The role of sub-continental mantle as both “sink” and “source” in deep Earth volatile cycles

• Published in: Geochimica et cosmochimica acta 2020-04, Vol.275, p.140-162
• Main Authors: Gibson, Sally A., Rooks, Eve E., Day, Jason A., Petrone, Chiara M., Leat, Phillip T.
• Format: Article
• Language: English
• Subjects: Global volatile cycles; Mantle; Peridotite; Pyroxenite; Volatiles
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2020.02.018

The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on “off-craton” sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low (