Platinum group element systematics of altered oceanic crust
|The deepest drillhole into oceanic crust (DSDP/OPD Hole 504B) was drilled by ODP drill ship Joides Resolution in several legs. We have worked on core material recovered from this hole to investigate the effects of hydrothermal alteration on the chemical composition of oceanic crust. (ODP)|
|In order to complement drill core material recovered from in-situ oceanic crust, we have recently started to investigate the platinum group element and Re-Os isotope geochemistry of obducted oceanic crust - the Oman ophiolite. (Carolina Lithgow-Bertelloni [UMICH])|
Wolfgang Bach (WHOI, now at Bremen University, Germany), Stan Hart (WHOI), Jerzy Blusztajn (WHOI), Prof. Peter Kelemen (Columbia University), Karen Hanghoj (Columbia University, now at the Danish Geological Survey)
In two NSF-sponsored research projects (NSF-OCE9811209 "Hydrothermal Redistribution of PGEs and U-Th-Pb in DSDP Site 504B; NSF-OCE0337677 "Rhenium-Osmium Isotope and Platinum Group Element Systematics of Lower Oceanic Crust), our group set out to investigate the geochemistry of platinum group elements (PGE) and the rhenium-osmium isotope system in altered oceanic crust. This research significantly improves our understanding of the effects of alteration of oceanic crust on the inventory of PGE in oceanic crust. As altered oceanic crust is subducted into the mantle, this research also contributes to our understanding of the long-term evolution of this important geochemical reseroir.
The altered upper oceanic crust in DSDP/ODP Hole 504B (about 7 Myr old) has PGE concentrations of: 21 pg Os/g 9 pg Ir/g 304 pg Pt/g 265 pg Pd/g 1526 pg Re/g and is characterized by an average 187Re/188Os of 353 and a 187Os/188Os of 0.213. In contrast, 118 Myr old altered oceanic crust in DSDP Sites 417 and 418 in the western Atlantic has a much more radiogenic 187Os/188Os of 1.042 (+- 0.0016, s.d.), but a nearly identical 187Re/188Os of 349. PGE concentrations in Holes 417/418 are: 33 pg Os/g 25 pg Ir/g 698 pg Ru/g 804 pg Pt/g 2001 pg Pd/g 2153 pg Re/g. Analytical data for more than 40 samples from DSDP/ODP Hole 504B are published in Peucker-Ehrenbrink et al., 2003, G-cubed 4, doi: 10.1029/2003GC000414. Additional major, trace element and isotope (Sr, O, S) data are published in Bach et al., 2003, G-cubed 4, doi: 10.1029/2002GC000419.
Comprehensive analytical work on a crustal section through the Oman ophiolite provides insights into the composition of the lower oceanic crust. In contrast to earlier data for mid-crustal gabbroic rocks recovered from ODP Hole 735B (Blusztajn et al., 2000), our data for a 4.7 km section of lower oceanic crust (Oman ophiolite) indicate that the lower oceanic crust is the main PGE reservoir. A model 6.5 km thick section of oceanic crust that consists of 1825 m DSDP 504-like upper oceanic crust and 4680 m Oman-like middle and lower crust contains: 44 pg Os/g 128 pg Ir/g 2828 pg Pt/g 1760 pg Pd/g 500 pg Re/g. Corrected for radiogenic ingrowth since emplacement, such crust has a 187Os/188Os of 0.144 and a 187Re/188Os of 80. It will develop 187Os/188Os values of 0.82, 1.51, 2.9 and 4.32 after 0.5, 1.0, 2.0, and 3.0 billion years, respectively, with Os concentrations of 47, 51, 59, and 67 pg/g. Mixtures of 30% of such subducted crust with 70% mantle-peridotite can evolve HIMU-type 187Os/188Os signatures within 2 billion years (Peucker-Ehrenbrink et al., Geology, accepted, 2011).