The H 2 O content of basalt glasses from Southwest Pacific back-arc basins

• Published in: Earth and Planetary Science Letters
• Main Authors: Danyushevsky, LV, Falloon, TJ, Sobolev, AV, Crawford, AJ, Carroll, M, Price, RC
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Science Bv 1993
• Subjects: Earth Sciences; Geology; Igneous and Metamorphic Petrology; Scotia Sea; Pacific; Ocean Island
• Online Access: https://doi.org/10.1016/0012-821X(93)90089-R; http://ecite.utas.edu.au/96750

The H 2 O 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 K 2 O vs. H 2 O the glass data define two distinct trends characterized by different slopes. Trend I, with a slope (K 2 O/H 2 O) of 0.25, can be explained by addition of a subduction-related component with K 2 O/H 2 O = 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 K 2 O/H 2 O∼ 1.5, to the same depleted mantle source. The K 2 O/TiO 2 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 K 2 O/H 2 O 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 H 2 O enriched BABB of Trend I partly overlap in terms of H 2 O and K 2 O content and H 2 O/TiO 2 and K 2 O/TiO 2 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 K 2 O/H 2 O vs. TiO 2 tectonic discriminan diagram of Muenow et al. [2] appears to be less useful than when originally proposed. The very low K 2 O/H 2 O value ( < 0.05) of the H 2 O-bearing phase involved in boninite genesis implies that it maybe a fluid derived from the subducted slab. The significantly higher K 2 O/H 2 O 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 K 2 O/H 2 >O value (0.25) is also of some interest, as the same value occurs in depleted MORB.