Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v33c2236t&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V33C-2236
Physics
1040 Radiogenic Isotope Geochemistry, 3060 Subduction Zone Processes (1031, 3613, 8170, 8413), 3610 Geochemical Modeling (1009, 8410), 3621 Mantle Processes (1038)
Scientific paper
Xenoliths from the Cretaceous Koidu kimberlite complex, Sierra Leone, West Africa, provide a rare opportunity to investigate the origin of rutile-bearing eclogites with variable, but superchondritic Nb/Ta, Nb/La and Ti/Zr ratios (Rudnick et al, 2000). Previous studies of the trace element and δ18O values of mineral separates and reconstructed whole rock compositions of two suites (high and low MgO) of eclogites lead to two inferred origins: as cumulates (high MgO eclogites, Barth et al., 2002) or residues of altered Archean oceanic crust (low MgO eclogites, Barth et al., 2001). We present the first Hf-Nd isotope data on clinopyroxenes (cpx) and garnets (gt) from Koidu eclogites. The reconstructed whole rock (cpx+gt) Hf isotopic compositions are heterogeneous, ranging in ɛHf from 0.2 to + 41 at the time of kimberlite eruption (85 Ma). Nd isotopic compositions are equally variable (ɛNd = 0.1 to +37), placing all eclogites at drastically more radiogenic values than terrestrial basalts. However, a significant part of the Hf- budget can reside in the rutile, which will most likely have a less radiogenic hafnium isotopic composition compared to the silicates. One can construct a limiting case by using mass balance arguments to calculate the hypothetical hafnium isotopic composition of the rutile. The bulk rock hafnium isotopic composition was constrained to be 4Ga old subducted oceanic crust. Even with a high modal abundance of rutile and a high Hf concentration in rutile, the calculated isotopic compositions are unrealistic and calculated rutile ages far exceed the age of the Universe. Using a younger age for the eclogite (2 or 3 Ga) does not significantly affect the calculated result. These first-order approximations show that the addition of rutile does not change the whole rock isotopic composition significantly from the radiogenic Hf-Nd compositions based on cpx and garnet alone. Our findings do not support geochemical models that predict 143Nd/144Nd and 176Hf/177Hf compositions of aged subducted oceanic crust to be less radiogenic in hafnium isotopic composition than the OIB array. Our measured compositions for the Koidu eclogites do, however, overlap with those of the Roberts Victor eclogites; the type locality for subducted Archean oceanic crust (Jacob et al., 2004). This result supports the conclusion that the isotopic composition of these eclogites represent small-scale heterogeneities in the Earth's mantle exaggerated by radiogenic in-growth following isolation in the subcratonic lithosphere. Eclogite cannot be treated as a homogeneous reservoir or end- member composition for global geochemical modeling.
Bizimis Michael
Rudnick Lawrence R.
Salters V. J.
Tibbetts N. J.
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