Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v51f..05s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V51F-05
Other
1025 Composition Of The Mantle, 1040 Radiogenic Isotope Geochemistry, 3036 Ocean Drilling, 3060 Subduction Zone Processes (1031, 3613, 8170, 8413), 3613 Subduction Zone Processes (1031, 3060, 8170, 8413)
Scientific paper
Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the Mariana forearc region have been studied for their platinum group element (PGE) concentrations and Os isotopic compositions. The samples allow a look at the slab fluid-modified subarc mantle immediately overlaying the actively subducting Pacific slab at depths of ~ 30 km. The average PGE (Os 2.3 ppb, Ir 1.5 ppb, Ru 5.4 ppb, Pd 1.6 ppb and Pt 16.3 ppb) and Re (60 ppt) abundances are comparable to those measured in other subarc mantle xenolith suites. The PGE and Re abundances are depleted up to 3 orders of magnitude relative to chondrites, with relative order of depletions Ru > Os > Ir> Pt> Re> Pd. The variable Pd contents (0.01-4.5 ppb) and the low Pd/Os (<2) in the Mariana forearc samples differ significantly from that of altered oceanic crust (Pd/Os~ 22), suggesting Os remained relatively unchanged during low temperature subduction-fluid additions. Serpentinitization of the harzburgites occurred in equilibrium with fluids that were both reducing and highly alkaline. Os in its original phases may be stabilized in such an environment, therefore can preserve evidence for ancient melting despite later slab interactions. Finally, the high Pt/Pd (ave. ~ 25) and low Re contents (ave.~ 60 ppt) in the Leg 195 forearc samples are complementary to those measured in boninites from the Izu-Bonin-Mariana arc-basin system, showing a genetic relationship. Our samples reveal an average 187Os/188Os of 0.123 [range = 0.119- 0.127], making them similar to abyssal peridotites from elsewhere. Radiogenic 187Os/188Os ratios would have been imparted to the serpentinites by slab fluids, so the subchondritic Os isotopic compositions implies that peridotite-slab fluid interactions did not alter the Os isotope systematics of the mantle protoliths. Low alumina abundances (<1 %) and trace element signatures (low HFSE; REE with U-shaped chondrite-normalized patterns and 2-3 times lower than chondrite meteorite abundances) imply that the serpentinites are highly melt-depleted. Depleted mantle model ages calculated from the Os isotopic compositions span the range from several million to more than billion years as was shown for other Mariana subarc mantle samples recovered ~ 1000 km to the north at Conical Seamount [Parkinson et al., Science, 1998]. The dramatic age variations and the trace element and isotope systematics in the subarc mantle of the Marianas reveal that 55 M.y. of active plate convergence in the Western Pacific did not entirely recycle the pre-subduction mantle. We suggest that remnants of old mantle remain trapped as small "plum"- like domains in the region between the Mariana Trench and the Mariana volcanic front.
Horan Mary F.
Mock T. D.
Savov Ivan P.
Shirey Steven B.
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