Synchrotron FTIR microanalysis of volatiles in melt inclusions and exsolved particles in ultramafic deep-seated garnets

Abstract

Silicon-rich deep-seated garnets uplifted in the Jagersfontein kimberlite pipe (South Africa) are unique samples from the asthenospheric deep mantle (more than 300 km: [Haggerty, S.E., Sautter, V., 1990: Ultradeep (greater than 300 Kilometers) Ultramafic upper mantle Xenoliths. Science 248 993–996.]; [Sautter, V., Haggerty, S.E., Field, S., 1991. Ultradeep (> 300 Kilometres) Ultramafic Xenoliths: petrological evidence from the transition Zone. Science 252 827–830.]). During their complex multistage ascent from the asthenosphere through the lithosphere, they have preserved evidence of a variety of fluid-rock interactions ranging from deep mantle conditions to metasomatized lithospheric root zones and, finally, superficial alteration. Synchrotron Fourier Transform Infrared Microspectrometry (SFTIR; [Guilhaumou, N., Dumas, P., Carr, G.L., Williams, G.P., 1998. Synchrotron Infrared Microspectrometry applied to petrography in the micron scale range: fluid chemical analysis and mapping. Applied Spectroscopy 52 (8) 176–184.]) of primary and secondary melt inclusions combined with optical microscopy allow the recognition of two markedly different fluids at the micron scale in two representatives garnet samples from Jagerfontain xenoliths. Some primary melt inclusions (P1) form cloudy areas scattered in the garnets, representing an initial hydrated degassed glassy material that could result from an Early Partial Melting of the garnet host. Glassy material is also found as clusters of melt inclusion rimming orthopyroxene and clinopyroxene exsolved in these garnets. These inclusions (P2) probably result from volume change of the hydrated pyroxenes during ascent. The pyroxene exsolutions themselves contain significant amounts of water currently present as OH in their structure. The detection of OH dissolved in P1 and P2 melt inclusions (contained in nominally anhydrous pyroxene) is evidence for an unusual level of hydroxyl incorporation in the primary ultradeep garnet before exsolution, recording uplift from 300 km depth to the base of the lithosphere. A later fluid event is recognized from secondary melt inclusions (S-type) sometimes containing mechanically entrapped clinopyroxene and displaying typical “edge pitch” trails in the garnets. The S-type melt inclusions nucleated preferentially on pre-existing dislocations in garnets by late chemical alteration. These inclusions consist of a highly hydrated glassy phase containing CO 3 − ions, associated with a vapour phase with CO 2 and H 2O. This fluid represents a carbonaceous magma rich in dissolved CO 2. The CO 2 density of these gaseous phases measured by Raman microspectrometry shows that they were entrapped at shallow depth (0,4 GPa for 1000 °C). Some of this fluid material may be derived from carbonaceous magma, i.e. the host kimberlite or precursor fluids that metasomatized the pyroxene–garnet interface to produce tremolite (recognized by SFTIR) whilst the xenolith sample was in the lithosphere.