The Competing Roles of Melting and Metasomatism on the Water Systematics of the Oceanic Lithosphere.
Abstract
Water (i.e. hydrogen calculated as H2O) contents in mantle rocks affects their physical and chemical behavior in the Earth's interior. We report H2O (by FTIR), major and trace element concentrations of minerals from peridotite xenoliths from oceanic island basalts (OIB): Hawaii (Oahu, Kauai), Canaries (Lanzarote) and Samoa (Savai'i), to test the effects of melting and metasomatism on the water systematics of the oceanic lithosphere.
Pyroxene and bulk rock H2O contents of Hawaiian peridotites correlate with depletion (e.g. OPX Al2O3, spinel Cr#), but not with metasomatism (e.g. La/Yb). Two trends are seen in the Hawaii samples: Pali (Oahu) and Kauai peridotites (trend P-K) have low bulk H2O contents (5 - 50 ppm) that form curved trends with depletion, consistent with melt residues of a MORB source with ~200 ppm H2O. Trend P-K may represent typical oceanic lithosphere unaffected by OIB metasomatism. Salt Lake Crater and Aliamanu peridotites (Oahu; trend SLC-A) have higher bulk H2O contents (50-100 ppm) for a given degree of depletion than the P-K samples. They form linear trends between H2O contents and depletion that cannot be explained by any reasonable melting model. Lanzarote peridotites are more depleted than the Hawaii samples (Fo>91), but with higher H2O contents (20-50 ppm) for a given degree of depletion than the P-K trend, and fall at the extension of the SLC-A trend. A positive correlation between H2O and CPX La/Yb and negative with Ti/Eu implies a link with carbonatitic metasomatism. The low H2O/Ce (<10) in CPX indicates dehydration, perhaps due to high solubility of H2O in carbonatite melts. Samoan harzburgites are the most depleted (Fo=91.5-92) and driest (most <10 ppm bulk H2O, 20-100 ppm H2O in OPX), with no clear correlation with indices of carbonatite or silicate melt metasomatism. These data imply that metasomatism does not overprint primary melting trends in the H2O systematics. Mineral chemistry may exert a key control on rehydration (by diffusion?) during melt rock reaction, thereby linking elements controlled by depletion and insensitive to metasomatism (e.g. Al) with H2O. Metasomatism of the oceanic lithosphere does not imply addition of volatiles, and plume-lithosphere interaction do not necessarily result in rehydration of the lithosphere.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0164B
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY