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The H2O content of basalt glasses from Southwest Pacific back-arc basins

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posted on 2023-05-18, 05:16 authored by Leonid Danyushevsky, Trevor FalloonTrevor Falloon, Sobolev, AV, Anthony CrawfordAnthony Crawford, Carroll, M, Price, RC

The H2O content of 35 glasses from Southwest Pacific back-arc basins (Lau, North Fiji, Woodlark and Manus) have been determined by infrared spectroscopy. On a plot of K2O vs. H2O the glass data define two distinct trends characterized by different slopes. Trend I, with a slope (K2O/H2O) of 0.25, can be explained by addition of a subduction-related component with K2O/H2O = 0.25 to a depleted mid-ocean ridge basalt mantle source (N- or D-MORB-like). Trend II, which coincides with the N- to E-MORB compositional spectrum, can be produced by addition of a non-subduction component, possibly an alkaline magma with K2O/H2O∼ 1.5, to the same depleted mantle source. The K2O/TiO2 and K/Nb values of E-MORB and back-arc basin basalts (BABB) of Trend II suggest that the enriched component involved in their genesis is not derived from a typical ocean island basalt (OIB, e.g. Hawaiian) mantle source. Our data show that the entire spectrum of BABB compositions can be explained by different degrees of mixing of a mantle source of either D-, N- or E-MORB composition with the subduction-related component, characterized by a K2O/H2O value of 0.25.

Different BABB types correlate with tectonic setting. Samples from the Trend II are associated with relatively stable spreading ridges, whereas those affected by the subduction-related component are always associated with more complex tectonic settings, or come from young or incipient back-arc basins. Pronounced E-MORB affinities of mantle sources are demonstrated only for samples from the Lau, North Fiji and Scotia Sea basins.

The most H2O enriched BABB of Trend I partly overlap in terms of H2O and K2O content and H2O/TiO2 and K2O/TiO2 values with island arc tholeiites. This suggests involvement of similar subduction-related components in the genesis of these two magma types. Because a larger database is now available, the K2O/H2O vs. TiO2 tectonic discriminan diagram of Muenow et al. [2] appears to be less useful than when originally proposed.

The very low K2O/H2O value ( < 0.05) of the H2O-bearing phase involved in boninite genesis implies that it maybe a fluid derived from the subducted slab. The significantly higher K2O/H2O value (0.25) of the subduction-related component involved in petrogenesis of BABB and some arc tholeiites indicates that it was a melt, rather than a fluid. This K2O/H2>O value (0.25) is also of some interest, as the same value occurs in depleted MORB.


Publication title

Earth and Planetary Science Letters










School of Natural Sciences


Elsevier Science Bv

Place of publication

Po Box 211, Amsterdam, Netherlands, 1000 Ae

Rights statement

Copyright 1993 Elsevier

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Socio-economic Objectives

Expanding knowledge in the earth sciences

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