K. W. W. Sims

Assessing the presence of garnet‐pyroxenite in the mantle sources of basalts through combined hafnium‐neodymium‐thorium isotope systematics

[1]xa0The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinel-garnet transition are different if garnet-pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet-peridotite will have significant 230Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts.

Aberrant youth: Chemical and isotopic constraints on the origin of off-axis lavas from the East Pacific Rise, 9°–10°N

[1]xa0We report measurements of U-series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and major and trace element abundances in a suite of well-located, off-axis MORBs that span the East Pacific Rise (EPR) ridge crest from 9°48′–52′N and across it for ∼4 km on either side. The geological context of the samples are well constrained as they were collected by submersible in an area that has been extensively imaged by remote sensing techniques. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off-axis N-MORB are identical to the axial lavas from this same region, suggesting that their sources are similar and that melting processes are the dominant influence in establishing the U-Th-Ra disequilibria and trace element fractionations. A majority of off-axis samples have U-Th and Th-Ra disequilibria that are larger, and model ages that are younger, than would be predicted from their off-axis distance and the time-integrated spreading rate. There are, however, a few off-axis samples with U-Th ages that are consistent with their spreading rate ages. It is likely that these samples erupted within or close to the axial summit trough (AST) and aged at a rate proportional to the spreading rate. The anomalously young ages determined for most of the off-axis lavas suggest that volcanic construction along this region is occurring over a zone that is wider (at least 4 km) than the AST (10s to 100s of meters). The combined observational, chemical and isotopic data support a model for the 9°0′N area that includes a significant component of crustal accumulation resulting from lavas that breach the AST and flow down the flanks of the EPR ridge crest. However, these data also require a minor component of off-axis eruptions that occur on distinct pillow mounds and ridges. This suggests that MOR construction involves several volcanic and tectonic processes acting in concert to form a complex patchwork of lava ages and compositions along, and across, this fast spreading ridge crest.