Melting Phase Relations and Element Partitioning in MORB to Lowermost Mantle Conditions
Shigehiko TatenoKei HiroseShuhei SakataKyoko YonemitsuHaruka OzawaTakafumi HirataNaohisa HiraoYasuo Ohishi
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Abstract Melting phase relations and crystal‐melt element partitioning in a mid‐oceanic ridge basalt bulk composition were studied to 135 GPa using laser‐heated diamond‐anvil cell techniques. Using field‐emission‐type electron microprobe (FE‐EPMA), transmission electron microscope (TEM), and laser ablation‐inductively‐coupled plasma mass spectrometer (LA‐ICP‐MS), we obtained comprehensive analyses of major and trace elements in coexisting melt and solid phases. CaSiO 3 ‐perovskite (Ca‐pv) was found to be the liquidus phase throughout the lower mantle pressure range. Whereas silica, followed by Mg‐perovskite, are the second and third crystallizing phases to pressures exceeding 100 GPa, postperovskite, closely followed by seifertite, succeed Ca‐pv at 135 GPa. The partitioning of trace elements between Ca‐pv and melts exhibited a strong pressure effect, possibly due to a combination of high compressibility of cations compared to the lattice site in Ca‐pv and melt compressional effects. The Ca‐pv/melt partition coefficients for Na and K ( D Na and D K ) increase with increasing pressure, with D Na close to unity and D K greater than unity at lowermost mantle pressures. Also, D Nd becomes larger (or identical within uncertainty) than D Sm in the deep lower mantle. Partial melt formed by 51% partial melting of mid‐oceanic ridge basalt at 135 GPa showed marked iron‐enrichment and should thus have negative buoyancy at the base of the mantle. The density of residual solid is almost identical to the PREM density, and therefore, it is likely to be involved in mantle convection and recycled to the surface.Keywords:
Trace element
Liquidus
Incompatible element
Fractional crystallization (geology)
Incompatible element
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Incompatible element
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Trace element
Fractional crystallization (geology)
Leucogranite
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Rare-earth element
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Zero‐age basalts dredged from the Kolbeinsey Ridge directly north of Iceland are mafic quartz tholeiites (MgO 6–10 wt. %), strongly depleted in incompatible elements. Fractionation‐corrected Na 2 O contents (“Nag”) are amongst the lowest found on the global ridge system, implying that the degree of partial melting at Kolbeinsey is amongst the highest for all mid‐ocean ridge basalt (MORB). In contrast, the basalts show large ranges of incompatible‐element ratios (e.g., K 2 O/TiO 2 of 0.01 to 0.12 and Nd/Sm of 2.1 to 2.9) not related to variations in radiogenic isotope ratios; this suggests recent enrichment/depletion events associated with small‐degree partial melting as their cause, rather than long‐lived source heterogeneity. Tholeiitic MORB from many regions globally show similar or more extreme variations in K 2 O/TiO 2 . Dynamic melting of an adiabatically upwelling source can reconcile these conflicting indications of the degree of melting. Through dynamic melting, the incompatible elements are partially separated into different melt fractions based on their bulk partition coefficients, more incompatible elements being concentrated in deeper, smaller‐degree partial melts. The final erupted magma is a mix of melts from all depths in the melting column. The concentration of highly incompatible elements in the mix will be very sensitive to the physical processes allowing the deep melts to separate and migrate to the site of mixing, and small fluctuations in the efficiency of the separation process can account for the large range of trace element ratios seen at Kolbeinsey. The major element chemistry of the erupted mix (and Na 8 ) is much more robust, depending mainly on the integrated total amount of melting. The large variations of incompatible element ratios seen at Kolbeinsey, and in MORB in general, therefore give no information about the total degree of melting occurring beneath the ridge, nor do they require a heterogeneous source.
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Trace element
Carbonatite
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Komatiites are ultra-hot ultramafic lavas, largely restricted to the Archaean. They represent an extreme endmember of terrestrial magmatism and challenge our understanding of how mantle melting operates. We briefly introduce this compositionally diverse group of lavas and critically evaluate constraints on their formation. Despite evidence for moderate water contents in some komatiites, the vast majority require an unusually hot mantle source and probably formed by critical melting in dry or ‘damp’ plumes. The low concentrations of incompatible trace elements in most komatiites cannot be explained by residual phases rich in these elements and instead reflect high degrees of partial melting. Constraining the melting pressures of komatiites is complicated by a lack of robust constraints. However, high MgO contents, high degrees of partial melting, and evidence of residual garnet in the formation of Al-depleted komatiites indicate that melting began at considerable depth in the upper mantle, if not within the lower mantle. We combine these constraints to present models for komatiite formation. Al-depleted komatiites are high pressure melts of fertile mantle; they segregated from sources containing residual garnet at pressures >7 GPa and possibly >10 GPa. Al-undepleted komatiites segregated at lower pressures and/or after reaching higher degrees of partial melting. They came from a depleted source that may have formed by low degrees of hydrous melting in the mantle transition zone. Al-enriched, or Ti-depleted komatiites originated from extremely depleted sources. Their melting pressures are difficult to ascertain, but evidence from the Commondale komatiites suggest at least some formed at pressures >10 GPa. Ti-enriched komatiites and post-Archaean komatiites were produced by smaller degrees of melting of variably enriched or depleted sources, with melting conditions comparable to those of modern picrites.
Ultramafic rock
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Primitive mantle
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The Proterozoic Bandai basic rocks, exposed in the Kulu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The majority of the basalts are characterised by high TiO2 (> 2wt %), Ti/Y, Ti/Zr, TiO2/K2O, and low Rb/Sr ratios. They are enriched with high-field-strength (HFS) elements (Nb, Zr, Ti) relative to low-field-strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are characterised by low TiO2 (< 2 wt%), Ti/Y and Ti/Zr, and high Rb/Sr and Rb/Ba ratios. The common factors of the Bandai basic rocks are their quartz-normative compositions and continental tholeiitic characteristics with Nb/La always less than 1. The compositional variations in the basalt types cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas by various degrees of partial melting. Although some of the characteristics of the basalts (especially high-Ti rocks), like low Mg number (~30) and high concentrations of some of the LFS incompatible elements point towards assimilation and fractional crystallization process, large variations in the incompatible element ratios like Ti/Y, Ti/Zr, Rb/Sr, and Nb/La provide evidence for the derivation of these rocks through variable degrees of partial melting from an enriched mantle source.
Furthermore, the Bandai basic rocks, apart from field settings, are geochemically similar to other Proterozoic basic bodies like the Rampur volcanics, Mandi- Darla volcanics, Garhwal volcanics, and Bhimtal- Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km2 in the Lesser Himalaya provides an evidence of plume activity in the region.
Petrogenesis
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Fractional crystallization (geology)
Asthenosphere
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Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed.
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Metasomatism
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Rare-earth element
Isotope Geochemistry
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