Is the African cratonic lithosphere wet or dry?

Thick continental lithosphere (tectosphere) beneath African cratons has been stable for ~2.5 b.y. despite its mechanical interaction with sublithospheric mantle. Water is known to have signifi cant infl uence on mechanical stiffness, and the depletion of water is often considered to be a key to preserving the thick lithosphere. Although water-rich environments indicated by the present water content of cratonic xenoliths appear to contradict this hypothesis, these water contents might have been modifi ed at later stages due to the high diffusivity of hydrogen in minerals. Deformation microstructures such as lattice-preferred orientation indicate water-poor conditions (<200 ppm H/Si) during long-term plastic deformation in the continental lithosphere. Analysis of convective instability further constrains the water content to be less than 100 ppm H/Si. We suggest that the continental tectosphere beneath southern Africa must have a low water content, at least one order of magnitude less than oceanic upper mantle, and that the present-day water content of cratonic xenoliths most likely refl ects localized metasomatism before eruption. *[email protected] Katayama, I., and Korenaga, J., 2011, Is the African cratonic lithosphere wet or dry?, in Beccaluva, L., Bianchini, G., and Wilson, M., eds., Volcanism and Evolution of the African Lithosphere: Geological Society of America Special Paper 478, p. 249–256, doi:10.1130/2011.2478(13). For permission to copy, contact editing@ geosociety.org. © 2011 The Geological Society of America. All rights reserved. INTRODUCTION Water has signifi cant infl uence on the viscosity of mantle minerals (e.g., Karato et al., 1986; Mei and Kohlstedt, 2000), and consequently mantle dynamics depend strongly on the distribution of water. Continental tectosphere, which is thick mantle lithosphere lying below Archean cratons, has a thermochemical structure that differs from average suboceanic mantle (Jordan, 1975). This special kind of lithosphere is believed to have been stable and not experienced major tectonic disruptions for the last 2 b.y. or so, whereas other regions have undergone intense tectonic episodes (Richardson et al., 1984; Pearson, 1999). Although chemical buoyancy caused by the extraction of basaltic or komatiitic melts may be partly responsible for the stability of continental tectosphere (Jordan, 1975), geodynamic studies indicate that high viscosity is the most essential factor for the preservation of the thick lithosphere (Doin et al., 1997; Shapiro et al., 1999; Lenardic and Moresi, 1999; Sleep, 2003). Water is preferentially partitioned into a melt phase during partial melting (e.g., Koga et al., 2003), and a relatively low water content is expected for the residual mantle. Pollack (1986) proposed that volatile loss due to magmatic events might have increased mechanical stiffness and thus stabilized the continental tectosphere. Cratonic mantle xenoliths, however, usually point to waterrich environments, as suggested by the common presence of hydrous minerals such as phlogopites (e.g., Pearson et al., 2003). Moreover, direct measurements of water content in cratonic xenoliths from southern Africa show a high amount of water, 250 Katayama and Korenaga ~800 ppm H/Si in olivine (Miller et al., 1987; Bell and Rossman, 1992; Kurosawa et al., 1997). This water content is actually higher than that in olivine from off-craton and from wedge mantle in subduction zones (Peslier and Luhr, 2006). If the mantle beneath cratons has more water than these mobile regions, then it could be very weak and easily disrupted by convection. The origin of these “wet” signatures is a key question regarding the bulk property of continental tectosphere. Usually, these signatures are interpreted to be of a secondary origin, i.e., associated with metasomatic events prior to the kimberlite magmatism that brought those xenoliths up to the surface. That is, the wet signatures are believed to be highly localized in space and not representative of the lithospheric mantle. On the other hand, because the wet signatures are so commonly observed in cratonic xenoliths, it is also possible to regard them as representative samples and argue for wet and weak continental lithosphere (Maggi et al., 2000; Jackson, 2002). Thus, interpretation of the water contents of cratonic xenoliths has been ambiguous. They are invaluable direct samples, but we also know that they could suffer from severe sampling biases (e.g., Artemieva, 2009). The very fact that they were brought up to the surface could mean that they are fundamentally different from the rest of the continental tectosphere. A frustrating situation is that, whereas dehydrated, stiff mantle appears to be required for the long-term stability, we do not have direct samples to prove this hypothesis. The purpose of this paper is twofold. First, we would like to point out that the deformation fabric of cratonic olivine can clearly dismiss the wet signatures of mantle xenoliths from the African lithosphere as a secondary origin and provide a direct constraint on the long-term water budget of continental tectosphere. Second, we will show such a constraint can be further tightened by a simple convective instability analysis incorporating xenolith data. We begin with a brief review on the present-day water content of cratonic olivine. PRESENT WATER CONTENT IN CRATONIC OLIVINE Trace amounts of water (hydrogen) in nominally anhydrous minerals have been widely detected by infrared spectroscopy and ion mass spectrometry (e.g., Rossman, 2006). Olivines from natural peridotites have been reported to contain ~10–1000 ppm H/Si (note that water content is expressed by atomic ratio of hydrogen and silicon, this corresponds to ~1–100 ppm H 2 O by weight); olivines in garnet peridotites usually contain more water than those from spinel-peridotites (Bell and Rossman, 1992). In the cratonic xenoliths that derived from continental lithosphere, olivines show a wide range of water concentration, with an average of ~800 ppm H/Si (Fig. 1). The water contents of olivines from the cratonic lithosphere are higher than those from offcratons (<600 ppm H/Si) and from mantle wedges (<500 ppm H/Si), both of which are tectonically active (Peslier and Luhr, 2006). Though these direct measurements provide a fi rst clue to the water content of the continental tectosphere, it is not clear whether the present water content refl ects the original value in the deep mantle because of the high diffusivity of hydrogen. The diffusion coeffi cient of hydrogen in olivine is more than ten orders of magnitude faster than oxygen or silicon diffusion (Mackwell and Kohlstedt, 1990), so that the water content of olivine can be easily modifi ed; it takes days or weeks to reequilibrate a millimeter-size olivine (Fig. 2). Cratonic xenoliths are usually trapped by volatile-rich kimberlite diapirs, and this transport mechanism could easily overprint the original water content. In fact, there are two types of geophysical data suggesting that the observed wet signatures of cratonic xenoliths are unlikely