U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia
Date
2007
Authors
Demidjuk, Z.
Turner, S.
Sandiford, M.
George, R.
Foden, J.
Etheridge, M.
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Journal article
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Earth and Planetary Science Letters, 2007; 261(3-4):517-533
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Zoe Demidjuk, Simon Turner, Mike Sandiford, Rhiannon George, John Foden and Mike Etheridge
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Abstract
Young (< 5 kyr) olivine- and clinopyroxene-phyric ne-hawaiites from Mounts Gambier and Schank in the Newer Volcanic Province in South Australia have been analysed for major and trace elements as well as for Sr and Nd isotopes and <sup>238</sup>U-<sup>230</sup>Th disequilibria in order to constrain the mantle melting processes responsible for their origin. The rocks are relatively primitive (6.9-9.1% MgO), incompatible trace element-enriched alkali basalts with <sup>87</sup>Sr/<sup>86</sup>Sr = 0.70398-0.70415 and <sup>143</sup>Nd/<sup>144</sup>Nd = 0.51280-0.51271. Trace element modelling suggests that they reflect 3-6% partial melting in the presence of 2-8% residual garnet. Trends towards low K/K<sup>*</sup> are accompanied by decreasing <sup>87</sup>Sr/<sup>86</sup>Sr and provide evidence for the involvement of hydrous phases during melting. <sup>230</sup>Th excesses of 12-57% cannot be simulated by batch melting of the lithosphere and instead require dynamic melting models. It is argued that the distinction between continental basalts bearing significant U-Th disequilibria and those in secular equilibrium reflects dynamic melting in upwelling asthenosphere, rather than static batch melting within the lithosphere or the presence or absence of residual garnet. Upwelling rates are estimated at ∼ 1.5 cm/yr. A subdued, localised topographic uplift associated with the magmatism suggests that any upwelling is more likely associated with a secondary mode localised to the upper mantle, rather than a broad zone of deeply-sourced (plume) upwelling. Upper mantle, 'edge-driven' convection is consistent with seismic tomographic and anisotropy studies that imply rapid differential motion of variable thickness Australian lithosphere and the underlying asthenosphere. In this scenario, melting is linked to a significant contribution from hydrous mantle that is envisaged as resulting either from convective entrainment of lithosphere along the trailing edge of a lithospheric keel, or inherited variability in the asthenosphere. © 2007 Elsevier B.V. All rights reserved.
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Copyright © 2007 Elsevier B.V. All rights reserved.