Intense magmatic heating of oceanic lithosphere revealed by Hawaiian xenoliths

Mantle peridotite thermobarometry has been extensively used to establish the thermal state of the lithosphere. The pressure and temperature (P-T) information from these rocks is routinely compared to model geotherms and the goodness of fit can be used to identify critical parameters of the thermal model. This method implies that mineral thermobarometers do not have sufficient time to re-equilibrate during their transport to the surface and therefore have preserved the ambient geotherm of the source region.

In this work we performed a systematic analysis on garnet peridotite xenoliths from Hawai’i. We employed well-established and new thermometers based on Ca-Mg, Cr-Al, Fe-Mg exchange reactions between opx-cpx, opx-grt and cpx-grt mineral pairs. Pressure was determined using the Al solubility between opx and grt. Our results reveal that the Hawaiian xenoliths fit oceanic geotherms that span from 45 to 25 Ma. This apparent fit is at odds with the well-established age of the oceanic lithosphere in the region (90 Ma).

The discrepancy between the two age groups can be perfectly explained by the fact that the lithosphere beneath Hawai’i has experienced intense thermal perturbation during the ascent of the magmas to the surface. Our interpretation is in agreement with published plate flexure models that call on magma-assisted flexural weakening of the lithosphere.