Pb–isotopic Features of Primitive Rocks from Hess Deep: Distinguishing between EPR and Cocos–Nazca Mantle Source(s)

Abstract:

We have acquired high–precision Pb–isotopic signatures of primitive lithologies (basalts/gabbros) recovered from IODP Expedition 345.The Hess Deep Rift, located in the vicinity of the Galapagos triple junction (Cocos, Nazca, and Pacific), is viewed as one the best–studied tectonic windows into fast–spreading crust because a relatively young (<1.5 Ma) cross section of oceanic crust. This allows for (1) characterization of the mantle source(s) at Hess Deep, (2) insight into the extent of isotopic homogeneity or heterogeneity in the area, and (3) constrain the relative contributions from the intruding Cocos–Nazca spreading center.

The observed Pb–isotopic variation at Hess Deep covers almost the entire range of EPR MORB (10°N to –5°S). Hess Deep samples range from ²⁰⁸Pb (37.3–38.25), ²⁰⁷Pb (15.47–15.58), ²⁰⁶Pb (17.69–18.91). These compositions suggest that this part of Hess Deep mantle is no more isotopically homogeneous than EPR mantle. Two distinct arrays are also observed: ²⁰⁸Pb–enriched (r²=0.985; n=30) and ²⁰⁸Pb–depleted (r²=0.988; n=6). The ²⁰⁸Pb/²⁰⁴Pb isotopes indicates that the Pb–source for some of the samples at Hess Deep had very low Th/U ratios, whereas other areas around the Galapagos microplate seem to have more “normal” ratios. These trends are less apparent when viewed with ²⁰⁷Pb–isotopes. Instead, the majority of basalts and gabbros follow the NHRL, however, at the depleted–end of this array a negative excursion to more enriched compositions is observed. This negative but linear trend could signify an alteration trend or mixing with an EMI–type mantle source, yet this mixing is not observed with ²⁰⁸Pb. This trend is also observed at Pito Deep, which has similar origins to Hess Deep (Barker et al., 2008; Pollack et al., 2009).

The Galapagos region has been considered a testing ground for mixing of HIMU, Enriched Mantle, and Depleted Mantle reservoirs (e.g., Schilling et al., 2002). According to our data, however, an EPR–component must also be considered. We model Hess Deep Pb–isotopes as a 4–component system. EPR–DM–EM comprise a ‘local’ reservoir, but the majority of samples contain a mixture of modified-HIMU–EM–EPR, a product of incoming plume material entrained within the Galapagos Spreading Center.