Formation of plagioclase-bearing peridotite and a peridotite-wehrlite-gabbro suite through melt-rock reaction: An experimental study
Abstract
Plagioclase-bearing peridotites are observed among abyssal peridotites, massif peridotites, and mantle sections of ophiolites of lherzolite subtype. Formation of plagioclase-bearing peridotites is often attributed to basalt impregnation into host harzburgite or lherzolite in a thermal boundary layer. During transport through asthenospheric mantle, melt generated in the deep mantle will inevitably interact with the overlying mantle column through reactive dissolution and may leave geochemical imprints on plagioclase-bearing peridotites. To assess the role of melt-rock reaction on the formation of plagioclase-bearing peridotites and its implications for lithosphere composition, we conducted dissolution experiments in which a 88% spinel lherzolite + 12% basalt starting mixture was juxtaposed against a primitive MORB in a graphite-lined molybdenum capsule. The reaction couples were run at 1300°C and 1 GPa for 1 or 24 hrs, and then stepped cooled to 1050°C and 0.7 GPa over the next several days. Cooling promotes in situ crystallization of interstitial melts, allowing us to better characterize the mineral compositional trends produced and observed by melt-rock reaction and crystallization. A gabbro and a plagioclase-bearing peridotite were observed in the two halves of the reaction couple after the experiments were completed. The peridotite from the 24 hr reaction experiment is mostly composed of subhedral to euhedral olivines (10-50 μm in size, Mg# 75-83), poikilitic clinopyroxene (~100 μm in size, Mg# 73-83) with olivine and spinel chadocrysts, and interstitial plagioclase (An# 68-78) and melt. In a control experiment quenched after a 24 hour reaction at 1300°C the basalt completely dissolved the pyroxenes and spinels leaving a residue of rounded olivine grains (10-100 μm in size) surrounded by a relatively large melt fraction. Textural results from the step-cooling experiments suggest the following crystallization sequence from the olivine+melt mush: olivine, spinel -> clinopyroxene -> plagioclase, consistent with previously defined phase relations. Major and minor element compositions of olivine, clinopyroxene, and plagioclase show systematic crystal fractionation trends as well as the influence of melt-rock reaction. These systematic variations in mineral composition are generally consistent with those observed in plagioclase-bearing peridotites among abyssal peridotites and mantle sections of ophiolites, highlighting the roles of melt-rock reaction and subsequent cooling and crystallization in producing the lithologic diversity of oceanic lithosphere. Results from this study have important implications for the genesis of the harzburgite-dunite-wehrlite-gabbro suite around the Moho transition zone, such as those observed in Hess Deep, East Pacific Rise, Atlantic massif, and Oman ophiolite.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V33C2890S
- Keywords:
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- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3621 MINERALOGY AND PETROLOGY / Mantle processes;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology