), a strong evidence-based behavioral parent training program used for young children with behavior issues, has been used in its traditional form with increasing frequency with children and families who have trauma histories, with clinicians tailoring PCIT to use with children who have experienced trauma. Trauma-Directed Interaction (TDI) is a new systematic adaption to the standard PCIT parent training program that has the potential to help treat trauma in younger children. TDI includes several trauma-informed techniques that are added to a course of standard PCIT treatment including psychoeducation regarding trauma, recognition of feelings, and emotional regulation. This case study illustrates the use of a manualized trauma adaptation to PCIT (TDI) with a three-year-old boy who had a history of child maltreatment and his caregiver. This case provides a summary of the progression of this intervention and the results obtained. Results from the case indicated that TDI treatment was effective in not only reducing child trauma and behavioral symptoms but also in reducing mild caregiver mental health concerns. The next steps for TDI treatment and need for further research are discussed.

Child abuse and neglect in very young children can lead to trauma-related stress symptoms that can be challenging to treat. Children exposed to multiple traumas occurring in the context of a caregiving relationship are sometimes more severely affected, evidenced by diverse negative behavioral, physical, social, and emotional consequences. Some of these children go on to develop post-traumatic stress disorder (PTSD). Parent–child interaction therapy (PCIT) is a dyadic play-based treatment for children with behavior problems and their parents or caregivers. There is limited research relating to the application of PCIT with very young children with PTSD with some studies actively excluding families where PTSD was present (Herschell et al., 2017). Additionally, there are no articles relating to treatment of a child restored to the care of a parent following out-of-home care (OOHC) and fewer still relating to a child restored to the care of her father. This case study illustrates the use of PCIT with a 3-year-old girl with PTSD, recently restored to her father’s care. It highlights how the use of PCIT in this case not only improved behavioral indicators of traumatic stress, but also afforded this child permanency and stability. The case study provides a summary of the progression of this intervention and the results obtained throughout treatment and 4 years post-intervention. The results indicated that PCIT, with trauma-informed tailoring, was an effective treatment in this case and contributed to safe and permanent care for this child.

There is remarkable variation in the rate at which genetic incompatibilities in molecular interactions accumulate. In some cases, minor changes—even single-nucleotide substitutions—create major incompatibilities when hybridization forces new variants to function in a novel genetic background from an isolated population. In other cases, genes or even entire functional pathways can be horizontally transferred between anciently divergent evolutionary lineages that span the tree of life with little evidence of incompatibilities. In this review, we explore whether there are general principles that can explain why certain genes are prone to incompatibilities while others maintain interchangeability. We summarize evidence pointing to four genetic features that may contribute to greater resistance to functional replacement: (1) function in multisubunit enzyme complexes and protein–protein interactions, (2) sensitivity to changes in gene dosage, (3) rapid rate of sequence evolution, and (4) overall importance to cell viability, which creates sensitivity to small perturbations in molecular function. We discuss the relative levels of support for these different hypotheses and lay out future directions that may help explain the striking contrasts in patterns of incompatibility and interchangeability throughout the history of molecular evolution.

Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures

Hydrogen is a rapidly diffusing monovalent cation in nominally anhydrous minerals (NAMs, such as olivine, orthopyroxene, and clinopyroxene), which is potentially re-equilibrated during silicate melt-rock and aqueous fluid-rock interactions in massif and abyssal peridotites. We apply a 3D numerical diffusion modeling technique to provide first-order timescales of complete hydrogen re-equilibration in olivine, clinopyroxene, and orthopyroxene over the temperature range 600–1200 °C. Model crystals are 1–3 mm along the c-axis and utilize H+ diffusion coefficients appropriate for Fe-bearing systems. Two sets of models were run with different boundary compositions: (1) “low-H models” are constrained by mineral-melt equilibrium partitioning with a basaltic melt that has 0.75 wt% H2O, and (2) “high-H models,” which utilize the upper end of the estimated range of mantle water solubility for each phase. Both sets of models yield re-equilibration timescales that are identical and are fast for all phases at a given temperature. These timescales have strong log-linear trends as a function of temperature (R2 from 0.97 to 0.99) that can be used to calculate the expected re-equilibration time at a given temperature and grain size. At the high end of the model temperatures (1000–1200 °C), H+ completely re-equilibrates in olivine, orthopyroxene, and clinopyroxene within minutes to hours, consistent with previous studies. These short timescales indicate that xenolith NAM mantle water contents are likely to be overprinted prior to eruption. The models also resolve the decoupled water-trace element relationship in Southwest Indian Ridge peridotites, in which peridotite REE abundances are reproduced by partial melting models whereas the relatively high NAM H2O contents require later re-equilibration with melt.At temperatures of 600–800 °C, which correspond to conditions of hydrothermal alteration of pyroxene to amphibole and talc, H+ re-equilibration typically occurs over a range of timescales spanning days to years. These durations are well within existing estimates for the duration of fluid flow in oceanic hydrothermal systems, suggesting that peridotite NAM water contents are susceptible to diffusive overprinting during higher temperature hydrothermal alteration. Thus, diffusion during aqueous fluid-rock interactions may also explain NAM H2O contents that are too high to reflect residues of melting. These relatively short timescales at low temperatures suggest that the origin of water contents measured in peridotite NAMs requires additional constraints on sample petrogenesis, including petro-graphic and trace element analyses. Our 3D model results also hint that H+ may diffuse appreciably during peridotite serpentinization, but diffusion coefficients at low temperature are unconstrained and additional experimental investigations are needed.

Summary coalescent methods offer an alternative to the concatenation (supermatrix) approach for inferring phylogenetic relationships from genome-scale datasets. Given huge datasets, broad congruence between contrasting phylogenomic paradigms is often obtained, but empirical studies commonly show some well supported conflicts between concatenation and coalescence results and also between species trees estimated from alternative coalescent methods. Partitioned support indices can help arbitrate these discrepancies by pinpointing outlier loci that are unjustifiably influential at conflicting nodes. Partitioned coalescence support (PCS) recently was developed for summary coalescent methods, such as ASTRAL and MP-EST, that use the summed fits of individual gene trees to estimate the species tree. However, PCS cannot be implemented when distance-based coalescent methods (e.g., STAR, NJst, ASTRID, STEAC) are applied. Here, this deficiency is addressed by automating computation of ‘partitioned coalescent branch length’ (PCBL), a novel index that uses iterative removal of individual gene trees to assess the impact of each gene on every clade in a distance-based coalescent tree. Reanalyses of five phylogenomic datasets show that PCBL for STAR and NJst trees helps quantify the overall stability/instability of clades and clarifies disagreements with results from optimality-based coalescent analyses. PCBL scores reveal severe ‘missing taxa’, ‘apical nesting’, ‘misrooting’, and ‘basal dragdown’ biases. Contrived examples demonstrate the gross overweighting of outlier gene trees that drives these biases. Because of interrelated biases revealed by PCBL scores, caution should be exercised when using STAR and NJst, in particular when many taxa are analyzed, missing data are non-randomly distributed, and widespread gene-tree reconstruction error is suspected. Similar biases in the optimality-based coalescent method MP-EST indicate that congruence among species trees estimated via STAR, NJst, and MP-EST should not be interpreted as independent corroboration for phylogenetic relationships. Such agreements among methods instead might be due to the common defects of all three summary coalescent methods.

Differences in tRNA expression have been implicated in a remarkable number of biological processes. There is growing evidence that tRNA genes can play dramatically different roles depending on both expression and post-transcriptional modification, yet sequencing tRNAs to measure abundance and detect modifications remains challenging. Their secondary structure and extensive post-transcriptional modifications interfere with RNA-seq library preparation methods and have limited the utility of high-throughput sequencing technologies. Here, we combine two modifications to standard RNA-seq methods by treating with the demethylating enzyme AlkB and ligating with tRNA-specific adapters in order to sequence tRNAs from four species of flowering plants, a group that has been shown to have some of the most extensive rates of post-transcriptional tRNA modifications. This protocol has the advantage of detecting full-length tRNAs and sequence variants that can be used to infer many post-transcriptional modifications. We used the resulting data to produce a modification index of almost all unique reference tRNAs inArabidopsis thaliana, which exhibited many anciently conserved similarities with humans but also positions that appear to be “hot spots” for modifications in angiosperm tRNAs. We also found evidence based on northern blot analysis and droplet digital PCR that, even after demethylation treatment, tRNA-seq can produce highly biased estimates of absolute expression levels most likely due to biased reverse transcription. Nevertheless, the generation of full-length tRNA sequences with modification data is still promising for assessing differences in relative tRNA expression across treatments, tissues or subcellular fractions and help elucidate the functional roles of tRNA modifications.

Transfer RNAs (tRNAs) remain one of the very few classes of genes still encoded in the mitochondrial genome. These key components of the protein translation system must interact with a large enzymatic network of nuclear-encoded gene products to maintain mitochondrial function. Plants have an evolutionarily dynamic mitochondrial tRNA population, including ongoing tRNA gene loss and replacement by both horizontal gene transfer from diverse sources and import of nuclear-expressed tRNAs from the cytosol. Thus, plant mitochondria represent an excellent model for understanding how anciently divergent genes can act as "interchangeable parts" during the evolution of complex molecular systems. In particular, understanding the integration of the mitochondrial translation system with elements of the corresponding machinery used in cytosolic protein synthesis is a key area for eukaryotic cellular evolution. Here, we review the increasingly detailed phylogenetic data about the evolutionary history of mitochondrial tRNA gene loss, transfer, and functional replacement that has created extreme variation in mitochondrial tRNA populations across plant species. We describe emerging tRNA-seq methods with promise for refining our understanding of the expression and subcellular localization of tRNAs. Finally, we summarize current evidence and identify open questions related to coevolutionary changes in nuclear-encoded enzymes that have accompanied turnover in mitochondrial tRNA populations.

The developmental consequences of childhood trauma for young children are extensive and impact a diverse range of areas. Young children require treatments that consider their developmental stage and are inclusive of caregiver involvement. Parent-Child Interaction Therapy (PCIT), with its dyadic focus and developmental sensitivity, is uniquely positioned to offer therapeutic support to young children and their families.The current study aimed to conduct a systematic review of the current literature on PCIT and trauma and determine treatment outcomes for children and caregivers.A systematic review of five electronic databases was undertaken. Studies that utilized PCIT to treat a population who had experienced trauma were included in the review regardless of study design.PCIT was used to treat a population who had experienced trauma in 40 studies. PCIT was an effective treatment in improving a variety of child and parent outcomes in this population including reduced parenting stress, child behavior problems, child trauma symptoms, parental mental health concerns, negative parenting strategies, and reducing potential risk of recidivism of abuse and neglect. These findings should be taken with caution given attrition rates and potential for bias in the study samples.Clinicians should consider PCIT as a potential treatment for children who have experienced trauma and their families. Future research should incorporate corroborative sources of information, assessment of caregiver and child trauma symptoms, examination of permanency outcomes, and consider standardization of PCIT modifications for child trauma to determine treatment in this population of children.

The number of tRNAs encoded in plant mitochondrial genomes varies considerably. Ongoing loss of bacterial-like mitochondrial tRNA genes in many lineages necessitates the import of nuclear-encoded eukaryotic counterparts that share little sequence similarity. Because tRNAs are involved in highly specific molecular interactions, this replacement process raises questions about the identity and trafficking of enzymes necessary for the maturation and function of newly imported tRNAs. In particular, the aminoacyl-tRNA synthetases (aaRSs) that charge tRNAs are usually divided into distinct classes that specialize on either organellar (mitochondrial and plastid) or cytosolic tRNAs. Here, we investigate the evolution of aaRS subcellular localization in a plant lineage (Sileneae) that has experienced extensive and rapid mitochondrial tRNA loss. By analyzing full-length mRNA transcripts (PacBio Iso-Seq), we found the predicted retargeting of many ancestrally cytosolic aaRSs to the mitochondrion and confirmed these results with colocalization microscopy assays. However, we also found cases where aaRS localization does not appear to change despite functional tRNA replacement, suggesting evolution of novel interactions and charging relationships. Therefore, the history of repeated tRNA replacement inSileneaemitochondria reveals that differing constraints on tRNA/aaRS interactions may determine which of these alternative coevolutionary paths is used to maintain organellar translation in plant cells.

In this essay, we explore a central problem of structural geology today, and in the foreseeable future, which is the determination of constitutive laws governing rock deformation to produce geologic structures. Although laboratory experiments provide much needed data and insights about constitutive laws, these experiments cannot cover the range of conditions and compositions relevant to the formation of geologic structures. We advocate that structural geologists address this limitation by interpreting natural experiments, documented with field and microstructural data, using continuum mechanical models that enable the deduction of constitutive laws. To put this procedure into a historical context, we review the founding of structural geology by James Hutton in the late 18th century, and the seminal contributions to continuum mechanics from Newton to Cauchy that provide the tools to model geologic structures. The procedure is illustrated with two examples drawn from recent and on-going field investigations of crustal and mantle lithologies. We conclude by pointing to future research opportunities that will engage structural geologists in the pursuit of constitutive laws during the 21st century.

The function and evolution of eukaryotic cells depend upon direct molecular interactions between gene products encoded in nuclear and cytoplasmic genomes. Understanding how these cytonuclear interactions drive molecular evolution and generate genetic incompatibilities between isolated populations and species is of central importance to eukaryotic biology. Plants are an outstanding system to investigate such effects because of their two different genomic compartments present in the cytoplasm (mitochondria and plastids) and the extensive resources detailing subcellular targeting of nuclear-encoded proteins. However, the field lacks a consistent classification scheme for mitochondrial- and plastid-targeted proteins based on their molecular interactions with cytoplasmic genomes and gene products, which hinders efforts to standardize and compare results across studies. Here, we take advantage of detailed knowledge about the model angiosperm Arabidopsis thaliana to provide a curated database of plant cytonuclear interactions at the molecular level. CyMIRA (Cytonuclear Molecular Interactions Reference for Arabidopsis) is available at http://cymira.colostate.edu/ and https://github.com/dbsloan/cymira and will serve as a resource to aid researchers in partitioning evolutionary genomic data into functional gene classes based on organelle targeting and direct molecular interaction with cytoplasmic genomes and gene products. It includes 11 categories (and 27 subcategories) of different cytonuclear complexes and types of molecular interactions, and it reports residue-level information for cytonuclear contact sites. We hope that this framework will make it easier to standardize, interpret, and compare studies testing the functional and evolutionary consequences of cytonuclear interactions.

Transfer RNAs are among the most ancient molecules of life on earth. Beyond their crucial role in protein synthesis as carriers of amino acids, they are also important players in a plethora of other biological processes. Many debates in term of biogenesis, regulation and function persist around these fascinating non-coding RNAs. Our review focuses on the first step of their biogenesis in eukaryotes, i.e. their transcription from nuclear genes. Numerous and complementary ways have emerged during evolution to regulate transfer RNA gene transcription. Here, we will summarize the different actors implicated in this process: cis-elements, trans-factors, genomic contexts, epigenetic environments and finally three-dimensional organization of nuclear genomes. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1099-1108, 2019.

Intermediate‐depth earthquakes (focal depths 70–300 km) are enigmatic with respect to their nucleation and rupture mechanism and the properties controlling their spatial distribution. Several recent studies have shown a link between intermediate‐depth earthquakes and the thermal‐petrological path of subducting slabs in relation to the stability field of hydrous minerals. Here we investigate whether the structural characteristics of incoming plates can be correlated with the intermediate‐depth seismicity rate. We quantify the structural characteristics of 17 incoming plates by estimating the maximum fault throw of bending‐related faults. Maximum fault throw exhibits a statistically significant correlation with the seismicity rate. We suggest that the correlation between fault throw and intermediate‐depth seismicity rate indicates the role of hydration of the incoming plate, with larger faults reflecting increased damage, greater fluid circulation, and thus more extensive slab hydration.

In most eukaryotes, transfer RNAs (tRNAs) are one of the very few classes of genes remaining in the mitochondrial genome, but some mitochondria have lost these vestiges of their prokaryotic ancestry. Sequencing of mitogenomes from the flowering plant genus Silene previously revealed a large range in tRNA gene content, suggesting rapid and ongoing gene loss/replacement. Here, we use this system to test longstanding hypotheses about how mitochondrial tRNA genes are replaced by importing nuclear-encoded tRNAs. We traced the evolutionary history of these gene loss events by sequencing mitochondrial genomes from key outgroups (Agrostemma githago and Silene [=Lychnis] chalcedonica). We then performed the first global sequencing of purified plant mitochondrial tRNA populations to characterize the expression of mitochondrial-encoded tRNAs and the identity of imported nuclear-encoded tRNAs. We also confirmed the utility of high-throughput sequencing methods for the detection of tRNA import by sequencing mitochondrial tRNA populations in a species (Solanum tuberosum) with known tRNA trafficking patterns. Mitochondrial tRNA sequencing in Silene revealed substantial shifts in the abundance of some nuclear-encoded tRNAs in conjunction with their recent history of mt-tRNA gene loss and surprising cases where tRNAs with anticodons still encoded in the mitochondrial genome also appeared to be imported. These data suggest that nuclear-encoded counterparts are likely replacing mitochondrial tRNAs even in systems with recent mitochondrial tRNA gene loss, and the redundant import of a nuclear-encoded tRNA may provide a mechanism for functional replacement between translation systems separated by billions of years of evolutionary divergence.

Seismic anisotropy is a key observational tool for mapping flow in Earth's upper mantle. However, interpreting patterns of anisotropy relies on a firm understanding of the microphysical mechanisms that generate anisotropy. Here we discuss our current understanding of the generation of intrinsic and extrinsic seismic anisotropy in upper-mantle rocks. Intrinsic anisotropy results from the elastic anisotropy of the constituent minerals. We address the role of thermochemical conditions in modifying the manner in which these minerals align with the deformation reference frame, controlling the macroscopic anisotropy. Extrinsic anisotropy results from the composite behavior of a material composed of multiple phases. We examine the influence of mineralogical layering and the presence of a melt phase on anisotropy, including the interaction between extrinsic and intrinsic anisotropy. Finally, we compare and contrast existing methods to forward model the development of anisotropy in the upper mantle and demonstrate that current predictions suggest most observed anisotropy results from intrinsic anisotropy when fluids are not present.

The oxygen fugacity (fO2) of the oceanic upper mantle has fundamental implications for the production of magmas and evolution of the Earth's interior and exterior. Mid-ocean ridge basalts and peridotites sample the oceanic upper mantle, and retain a record of oxygen fugacity. While fO2 has been calculated for mid-ocean ridge basalts worldwide (>200 locations), ridge peridotites have been comparatively less well studied (33 samples from 11 locations), and never in the same geographic location as basalts. In order to determine whether peridotites and basalts from mid-ocean ridges record congruent information about the fO2 of the Earth's interior, we analyzed 31 basalts and 41 peridotites from the Oblique Segment of the Southwest Indian Ridge. By measuring basalts and peridotites from the same ridge segment, we can compare samples with maximally similar petrogenetic histories. We project the composition and oxygen fugacity of each lithology back to source conditions, and evaluate the effects of factors such as subsolidus diffusion in peridotites and fractional crystallization in basalts. We find that, on average, basalts and peridotites from the Oblique Segment both reflect a source mantle very near the quartz–fayalite–magnetite (QFM) buffer. However, peridotites record a significantly wider range of values (nearly 3 orders of magnitude in fO2), with a single dredge recording a range in fO2 greater than that previously reported for mid-ocean ridge peridotites worldwide. This suggests that mantle fO2 may be heterogeneous on relatively short length scales, and that this heterogeneity may be obscured within aggregated basalt melts. We further suggest that the global peridotite fO2 dataset may not provide a representative sample of average basalt-source mantle. Our study motivates further investigation of the fO2 recorded by ridge peridotites, as peridotites record information about the fO2 of the Earth's interior that cannot be gleaned from analysis of basalts alone.

Alteration mineral assemblages in abyssal peridotites offer insights into the temperature and pressure conditions during hydrothermal fluid circulation in the oceanic lithosphere. Abundant hydrothermal venting has been documented along the ultraslow spreading Gakkel Ridge and peridotites have been extensively sampled from the ridge, yet these peridotites have not been systematically evaluated for the nature and extent of their alteration. Here, we present an analysis of the alteration of 40 Gakkel peridotites from 19 dredges spanning ~600 km of the ridge within the Sparsely Magmatic and Eastern Volcanic Zones. We use a petrographic-based alteration scoring system to assign a rank to each sample of 1 (unaltered) to 5 (altered) in 0.5 step intervals, based on the alteration scale of Birner et al. (2016). Gakkel peridotites cover the full range of alteration, from essentially unaltered to completely serpentinized, but are generally moderately to highly altered (average score of 3.5). The extent of alteration is independent of peridotite lithology and is not systematically different between the two zones. Serpentine is present in all samples, magnetite occurs in all samples with alteration scores >2.5, and 27% of samples contain carbonate veins that cross-cut all other minerals. In 20% of samples, all with alteration scores >2.5, tremolite + talc occurs as rims around pyroxene, while chlorite occurs as haloes around spinel. A gabbro-veined lherzolite also contains pargasite replacing pyroxene, with later overprinting by tremolite-talc intergrowths. The abundance of tremolite, chlorite, and talc in Gakkel samples is much lower than reported for talc-tremolite-chlorite schists from oceanic core complexes, where fluid flow through the lower crust leads to open-system behavior. At Gakkel, our petrological observations suggest that alteration was nearer to closed-system conditions, which can be explained by the absence of crust along some sections of the ridge. Pseudosection modeling with Perple_X, which assumes closed-system behavior, indicates that pyroxene reacts to form tremolite + chlorite + talc at temperatures >500 °C, corresponding to fluid flow to depths >20 km for the Gakkel Ridge. The sample with pargasitic amphibole requires temperatures >750 °C to form. Overall, our results suggest that hydrothermal alteration of Gakkel peridotites, and more generally of the lithospheric mantle in regions that lack significant crust, results in different alteration mineral abundances and relationships than in regions where the mafic crust is involved in alteration.

Mid-ocean ridge peridotites record significantly greater variability in major and trace elements, isotopic compositions, and thermodynamic properties such as oxygen fugacity (fO2) than do their basaltic counterparts. This variability may derive from modern ridge processes related to melting and melt-rock interaction or from long-lived source heterogeneity related to recycled material or ancient melting events. In this study, we investigate variations in spinel geochemistry as well as silicate major and trace element chemistry and oxygen fugacity of a suite of peridotites from a single segment of the Southwest Indian Ridge (SWIR). We present new petrographic analysis and trace element data for samples with previously-published fO2 results and combine this with new data for a suite of SWIR gabbro-veined peridotites. We find that SWIR residual lherzolites record low spinel Cr# (Cr# = 100*Cr/(Cr+Al) 30) record both melt extraction as well as melt-rock interaction. In these samples, spinel Cr# has been substantially elevated by reaction of spinel to form plagioclase during melt addition, complicating the use of spinel Cr# in mid-ocean ridge peridotites as a proxy for degree of melt extraction alone. While spinel Cr# remains a robust proxy for melt extraction within residual, non-melt-influenced samples, mid-ocean ridge peridotites must first be evaluated to ensure that modification by melt-rock reaction has not occurred. Although addition of MORB melt to a peridotite residue modifies spinel Cr#, this melt addition does not result in significant changes to the fO2 recorded by the peridotite. Residual SWIR lherzolites record fO2 of 0.66±0.39 relative to the quartz-fayalite-magnetite buffer (QFM), statistically indistinguishable from melt-influenced and veined SWIR samples (QFM+1.13±0.61). In contrast to other tectonic settings, such as subduction zones, ocean islands, and continental cratons—locations where peridotite is oxidized by petrogenetically unrelated, presumably high-fO2 melts/fluids—ridge peridotites interact with MORB, which has little to no oxidizing power over its own mantle residues. Thus, modern processes beneath the ridge modify peridotite major and trace elements, but do not generate variability in oxygen fugacity.

The elevated oxygen fugacity recorded by subduction-related lavas and peridotites, relative to their mid-ocean ridge counterparts, fundamentally influences the petrogenesis of arc magmas. However, the timing, process, and spatial extent of oxidizing mass transfer at subduction zones remain unknown. Forearc peridotites, which are sometimes exposed on the trench wall of the overriding plate, record chemical fingerprints of the melting and melt–rock interaction processes that occur during and following subduction initiation, and thus provide insight into the spatial and temporal evolution of this oxidized signature. In this study, we present new major element, trace element, and oxygen fugacity data for a suite of forearc peridotites recovered from the Tonga Trench, in addition to a new assessment of literature data for previously studied forearc peridotites. For Tonga samples and literature data for forearc, ridge, and subduction-zone peridotites, we calculate oxygen fugacity (fO2) using an updated method. In contrast to previous studies, we find that spinel Cr#, a proxy for extent of melt extraction, does not correlate with oxygen fugacity, such that many forearc peridotites with high spinel Cr# do not record oxygen fugacity higher than the mid-ocean ridge peridotite array. Combining these observations with trace element modeling, we conclude that forearc peridotites are less pervasively influenced by oxidation owing to subduction processes than previously reported. The oxygen fugacity recorded by Tonga forearc peridotites is heterogeneous between dredges and homogeneous within dredges. To explore these variations, we grouped the dredges into two categories. Group I peridotites have high spinel Cr#, extremely depleted trace element compositions and oxygen fugacity values consistent with the mid-ocean ridge peridotite array. We interpret these to be the residues of large degrees of fractional melting, with little influence from arc-like melts or fluids, formed during the first stages of subduction initiation. Group II peridotites have lower spinel Cr#, enriched light rare earth elements, and oxygen fugacity elevated by ≥1 log unit above the mid-ocean peridotite array. We interpret these peridotites to be the residues of flux melting, initiated once corner flow is established in the young subduction zone. We conclude that the forearc mantle is not pervasively oxidized relative to mid-ocean ridge mantle, and that the asthenospheric mantle in the proto-subduction zone region is not oxidized prior to subduction initiation. As the oxidized signature in Group II peridotites accompanies geochemical evidence of interaction with subduction-related fluids and melts, this suggests that the sub-arc mantle is oxidized concurrently with addition of subduction fluids to the mantle wedge.

Uncertainty in constitutive equations for brittle-ductile deformation limits our understanding of earthquake nucleation and propagation at the base of the seismogenic lithosphere. To reduce this uncertainty, we investigate exhumed strike-slip faults and related deformation features in the Lake Edison granodiorite (central Sierra Nevada, CA) that developed at 250-500°C and ~250 MPa. The Seven Gables outcrop contains a 10-cm wide contractional fault step separating two meter-scale left-lateral faults. Within the step, a ~4 cm-thick leucocratic dike is stretched and rotated, thus constraining the kinematics of deformation, and the dike and surrounding granodiorite are strongly mylonitized. Petrographic and electron backscatter diffraction analyses reveal evidence for brittle and plastic deformation mechanisms, including dislocation creep, diffusion creep, microfracturing, and cataclasis. We present a 2D finite element model of the Seven Gables outcrop that tests a series of candidate constitutive equations: Von Mises elastoplasticity, Drucker-Prager elastoplasticity, power-law creep viscoelasticity, two-layer elastoviscoplasticity, and coupled elastoviscoplasticity. Models based on Von Mises yielding most accurately match the outcrop deformation. Frictional plastic yield criteria (i.e., Drucker-Prager) are incapable of reproducing the outcrop deformation due to the elevated mean compressive stress and reduced plastic yielding within the model fault step. Furthermore, the power-law creep viscoelastic model requires a high strain rate (~10-4 s-1) to resolve slip on faults and fails to localize strain within the step region. Comparing model results and elastic stress fields with field observations suggests that deformation localizes in regions of elevated mean compressive stress and Mises equivalent stress.

Some human populations interbred with Neanderthals and Denisovans, resulting in substantial contributions to modern-human genomes. Therefore, it is now possible to use genomic data to investigate mechanisms that shaped historical gene flow between humans and our closest hominin relatives. More generally, in eukaryotes, mitonuclear interactions have been argued to play a disproportionate role in generating reproductive isolation. There is no evidence of mtDNA introgression into modern human populations, which means that all introgressed nuclear alleles from archaic hominins must function on a modern-human mitochondrial background. Therefore, mitonuclear interactions are also potentially relevant to hominin evolution. We performed a detailed accounting of mtDNA divergence among hominin lineages and used population-genomic data to test the hypothesis that mitonuclear incompatibilities have preferentially restricted the introgression of nuclear genes with mitochondrial functions. We found a small but significant underrepresentation of introgressed Neanderthal alleles at such nuclear loci. Structural analyses of mitochondrial enzyme complexes revealed that these effects are unlikely to be mediated by physically interacting sites in mitochondrial and nuclear gene products. We did not detect any underrepresentation of introgressed Denisovan alleles at mitochondrial-targeted loci, but this may reflect reduced power because locus-specific estimates of Denisovan introgression are more conservative. Overall, we conclude that genes involved in mitochondrial function may have been subject to distinct selection pressures during the history of introgression from archaic hominins but that mitonuclear incompatibilities have had, at most, a small role in shaping genome-wide introgression patterns, perhaps because of limited functional divergence in mtDNA and interacting nuclear genes.

Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5 Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history. Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356 ± 21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361 ± 20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292 ± 25), and 186Os/188Os of 0.1198388 ± 29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.

Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity f O 2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe 3+ /ΣFe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe 3+ /ΣFe ratios that relies on a reported correlation between the difference in Fe 3+ /ΣFe ratio by Mossbauer spectroscopy and by electron microprobe (ΔFe 3+ /ΣFe Moss-EPMA ) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mossbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe 3+ /ΣFe Moss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe 3+ /ΣFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = 0.031 and corrected measurements have a mean ΔFe 3+ /ΣFe Moss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe 3+ /ΣFe ratios despite the absence of a correlation between ΔFe 3+ /ΣFe Moss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe 3+ /ΣFe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive f O 2 . Because there is a logarithmic relationship between the activity of magnetite and log f O 2 , the uncertainty in f O 2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe 3+ /ΣFe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe 3+ /ΣFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative f O 2 . We also document and correct errors that appear in the literature when formulating f O 2 that, combined, could yield errors in absolute f O 2 of greater than 0.75 log units—even with perfectly accurate Fe 3+ /ΣFe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe 3+ /ΣFe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of f O 2 from spinel peridotite mineral compositions.

The function and evolution of eukaryotic cells depends upon direct molecular interactions between gene products encoded in nuclear and cytoplasmic genomes. Understanding how these cytonuclear interactions drive molecular evolution and generate genetic incompatibilities between isolated populations and species is of central importance to eukaryotic biology. Plants are an outstanding system to investigate such effects because of their two different genomic compartments present in the cytoplasm (mitochondria and plastids) and the extensive resources detailing subcellular targeting of nuclear-encoded proteins. However, the field lacks a consistent classification scheme for mitochondrial- and plastid-targeted proteins based on their molecular interactions with cytoplasmic genomes and gene products, which hinders efforts to standardize and compare results across studies. Here, we take advantage of detailed knowledge about the model angiospermArabidopsis thalianato provide a curated database of plant cytonuclear interactions at the molecular level. CyMIRA (CytonuclearMolecularInteractionsReference forArabidopsis) is available athttp://cymira.colostate.edu/andhttps://github.com/dbsloan/cymiraand will serve as a resource to aid researchers in partitioning evolutionary genomic data into functional gene classes based on organelle targeting and direct molecular interaction with cytoplasmic genomes and gene products. It includes 11 categories (and 27 subcategories) of different cytonuclear complexes and types of molecular interactions, and it reports residue-level information for cytonuclear contact sites. We hope that this framework will make it easier to standardize, interpret and compare studies testing the functional and evolutionary consequences of cytonuclear interactions.

Abyssal peridotites are ultramafic rocks collected from mid-ocean ridges that are the residues of adiabatic decompression melting. Their compositions provide information on the degree of melting and melt–rock interaction involved in the formation of oceanic lithosphere, as well as providing constraints on pre-existing mantle heterogeneities. This review presents a compilation of abyssal peridotite geochemical data (modes, mineral major elements, and clinopyroxene trace elements) for >1200 samples from 53 localities on 6 major ridge systems. On the basis of composition and petrography, peridotites are classified into one of five lithological groups: (1) residual peridotite, (2) dunite, (3) gabbro-veined and/or plagioclase-bearing peridotite, (4) pyroxenite-veined peridotite, and (5) other types of melt-added peridotite. Almost a third of abyssal peridotites are veined, indicating that the oceanic lithospheric mantle is more fertile, on average, than estimates based on residual peridotites alone imply. All veins appear to have formed recently during melt transport beneath the ridge, though some pyroxenites may be derived from melting of recycled oceanic crust.A limited number of samples are available at intermediate and fast spreading rates, with samples from the East Pacific Rise indicating high degrees of melting. At slow and ultra-slow spreading rates, residual abyssal peridotites define a large (0–15% modal clinopyroxene and spinel Cr#=0.1–0.6) compositional range. These variations do not match the prediction for how degree of melting should vary as a function of spreading rate. Instead, the compositional ranges of residual peridotites are derived from a combination of melting, melt–rock interaction and pre-existing compositional variability, where melt–rock interaction is used here as a general term to refer to the wide range of processes that can occur during melt transport in the mantle. Globally, ~10% of abyssal peridotites are refractory (0% clinopyroxene, spinel Cr#>0.5, bulk Al2O3

Mantle sulfides (Fe–Ni–Cu-rich base metal sulfides or BMS; Fig. 1) play a crucial role in the distribution of Re, Os, and Pb in mantle rocks and are thus fundamental to obtaining absolute ages by direct geochronology using the Re–Os and Pb–Pb isotope systems on mantle samples. Mantle samples exist as hundreds of exposures of peridotites, pyroxenites and diamonds, either brought to the surface as accidental xenoliths and xenocrysts during kimberlitic or alkali basaltic volcanism (for comprehensive reviews, see Pearson et al. 2014; Aulbach et al. 2016, this volume; Luguet and Reisberg 2016, this volume), or as orogenic, ophiolitic and abyssal peridotite obducted at convergent margins and drilled / dredged from oceanic basins (e.g., Bodinier and Godard 2014; Becker and Dale 2016, this volume). This chapter reviews the occurrence of BMS in mantle samples and the role that they play in controlling the Re–Os and Pb isotope systematics of the mantle. Included in this review is a discussion of the role BMS plays in recording the multiple depletion / enrichment / metasomatic events that the mantle has undergone and the preservation of chemical heterogeneities that are inherently created by these processes. Along with discussions of the utility of Re–Os and Pb isotope measurements, this review will also consider the potential pitfalls and some of the surprises that can arise when analyzing these BMS micro-phases. Specifically excluded from this review is the extensive literature on Re–Os and Pb for the geochronology of sulfide systems in magmatic ores. This study is another field entirely from the study of sulfides in their native mantle hosts because of the complicated magmatic concentration processes occurring at crustal levels. Figure 1 Backscattered electron and chemical maps of typical mantle BMS grains. (a) Enclosed; (b) interstitial BMS, both from Mt Gambier peridotites, SE Australia (Alard …

Fluid-rock interaction on oceanic transform faults (OTFs) is important for both the deformation behavior of the lithosphere and volatile cycling in the Earth. Rocks deformed and exhumed at OTFs preserve information about the depth extent of fluid percolation and the nature of fluid-rock interactions within these fault zones. In this study, we focus on five dredges from the Shaka and Prince Edward OTFs on the ultraslow spreading Southwest Indian Ridge that recovered significant volumes of deformed mantle rocks. Samples are predominantly mylonites that have been deformed to high strains in the fault zone, but also contain several generations of fractures. Based on the mineral assemblages in fractures and shear bands combined with thermobarometry analysis, we identified three distinct temperature ranges of fluid-mantle interactions associated with deformation. At low temperature (LT), this leads to crystallization of serpentine (± talc ± amphibole ± chlorite) at

Genomic datasets sometimes support unconventional or conflicting phylogenetic relationships when different tree-building methods are applied. Coherent interpretations of such results are enabled by partitioning support for controversial relationships among the constituent genes of a phylogenomic dataset. For the supermatrix (= concatenation) approach, several simple methods that measure the distribution of support and conflict among loci were introduced over 15 years ago. More recently, partitioned coalescence support (PCS) was developed for phylogenetic coalescence methods that account for incomplete lineage sorting and use the summed fits of gene trees to estimate the species tree. Here, we automate computation of PCS to permit application of this index to genome-scale matrices that include hundreds of loci. Reanalyses of four phylogenomic datasets for amniotes, land plants, skinks, and angiosperms demonstrate how PCS scores can be used to: 1) compare conflicting results favored by alternative coalescence methods, 2) identify outlier gene trees that have a disproportionate influence on the resolution of contentious relationships, 3) assess the effects of missing data in species-trees analysis, and 4) clarify biases in commonly-implemented coalescence methods and support indices. We show that key phylogenomic conclusions from these analyses often hinge on just a few gene trees and that results can be driven by specific biases of a particular coalescence method and/or the extreme weight placed on gene trees with high taxon sampling. Attributing exceptionally high weight to some gene trees and very low weight to other gene trees counters the basic logic of phylogenomic coalescence analysis; even clades in species trees with high support according to commonly used indices (likelihood-ratio test, bootstrap, Bayesian local posterior probability) can be unstable to the removal of only one or two gene trees with high PCS. Computer simulations cannot adequately describe all of the contingencies and complexities of empirical genetic data. PCS scores complement simulation work by providing specific insights into a particular dataset given the assumptions of the phylogenetic coalescence method that is applied. In combination with standard measures of nodal support, PCS provides a more complete understanding of the overall genomic evidence for contested evolutionary relationships in species trees.

The partitioning of genetic material between the nucleus and cytoplasmic (i.e., mitochondrial and plastid) genomes within eukaryotic cells necessitates coordinated integration between these genomic compartments, with important evolutionary and biomedical implications. Classic questions persist about the pervasive reduction of cytoplasmic genomes via a combination of gene loss, transfer, and functional replacement — and yet why they are almost always retained in some minimal form. One striking consequence of cytonuclear integration is the existence of ‘chimeric’ enzyme complexes composed of subunits encoded in two different genomes. Advances in structural biology and comparative genomics are yielding important insights into the evolution of such complexes, including correlated sequence changes and recruitment of novel subunits. Thus, chimeric cytonuclear complexes provide a powerful window into the mechanisms of molecular coevolution.

Lithospheric structure changes at low spreading rates (

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved. Fil: Garber, Joshua M.. University of California at Davis; Estados Unidos Fil: Roeske, Sarah M.. University of California at Davis; Estados Unidos Fil: Warren, Jessica. University of Stanford; Estados Unidos Fil: Mulcahy, Sean R.. University of California at Berkeley; Estados Unidos Fil: McClelland, William C.. University of Iowa; Estados Unidos Fil: Austin, Lauren J.. University of Oregon; Estados Unidos Fil: Renne, Paul R.. University of California at Berkeley; Estados Unidos Fil: Vujovich, Graciela Irene. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Laboratorio de Tectónica Andina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Rock deformation often is investigated using kinematic and/or mechanical models. Here we provide a direct comparison of these modeling techniques in the context of a deformed dike within a meter-scale contractional fault step. The kinematic models consider two possible shear plane orientations and various modes of deformation (simple shear, transtension, transpression), while the mechanical model uses the finite element method and assumes elastoplastic constitutive behavior. The results for the kinematic and mechanical models are directly compared using the modeled maximum and minimum principal stretches. The kinematic analysis indicates that the contractional step may be classified as either transtensional or transpressional depending on the modeled shear plane orientation, suggesting that these terms may be inappropriate descriptors of step-related deformation. While the kinematic models do an acceptable job of depicting the change in dike shape and orientation, they are restricted to a prescribed homogeneous deformation. In contrast, the mechanical model allows for heterogeneous deformation within the step to accurately represent the deformation. The ability to characterize heterogeneous deformation and include fault slip – not as a prescription, but as a solution to the governing equations of motion – represents a significant advantage of the mechanical model over the kinematic models.

The concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H2O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high-temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60–670 ppm H2O, while orthopyroxenes have 10–300 ppm, which gives calculated olivine concentrations of 8–34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20–220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

Abyssal peridotites and mid-oceanic ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts from different mantle depths, their isotopic signature does not directly describe the isotopic composition of the mantle source, but instead describes the local average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on the distribution of isotopic heterogeneities. We analyzed Pb isotopic compositions in sulfide grains from the Southwest Indian Ridge and the Gakkel Ridge (Arctic Ocean) using the high-resolution Cameca 1280 ion microprobe. Sulfide Pb isotope ratios show very large variations, with 16 grains from 1 sample covering ∼25% of the entire range observed in the oceanic mantle. Pb isotopes in sulfides preserve a record of mantle compositions not seen in whole-rock MORBs from the same area. Sulfides from the Atlantis II Fracture Zone (Southwest Indian Ridge) confirm the presence of ancient refractory material scatter in the oceanic upper mantle. Gakkel Ridge sulfides define a high degree of isotopic variability, suggesting that oceanic mantle, not subcontinental lithospheric mantle, is the main source of such heterogeneity. Our results confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous and that other mantle end-member components are intimately mixed in. In-situ sulfide analysis is a powerful tool to detect the isotopic diversity of the MORB mantle source.

We present data from the Josephine Peridotite (SW Oregon, USA) that constrain the underlying physical processes responsible for the initiation of shear localization and the evolution of ductile shear zones in Earth's mantle. Field measurements of narrow (2–60 m wide) ductile shear zones in harzburgite were used to construct strain profiles, which have maximum shear strains ranging from γ=5.25 to γ>20. Measurements of pyroxene water concentrations from harzburgite samples within and immediately adjacent to the shear zones indicate that gradients in water concentration exist on a 10–100 m scale, even after exhumation. Water concentration measurements are correlated with olivine lattice-preferred orientation (LPO), corroborating experimental results on the influence of water on slip system activity. Using empirical olivine flow laws and the diffusivity of water in olivine, we model initiation of a ductile shear zone through localized water weakening. We demonstrate that this mechanism can readily generate spatial perturbations in both effective viscosity and strain. However this model is not able to reproduce both the observed shear strain gradients and water concentration data from the Josephine shear zones. We evaluate other plausible localization mechanisms, which may amplify this initial strain perturbation. The most relevant at these conditions is the development of viscous anisotropy associated with the evolution of olivine LPO. Using recent experimental results, we demonstrate that progressive rotation of olivine LPO into the shear plane enhances deformation within a shear zone. We conclude that feedback between at least two microphysical processes is needed to account for observed outcrop-scale shear localization.

Terrestrial oceanic and continental basalts are enriched by approximately +0.1‰ in 56Fe/54Fe ratio relative to primitive, undifferentiated meteorites (chondrites). The δ56Fe values of terrestrial basalts are also distinct from those of basalts from Mars and asteroid Vesta, which have chondritic Fe isotopic compositions. The processes responsible for the isotopic enrichment of terrestrial basalts are debated, in part because the Fe isotopic composition of the mantle source of terrestrial basalts is unknown. Here we report Fe isotopic measurements of abyssal peridotites, which are the residues of limited melting at oceanic ridges and are thus the best proxies for the composition of the convective portion of the mantle. Our data show that abyssal peridotites have a mean δ56Fe value of +0.010±0.007‰ (relative to IRMM-014), which is indistinguishable from chondrites. After correcting this data for seafloor weathering and mantle melting, we estimate the average Fe isotopic composition of the terrestrial mantle to be δ56Fe=+0.025±0.025‰, which is also indistinguishable from chondrites, within current analytical precision. We determine that the maximum shift in δ56Fe for peridotite residues during partial mantle melting is 0.01‰. Our results argue against isotopic fractionation during core–mantle differentiation or iron vaporization during the Moon-forming giant impact, because both processes would yield a bulk mantle δ56Fe value that is non-chondritic. In addition, our results suggest that disproportionation of mantle Fe2+–Fe3+ in perovskite and Fe0 metal and segregation of metal to the core could not have been a driver for Fe isotopic fractionation in the silicate mantle. Instead, the different iron isotopic compositions of abyssal peridotites and MORBs support mounting evidence for iron isotopic fractionation of melts but not residues during the formation of oceanic and continental crust.

A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6 m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (>80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (γ>20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of ∼4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle. The present data also characterize the isotopic composition of the Josephine Peridotite: 3He/4He=6.7±0.2 Ra (n=33, between 6.3 and 7.1 Ra). The presence of cosmogenic 3He in the matrix is indicated by the helium isotopic composition released by melting: 3He/4He=8.5±0.3 Ra (n=10; from 7.9 to 10.9). This corresponds to an exposure age of 10 Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U]

Single sulfides from abyssal peridotites have been analyzed for Pb and Re-Os to constrain the evolution of oceanic mantle composition. These represent the first analyses of Pb and Os isotopic compositions in the same sulfide grain. The sulfides are from Gakkel and Southwest Indian ridge peridotites, occur at 0.1 % modal abundances, and contain 0.001–0.4 ppm Re, 0.003–5 ppm Os, and 0.12–12 ppm Pb. Sulfide Pb isotopic compositions extend from depleted (e.g., 206Pb/204Pb=17.0) to enriched (19.6), covering a larger range than associated basalts. The Os isotopic range of sulfides is more restricted, but extends from depleted (187Os/188Os=0.116) to enriched (0.150). Pb and Os concentrations and isotopic compositions co-vary, with correlation coefficients of 0.76–0.94. Both Pb and Re–Os isotopic data follow ∼ 2 Ga isochrons, with isotopic compositions varying down to small ( ⪡ 1 km ) length-scales and some sulfides containing supra-chondritic 187Re/188Os and 187Os/188Os. These observations are best explained by long-term recycling of oceanic lithosphere combined with melt extraction and refertilization at ancient ocean ridges, rather than a specific event at 2 Ga. The concentration of Pb in sulfides indicates that they host 4 % of the mantle Pb budget. A re-evaluation of the mass balance of Pb in peridotites indicates that most mantle Pb is stored in silicate phases. The Pb partition coefficient between sulfide melt and silicate melt is estimated to be ∼ 3 , based on the correlation of Pb–Os concentrations in this study and measured Os partition coefficients from the literature. These observations indicate that sulfides do not exert a strong control on the fractionation of Pb during mantle melting, but they can be used to constrain mantle Pb isotopic composition. Sulfides in this study, combined with literature data for Pb isotopes in peridotite whole rocks and pyroxene separates, provide evidence for ultra-depleted mantle, as 24% of peridotites are unradiogenic (i.e., 207Pb/204Pb 15.440 and 206Pb/204Pb 17.726 ), compared to only 3% of ridge basalts. This suggests that the mantle contains volumetrically significant reservoirs of ultra-depleted material, probably derived from recycled oceanic lithospheric mantle. These depleted reservoirs contribute in only small amounts to oceanic crust generation, both due to a limited ability to melt and to the dilution of any melt by more enriched melts during crust formation.

Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fun-damental drivers in a wide range of large-scale processes. The operation of plate tectonics in gen-eral depends intimately on the manner in which lithospheric plates couple to the convecting inte-rior. Current debate centers on whether the transition from rigid lithosphere to flowing astheno-sph ere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are instead intra-lithospheric features reflecting a compositional boundary established at mid-ocean ridges, not a rheological boundary.

The significant viscous anisotropy that results from crystallographic alignment (texture) of olivine grains in deformed upper-mantle rocks strongly influences a large variety of geodynamic processes. Our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution. Unfortunately, existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths, making them less general than desired for many large-scale geodynamic scenarios. Here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories. For the mechanical behavior, we explore two extreme scenarios, one in which each grain experiences the same stress tensor (Sachs model) and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate (pseudo-Taylor model). For the textural evolution, we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation. Only recently has enough laboratory data on the deformation of olivine become available to calibrate these models. We use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models. These inversions demonstrate that the calibrated pseudo-Taylor model best reproduces the mechanical observations. Additionally, the pseudo-Taylor textural evolution model can reasonably reproduce the observed texture strength, shape, and orientation after large and complex deformations. A quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution. In addition, we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large-strain torsion tests. This set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large-scale numerical simulations.