Ancient refractory asthenosphere revealed by mantle re-melting at the Arctic Mid Atlantic Ridge

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle

Geo-neutrinos: A systematic approach to uncertainties and correlations

Geo-neutrinos emitted by heat-producing elements (U, Th and K) represent a unique probe of the Earth interior. The characterization of their fluxes is subject, however, to rather large and highly correlated uncertainties. The geochemical covariance of the U, Th and K abundances in various Earth reservoirs induces positive correlations among the associated geo-neutrino fluxes, and between these and the radiogenic heat. Mass-balance constraints in the Bulk Silicate Earth (BSE) tend instead to anti-correlate the radiogenic element abundances in complementary reservoirs. Experimental geo-neutrino observables may be further (anti)correlated by instrumental effects. In this context, we propose a systematic approach to covariance matrices, based on the fact that all the relevant geo-neutrino observables and constraints can be expressed as linear functions of the U, Th and K abundances in the Earth's reservoirs (with relatively well-known coefficients). We briefly discuss here the construction of a tentative "geo-neutrino source model" (GNSM) for the U, Th, and K abundances in the main Earth reservoirs, based on selected geophysical and geochemical data and models (when available), on plausible hypotheses (when possible), and admittedly on arbitrary assumptions (when unavoidable). We use then the GNSM to make predictions about several experiments ("forward approach"), and to show how future data can constrain - a posteriori - the error matrix of the model itself ("backward approach"). The method may provide a useful statistical framework for evaluating the impact and the global consistency of prospective geo-neutrino measurements and Earth models.Comment: 17 pages, including 4 figures. To appear on "Earth, Moon, and Planets," Special Issue on "Neutrino Geophysics," Proceedings of Neutrino Science 2005 (Honolulu, Hawaii, Dec. 2005

Development and evolution of detachment faulting along 50 km of the Mid-Atlantic Ridge near 16.5N

This is the accepted manuscript. An edited version of this paper was published by AGU. Copyright 2014 American Geophysical Union.A multifaceted study of the slow-spreading Mid-Atlantic Ridge (MAR) at 16.5ºN provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ~50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, non-corrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal one other detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multi-scale corrugations are observed 2-3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6º) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low-angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust > 2 Ma indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.This work was supported by the National Science Foundation grant number OCE-1155650

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The editorial board of AGU Advances thanks the individuals who reviewed for the journal in 2021. © 2022. The Authors. AGU Advances published by Wiley Periodicals LLC on behalf of American Geophysical Union

Cumulate causes for the low contents of sulfide-loving elements in the continental crust

Despite the economic importance of chalcophile (sulfide-loving) and siderophile (metal-loving) elements (CSEs), it is unclear how they become enriched or depleted in the continental crust, compared with the oceanic crust. This is due in part to our limited understanding of the partitioning behaviour of the CSEs. Here I compile compositional data for mid-ocean ridge basalts and subduction-related volcanic rocks. I show that the mantle-derived melts that contribute to oceanic and continental crust formation rarely avoid sulfide saturation during cooling in the crust and, on average, subduction-zone magmas fractionate sulfide at the base of the continental crust prior to ascent. Differentiation of mantle-derived melts enriches lower crustal sulfide- and silicate-bearing cumulates in some CSEs compared with the upper crust. This storage predisposes the cumulate-hosted compatible CSEs (such as Cu and Au) to be recycled back into the mantle during subduction and delamination, resulting in their low contents in the bulk continental crust and potentially contributing to the scarcity of ore deposits in the upper continental crust. By contrast, differentiation causes the upper oceanic and continental crust to become enriched in incompatible CSEs (such as W) compared with the lower oceanic and continental crust. Consequently, incompatible CSEs are predisposed to become enriched in subduction-zone magmas that contribute to continental crust formation and are less susceptible to removal from the continental crust via delamination compared with the compatible CSEs

The Problematization of Sexuality among Women Living with HIV and a New Feminist Approach for Understanding and Enhancing Women’s Sexual Lives

In the context of HIV, women’s sexual rights and sexual autonomy are important but frequently overlooked and violated. Guided by community voices, feminist theories, and qualitative empirical research, we reviewed two decades of global quantitative research on sexuality among women living with HIV. In the 32 studies we found, conducted in 25 countries and composed mostly of cis-gender heterosexual women, sexuality was narrowly constructed as sexual behaviours involving risk (namely, penetration) and physiological dysfunctions relating to HIV illness, with far less attention given to the fullness of sexual lives in context, including more positive and rewarding experiences such as satisfaction and pleasure. Findings suggest that women experience declines in sexual activity, function, satisfaction, and pleasure following HIV diagnosis, at least for some period. The extent of such declines, however, is varied, with numerous contextual forces shaping women’s sexual well-being. Clinical markers of HIV (e.g., viral load, CD4 cell count) poorly predicted sexual outcomes, interrupting widely held assumptions about sexuality for women with HIV. Instead, the effects of HIV-related stigma intersecting with inequities related to trauma, violence, intimate relations, substance use, poverty, aging, and other social and cultural conditions primarily influenced the ways in which women experienced and enacted their sexuality. However, studies framed through a medical lens tended to pathologize outcomes as individual “problems,” whereas others driven by a public health agenda remained primarily preoccupied with protecting the public from HIV. In light of these findings, we present a new feminist approach for research, policy, and practice toward understanding and enhancing women’s sexual lives—one that affirms sexual diversity; engages deeply with society, politics, and history; and is grounded in women’s sexual rights

Role of ancient, ultra-depleted mantle in Mid-Ocean-Ridge magmatism

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets