Mineralogy of subducted clay and clay restite in the lower mantle

Seismic tomography indicates that subducting oceanic lithosphere often penetrates the transition zone and eventually the lower mantle [e.g. 1, 2]. While mineralogical changes in the mafic and ultramafic portions of slabs have been well documented experimentally, the phase relations of overlying sediments at pressures above 25 GPa remain poorly studied. This is in part because sediments are expected to partially melt at sub-arc depth (P~2.5-4.5 GPa), and contribute to the genesis of arc magmas. Sediment restites left behind after the extraction of low pressure melts undergo major chemical changes, according to the melting reaction: Coe+Phen+Cpx+H2O = Grt+Ky+Melt [3]. However, sediments may not always melt depending on the thermal regime and volatile availability and composition [3]. Hence, chemically unmodified sediments as well as restites may be entrained to greater depths and contribute to compositional heterogeneity in the deep mantle. Indeed, mineral inclusions with compositions indicative of subducted sedimentary protoliths (CAS-phase; K-hollandite; stishovite) have been reported in 'ultradeep' diamonds and suggest that deep subduction and survival of sediments occurs to at least transition zone depths [4]. With this in mind, we have performed laser heated diamond anvil cell experiments at pressures of 8-80 GPa on two anhydrous glass starting materials: a marine clay and the restite that is left after 50% melt extraction of this clay at 3 GPa and 800 °C [3]. We chose to work with an anhydrous version of the marine clay given that the investigated pressure range exceeds that of phengite stability [5], and phengite is the only hydrous phase in subducted sediments at UHP conditions. The clay was heated along a P-T path representative of a cold subduction geotherm, whereas the clay restite was heated along a hotter subduction geotherm consistent with low pressure melting. Phases were identified by synchrotron X-ray micro-diffraction at beamline I15 of the Diamond Light Source in Didcot, England. Preliminary analysis of diffraction data collected at ambient pressure indicates the following phase assemblages. At transition zone conditions the clay produces an assemblage of St+K-Holl+Gt+Cpx+CAS-phase, consistent with multi-anvil results [6]. CAS-phase is absent by 30 GPa, and K-Holl disappears from the assemblage between 40 and 50 GPa. At >30 GPa the assemblage consists of St+NAL+CF-structured phase±K-Holl. In the restite composition the assemblage at 19-24 GPa is St+Cor+Gt, and at higher pressures is mainly St+Mg-perovskite with minor unknown peaks. Further analysis of diffraction patterns and FEG-EPMA analysis of ion-milled samples are in progress to elucidate phase relations to 80 GPa. [1] van der Hilst et al., Nature 1997. 386:578-584 . [2] Fukao et al., Annu. Rev. Earth Planet. Sci. 2009. 37:19-46. [3] Skora & Blundy, J. Petrol., 2010. 51:2211-2243. [4] Bulanova et al., Contrib. Mineral. Petrol. 2010. DOI: 10.1007/s00410-010-0490-6. [5] Domanik & Holloway, GCA 1996. 60: 4133-1450. [6] Irifune et al., Earth Planet. Sci. Lett., 1994. 126:351-368.