Computer Science
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....11097d&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #11097
Computer Science
Scientific paper
Oceanic crust has a high Re/Os ratio and, over time, develops a high 187Os/188Os ratio; the system is therefore an excellent tracer for subducted crustal material in the mantle. Some ocean island basalts (OIB) possess high 187Os/188Os isotopic ratios and these are thought to result from a component of subducted oceanic crust in the mantle source (e.g. HIMU). However, the available data from basalts suggest that up to 60% of Re may be lost during high pressure metamorphism, resulting in lower Re/Os ratios in the subducted slab [1]. In this case, large quantities of oceanic crust or very long residence times in the mantle source would be required to generate the observed OIB 187Os/188Os ratios. This study focuses on gabbros, which constitute the major part of oceanic crust, and are therefore crucial to the bulk Re-Os budget of subducted material. The Allalin gabbro of the Zermatt - Saas ophiolite zone (Switzerland) was formed at 163.5 +/- 1.8 Ma [2]. It was subsequently subjected to high-pressure metamorphism and contains both metastable gabbros and corresponding eclogites. The latter were formed during peak metamorphism at 2.0-2.2GPa and ~600°C around 40Ma. These samples allow a direct assessment of the Re-Os systematics of the gabbro-eclogite transformation during subduction-type high pressure metamorphism. Gabbroic samples have a mean Os abundance of 15ppt, and 300ppt Re. Eclogites have mean Os of 16ppt and 287ppt Re. A plot of 187Re/188Os against 187Os/188Os reveals a regression line for gabbros indicating an age of 164 Ma with an initial 187Os/188Os ratio within error of the mean upper mantle value. Eclogites display a regression line with an age of 153 Ma which is within error of the gabbro age. These data suggest that Re-Os has essentially behaved as a closed system, and there appears to have been no significant Re loss during transformation of these gabbros to eclogite facies at the temperatures and pressures observed. This implies that the high Re/Os ratios may, at least in some cases, be retained through subduction and recycled into the mantle. [1] Becker, H. (2000), Earth Planetary Sci. Lett. 177, 287-300. [2] Rubatto, D., Gebauer, D. and Fanning, M. (1998), Contrib. Mineral Petrol. 132, 269-287.
Argles T. W.
Burton Kevin W.
Dale Christopher W.
Gannoun Abdelmouhcine
Parkinson Ian J.
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