Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic fractionation by diffusion during mantle melting

Abstract

The Trinity peridotite (northern CA) contains numerous lithologic sequences consisting of dunite to harzburgite to spinel lherzolite to plagioclase lherzolite. Previous workers have documented geochemical gradients in these sequences consistent with melt-rock reaction processes occurring around dunites, interpreted to reflect conduits for melt ascent. We have undertaken a study of Li isotope compositions of clinopyroxene and some olivine within these sequences using ion probe techniques to test the hypothesis that the geochemical gradients are related to diffusive fluxing of alkali elements into or away from the melt conduit.Results show large variations in 7 Li/ 6 Li occurring in a consistent pattern across three transects from dunite to plagioclase lherzolite within the Trinity peridotite. Specifically, measurements of average ␦ 7 Li for single thin sections along the traverse reveal a low in ␦ 7 Li in the harzburgite adjacent to the dunite returning to higher values farther from the dunite with a typical offset of ϳ10 per mil in the low ␦ 7 Li trough. This pattern is consistent with a process whereby Li isotopes are fractionated during diffusion through a melt either from the dunite conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit. The patterns in 7 Li/ 6 Li occur over a length scale similar to the decrease in REE concentration in these same samples. Explaining both the trace element and Li isotopic gradients requires a combined process of alkali diffusion and melt extraction.We develop a numerical model and examine several scenarios of the combined diffusion-extraction process. Using experimentally constrained values for the change in Li diffusion coefficient with isotope mass, large changes in ␦ 7 Li as a function of distance can be created in year to decade timescales. The addition of the melt extraction term allows larger Li concentration gradients to be developed and thus produces larger isotopic fractionations than diffusion only models. The extraction aspect of the model can also account for the observed decrease in rare earth element concentrations across the transects.

Multi-mode Li diffusion in natural zircons: Evidence for diffusion in the presence of step-function concentration boundaries

“…Richter et al (2003) first documented diffusioninduced Li isotopic fractionation in an experimental study that measured a Li isotopic profile across the interface of two compositionally distinct end-member lithologies, molten basalt and rhyolite, and found that 6 Li diffuses ∼3% faster than 7 Li. Later, Lundstrom et al (2005) reported diffusive fractionation of Li isotopes in the Trinity Ophiolite. Following this, a series of studies reported Li diffusion within the country rocks adjacent to Li-rich intrusions (Teng et al, 2006;Marks et al, 2007;Liu et al, 2013), finding large and systematic Li isotopic variation over tens of meters and extremely low δ 7 Li (down to −20❤) values, which they interpreted to be the result of diffusion-driven fractionation of Li isotopes.…”

The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

“…The only previous numerical model to explain Li isotope compositions in a tectonically 505 emplaced peridotite was reported by Lundstrom et al (2005) for the Trinity Ophiolite. 506 Lundstrom et al (2005) reported troughs in δ 7 Li in the harzburgitic margins of dunite 507 channels though host lherzolites.…”

“…Highly variable 59 differences in δ 7 Li between different bulk mineral analyses from xenoliths indicate isotopic 60 disequilibrium likely driven by differential rates of Li diffusion (Jeffcoate et al, 2007;61 Rudnick Tang et al, 2007). Systematic changes in δ 7 Li with macroscopic 62 sampling position also implicate diffusional perturbation over a longer length-scale 63 (Lundstrom et al, 2005;Teng et al, 2006). 64…”

Diffusive fractionation of Li isotopes in wet, silicic melts

“…Experimental demonstrations have shown that Li diffusion in melts and minerals is subject to a large mass effect (Richter et al, 2003(Richter et al, , 2014(Richter et al, , 2017Dohmen et al, 2010). In natural samples, diffusive fractionation of Li isotopes has been recorded at the µm to m scale (e.g., Barrat et al, 2005;Lundstrom et al, 2005;Teng et al, 2006;Jeffcoate et al, 2007;Gao et al, 2011). The relative diffusivities of the two isotopes of Li are expressed by the empirical constant β in the equation where D is the diffusivity of the individual isotopes (Richter et al, 1999); the larger the value of β, the more sensitive the diffusivity is to isotope mass.…”