Rutile saturation in magmas: implications for Ti-Nb-Ta depletion in island-arc basalts

Details Rutile saturation in magmas: implications for Ti-Nb-Ta depletion in island-arc basalts Rutile saturation in magmas: implications for Ti-Nb-Ta depletion in island-arc basalts

Dec 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..86..225r&link_type=abstract

Earth and Planetary Science Letters, Volume 86, Issue 2-4, p. 225-239.

Computer Science

127

Scientific paper

The TiO2 contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8-30 kbar and T = 1000-1300°C under hydrous, CO2-saturated, and volatile-absent conditions. Dissolved TiO2 is positively correlated with T and not strongly dependent on Ptotal. For fixed P and T, TiO2 content decreases markedly as the melts become more felsic. The distribution of TiO2 between rutile and liquid, expressed as a wt.% concentration ratio, D(rut/liq), is given by: In D = -3.16 + (9373/T) + 0.026P - 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na + K + 2(Ca + Fe + Mg)]/Al . 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO2 on SiO2 content. There is no apparent dependence of rutile solubility on water content.
For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7-9, 5-7, and 1-3 wt.% TiO2, respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region.
We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.

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