Trace element mapping of high-pressure, high-temperature experimental samples with laser ablation ICP time-of-flight mass spectrometry – illuminating melt-rock reactions in the lithospheric mantle

Abstract Laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) is a fast, relatively high-spatial resolution method which allows to measure and visualize major, minor and trace elements in fine-grained samples. Thus, it is a promising tool for the investigation of high-pressure, high-temperature (HP-HT) reaction experiments that typically produce small grain sizes. Here, we apply LA-ICP-TOFMS mapping to HP-HT reaction experiments by Gervasoni et al. 2017 between a) hydrous eclogite and fertile peridotite, which simulates metasomatism in subduction zones, and b) ultramafic silicate-carbonate melt and peridotite, which simulates metasomatism at the base of cratonic lithospheric mantle. A spatial resolution (i.e., laser spot size) of 5 μm and an acquisition rate of 20 multi-element pixels per second (i.e., laser repetition rate of 20 Hz) were used for mapping of the experimental samples. We demonstrate the applicability of LA-ICP-TOFMS by comparison of our results to maps created with electron probe micro-analysis (EPMA) and elemental concentrations measured by conventional (i.e., crater-drilling) LA-ICP-MS. Our new data suggest that metasomatism in subduction zones is expected to result in the formation of Al-Ti-rich amphiboles that strongly fractionate the rare earth elements (REE) and high-field strength elements (HFSE) from other trace elements. In contrast, metasomatism of the cratonic lithosphere involves fractionation in the light and middle REE, large ion lithophile elements (LILE) and HFSE, leaving Ni and heavy REE behind in olivine and garnet, respectively. The trace element enrichment patterns of the resulting melt bear a general resemblance to those of natural Group I kimberlites.