Robert L. King

Hafnium and neodymium isotope variations in NE Atlantic seawater

The 176Hf/177Hf ratio of seawater is measured directly with 143Nd/144Nd in a composite vertical profile in the NE Atlantic. The value of ɛHf of intermediate and deep water exhibits little variation and averages +1.0 ± 0.8; surface water shows a wider range, −5.7 to +3.3. While intermediate and deep water samples plot on, or close to, the seawater Hf-Nd isotope array, defined by ferromanganese deposits, near-surface seawater samples plot between the seawater and terrestrial rock ɛHf − ɛNd arrays. It is likely that the shallow water variability in ɛHf arises from a short residence time of Hf in surface water and isotopically heterogeneous inputs. The difference between surface and intermediate-deep water ɛHf may be maintained by the release of surface scavenged Hf at depth from a subset of sinking particles; the composition of eNd at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter. Uniform ɛHf values below 1000 m suggest the residence time of Hf may be longer than that of Nd. The radiogenic isotopic composition of seawater Hf relative to Nd and terrestrial rocks suggests that minerals with high Lu/Hf ratios may be preferentially weathered, providing a source of radiogenic Hf to the oceans.

Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by multicollector inductively coupled plasma–mass spectrometry. Lawsonite 176 Lu/ 177 Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 ± 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins.