Platinum group element systematics of altered oceanic crust

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).