Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt
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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.Keywords:
Incompatible element
Trace element
Radiogenic nuclide
Fractional crystallization (geology)
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Leucogranite
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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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We present a model for computing the total melt production rate from the decompression partial melting region beneath a mid-ocean ridge, and the maximum oceanic crustal thickness created at the ridge axis assuming an ideal melt migration mechanism. The calculations are based on a self-consistent numerical model for the thermal structure and steady-state mantle flow field at a mid-ocean ridge. The model includes the effect of decreasing the melt production rate within the partial melting region by melt extraction as the residual mantle matrix becomes increasingly difficult to melt. Thus the melt fraction depends not only on temperature and pressure determined by the location beneath the ridge axis (the Eulerian description) but also on the accumulated melt extraction since the upwelling mantle matrix enters the partial melting region determined by the location along the flow-line path (the Langrangian description). This effect has been neglected by previous models. The model can predict the size of the melting region and the locations of the boundaries between mantle, residual mantle, and the partial melting region for a given spreading rate, also the distribution of the melt depletion and the mean melting depth. Given the observed average thickness of oceanic crust (~6 km), which is relatively independent of spreading rate, the model results also provide a constraint on the overall efficiency of melt migration to the ridge axis; the efficiency must decrease from 100% at 10 mm/yr to about 60% at fast spreading rates (> 50 mm/yr). Although this reduction may be partially due to the increasing size of the melting region with increasing spreading rate, it still requires less efficient melt migration near the ridge axis at fast spreading rate. We found that the calculated crustal thickness is very sensitive to the mantle temperature. For a normal mantle temperature of 1350°C, the model can generate the observed 6 km oceanic crust over the global range of spreading rates, while the anomalous thicker crusts of the Iceland hotspot and the Reykjanes Ridge are related to higher mantle temperatures associated with the hotspot. Finally, by comparing our model results with previous ones we found that neglecting variations of the melting relations of the residual mantle matrix with melt removal will overestimate the crustal thickness by at least a factor of 1.7.
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Amphibole
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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.
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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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