V44A-05
Sediment Subduction and the Generation of Extreme Geochemical Enrichment in Lavas Erupted at Oceanic Hotspots

Thursday, 17 December 2015: 17:00
310 (Moscone South)
Matthew G Jackson1, Jason Schmidt1, Frank J Spera2, Andrew R Kylander-Clark2 and Katherine Sheppard2, (1)University of California Santa Barbara, Department of Earth Sciences, Santa Barbara, CA, United States, (2)University of California Santa Barbara, Santa Barbara, CA, United States
Abstract:
Large quantities of terrigenous sediment have been injected into the mantle at subduction zones over geologic time. During subduction, the sediment on top of the downgoing slab is likely melted and incorporated into the mantle wedge peridotite. The sediment-metasomatized mantle wedge then enters the convecting mantle. After residing in the mantle for long time periods, the sediment-enriched mantle domain is returned to the surface in upwelling mantle plumes and melted beneath hotspots.

The Samoan hotspot provides a compelling case for sediment recycling from subduction zones to hotspots. Analyses of lavas from Samoa reveal Sr-isotopic ratios of up to 0.7205. The elevated Sr-isotopic ratios, together with unique trace element patterns similar to upper continental crust, provide powerful evidence that Samoan lavas host a component of recycled sediment. The most extreme Sr-isotopic ratio (0.7216) identified in a Samoan lava was measured in clinopyroxenes (cpx) phenocrysts separated from a lava with highly radiogenic Sr-isotopic ratios: Hundreds of visibly-heterogeneous cpx phenocrysts were pooled for a single Sr-isotopic analysis; this approach likely masked the full range of Sr-isotopic ratios sampled by individual cpx phenocrysts. For example, preliminary laser ablation ICP-MS analyses of zoned plagioclase from this lava reveal heterogeneous Sr-isotopic ratios that range above that found in the cpx. Melt inclusions hosted in cpx from the a lava with extreme high Sr-isotopic ratios promise another frontier for discovering even more extreme endmember compositions. Whole rock Sr-isotopic ratios correlate with silica in the suite of enriched lavas from Samoa, and the most enriched lavas have SiO2 up to ~61 wt.%. Cpx-hosted melt inclusions from a geochemically-enriched lava have SiO2 ranging from 65-68 wt.%. The inclusions may represent a new extreme EM2 mixing endmember sampled in diluted form in Samoan lavas.

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