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High pressure partial melting of gabbro and its role in the Hawaiian magma source

Gregory M Yaxley1 and Alex. V. Sobolev2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2Vernadsky Institute of Geochemistry, Russian Academy of Sciences, Moscow, Russia

We conducted high-pressure experiments on a natural oceanic gabbro composition (Gb108), with the aim of testing recent suggestions that Sr-enrichment in rare primitive melt inclusions from Mauna Loa, Hawaii, may have derived from melting of garnet pyroxenite formed in the magma source regions by reaction of peridotite with siliceous, Sr-enriched partial melts of eclogite of gabbroic composition.

Gb108 is a natural, Sr-enriched olivine gabbro, with a strong positive Sr anomaly superimposed on an overall depleted incompatible trace element pattern, reflecting its origin as a plagioclase-rich cumulate. At high pressures it crystallises as a coesite eclogite assemblage, with the solidus between 1300 and 1350°C at 3.5 GPa and 1450 and 1500°C at 4.5 GPa. Low degree partial melts are highly siliceous in composition, resembling dacites. Coesite is eliminated between 50-100°C above the solidus. The whole-rock Sr-enrichment is primarily hosted by clinopyroxene. This phase dominates the mode (>75 wt%) at all investigatd PT conditions, and is the major contributor to partial melts of this eclogite composition. Hence the partial melts have trace element patterns sub-parallel to those of clinopyroxene with ≈10x greater overall abundances and with strong positive Sr anomalies.

Recent studies of primitive Hawaiian volcanics have suggested the incorporation into their source regions of eclogite, formerly gabbroic material recycled through the mantle at subduction zones. The models suggest that formerly gabbroic material, present as eclogite in the Hawaiian plume, partially melted earlier than surrounding peridotite (i.e. at higher pressure) because of the lower solidus temperature of eclogite compared with peridotite. This produced highly siliceous melts which reacted with surrounding peridotite producing hybrid pyroxene + garnet lithologies. The Sr-enriched nature of the formerly plagioclase-rich gabbro was present in the siliceous partial melts, as demonstrated by our experiments, and was transferred to the reactive pyroxenite. These in turn partially melted, producing Sr-enriched picritic liquids which mixed with normal picritic partial melts of peridotite before eruption. On rare occasions these mixed, relatively Sr-rich melts were trapped as melt inclusions in primitive olivine phenocrysts.

Figure 1.  Back-scattered image of Gb108 experiment run at 3.5 GPa and 1400°C.
The lightest phase is garnet, the mid-grey phase is omphacitic clinopyroxene and the darkest,
interstitial phase is glass, which quenched from a siliceous liquid at the termination of the experiment.