ALBERT

Description: Tabular dunite bodies are thought to represent remnants of high-porosity pathways for efficient melt extraction from the mantle. They form by melt–rock reaction, an important physical process that affects the compositions of dunite-hosted basaltic melts and the mantle they originate from. To better understand melt–rock interactions in dunite channels, we analyzed clinopyroxene and orthopyroxene in samples collected across an ~20 m wide dunite–harzburgite–lherzolite–plagioclase lherzolite sequence in the previously well-studied Trinity ophiolite. We found spatial variation and fractionation in minor and trace elements in the constituent minerals. Rare earth element (REE) and high field strength element concentrations increase in unison about 9 m from the dunite–harzburgite contact. Minor elements in clinopyroxene also increase ~9 m from the dunite–harzburgite contact, and NiO contents in olivine increase ~3 m from the dunite–harzburgite contact. Clinopyroxene grains in plagioclase lherzolite samples farthest from the dunite–harzburgite contact exhibit core-to-rim variations in minor and trace elements that mimic the outcrop-scale chemical trends. Collectively, the lithological sequence and major and trace element concentration gradients suggest that a two-stage history of evolution is preserved at Trinity. In the first stage, a cooling melt infiltrated a harzburgitic residue of partial melting, precipitating plagioclase and pyroxene and forming plagioclase lherzolite. In the second stage, a trace element depleted, pyroxene- and plagioclase-undersaturated melt migrated from the dunite channel into the plagioclase lherzolite, forming a hybridized composition by reaction with the plagioclase lherzolite. Because Ni is relatively fast diffusing and compatible in olivine, it was chromatographically fractionated from other trace elements during the infiltration event. Orthopyroxene-saturated melt precipitated new clinopyroxene with depleted major and trace element compositions as it cooled in the dunite, harzburgite, and lherzolite. The REE abundances of the melts in equilibrium with dunite, harzburgite, and lherzolite are similar to those of boninitic dikes that cut crustal units at Trinity, and the infiltrating melts may be genetically related to the dikes. Dunite–harzburgite–lherzolite–plagioclase lherzolite sequences from Trinity and other peridotites probably formed by similar processes. The infiltration of dunite-hosted melts into peridotitic host-rock may be common, providing an explanation for the wide array of melt–peridotite interactions observed in abyssal peridotites and some ophiolites. This outcrop demonstrates that dunite channels can be sources of melt infiltration as well as melt extraction pathways.