Taking the Temperature of Ocean Islands: A Petrological Approach

Abstract

<p>Estimating mantle temperature is essential for understanding mantle convection and circulation. One route to constraining mantle temperature is via petrological observations, from combining estimates of magmatic temperature with models of melt generation. However, a key factor that has been less emphasised in previous work is the interplay between the temperature of primitive magmas and the composition of the mantle they are derived from. In practice, both of these are unknown and require simultaneous inference from the data. Here we report new estimates of magmatic temperatures for 17 ocean islands and a mid-ocean ridge setting (Siqueiros Fracture Zone) using olivine-spinel thermometry. With the acquired crystallisation temperature estimates, we calculate primary magmatic temperatures using a reverse fractionation model based on the most forsteritic populations of olivine and then apply a multi-lithological mantle melting model to invert for mantle potential temperature (Tp). We find that the most forsteritic olivines investigated in this study have forsterite (Fo) contents between 83.5 and 88.5 and crystallisation temperatures between 1130°C and 1340°C (±23-43°C). Calculations using a multi-element diffusion model show that diffusive resetting of olivine Fo content during magma storage may be prevalent under ocean islands. Considering that the observed maximum Fo contents are several units lower than those of the presumed primary mantle olivines (Fo > 90), a correction on is required to calculate that of the primary melts and Tp. Here we calculate an olivine-controlled liquid line of descent (LLD) with its one end fixed by the average Fo- (considering possible diffusive resetting) of the most forsteritic population at individual islands, and the other end lying on the (unknown) primary olivine Fo and crystallisation temperature. Mantle potential temperatures calculated from a fixed primary Fo91 (=1326-1661°C) show overlap with values reported by previous studies and are regarded as reliable temperature estimates. Using Tp = 1350 (±12) °C calculated for Siqueiros using the same approach (but with additional constraints from crustal thickness and magma chemistry), we obtain the plume excess temperature for ocean islands with high-Fo olivines (Fo > 85) as -23°C to 202°C, which are comparable with results reported by recent seismic tomography studies and show correlations with plume buoyancy fluxes from the literature (especially at hotspots providing olivines Fo > 87). We obtain =229-311°C for localities having lower olivine Fo (83-85), which likely indicates an overestimation of plume temperatures. The petrological approaches in this study can be applied to other ocean islands to constrain mantle temperature and identify key geophysical and petrological constraints that may contribute to more reliable mantle temperature estimates in future.</p>