A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations |
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| Authors: | Emily Tursack Yan Liang |
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| Institution: | (1) Department of Geological Sciences, Brown University, Providence, RI 02912, USA; |
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| 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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