[Eoas-seminar] Last EOAS Colloquium speaker of 2018-2019 this Fri 3:30 in CAR 101

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Mon Apr 22 10:07:10 EDT 2019

Our last speaker of the 2018-2019 EOAS Colloquium series will be this Fri at 3:30 in CAR 101:

Dr Jim Gill of UC Santa Cruz
Melting bi-lithologic mantle at an enriched mid-ocean ridge unrelated to a plume:
at high spatial and temporal resolution

Mantle melting beneath mid-ocean ridges can occur in discrete batches at century scale, and the melt batches can differentiate quickly in largely closed systems within the crust. These conclusions result from high-resolution mapping, sampling, dating, and tephra stratigraphy of basalts at the Endeavour segment of the intermediate rate Juan de Fuca Ridge. Chemically diverse basalts erupted within hundreds of meters of one another, and hundreds of years apart. Because they differ by factors of 5 in K2O/TiO2 and 3 in Nb/Zr, their major and trace elements and isotope define several basalt types and stratigraphic units. Ratios of highly incompatible trace elements, and radiogenic isotopes, are typical of MORB globally and define binary mixing trends. However, ratios of moderately incompatible elements such as La/Sm and K/Ti define two separate mixing trends relative to Pb and Hf isotopes. These element ratios are higher relative to isotope ratios when the ridge axis was inflated from 25 to 4 ka, and lower after a graben formed ~2.3 ka. Th/U and excess 230Th also increased after graben formation. These changes are interpreted as a result of a greater degree and rate of melting of a mafic (“pyroxenitic”) component during the earlier, inflated period. Although magma mixing in the mantle or in crustal melt lenses would be required to relate all the Endeavour basalts to single source, closed system fractional crystallization at about the depth of the seismically observed melt lens can explain magma evolution within single stratigraphic units. About 15-30% crystallization of roughly equal amounts of plagioclase and clinopyroxene, with 0.2-0.4 wt% H2O and fO2, ~QFM,is required. The depth of crystallization may have shoaled when the graben formed. Geological control adds much to igneous petrology, even at mid-ocean ridges.
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