Abstract
Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.
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