2106 Goldschmidt Conference Abstracts Focusing of upward fluid migration Metal-silicate partitioning of Mo and due to mineral grain size variation W at high pressures and I. WADA1*, M.D. BEHN1, E.M. PARMENTIER2 AND temperatures: Applications to core A.M. SHAW1 formation on Earth and Mars 1 Woods Hole Oceanographic Institution, Woods Hole, MA, 1 2 J. WADE AND B. J. WOOD 102543, USA (*correspondence: [email protected], [email protected], [email protected]) 1Dept of Earth Sciences, University of Oxford, South Parks 2Brown University, Providence, RI, 02912, USA Rd, Oxford, OX1 3AN. (*[email protected]) ([email protected]) 2Dept of Earth Sciences, University of Oxford, South Parks Rd, Oxford, OX1 3AN. ([email protected]) In this study, we use numerical models to quantify the effect of mineral grain size on the migration path of aquous In order to place better constraints on the conditions of fluids in the mantle wedge. Grain size affects grain-scale core formation on Earth and other terrestrial planetary bodies permeability of the mantle and fluid migration, which is an we have performed experiments to determine the partitioning important factor that controls the location of hydrous melting of Mo and W between liquid Fe-rich metal and liquid silicate in the wedge. By coupling a subduction zone thermal model at pressures of 1.5-24 GPa and temperatures of 1803-2723K. with a laboratory-derived grain size evolution model, we Experiments performed in MgO capsules at 1.5 GPa/1923 K predict that the spatial variation in grain size in the flowing indicate that Mo is in the +4 oxidation state in the silicate at part of the mantle wedge is large; grain size increases from oxygen fugacities >2 log units below the IW (Fe-FeO) buffer. 10–100 µm in the shallowest part of the region beneath the In contrast W6+ is the dominant tungsten oxidation state in the forearc to a few cm in the hottest part of the mantle beneath silicate at 1.9-3.2 log units below the IW buffer the arc. Based on our preliminary modeling results, we find Mo metal/silicate partitioning is strongly dependent on that aqueous fluids that migrate into the shallow fine-grain- pressure and silicate melt composition, but temperature has no size region become trapped in the down-going mantle due to detectable effect. In contrast, we find that W partitioning is low permeability and dragged downdip until permeability strongly dependent on silicate melt composition and becomes high enough for the fluids to migrate upward. Thus, temperature, but the role of pressure is minor. the grain size distribution can play an important role in Applying these and earlier results to the Earth and Mars controlling the location of hydrous melting by focusing the indicates that the Mo content of the terrestrial mantle is upward fluid migration. We plan to further develop our model consistent with core segregation at pressures of 20-40 GPa, in by incorporating the effect of dynamic pressure gradients and agreement with earlier work on Ni and Co partitioning. The accounting for the variation in fluid influx at the wedge base. Mo content of silicate Mars is about half that of silicate Earth Our modelling results will then be compared with the and is consistent with much lower pressures of core formation locations and degrees of hydrous melting inferred from (~11 GPa) on the smaller planet. In contrast to these results, geophysical and geochemical data for various arcs worldwide. the W content of silicate Mars (~50 ppb) is insensitive to conditions of core formation while that of silicate Earth (~12 ppb) is inconsistent with a single stage of core formation at any pressure. Since metal-silicate partitioning of W is strongly influenced by light element (S, Si, O) contents of the metal, we consider it likely that the “light” element in the core is largely responsible for the inconsistency in core-mantle partitioning of this element. Mineralogical Magazine www.minersoc.org Downloaded from http://pubs.geoscienceworld.org/minmag/article-pdf/75/3/2106/2919301/gsminmag.75.3.23-W.pdf by guest on 28 September 2021 Goldschmidt Conference Abstracts 2107 REE and stable isotope constraints on From anoxia to oxic conditions in the formation of metamorphic quartz aftermath of oceanic anoxic event 2 veins: A case study from the Rhenish (Late Cretaceous) Massif (Germany) M. WAGREICH T. WAGNER1*, A.J. BOYCE2, J. ERZINGER3 University of Vienna, Center for Earth Sciences, Althanstrasse 14, 1090 Vienna, Austria, 1Geochemistry and Petrology, ETH Zurich, Switzerland ([email protected]) (*correspondence: [email protected]) 2 Scottish Universities Environmental Research Center, Sections in the Ultrahelvetic units of the Eastern Alps Glasgow, UK 3 (Austria) record oceanic anoxic event 2 (OAE 2) at the distal GeoForschungsZentrum, Potsdam, Germany European continental margin of the western Tethys [1, 2]. Upper Cenomanian marl-limestone cycles are overlain by We have investigated fluid-rock reactions during black, organic-rich (5% TOC, kerogen type II) layers, formation of metamorphic quartz veins in the fold-and-thrust followed by Lower/Middle Turonian light grey to reddish belt of the Rhenish Massif (Germany). The veins record two marly limestones. Carbon isotope values display the well assemblages that were formed in an evolving fluid-rock documented positive shift. The appearance of red-colored system, which are (1) massive vein filling (elongate-blocky carbonates (CORB - Cretaceous Oceanic Red Beds) indicates quartz, chlorite, apatite, albite) and (2) open space filling a total time span of about 1.5 my for oxic bottom waters to (euhedral quartz crystals, carbonates, sulfides). We performed become dominant. Orbital cycles of 400 kyr and 100 kyr a detailed REE and stable isotope study of vein minerals, frequencies are identified. Benthic foraminifera associations altered wall rocks and precursor host rock metapelites. The indicate repeated phases of enhanced organic matter flux and REE and oxygen isotope data of vein quartz and altered wall less aerated bottom waters during the transitional interval [3]. rocks, combined with mass balance analysis, support that local Sedimentation of red layers was controlled by periods of well mobilization of material was dominant during formation of the oxygenated bottom waters, reduced sedimentation rates and early massive vein assemblage, but that contributions from degradation of organic matter in the underlying sediments. advecting fluids were also important. The strong shift in K/Na Principal component analysis of carbonate chemical data ratios in altered wall rocks and model fluid temperatures that showed that the development of red coloured pelagic are higher (350-400 °C) than estimates for the host rocks point sediments is accompanied by a shift towards highly to substantial fluid advection. Formation of the veins can be oligotrophic conditions in the surface ocean as well as a explained by a crack-flow-seal model, with multiple repetition decrease in hydrothermal activity [4]. of vein opening, fluid advection and vein sealing events. Each Higher up in the section, red limestone-marl cycles are cycle was initiated with vein opening, resulting in enhanced present. Enhanced input of nutrient- like trace metals during permeability and considerable fluid advection and episodes of higher volcanic activity is inferred, terrigeneous hydrothermal alteration of wall rocks. Conditions during each elements (Al, Li, Rb, Be) decrease upwards. Iron speciation cycle evolved towards a decrease in fluid advection, coupled data for marl and limestone layers attest to oxic early with substantial diffusional leaching of silica and precipitation diagenesis during marl deposition compared to limestone in the veins. The formation of the later open space filling episodes. Low sediment accumulation rates (2.5 mm/ka) are assemblage records transition from an advection- to a reconstructed. Geochemistry and stable isotope data indicate a diffusion-dominated regime. This is supported by vein mineral highly oligotrophic environment with efficient recycling of and fluid inclusion textures recording conditions of organic matter and nutrients in the upper water column. undisturbed mineral growth, fluid inclusion data that point to a Nutrient availability varied and resulted in periods of higher thermally equilibrated state (150-200 °C), and stable isotope primary production. Iron oxides cause the red color in CORBs. data that demonstrate a local source for the vein minerals. The main fraction of iron in CORB sediments is fixed in silicate lattices and immobile. [1] Neuhuber et al. (2007) PPP 251, 222-238. [2] Wagreich et al. (2008) Cret. Res. 29, 965-975. [3] Wendler et al. (2009) SEPM Spec. Publ. 91, 209-221. Neuhuber & Wagreich (2011) Sediment. Geol. 235, 72-78. Mineralogical Magazine www.minersoc.org Downloaded from http://pubs.geoscienceworld.org/minmag/article-pdf/75/3/2106/2919301/gsminmag.75.3.23-W.pdf by guest on 28 September 2021 2108 Goldschmidt Conference Abstracts Uniform Os isotopic composition in Nuclear forensic analysis of trinitite early-formed planetesimals at high spatial resolution R.J. WALKER C.M. WALLACE*, A. SIMONETTI, AND P.C. BURNS Dept. of Geology, University of Maryland, College Park, MD Department of Civil Engineering and Geological Sciences, 20742, USA ([email protected]) University of Notre Dame, Notre Dame, IN 46556, USA (*correspondence: [email protected]) The isotopic compositions of some elements, such as W, Ru and Mo, vary among early-formed planetesimals. These The world’s first atomic bomb, the Trinity “gadget” was variations likely reflect incorporation of different proportions detonated on July 16, 1945. The explosion resulted in partial of matter from diverse nucleosynthetic sources, and could be melting of the surrounding desert sand, which subsequently the result of accretion from a poorly mixed nebula, fused into blast-melt glass known as trinitite. Recent accretionary processes that favored isotopically distinct investigations of trinitite have been conducted using a variety components, and/or late injection of isotopically diverse of analytical techniques, including EMPA, SEM, SIMS, XRF, matter to the nascent Solar System.