On the Origin of High-Alumina Arc Basalt and the Mechanics of Melt Extraction

Most models of high-alumina arc basalt petrogenesis rely heavilyon the supposition that the abundances of certain trace elements,in particular the relatively unfractionated Rare Earth Element(REE) patterns and the unusually high concentrations of K, Rb,Sr, and Ba are incompatible with a garnet-bearing subductedoceanic crustal (quartz eclogite) source rock. We have carefullyexamined this apparently unequivocal evidence in light of recentprogress on the physics of melt extraction and the heat transferand mechanics of magma ascent. The weakest element of all traceelement models involving a quartz eclogite source is the assumptionthat the element concentrations are fixed at the source andonly later modified in the near-surface environment. We expandon such models by monitoring the concentrations of REE and majorand trace elements during initial melting, ascent, and extractionof magma. This is done by combining calculated cooling curvesfor ascending magmatic bodies with high pressure phase equilibria.The amount that each phase contributes to the melt is monitoredalong with the composition of the melt and residual solids.With quartz eclogite, initial melting initiates gravitationalinstability of the entire source material (melt plus solids)before melt extraction can occur. During ascent of this mush,melting increases until the solids can be repacked to free themelt. This extraction takes place some 15–20 km abovethe slab, after about 50 per cent melting, at which point themelt has a pattern of REE and other trace element concentrationsalmost identical to those observed in high-alumina arc basalts,assuming an initial composition equivalent to altered oceaniccrust plus 5 per cent pelagic sediment. Sr abundances are theonly ones which are not well-matched by this process. The majorelement concentrations of the extracted melt also closely matchthose of high-alumina arc basalt. A similar, but less detailedevaluation of both fertile and depleted peridotite source rocksyields good agreement for the REE and other trace element concentrationsassuming a LREE-enriched source rock strongly enriched in K,Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied onlythrough substantial low pressure fractionation of mafic phases,in particular olivine. Though not rigorously tested, such aprocess may be compatible with the observed major element concentrationsof high-alumina basalt. However, the experimentally verifiedfact that high-alumina basalts could never have been in equilibriumwith either an olivine-bearing magma or source rock eliminatesthis possibility altogether. Thus, the simultaneous considerationof the mechanics of ascent and melt extraction along with phaseequilibria clearly shows that partial melting of quartz eclogitebest satisfies the chemical constraints of major, trace, andREE characteristics of high-alumina arc basalts.