Trace-element partitioning between gregoryite, nyerereite, and natrocarbonatite melt: implications for natrocarbonatite evolution

Fundamental issues regarding the origin and evolution of primitive shoshonitic magmas are addressed using mineralogical, melt inclusion and geochemical data from Fijian shoshonites. Melt inclusions in olivine phenocrysts from primitive Fijian shoshonites are used to critically assess the issues relating to melt sampling by high Fo olivine phenocrysts and address to what degree trapped melt compositions reflect the larger volume magmatic system as a whole. Shoshonitic magmas erupted in Fiji during the Pliocene (5-3Ma) from 11 main volcanic centres along three broad ENE and NNW trending lineaments. The most mafic shoshonitic lavas (absarokites) range from 8.4-15.2wt% MgO and are highly porphyritic, containing phenocrysts of olivine (to F093.2) and clinopyroxene (to Mg# 93.3). The vast majority of melt inclusions in high-Fo-olivine phenocrysts from Fijian shoshonites have anomalous major element composition, mainly characterised by high CaO contents and high-CaO/Al203. Anomalous melt inclusion compositions are interpreted to reflect localised, grain-scale dissolution-reaction-mixing processes within the magmatic plumbing system where hot, primitive magma comes in contact with wall-rocks and/or pre-existing semi-solidified mush zones. Injection of hot primitive melt causes partial dissolution of the mush-zone phases, which are not in equilibrium with the primitive melt and mixing of the reaction products with the primitive magma. Rapid cooling at the margins of the magma body induces fast crystallisation and efficient trapping of numerous and large melt inclusions, with anomalous major element composition. Populations of melt inclusions in high-Fo olivine phenocrysts from Fijian shoshonites, and arguably many other subduction related suites, are naturally biased toward anomalous compositions.

Phenocryst

Melt inclusions

Magma chamber

Porphyritic

Incompatible element

Source

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Citations (3)

Geochemical and strontium isotopic characteristics of parental Aleutian arc magmas: evidence from the basaltic lavas of Atka

Contributions to Mineralogy and Petrology (1986)

James D. Myers B. D. Marsh Alok Sinha

Phenocryst

Porphyritic

Incompatible element

Trace element

10.1007/bf00371221

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Citations (58)

Geochemical effects of dynamic melting beneath ridges: Reconciling major and trace element variations in Kolbeinsey (and global) mid‐ocean ridge basalt

Journal of Geophysical Research Atmospheres (1994)

Colin W. Devey Dieter Garbe‐Schönberg P. Stoffers Catherine Chauvel Dieter F. Mertz

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.

Incompatible element

Trace element

Radiogenic nuclide

10.1029/93jb03364

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Citations (77)

Near-primary melt inclusions in anorthite phenocrysts from the Galapagos Platfrom

Earth and Planetary Science Letters (1993)

Chris W. Sinton David M. Christie Valerie L. Coombs Roger L. Nielsen Martin Fisk

Phenocryst

Anorthite

Melt inclusions