Light Element Systematics of Metamorphic Clasts from Odp Legs 125 and 195, South Chamorro and Conical Seamounts, Mariana Forearc.
Abstract
ODP Legs 125 and 195 drilled the summits and flanks of two actively fluid-venting mud volcanoes in the Mariana Forearc: the Conical and the S.Chamorro Seamounts. The recovered lithologies include serpentinized harzburgites and dunites as well as metamorphic clasts, both enclosed in a serpentine mud matrix. The metamafic clasts are schists with highly variable diagnostic mineral assemblages - chlorite, serpentine, talc; tremolite, actinolite and glaucophane; albite, quartz, prhenite, epidote and zeolites. While the major and trace element geochemistries of the metamorphic clasts are highly variable, two protolith groups can be identified. The first group includes serpentinites/mantle-derived source rocks with high LOI, high Ni (avg. 2280 ppm), high Cr (1040-3950 ppm), and low TiO2 (0.04wt%- 0.4 wt%) abundances. The second group includes metamorphosed MORB/sedimentary materials with low LOI, high TiO2 (0.97- 3.58 wt%), low Ni (avg. 470 ppm) and Cr (14- 340 ppm). Samples in this group have MORB-like REE patterns ([Sm/Yb]N ~ 1; [La/Sm]N ~ 0.6-1), as well as non-serpentinite Y abundances (avg: 30 ppm) and Si/Mg ratios ranging from 2.1 to 8.9. Light element/immobile element ratios of the clasts show extreme enrichments of fluid mobile elements (FME). Clasts from both seamounts have very high B/Be (8.2-2715; avg: 358) and B/Li (avg: 2.9). As noted by Bebout et al.(1993, 1999), metamorphism at the slab-mantle interface may prevent the deep subduction of highly volatile components such as H2O, B, As, Cs and Sb. Although only 5-10% of the recovered clasts are slab-derived metamorphic rocks, major and trace element signatures of the serpentinite muds suggest the admixture of a slab crustal component. The extreme FME enrichments preserved in these crustal rocks may thus indicate a large shallow FME outflux.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.V51A..08G
- Keywords:
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- 1010 Chemical evolution;
- 1025 Composition of the mantle;
- 1065 Trace elements (3670)