Delamination and recycling of Archaean crust caused by gravitational instabilities

Mantle temperatures during the Archaean eon were higher than today. As a consequence, the primary crust formed at the time is thought to have been extensive, thick and magnesium rich, and underlain by a highly residual mantle1. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle2. Furthermore, Archaean crust exposed today is composed mostly of tonalite–trondhjemite–granodiorite, indicative of a hydrated, low-magnesium basalt source3, suggesting that they were not directly generated from a magnesium-rich primary crust. Here we present thermodynamic calculations that indicate that the stable mineral assemblages expected to form at the base of a 45-km-thick, fully hydrated and anhydrous magnesium-rich crust are denser than the underlying, complementary residual mantle. We use two-dimensional geodynamic models to show that the base of magmatically over-thickened magnesium-rich crust, whether fully hydrated or anhydrous, would have been gravitationally unstable at mantle temperatures greater than 1,500–1,550?°C. The dense crust would drip down into the mantle, generating a return flow of asthenospheric mantle that melts to create more primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to source tonalite–trondhjemite–granodiorite melts. The residues of these processes, with an ultramafic composition, must now reside in the mantle

Constraining fault friction in oceanic lithosphere using the dip angles of newly-formed faults at outer rises

We investigate the mechanical properties of the oceanic lithosphere using earthquake focal mechanisms from subduction zone outer rises. We study regions where faulting oblique to the pre-existing mid-ocean ridge fabric implies the formation of new faults. The nodal-plane dips of dip-slip earthquakes on these faults are dominantly in the range 30–60°, with a strong peak concentrated around 45°. This distribution is inconsistent with the pattern that would result from high coefficients of friction (e.g. 0.6, equivalent to normal faults forming at 60° and thrust faults at 30°). We instead suggest that the observed distribution of dips implies that faults in the oceanic lithosphere have a low coefficient of friction, due to either low-friction clay minerals formed by hydrothermal alteration at the ridge, or due to an intrinsic level of friction that is lower than that suggested by laboratory studies

Worldview Conflict in Daily Life

Building on laboratory- and survey-based research probing the psychology of ideology and the experience of worldview conflict, we examined the association between worldview conflict and emotional reactions, psychological well-being, humanity esteem, and political ideology in everyday life using experience sampling. In three combined samples (total N = 328), experiencing disagreement compared to agreement was associated with experiencing more other-condemning emotions, less well-being, and less humanity esteem. There were no clear associations between experiencing disagreement and experiencing self-conscious emotions, positive emotions, and mental stress. None of the relationships were moderated by political ideology. These results both replicate and challenge findings from laboratory- and survey-based research, and we discuss possible reasons for the discrepancies. Experience sampling methods can help researchers get a glimpse into everyday worldview conflict

GORE MOUNTAIN GARNET AMPHIBOLITE RECORDS UHT CONDITIONS: IMPLICATIONS FOR THE RHEOLOGY OF THE LOWER CONTINENTAL CRUST DURING OROGENESIS

The Gore Mountain Garnet Amphibolite (GMGA), part of the Mesoproterozoic Grenville Province in the Adirondack Highlands, NY, USA, is an iconic rock type known for hosting the world’s largest garnets (up to 1 m diameter). We present a new detailed petrographic study of these rocks. Field relations, whole-rock, and mineral major and trace element chemistry suggest that these rocks formed via a prograde hydration reaction of a metagabbro during an increase in pressure and temperature. Laser ablation inductively coupled plasma mass spectrometry U–Pb geochronology applied to zircon interpreted to be metamorphic in origin dates this reaction to 1053·9 ± 5·4 Ma (2σ; MSWD = 0·94), during the Ottawan Orogeny (1090–1020 Ma). Our results on peak metamorphic P–T conditions based on thermobarometry, diffusion models, and thermodynamic modelling indicate that these rocks formed at ultrahigh-temperature (>900 °C) conditions (P = 9–10 kbar, T = 950 ± 40 °C), significantly hotter than previously estimated. Diffusion models pinned by nearby cooling ages require the GMGA to initially cool quickly (9·1 °C Ma–1), followed by slower cooling (2·6 °C Ma–1). The two-stage cooling history for the GMGA could reflect initial advection-dominated cooling followed by conduction-dominated cooling once flow ceases. Our results suggest that the region was hot enough to undergo topography-driven lower crustal flow similar to that hypothesized for modern Tibet for 20–0 Ma (25–0 Ma when the effects of melt are included).NSF (Grant EAR-1722935

The Composition of the Lower Oceanic Crust in the Wadi Khafifah Section of the Southern Samail (Oman) Ophiolite

International audienceThe composition of the intrusive gabbroic lower oceanic crust remains poorly characterized in comparison to the extrusive portion of the oceanic crust, especially for intermediate-fast spreading mid-ocean ridges. This is a consequence of limited exposures of extant lower oceanic crust or ophiolites similar to mid-ocean ridge crust. One of the best analogues for mid-ocean ridge crust is the southern Samail ophiolite that formed during a period of rapid seafloor spreading above a nascent subduction zone. Here, we focus on the geochemical stratigraphy (whole rock and mineral major and trace element compositions) of the 5,200 m-thick, lower crustal, Wadi Khafifah section of the Wadin Tayin massif in the southern Samail (Oman) ophiolite. Gabbros from the lowermost 3,700 m of this section (the “lower gabbros”) show no systematic changes in composition with height above the Mantle Transition Zone. In contrast, gabbros from the uppermost 1,500 m (the “upper gabbros”) display marked increases in incompatible trace element concentration with increasing height. Liquids in equilibrium with the lower gabbros have major and trace element compositions that overlap with those measured in the upper gabbros and sheeted dikes. Upper gabbros preserve mineral cores with primitive major element compositions that overlap with the range of lower gabbros; however, upper gabbro whole rock compositions are significantly more enriched in incompatible trace elements relative to the lower gabbros. Our data reveal that the upper gabbros are a composite of accumulated minerals derived from primitive melts and a large fraction of evolved melts derived from the fractionation of the lower gabbros. We propose a new “Full Sheeted Sills” model for the lower oceanic crust in which primitive magmas from the mantle are emplaced throughout the lower crust and crystallized in situ. After diking events, evolved magmas leave the lower gabbros and replenish the upper gabbros, thereby contributing to the higher incompatible trace element budget in the upper gabbros relative to the lower gabbros. Our reconstructed bulk compositions of the lower plutonic crust and the bulk oceanic crust from the Wadi Khafifah section yield a plausible primary mantle-derived magma composition in equilibrium with depleted mid-ocean ridge basalts mantle

Dating the Bushveld Complex: Timing of Crystallization, Duration of Magmatism, and Cooling of the World’s Largest Layered Intrusion and Related Rocks

The Paleoproterozoic Bushveld Complex, including the world’s largest layered intrusion and host to world-class stratiform chromium, platinum group element, and vanadium deposits, is a remarkable natural laboratory for investigating the timescales of magmatic processes in the Earth’s crust. A framework for the emplacement, crystallization, and cooling of the Bushveld Complex based on integrated U–Pb zircon–baddeleyite–titanite–rutile geochronology is presented for samples of different rock types from the Bushveld Complex, including ultramafic and mafic cumulates, mineralized horizons, granitic rocks from the roof, and a carbonatite from the nearby alkaline Phalaborwa Complex. The results indicate that (1) the Bushveld Complex was built incrementally over an ∼5 Myr interval from 2060 to 2055 Ma with a peak in magma flux at c. 2055–2056 Ma, (2) U–Pb zircon crystallization ages do not decrease in an uninterrupted systematic manner from the base to the top of the intrusion, indicating that the Bushveld Complex does not represent the crystallized products of a single progressively filled and cooled magma chamber, and (3) U–Pb rutile dates constrain cooling of the intrusion at the level of the Critical Zone through ∼500 °C by 2053 Ma. The c. 2060 Ma Phalaborwa Complex (pyroxenite, syenite, carbonatite + Cu–Fe-phosphate–vermiculite deposits) represents one of the earliest manifestations of widespread Bushveld-related magmatism in the northern Kaapvaal craton. The extended range and out-of-sequence U–Pb zircon dates determined for a harzburgite from the Lower Zone (c. 2056 Ma), an orthopyroxenite from the Lower Critical Zone (c. 2057 Ma), and orthopyroxenites from the Upper Critical Zone (c. 2057–2060 Ma) are interpreted to indicate that the lower part of the Bushveld Complex developed through successive intrusions and accretion of sheet-like intrusions (sills), some intruded at different stratigraphic levels. Crystallization of the main volume of the Bushveld Complex, as represented by the thick gabbroic sequences of the Main Zone and Upper Zone, is constrained to a relatively narrow interval of time (∼1 Myr) at c. 2055–2056 Ma. Granites and granophyres in the roof, and a diorite in the uppermost Upper Zone, constitute the youngest igneous activity in the Bushveld Complex at c. 2055 Ma. Collectively, these results contribute to an emerging paradigm shift for the assembly of some ultramafic–mafic magmatic systems from the conventional ‘big tank’ model to an ‘amalgamated sill’ model. The volume–duration relationship determined for magmatism in the Bushveld Complex, when compared with timescales established for the assembly of other layered intrusions and more silica-rich plutonic–volcanic systems worldwide, is distinct and equivalent to those determined for Phanerozoic continental and oceanic flood basalts that constitute large igneous provinces. Emplacement of the 2055–2060 Ma Bushveld Complex corresponds to the end of the Lomagundi–Jatuli Event, the largest magnitude positive carbon isotope excursion in Earth history, and this temporal correlation suggests that there may have been a contribution from voluminous Bushveld ultramafic–mafic–silicic magmatism to disruptions in the global paleoenvironment

Does Forgiveness Require Interpersonal Interactions? Individual Differences in Conceptualization of Forgiveness

The present studies examined whether victims believed that forgiveness requires interpersonal interaction between the victim and offender. Having an interpersonal conceptualization of forgiveness was positively related to a collectivistic worldview. Also, an interpersonal conceptualization of forgiveness was positively related to forgiveness measures that stressed interpersonal interactions with the offender. However, an interpersonal conceptualization of forgiveness was positively related to a general measure of forgiveness only for those participants who were in a continuing relationship with the offender. The current research is an important step in understanding the contexts in which one\u27s conceptualization of forgiveness may influence its practice. (C) 2012 Elsevier Ltd. All rights reserved

Clinical Laboratory Values as Early Indicators of Ebola Virus Infection in Nonhuman Primates

The Ebola virus (EBOV) outbreak in West Africa during 2013–2016 demonstrated the need to improve Ebola virus disease (EVD) diagnostics and standards of care. This retrospective study compared laboratory values and clinical features of 3 nonhuman primate models of lethal EVD to assess associations with improved survival time. In addition, the study identified laboratory values useful as predictors of survival, surrogates for EBOV viral loads, and triggers for initiation of therapeutic interventions in these nonhuman primate models. Furthermore, the data support that, in nonhuman primates, the Makona strain of EBOV may be less virulent than the Kikwit strain of EBOV. The applicability of these findings as potential diagnostic and management tools for EVD in humans warrants further investigation