Olivine and plagioclase-hosted melt inclusions and their application to determining parental arc magmas Public Deposited

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  • Three broadly interrelated problems of critical significance to the proper interpretation of melt inclusion (MI) data are addressed in this thesis. The first issue, the petrogenesis of anorthitic plagioclase phenocrysts in MORB lavas, has relevance due to the presence of numerous MI in many high-An feldspars. This problem was addressed experimentally using basaltic starting compositions saturated with anorthite and forsterite at 1 atm. The resulting liquids were in equilibrium with Al-spine! (A1203 of 6 1-68 wt%) at 1290°C and with both >Fo89 olivine and >An85 feldspar at temperatures of 1230° and 12 10°. Melt compositions were similar to natural MORB glasses with Mg#s of 63 to>85. The results suggest that dry, anorthite-bearing basaltic magmas may develop from interaction between primary melt and Al-spine! bearing upper mantle, and consequently MI in natural high-An feldspars may not represent primary magmas. It has been proposed that the mechanisms of MI formation could modify the entrapped melt composition. This problem wa addressed through another experimental series using anorthite/forsterite saturated anhydrous mafic liquids. Charges were cooled from 1300° to 1230° and 1210° C at rates of 1°-l0°/min. followed by 0-24 hour isothermal periods. Hopper and skeletal crystal morphologies developed during the cooling period, and planar overgrowth of these textures during isothermal periods formed most inclusions. In general, inclusion compositions were uniform and similar to the host glass, indicating that boundary layers were not entrapped and inclusion formation by this mechanism did not modify entrapped melts. The third problem addressed the usefulness of MI in identifying primary arc magmas. Melt trapped within olivine phenocrysts in a Mariana arc picrite lava (Mg# - 76) were the focus of this study. These had compositions of up to 21.7 wt% MgO, 0.4- 0.8 wt% Ti02, 43.2-47.0 wt% Si02, and 1.7-3.4 wt% total alkalis. Major, trace element and volatile data from MI within Fo₉₂ olivine indicate that these MI were samples of water-poor primary arc melts. Trace element patterns displayed an arc signature, but at lower total abundances relative to most arc melt inclusions. We propose that the MI do represent primary magmas from both fluxed and decompression melting of the mantle wedge.
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