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The Sources and Origin of Magmas
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
E.V. Sharkov, O.A. Bogatikov, V.I. Kovalenko
Another, more significant achievement of ulta-high-pressure experimental petrology was the discovery of the higher compressibility of the melts relative to restite minerals (Nisbet and Walker, 1982; Rigden et al., 1984; etc.). Hence, at depths of more than 250–300 km, the density of ultrabasic magmas is higher than that of olivine and pyroxene, but lower than that of majorite (Fig. 9.9b). Accordingly, only the latter can be fractionated from the melt, whereas Ol and Px should form a suspension in the magma generation zone. This should create different variants when uprising mantle diapirs, which led to the production of both Al-depleted and Al-enriched magmas, are melted. In diapirs originating at shallower depths (< 400–300 km), the olivine represents the liquidus phase and the melts are not depleted in Al (Ohtani, 1990).
Petrology and petrogenesis of an intraplate alkaline lamprophyre-phonolite-carbonatite association in the Alpine Dyke Swarm, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
The validity of the proposal that phonolite-carbonatite pairs in the ADS represent immiscible conjugate liquids could also be tested against geochemical constraints determined from experimental petrology. A major caveat is that ADS carbonatites do not represent magma compositions, having lost significant but unquantifiable components to a fenitising fluid. For the phonolite-carbonatite pairs OU86382-86396 and OU 28983-28122, partition coefficients (D = concentration in carbonatite melt/concentration in silicate melt) for La of 31 and 15; Lu 7 and 3.8; Y 12.8 and 5.8; Sr 22.9 and 39.2; Ba 8.2 and 7.7; and Th 2.7 and 7.84 are all >1. Despite the major differences in the REE distributions of the two rock pairs (Figure 7), these elements are consistently enriched in carbonatite. Conversely Rb, Zr, Ga, Hf, Ta, Nb, Si, Ti, Al are preferentially partitioned into phonolite. A very similar enrichment/depletion pattern has been established for experimental conjugate liquids formed by immiscibility from a hydrous silicate melt with high degree of polymerisation (Martin et al. 2013). Significantly, such enrichment in the REE does not occur experimentally during immiscibility under anhydrous conditions.