Anhydrous Partial Melting Experiments on MORB-like Eclogite: Phase Relations, Phase Compositions and Mineral-Melt Partitioning of Major Elements at 2-3 GPa

We present melt and mineral compositions from nominally anhydrous partial melting experiments at 2-3 GPa on a quartz eclogite composition (G2) similar to average oceanic crust. Near-solidus partial melts at 3 GPa, determined with melt traps of vitreous carbon spheres, have 55-57 wt % SiO2, rather less silica than the dacitic compositions that are generally assumed for near-solidus eclogite partial melts. At 2 GPa, equivalent near-solidus partial melts are less silicic (≤52 wt % SiO2). The 3 GPa near-solidus partial melts (up to melt fractions of ∼3%) are saturated in rutile and have 5·7-6·7 wt % TiO2. The G2 composition is K2O-poor (0·03 wt %), but a modified composition with 0·26 wt % K2O (G2K) produces dacitic near-solidus melts with 61-64 wt % SiO2. Rutile saturation for G2K extends to higher melt fraction (∼13%) and occurs at lower TiO2 melt contents (3·3 wt %) than for G2. These results can be understood in terms of a simplified thermodynamic model in which alkalis increase the SiO2 content of liquids saturated in quartz, which in turn diminishes the TiO2 concentrations required to maintain rutile saturation. Additionally, the mode of residual garnet and generation of silicic liquids by partial melting of anhydrous eclogite are linked, as garnet is required to mass-balance formation of appreciable SiO2-rich melt. Partitioning of Na between clinopyroxene and melt shows significant increases with pressure, but only modest shifts with changing temperature. In contrast, partitioning of Ti between cpx and melt, as well as between cpx and garnet, shows pronounced dependence on temperature for compositions relevant to anhydrous partial melting of eclogite. Mixtures between partial melts of eclogite and primitive picritic Hawaiian magmas are similar to magnesian, SiO2-rich compositions inferred from melt inclusions from the Koolau volcano. However, in detail, no eclogitic partial melt has been identified that is capable of explaining all of the compositional features of the exotic Koolau component. Based on phase compositions in our experiments, the calculated density of near-solidus eclogite is 3440 kg/m3, notably less than commonly assumed. Therefore, the excess temperature required for a plume to support a given proportion of eclogite in the upper mantle may be less than previously assume

High-pressure Partial Melting of Mafic Lithologies in the Mantle

We review experimental phase equilibria associated with partial melting of mafic lithologies (pyroxenites) at high pressures to reveal systematic relationships between bulk compositions of pyroxenite and their melting relations. An important aspect of pyroxenite phase equilibria is the existence of the garnet-pyroxene thermal divide, defined by the enstatite-Ca-Tschermaks pyroxene-diopside plane in CaO-MgO-Al2O3-SiO2 projections. This divide appears at pressures above ∼2 GPa in the natural system where garnet and pyroxenes are the principal residual phases in pyroxenites. Bulk compositions that reside on either side of the divide have distinct phase assemblages from subsolidus to liquidus and produce distinct types of partial melt ranging from strongly nepheline-normative to quartz-normative compositions. Solidus and liquidus locations are little affected by the location of natural pyroxenite compositions relative to the thermal divide and are instead controlled chiefly by bulk alkali contents and Mg-numbers. Changes in phase volumes of residual minerals also influence partial melt compositions. If olivine is absent during partial melting, expansion of the phase volume of garnet relative to clinopyroxene with increasing pressure produces liquids with high Ca/Al and low MgO compared with garnet peridotite-derived partial melt

Partial melting experiments on a MORB-like pyroxenite between 2 and 3 GPa: constraints on the presence of pyroxenite in basalt source regions from solidus locations and melting rate,

[1] We present partial melting experiments at 2-3 GPa on a basaltic pyroxenite (G2) similar in composition to typical oceanic crust. The 3.0 GPa solidus is located at 1310 ± 12°C and the liquidus is 1500-1525°C. Clinopyroxene, garnet, quartz, and rutile are subsolidus phases. Garnet, quartz, and rutile are absent above 1475°C, 1365°C, and 1335°C, respectively. At the solidus, the garnet mode is low (18 wt.%) because clinopyroxene is unusually aluminous (13.8-15.5 wt.% Al 2 O 3 ). In adiabatically upwelling mantle near 2-3 GPa, G2-like pyroxenite begins melting 35-50 km deeper than peridotite. The calculated near-solidus adiabatic productivity for G2 is $13$59%/GPa through the melting interval, suggesting substantial partial melting deep in basalt source regions: G2 is $60$<2% of the MORB source. Owing to high extents of partial melting, the effect of modest amounts of pyroxenite on Sm/Yb ratios of aggregated basalts is limited and depends largely on the average bulk composition of the pyroxenite source. Low near-solidus adiabatic productivities could allow small ($1-2%) proportions of basaltic pyroxenite to enhance

Distribution of recycled crust within the upper mantle : insights from the oxygen isotope composition of MORB from the Australian-Antarctic Discordance

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 10 (2009): Q12004, doi:10.1029/2009GC002728Geochemical heterogeneity within the mantle has long been recognized through the diversity of trace element and radiogenic isotopic compositions of mantle-derived rocks, yet the specific origin, abundance, and distribution of enriched material within the mantle have been difficult to quantify. In particular, the origin of the distinctive geochemical characteristics of Indian mantle has been debated for decades. We present new laser fluorination oxygen isotope measurements of mid-ocean ridge basalt from the Australian-Antarctic Discordance (AAD), an area where a particularly abrupt transition occurs between Pacific-type mid-ocean ridge basalts (MORB) and Atlantic-type MORB. These data show no distinction in average δ18O between Pacific- and Atlantic-type MORB, indicating that the origin of Indian-type mantle cannot be attributed to the presence of pelagic sediment. The combined radiogenic isotope, δ18O, and trace element characteristics of Indian-type MORB at the AAD are consistent with contamination of the Indian upper mantle by lower crustal material. We also present a compilation of available laser fluorination δ18O data for MORB and use these data to evaluate the nature and percentage of enriched material within the upper mantle globally. Data for each ocean basin fit a normal distribution, with indistinguishable means and standard deviations, implying that the variation in δ18O of MORB reflects a stochastic process that operates similarly across all ocean basins. Monte Carlo simulations show that the mean and standard deviation of the MORB data are robust indicators of the mean and standard deviation of the parent distribution of data. Further, although some skewness in the data cannot be ruled out, Monte Carlo results are most consistent with a normal parent distribution. This similarity in characteristics of the δ18O data between ocean basins, together with correlations of δ18O with radiogenic isotope and trace element characteristics of subsets of the data, suggest that the upper mantle globally contains an average of ∼5–10% recycled crustal material and that the depleted mantle in the absence of this component would have δ18O of ∼5.25‰. The Monte Carlo simulations also suggest that additional oxygen isotope data may be used in the future to test the ability of geodynamical models to predict the physical distribution of enriched domains within the upper mantle

Geochemical stages at Jasper Seamount and the origin of intraplate volcanoes

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 19 (2009): Q02001, doi:10.1029/2008GC002236.Ocean intraplate volcanoes (OIVs) are formed in a sequence of stages, from large to small, that involve a systematic progression in mantle melting in terms of volumes and melt fractions with concomitant distinct mantle source signatures. The Hawaiian volcanoes are the best-known example of this type of evolution, even though they are extraordinarily large. We explore the Pb-Sr-Nd-Hf isotopic evolution of much smaller OIVs in the Fieberling-Guadalupe Seamount Trail (FGST) and small, near-ridge generated seamounts in the same region. In particular, we investigate whether we can extend the Hawaiian models to Jasper Seamount in the FGST, which displays three distinct volcanic stages. Each stage has characteristic variations in Pb-Sr-Nd-Hf isotopic composition and trace element enrichment that are remarkably similar to the systematics observed in Hawaii: (1) The most voluminous, basal “shield building” stage, the Flank Transitional Series (FTS), displays slightly isotopically enriched compositions compared to the common component C and the least enriched trace elements (143Nd/144Nd: 0.512866–0.512909, 206Pb/204Pb: 18.904–19.054; La/Sm: 3.71–4.82). (2) The younger and substantially less voluminous Flank Alkalic Series (FAS) is comparatively depleted in Sr, Nd, and Hf isotope compositions plotting on the side of C, near the least extreme values for the Austral Islands and St. Helena. Trace elements are highly enriched (143Nd/144Nd: 0.512912–0.512948, 206Pb/204Pb: 19.959–20.185; La/Sm: 9.24). (3) The Summit Alkalic Series (SAS) displays the most depleted Sr, Nd, and Hf isotope ratios and is very close in isotopic composition to the nearby near-ridge seamounts but with highly enriched trace elements (143Nd/144Nd: 0.512999–0.513050, 206Pb/204Pb: 19.080–19.237; La/Sm: 5.73–8.61). These data fit well with proposed multicomponent melting models for Hawaii, where source lithology controls melt productivity. We examine the effect of melting a source with dry peridotite, wet peridotite, and pyroxenite, calculating melt productivity functions with depth to evaluate the effect of potential temperature and lithospheric thickness. This type of melting model appears to explain the isotopic variation in a range of small to large OIVs, in particular for OIVs occurring far from the complicating effects of plate boundaries and continental crust, constraining their geodynamic origin.JBT acknowledges financial support from the French Institut National des Sciences de l’Univers. The isotope work at SDSU was made possible by NSF and Keck grants to BBH

The management of acute venous thromboembolism in clinical practice. Results from the European PREFER in VTE Registry

Venous thromboembolism (VTE) is a significant cause of morbidity and mortality in Europe. Data from real-world registries are necessary, as clinical trials do not represent the full spectrum of VTE patients seen in clinical practice. We aimed to document the epidemiology, management and outcomes of VTE using data from a large, observational database. PREFER in VTE was an international, non-interventional disease registry conducted between January 2013 and July 2015 in primary and secondary care across seven European countries. Consecutive patients with acute VTE were documented and followed up over 12 months. PREFER in VTE included 3,455 patients with a mean age of 60.8 ± 17.0 years. Overall, 53.0 % were male. The majority of patients were assessed in the hospital setting as inpatients or outpatients (78.5 %). The diagnosis was deep-vein thrombosis (DVT) in 59.5 % and pulmonary embolism (PE) in 40.5 %. The most common comorbidities were the various types of cardiovascular disease (excluding hypertension; 45.5 %), hypertension (42.3 %) and dyslipidaemia (21.1 %). Following the index VTE, a large proportion of patients received initial therapy with heparin (73.2 %), almost half received a vitamin K antagonist (48.7 %) and nearly a quarter received a DOAC (24.5 %). Almost a quarter of all presentations were for recurrent VTE, with >80 % of previous episodes having occurred more than 12 months prior to baseline. In conclusion, PREFER in VTE has provided contemporary insights into VTE patients and their real-world management, including their baseline characteristics, risk factors, disease history, symptoms and signs, initial therapy and outcomes

Persistence of strong silica-enriched domains in the Earth's lower mantle

The composition of the lower mantle—comprising 56% of Earth’s volume—remains poorly constrained. Among the major elements, Mg/Si ratios ranging from ∼0.9–1.1, such as in rocky Solar-System building blocks (or chondrites), to ∼1.2–1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing, and thus homogenous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity associated with a 20-fold change in viscosity, such as due to the dominance of intrinsically strong (Mg, Fe)SiO3–bridgmanite in low-Mg/Si domains, is sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize mantle convection patterns, and coherently persist at depths of about 1,000–2,200 km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such bridgmanite-enriched ancient mantle structures (BEAMS) may reconcile the geographical fixity of deep-rooted mantle upwelling centres, and geophysical changes in seismic-tomography patterns, radial viscosity, rising plumes and sinking slabs near 1,000 km depth. Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures

Anhydrous Partial Melting Experiments on MORB-like Eclogite: Phase Relations, Phase Compositions and Mineral–Melt Partitioning of Major Elements at 2–3 GPa

ISSN:0022-3530ISSN:1460-241