Melting systematics in mid-ocean ridge basalts : application of a plagioclase-spinel melting model to global variations in major element chemistry and crustal thickness

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 120 (2015): 4863–4886, doi:10.1002/2015JB011885.We present a new model for anhydrous melting in the spinel and plagioclase stability fields that provides enhanced predictive capabilities for the major element compositional variability found in mid-ocean ridge basalts (MORBs). The model is built on the formulation of Kinzler and Grove (1992) and Kinzler (1997) but incorporates new experimental data collected since these calibrations. The melting model is coupled to geodynamic simulations of mantle flow and mid-ocean ridge temperature structure to investigate global variations in MORB chemistry and crustal thickness as a function of mantle potential temperature, spreading rate, mantle composition, and the pattern(s) of melt migration. While the initiation of melting is controlled by mantle temperature, the cessation of melting is primarily determined by spreading rate, which controls the thickness of the lithospheric lid, and not by the exhaustion of clinopyroxene. Spreading rate has the greatest influence on MORB compositions at slow to ultraslow spreading rates (<2 cm/yr half rate), where the thermal boundary layer becomes thicker than the oceanic crust. A key aspect of our approach is that we incorporate evidence from both MORB major element compositions and seismically determined crustal thicknesses to constrain global variations in mantle melting parameters. Specifically, we show that to explain the global data set of crustal thickness, Na8, Fe8, Si8, Ca8/Al8, and K8/Ti8 (oxides normalized to 8 wt % MgO) require a relatively narrow zone over which melts are pooled to the ridge axis. In all cases, our preferred model involves melt transport to the ridge axis over relatively short horizontal length scales (~25 km). This implies that although melting occurs over a wide region beneath the ridge axis, up to 20–40% of the total melt volume is not extracted and will eventually refreeze and refertilize the lithosphere. We find that the temperature range required to explain the global geochemical and geophysical data sets is 1300°C to 1450°C. Finally, a small subset of the global data is best modeled as melts of a depleted mantle source composition (e.g., depleted MORB mantle—2% melt).Funding was provided by NSF grants OCE-1458201 (M.D.B) and OCE-1457916 (T.L.G) and to M.D.B by the Deep Ocean Exploration Institute at Woods Hole Oceanographic Institution and the Deep Carbon Observatory.2016-01-2

Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant ^(238)U/^(235)U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from −0.17 to −1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10^(−6) at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ^(238)U values are consistent with fractionationduring magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction

Mantle melting as a function of water content beneath back-arc basins

Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings

Melting processes and mantle sources of lavas on Mercury

peer reviewe

A subduction origin for komatites and cratonic lithospheric mantle

We present a model in which the generation of komatiites in Archaean subduction zones produced depleted mantle residues that eventually formed the highly depleted portions of the Kaapvaal lithospheric mantle. The envisioned melting process is similar to that which has formed boninites in Phanerozoic subduction zones such as the Izu-Bonin-Mariana arc. The primary differences between the Archaean and Phanerozoic melting regimes are higher mean melting temperatures (1450 versus 1350 °C) and higher mean melting pressures (2.5 versus 1.5 GPa) for the komatiites. The komatiites from the Komati Formation in the Barberton greenstone belt are mafic enough to have produced the depletion seen in most Kaapvaal granular peridotite xenoliths. However, the most highly depleted Kaapvaal xenoliths require an even more Mg-rich magma than the Komati komatiites (Kk). Samples of boninite mantle residues from the fore-arc of the Marianas subduction zone are nearly as depleted as the Kaapvaal cratonic mantle, indicating that buoyant, craton-like mantle is being produced today. We speculate that production rates of cratonic mantle were greater in the Archaean due to the greater depth of melting for komatiites (relative to boninites) and greater worldwide arc length. The high production rates and high buoyancy of the komatiite mantle residues gave rise to the rapid growth and stabilization of the Kaapvaal craton in the Archaean

Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant ^(238)U/^(235)U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from −0.17 to −1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10^(−6) at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ^(238)U values are consistent with fractionationduring magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction

Neutralizing Antibody-Resistant Hepatitis C Virus Cell-to-Cell Transmission

Hepatitis C virus (HCV) can initiate infection by cell-free particle and cell-cell contact-dependent transmission. In this study we use a novel infectious coculture system to examine these alternative modes of infection. Cell-to-cell transmission is relatively resistant to anti-HCV glycoprotein monoclonal anti- bodies and polyclonal immunoglobulin isolated from infected individuals, providing an effective strategy for escaping host humoral immune responses. Chimeric viruses expressing the structural proteins rep- resenting the seven major HCV genotypes demonstrate neutralizing antibody-resistant cell-to-cell trans- mission. HCV entry is a multistep process involving numerous receptors. In this study we demonstrate that, in contrast to earlier reports, CD81 and the tight-junction components claudin-1 and occludin are all essential for both cell-free and cell-to-cell viral transmission. However, scavenger receptor BI (SR-BI) has a more prominent role in cell-to-cell transmission of the virus, with SR-BI-specific antibodies and small-molecule inhibitors showing preferential inhibition of this infection route. These observations highlight the importance of targeting host cell receptors, in particular SR-BI, to control viral infection and spread in the liver