Distribution of lithium in the Cordilleran Mantle wedge

Abstract:

Enriched fluid–mobile element (i.e., B, Li, Be) concentrations in peridotites from the Coast Range ophiolite are compelling evidence that this ophiolite originated in a subduction environment. A new method presented in Shervais and Jean (2012) for modeling the fluid enrichment process, represents the total addition of material to the mantle wedge source region and can be applied to any refractory mantle peridotite that has been modified by melt extraction and/or metasomatism. Although the end–result is attributed to an added flux of aqueous fluid or fluid–rich melt phase derived from the subducting slab, in the range of tens of parts per million – the nature and composition of this fluid could not be constrained. To address fluid(s) origins, we have analyzed Li isotopes in bulk rock peridotite and eclogite, and garnet separates, to identify possible sources, and fluid flow mechanisms and pathways. Bulk rock Li abundances of CRO peridotites (δ⁷Li = –14.3 to 5.5‰; 1.9–7.5 ppm) are indicative of Li addition and δ⁷Li–values are lighter than normal upper mantle values. However, Li abundances of clino– and orthopyroxene appear to record different processes operating during the CRO–mantle evolution. Low Li abundances in orthopyroxene (<1 ppm) suggest depletion via partial melting, whereas high concentrations in clinopyroxenes (>2 ppm) record subsequent interaction with Li–enriched fluids (or melts). The preferential partitioning of lithium in clinopyroxene could be indicative of a particular metasomatic agent, e.g., fluids from a dehydrating slab. Future in-situ peridotite isotope studies via laser ablation will further elucidate the fractionation of lithium between orthopyroxene, clinopyroxene, and serpentine. To obtain a more complete picture of the slab to arc transfer processes, we also measured eclogites and garnet separates to δ⁷Li= –18 to 3.5‰ (11.5–32.5 ppm) and δ⁷Li= 1.9 to 11.7‰ (0.7–3.9 ppm), respectively. In connection with previous studies focused on high–grade metamorphic assemblages within the Franciscan complex, an overall framework exists to reconstruct the Li architecture of the Middle Jurassic–Cordilleran subduction zone.