Slab-derived fluid phase precipitation at high pressures

Remnants of the fluid phase at ultrahigh pressure (UHP) in subduction environments may be preserved as primary polyphase inclusions in UHP minerals. These inclusions show the same textural features as fluid inclusions and locally may enclose UHP daughter minerals. Detailed textural studies, as well as experimental attempts to homogenise such inclusions, demonstrate that polyphase inclusions derive from solute-rich aqueous fluids or hydrous melts. However, the mode of precipitation of daughter minerals from these solute-rich hydrous fluids has never been investigated yet. A case study is represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China). They derive from harzburgite precursors metasomatised at ~ 4 GPa, 750 °C by a silica- and incompatible trace element-rich fluid phase, sourced from the associated crustal rocks (Malaspina et al., 2006). This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid polyphase primary inclusions in garnet display negative crystal shapes and constant volume ratios of infilling minerals (spinel: 10–20 vol.%; amphibole, chlorite, talc, mica: 80– 90 vol.%). Experimental homogenisation of polyphase inclusions demonstrated that derive from trapped solute-rich aqueous fluids (Malaspina et al., 2006). To constrain the possible mode of precipitation of daughter minerals, significant contribution could be given by the analysis of the growth relationships between host and infillings. As demonstrated by Nestola et al. (2013) for the case of diamond and their olivine inclusions, the use of ‘state of the art‘ single-crystal X-ray diffraction to collect data symoultaneously on both host and inclusion phases, allows to obtain “orientation matrixes” of both phases thus retrieving their reciprocal crystallographic orientation. We performed a single-crystal X-ray diffraction experiment by Synchrotron Radiation at DLS-Diamond Light Source. Combination of the EMPA analyses, Raman Spctroscopy and X- ray diffraction allowed unique identification of each mineral phase and its orientation with respect to the host. Applying this methodology we have been able to infer the possible epitaxial growth of spinel-garnet; gedrite-pargasite and spinel- gedrite pairs which share preferred crystallographic orientation. Such information is crucial to evaluate any recurrent orientation, ruling out epitaxial growth relationships, sheding light on the temporal growth relations between host and inclusion that could not be otherwise evaluated. Malaspina N., Hermann J., Scambelluri M., Compagnoni R. 2006. Polyphase inclusions in garnet–orthopyroxenite (Dabie Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite. Earth. Planet. Sci. Lett., 249, 173–187. Nestola F., Nimis P., Milani S., Angel R.J., Bruno M., Harris J.W. 2013. Crystallographic Relationships between Diamond and its Olivine Inclusions. An Update. Min. Mag., 77(5) 1840.