Garnet Xenocryst in a Petit Spot Lava: Recycling or Direct Formation in Oceanic Lithosphere?

Abstract:

Xenoliths and xenocrysts present in petit spot lavas can provide information on the nature and processes occurring in the oceanic lithosphere. Petit spot volcanoes represent small-scale seamounts formed by the extraction of low-degree melts from the base of the lithosphere in response to plate flexure and/or crack propagation. Here, we report the discovery of a garnet xenocryst present in a petit spot sample from east of Japan. The xenocryst is pyrope-rich (Py₆₀, Gr₂₀, Al₁₆), has low Cr and Ti contents, depleted LREE, a slight positive Eu anomaly and flat HREE with primitive mantle normalized values of around one.

The presence of garnet in the oceanic lithosphere could originate from gt-peridotite but such garnet should have higher Mg# and Cr content than observed in our sample. Garnet in oceanic lithosphere could also be linked to the presence of gt-pyroxenite. Mantle pyroxenite xenoliths are frequently interpreted as subducted oceanic crust (eclogite). Although we cannot exclude this hypothesis, the presence of eclogite in the Pacific oceanic lithosphere would require long tramsport time without melting at the mid-oceanic ridge. In addition, the pyrope content of the xenocryst is relatively high and the HREE are low compared to garnet from typical eclogites.

Gt-pyroxenite could also form by direct crystallization from a melt percolating through the lithosphere. However, the REE pattern of the garnet xenocryst excludes direct crystallization from a basaltic melt. We hypothesize, in order to explain the major and trace element composition of this garnet, that it is formed by subsolidus reaction of ol-pl-cpx cumulates during the cooling of the lithosphere at constant pressure (~1-1.5 GPa). This cumulate is interpreted to have formed during the percolation and differentiation of basaltic melts in the periphery of a mid-oceanic ridge. The presence of this garnet xenocryst supports the hypothesis that oceanic lithosphere is re-enriched by metasomatic processes.