Beyond Plagioclase-Peridotite: Unravelling the melt-rock interaction history of the Santa Elena Ophiolite, NW Costa Rica

The Santa Elena Ophiolite (SEO), located on the northwestern coast of Costa Rica, consists primarily of preserved oceanic mantle and crustal rocks which are predominantly characterized by mantle peridotites (i.e., spinel lherzolite with minor harzburgite and dunite). Previous investigations of the SEO focused on characterizing the tectonic origin of the complex have resulted in ambiguity, in part, due to the presence of abundant textural features which suggest the complex has experienced multiple melt-rock interaction events (e.g., disseminated plagioclase and plagioclase-spinel, CPX-spinel, and plagioclase-CPX symplectites). To further characterize and determine the relative timing of these events, we have begun a detailed investigation of several key samples from the SEO suite which best preserve evidence for one or more of these events. Fresh plagioclase in textural disequilibrium with the primary peridotite phases is inferred to be indicative of at least one of the reaction events. Though the abundant altered plagioclase found in many of the SEO peridotites might be related to the same event, the presence of coexisting, texturally distinct, altered and fresh plagioclase in many of the samples suggests that there may be more than one plagioclase-forming event preserved in the SEO. Further analysis has also revealed the presence of reactive pargasitic amphibole ± spinel in association with optically similar CPX in several samples, including two pyroxenites. Amphibole forms both as an intergranular, reactive phase in pyroxene porphyroclasts and as small grains along the cleavage planes of large CPX grains. Though trace quantities of high-Ti pargasite can be found associated with fresh plagioclase, the presence of pargasitic amphibole in the samples is suggested to be an indicator of another separate event. Critically, we observe major differences in the composition and zonation of key elements (e.g., Cr, Ti, Al, Na) in spinel and CPX depending on which reactive phase(s) they are associated with. Based on our observations of the interplay between the primary and reactive phases, we infer that plagioclase and amphibole may both play an equally significant role in the development of heterogeneity within mantle peridotite.