Hydrogen isotope heterogeneities in the mantle from ion probe analysis of amphiboles from ultramafic rocks

Abstract

An ion probe analytical procedure has been established to measure in-situ DH ratios in hydroxylated minerals on the scale of a few tens of μm with a precision of ±10‰. This technique has been applied to determine the DH ratio of amphiboles occurring as rare disseminated minerals in peridotites. These amphiboles include pargasite from the Lherz peridotite, Pyre´ne´es (France), and kaersutite and pargasite in ultramafic xenoliths and megacrysts from the Massif Central (France), Nunivak Island (Alaska) and Salt Lake Crater (Hawaii). The δD value measured in Lherz (−92 and −65) and Nunivak Island (−93 to −69) are in the accepted range for upper mantle minerals. The higher δD (−59 to −28) from the Massif Central xenoliths may involve contamination of the subcontinental lithosphere by subduction of seawater-altered oceanic crust. The very low δD (down to −125) observed for Hawaiian xenoliths could reflect the presence of a previously unidentified deep hydrogen component in the mantle. For the Massif Central and Salt Lake Crater, δD smow variations up to 70‰ were measured both on the scale of single crystals (≤200 μm) and among crystals of the same sample. Chemical variations are minor and not correlated with δD . Combining the diffusion data for hydrogen in amphiboles with these results implies that the time interval between the onset of exchange and volcanism was shorter than a few hundred years. The δD heterogeneity observed for the latter two localities probably results from incomplete equilibration between xenoliths and fluids or hydrous magmas rather than from fractionation processes.