Polyphase inclusions in garnet pyroxenites from Sulu (China) as carriers of seawater at ultrahigh pressure

Unravelling processes of fluid-mediated element exchange between slab lithologies and the mantle wedge is of primary importance in understanding element mobility in subduction zones. Several studies have addressed element transfer related to fluid release during prograde metamorphism in subduction zones [1-4]. Nevertheless, detailed studies documenting interactions between felsic, mafic and ultramafic rocks at ultrahigh-pressure (UHP) are still scarce [5]. For this reason, UHP metasomatised rocks represent ideal materials to study the element exchange at pressures corresponding to sub-arc depths in subduction zones. We present preliminary results of Ca-rich garnet–clinopyroxenites from Suolushu, occurring as layers in a large serpentinite body at Hujialin, Rizhao County, in the Sulu UHP metamorphic terrane (eastern China). Both clinopyroxenites and hosting serpentinites are intercalated with coesite-eclogites and hosted by coesite-bearing gneiss. Similar garnet–clinpyroxene layers from Hujialin have been studied by [6] and interpreted as cumulates crystallised from a hydrous, subduction related magma at ~ 1 GPa and 1000 °C. They were subjected to minor Ca enrichment coeval with serpentinisation of the host ultramafic rocks and then subducted at UHP (4.8 ±0.6 GPa and 750 ±50 °C). Ca-rich garnet–clinopyroxenites are composed of centimeter-sized garnet porphyroblasts in a matrix of fine-grained green diopside, opaque minerals associated with green spinel, and garnet. Garnet porphyroblasts include rounded clinopyroxene, opaque minerals, and/or spinel grains. Aggregates of magnetite and spinel are abundant in some samples. Peak porphyroblastic garnets preserve primary polyphase inclusions in their cores, consisting of amphibole(s), chlorite, pyroxene, micas and spinel. We studied these inclusions with the Transmission Electron Microscope at the University of Milano. They show an inner part formed by amphibole and clinopyroxene surrounded by Al rich Mg-silicates. Amphibole and pyroxene grow coherently by sharing the [001] direction, the one parallel to the tetrahedral chains. At the grain boundary between amphiboles and pyroxene, or amphiboles and Al rich Mg silicates, smaller amphibole grains extremely enriched in both Cl (up to 8 at.%) and Sr (up to 1.5 at.%) occasionally occur. Such Cl-amphiboles grow coherently with the neighbouring amphibole. The Al rich Mg-silicate phases show electron diffraction patterns with several streaking, indicating possible polytypic disorder. They exhibit periodicities of 14.1 Å, characteristic of chlorite. These phases form a rim between the inclusion precipitates and the hosting garnet, whereas a direct contact between garnet and amphibole or pyroxene have never been observed. Serpentinites act as carriers of oceanic Cl, B, Sr, Rb, Cs, and alkalis which are recycled into variably saline fluids within the stability field of antigorite serpentine [7]. Polyphase inclusions studied in Hujalin clinopyroxenites likely derive from the interaction with the hosting serpentinites at HP-UHP and could represent a snapshot of such fluid-mediated element recycling occurring in the slab at sub-arc depths. References. [1] Bebout, G.E., Ryan, J.G., Leeman, W.P., Bebout, A.E. (1999): Earth Planet. Sci. Lett, 171, 63–81; [2] Becker, H., Jochum, K.P., Carlson, R.W. (2000): Geology, 163, 65–99; [3] Scambelluri, M., Philippot, P. (2001): Lithos, 55, 213–227; [4] Spandler, C.J., Hermann, J., Arculus, R.J., Mavrogenes, J.A. (2003): Contrib. Mineral. Petrol., 146, 205–222; [5] Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006): Lithos, 90, 19–42; [6] Yang, J.J. (2006): J. Petrol., 47, 965–990; [7] Scambelluri, M., Fiebig, J., Malaspina, N., Müntener, O., Pettke, T. (2004): Int. Geol. Rev., 46, 595–613.