The origin of mid-ocean ridge basalts : insights from trace element contents in anorthite, anorthite-hosted melt inclusions, and ocean core complexes Public Deposited

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  • The primary goal of this study is to access the processes that alter primary melts after segregation from a mantle source and ultimately form petrologic Layers 2 and 3 of the Ocean Crust. Mineral, melt inclusion, and whole rock chemical compositions are utilized to further the understanding of 1) the behavior of trace element partitioning between high anorthite plagioclase and basalt melt, 2) the nature of the record preserved by melt inclusions in anorthitic plagioclase in MORB, 3) the variability of melt compositions generated at slow spreading ridges, and 4) the composition of melt and crystals present in an axial magma chamber extrapolated from plagioclase-ultraphyric basalts. The major and trace element compositions of melt inclusions hosted in high anorthite feldspar from Gorda Ridge and Southeast Indian Ridge are used to evaluate the degree to which melt inclusions represent original melts trapped at the time of crystallization versus melts that have been affected by entrapment processes or modified by post-entrapment processes. Melt inclusions that are 'real' melts will display trace element partitioning relationships with the adjacent mineral host that fit known models. Melt inclusions that have been modified by inclusion-specific processes, will not conform to known partitioning models. Using anorthitic plagioclase from pillow basalts dredged from the Gorda Ridge and the Southeast Indian Ridge, our results indicate that there is no apparent correlation between the calculated mineral-inclusion partition coefficients and the size or composition of the melt inclusion. The data is generally consistent with experimentally determined partition coefficients for high anorthite feldspar. However, some samples contain trace element patterns that are consistent with some degree of diffusive re-equilibrium of compatible elements, i.e., Sr, Eu, and Ba. Therefore, although plagioclase-hosted melt inclusions are broadly representative of the magmas from which with anorthite phenocrysts formed, these melts do not represent the unmodified array of primary mantle melts, and are produced by a complex set of processes. The variability of magmas produced at a slow-spreading plate boundary is evaluated using mineral analyses, combined with whole rock compositions of gabbroic and basaltic lithologies and is conducted within a spatial context. Samples are drill cores halves obtained during IODP Legs 304/5 from Sites 1309, 1310, and 1311 that are located on Atlantis Massif, at 30°N on the Mid-Atlantic Ridge. Gabbroic samples span the full range of compositions collected from ocean ridges: primitive troctolites through evolved leuco- and oxide- gabbros and whole rock Mg #s (molar Mg/Mg+Fe²⁺) range from 36 to 89 while basalts span a more limited range (50-67). Gabbroic major and trace element compositions are consistently modeled as representing nearly-pure crystal residuum. Trace elements contents of gabbroic clinopyroxenes are generally in equilibrium with basalt samples, although they span a larger range of equilibrium concentrations than the very narrow range exhibited by the basalts. These findings suggest that gabbros are very pure cumulates and that basaltic melt can be efficiently separated from the residual crystals. Plagioclase ultra-phyric basalts from Southwest Indian Ridge contain greater than 20% plagioclase phenocrysts. Plagioclase An content is generally higher than (molar Ca/[Ca+Na+K] ranges from An94 to An84) the calculated equilibrium composition for respective host melts. Additionally, crystals within a single sample exhibit a variety of zoning patterns, morphologies, and textures. Crystal size distributions of 3 samples are linear and suggest that phenocrysts within a single sample grew in a similar thermal environment. Similar CSD slopes are obtained from each sample indicative of a fairly continuous thermal environment along ridge axis. We make an argument that the best fitting model for the petrogenesis of plagioclase ultra-phyric basalts invokes a magma chamber where convection physically separates low density plagioclase phenocrysts and gradients in intrinsic variables (temperature, pressure, and composition) produce the suite of textures and zoning exhibited by individual crystals within a sample. The bimodal trace element composition of crystals from one sample indicates that melts can exist in chemical isolation until just prior to eruption.
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