HETEROGENITY OF HAFNIUM ISOTOPES IN OCEANIC MANTLE RECORDED BY ZIRCONS FROM GABBROS IN SLOW-SPREADING RIDGES

Ocean drilling near the 15°20’ and Atlantis (30°N) Fracture Zones along the Mid-Atlantic Ridge (MAR) reveals discontinuous gabbroic crust and mantle peridotite exposed on the seafloor. Published core observations, geochronology, and geochemical evidence suggest that these crustal sections were built by protracted intrusion of small melt bodies (10’s of m to cm’s thick). We report ɛHf values from 134 zircons hosted by 17 evolved rocks from 3 cores drilled in these locations in order to further examine mechanisms for igneous construction of the lower oceanic crust and investigate the upper mantle source of the crust-forming magmas.

Hafnium isotope compositions were determined by LA-ICP-MS at the University of Florida. The zircon standard FC-1 was analyzed throughout the session and gave an external precision of ±1.4 (ɛHf; 2 s.d.; n=55). Ocean crustal zircons display a broad range of ɛHf values from 13.3 to 22.1. Zircons within single cores differ by up to 7.4 epsilon units (ɛHf = 13.3 - 20.7, 1.4 km core 1309D, 30°N). Core 1275D (209 m, 15°44’N) yields ɛHf values from 15.4 to 22.1 (n=44). Two samples from core 1270D (57 m, 244 km south of 1275D) have uniform ɛHf values from 13.3 to 16.8 (n=16). These single cores are more variable than published zircon values from the Markov Deep (ɛHf = 18.8 ± 0.3; Kostitsyn et al., 2009) and similar to the range of ɛHf reported for basalts extending ~200 km along this portion of the ridge.

The overall range and downhole distribution of ɛHf in cores 1275D and 1309D are consistent with crustal construction from discreet pulses of magma derived from local heterogeneously depleted mantle sources. It is inferred that small-scale regions of the mantle source preserve varying degrees of isotopic depletion (also indicated by harzburgites on the seafloor near the 15º20’ FZ), resulting from differential melt extraction or refertilization in the geologic past. The ɛHf range arises from segregation and isolation of mantle sources and is dependent on ¹⁷⁶Lu/¹⁷⁷Hf ratio and time. If it is assumed that heterogeneity developed from an average depleted MORB mantle at the time indicated by the NHRL Pb ‘age’ (~1.8 Ga), a spread in ¹⁷⁶Lu/¹⁷⁷Hf of the mantle sources of 0.0365 to 0.0435 would be required to develop the observed ɛHf ranges. More recent development of local heterogeneities would imply larger differences in Lu/Hf.