Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

Uncertainty regarding the evolution of the oxygen isotopic composition of seawater casts doubt on past temperature reconstructions. Here, the authors present a new, precise δ18O value for the Neoproterozoic, and propose that ocean temperatures on the eve of the Sturtian glaciation were 15–30 °C warmer than present

Geochemical Characterization of the Oman Crust-Mantle Transition Zone, OmanDP Holes CM1A and CM2B

International audienceThe transition from the gabbroic oceanic crust to the residual mantle harzburgites of the Oman ophiolite has been drilled at Holes CM1A and CM2B (Wadi Tayin massif) during Phase 2 of the International Continental Scientific Drilling Program Oman Drilling Project (November 2017-January 2018). In order to unravel the formation processes of ultramafic rocks in the Wadi Tayin massif crust-mantle transition zone and deeper in the mantle sections beneath oceanic spreading centers, our study focuses on the whole rock major and trace element compositions (together with CO2 and H2O concentrations) of these ultramafic rocks (56 dunites and 49 harzburgites). Despite extensive serpentinization and some carbonation, most of the trace element contents (REE, HFSE, Ti, Th, U) record high temperature, magmatic process-related signatures. Two major trends are observed, with good correlations between (a) Th and U, Nb and LREE on one hand, and between (b) heavy REE, Ti and Hf on the other hand. We interpret the first trend as the signature of late melt/peridotite interactions as LREE are known to be mobilized by such processes (``lithospheric process'') and the second trend as the signature of the initial mantle partial melting (``asthenospheric process''), with little or no overprint from melt/rock reaction events

Spatial Mismatch, Poverty, and Vulnerable Populations

International audienceSpatial mismatch relates the unemployment and poverty of vulnerable population groups to their remoteness from job opportunities. Although the intuition initially applied to African Americans in US inner cities, spatial mismatch has a broader validity beyond the sole US context. In light of a detailed presentation of the mechanisms at work, we present the main results from various empirical tests of the spatial mismatch theory. Since key aspects of that theory remain to be tested, we also discuss methodological approaches and provide guidance for further research. We derive lessons for policy implications and comment on the appropriateness of related urban policies

Spatial Mismatch, Poverty, and Vulnerable Populations

International audienc

Cr-spinel records metasomatism not petrogenesis of mantle rocks

Mantle melts provide a window on processes related to global plate tectonics. The composition of chromian spinel (Cr-spinel) from mafic-ultramafic rocks has been widely used for tracing the geotectonic environments, the degree of mantle melting and the rate of mid-ocean ridge spreading. The assumption is that Cr-spinel’s core composition (Cr# = Cr/(Cr + Al)) is homogenous, insensitive to post-formation modification and therefore a robust petrogenetic indicator. However, we demonstrate that the composition of Cr-spinel can be modified by fluid/melt-rock interactions in both sub-arc and sub-mid oceanic mantle. Metasomatism can produce Al-Cr heterogeneity in Cr-spinel that lowers the Cr/Al ratio, and therefore modifies the Cr#, making Cr# ineffective as a geotectonic and mantle melting indicator. Our analysis also demonstrates that Cr-spinel is a potential sink for fluid-mobile elements, especially in subduction zone environments. The heterogeneity of Cr# in Cr-spinel can, therefore, be used as an excellent tracer for metasomatic processes