ASSESSING THE LIGHT ELEMENT CYCLE: THE FORE-ARC CONNECTION

One of the most significant cycles that define Earth’s various chemical systems are those that include the light elements. Li, Be, and B are the three most important elements that fall into this category. It is known that altered oceanic mantle is a repository for these elements, and subduction zones carry these elements into the lower mantle - a plausible explanation for the HIMU-isotopic signature. However, the concentrations of these elements have hardly been explored or quantified in mantle peridotite. Estimates of the light element content of primitive mantle and depleted mantle imply very low abundances in whole rock samples and major mantle phases - around 1 ppm for Li and B, consistent with C1-chondrite, and Be being extremely low (< 0.07 ppm).

The Coast Range Ophiolite, a Jurassic fore-arc ophiolite that preserves mantle lithologies formed in response to hydrous melting, light element compositions are highly enriched, e.g., Li up to 10x DMM, B up to 600x DMM, and Be are also present above detection limits. The concentrations for most of the light elements should effectively be zero after partial melting, however, upon analysis were enriched by factors of up to ~10⁵ to ~10⁴⁰; therefore any enrichment observed in our samples must be due solely to fluid addition within the mantle wedge. To model this enrichment a new algorithm was derived, which modeled the fluid enrichment process and represents the total addition of material to the mantle wedge source region. This calculation can be applied to any refractory mantle peridotite that has been modified by melt extraction and/or metasomatism. Application of this new method shows that the amount of fluid added to a DMM-source was in the 10’s of ppm range for each light element.

It has been demonstrated that other fore-arc ophiolites, e.g., Oman, Troodos, Bay of Islands, among others, display enriched light element signatures. These ophiolites, however, have been shown to have subduction origins. One of the critical issues to understanding this cycle is finding other repositories for light elements. We posit that lower oceanic crust (peridotites and gabbros) and eclogites could represent such reservoirs. Demonstrating enriched light elements in these two domains would be an important component for understanding mantle dynamics and the cycling of light elements since accretion.