Melt-rich lithosphere-asthenosphere boundary inferred from petit-spot volcanoes

Young basaltic knolls have been discovered on the old oceanic lithosphere, namely petit-spot volcanoes. Based on their geochemical signatures, they have presumably originated from partial melts in the asthenosphere. However, there is no direct information on the depth provenance of petit-spot formation. Here we report new geothermobarometric data of rare mantle xenoliths discovered from petit-spot lavas exhibiting a geotherm much hotter than expected for the ca. 140 Ma seafloor on which petit-spots were formed. Such an anomalously hot geotherm indicates that melt porosity around the lithosphere-asthenosphere boundary (LAB) must be as high as a few percent. Such high melt porosity would be possible by continuous melt replenishment. Excess pressure induced by the outer-rise topography enables horizontal melt migration along the LAB and sustains a continuous melt supply to petit-spot magmatism. Given the general age-depth relationship of ocean basins, a melt-rich boundary region could also be a global feature. INTRODUCTION Debates on the presence of partial melts in the asthenosphere have intensified recently. The occurrences of a seismic low-velocity zone and a high electric-conductive layer in upper mantle, for example, have provided supporting evidence for partial melting in the asthenosphere (e.g., Sifré et al., 2014), which is consistent with the experimentally determined peridotite solidus in the presence of H2O and CO2 (Wyllie, 2012). However, recent studies suggest that partial melts are not required to explain seismic observations (Faul and Jackson, 2005; Stixrude and Lithgow-Bertelloni, 2005; Priestley and McKenzie, 2006). In order to assess the presence of the melt, petit-spot volcanism deserves attention. Petit-spot volcanoes are tiny seamounts erupting off the fore-bulge of the downgoing oceanic plate (Hirano et al., 2004, 2006). Petit-spots have been successively discovered at many sites in the world (e.g., Hirano et al., 2006, 2008, 2013), suggesting that petit-spot magmatism is ubiquitous at areas of plate flexure. Because of their isotopic similarities to mid-ocean-ridge basalts, petit-spot magmas are likely to be derived from partial melts in the ambient asthenosphere (Hirano et al., 2006). Therefore, petit-spots are invaluable samples of asthenospheric partial melts beneath a mature oceanic lithosphere; a good understanding of how petit-spots actually form can provide entirely new insight into the physical state of the normal upper mantle. So far, about a dozen mantle xenoliths have been recovered from petit-spot lavas (Yamamoto et al., 2009). The occurrence of the mantle xenoliths provides essential information on the host magma, such as the origin and ascent rate of the magma. In addition, these xenoliths can constrain how the oceanic lithosphere is affected by petit-spot formation. As long-standing volcanism at an old oceanic plate is expected to affect the geotherm, the thermal structure of the oceanic plate deduced from the mantle xenoliths could potentially test the occurrence of melt at the base of the lithosphere. THERMAL STRUCTURE OF THE OCEANIC LITHOSPHERE We used five xenoliths recovered from petit-spot basalts erupting on subducting Pacific Plate off the Japan Trench (Fig. DR1 in the GSA Data Repository1). To infer the thermal structure of the oceanic plate from which these xenoliths are supposed to have originated, we need geothermobarometric data of the mantle xenoliths. The temperature was estimated using the two-pyroxene thermometer proposed by Wells (1977). The range of temperature is 771–1129 °C (Tables DR1 and DR2 in the Data Repository). Regarding the pressure, we conducted a thorough investigation of the depth provenance of the xenoliths based on pressure information recorded in the mantle xenoliths such as thermodynamic geobarometry, mineralogy, and the residual pressure of fluid inclusions and their stability conditions. We summarize the essence of the investigation here. The xenoliths are spinel peridotites, which are stable in mantle at 1.0–2.2 GPa (O’Neill, 1981; Borghini et al., 2010). In addition, there are abundant CO2 inclusions in the xenoliths. Taking the stability conditions of CO2 fluid in olivine into consideration (Koziol and Newton, 1998), the present spinel peridotites should be derived from the pressure-temperature (P-T) conditions depicted by a shaded area in Figure 1. There is geobarometry based on residual pressure of fluid inclusions, which can be applied to mantle xenoliths (e.g., Miller and Richter, 1982). The estimated pressure and temperature are shown in Figure 1 (see the Data Repository for data and analytical procedures). There is considerable validity to the pressure estimated by the fluid inclusion geobarometry because the P-T values are well supported by the stability conditions of both aluminum-bearing minerals and CO2 fluid in olivine. The estimated pressures have a positive correlation with the temperatures. The most plausible explanation for the correlation is that they reflect the geotherm beneath the sampling sites, but the geothermobarometric data of these mantle xenoliths imply a very hot geotherm corresponding to young (ca. 18 Ma) oceanic lithosphere (Fig. 1). Such a geotherm must be a localized feature, restricted to the vicinity of these petit-spots; if it represented a regional geotherm, an isostatic adjustment would have raised the seafloor by as much as ~2 km (Turcotte and Schubert, 2002). This view of a highly localized hot geotherm is also consistent with the spotty distribution of anomalously high heat-flow data reported from this region (Yamano et al., 2008). GENERATION MECHANISM OF PETIT-SPOT MAGMA The existence of localized thermal anomalies in the old oceanic lithosphere thus appears to be feasible, and as discussed below, two lines of reasoning suggest that such thermal anomalies may have profound implications for the origin of petit-spots as well as the nature of the lithosphereasthenosphere boundary. First, the amount of excess heat required to generate the thermal anomaly may be estimated as p m = ρ ∆E C ∆T HA, (1) 1GSA Data Repository item 2014342, supplementary information about the samples and the accuracy of the geobarometry, is available online at www .geosociety.org /pubs /ft2014.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, November 2014; v. 42; no. 11; p. 967–970; Data Repository item 2014342 | doi:10.1130/G35944.1 | Published online 26 September 2014 © 2014 eological Soci ty of America. For ermission to copy, contact [email protected]. 968 www.gsapubs.org | November 2014 | GEOLOGY where Cp is the specific heat for mantle peridotite (1 kJ kg -1 K-1), DT is the average temperature excess (~300 K; average temperature difference between the 18 Ma and 140 Ma geotherm), rm is the mantle density (3300 kg m-3), H is the thickness of the lithosphere (~100 km), and A is the cross-sectional area of an affected region. For an area of 1 km2, for example, the excess heat would be ~1020 J, equivalent to the latent heat of solidification released from ~75 km3 of melt. A typical petit-spot is only ~50 m high, with a footprint of 1 km2 (Hirano et al., 2006), so the melt volume required for the excess heat could be greater than the erupted volume by more than three orders of magnitude. This inference is admittedly crude, being sensitive to the assumed cross-sectional area. Pyroxenes in two mantle xenoliths, however, do not show notable zoning, and given the diffusion rate of ~10-20 m2 s-1 for the inter-diffusion of elements relevant for geothermometry (Zhang et al., 2010), heating must have continued for at least 1 m.y. to achieve grain-scale chemical re-equilibration, during which a thermal anomaly could have spread for a few kilometers. The cross-sectional area thus must be greater than a few square kilometers. Even though petit-spots appear to be volumetrically trivial, therefore, the associated thermal anomalies in the lithosphere suggest that these surface expressions may well be just “the tip of the iceberg.” Second, the formation of petit-spots requires melt migration through thick oceanic lithosphere, and a quantitative consideration suggests a rather specific circumstance, beyond a favorable stress state caused by nearby plate bending (Hirano et al., 2006). To avoid freezing within the lithosphere, melt has to travel sufficiently fast, or the Peclet number must be substantially greater than unity (Turcotte and Schubert, 2002): Pe vh 1 = κ , (2) where v is vertical melt velocity, h is a channel width, and k is thermal diffusivity (10-6 m2 s-1). A primary rate-limiting factor for melt ascent is the supply rate through the base of the lithosphere, which may be seen in the following mass conservation equation: