Volatile and trace elements in basaltic glasses from Samoa: Implications for water distribution in the mantle

Abstract

We report volatile (H 2 O, CO 2 , F, S, Cl) and trace element data for submarine alkalic basalt glasses from the three youngest Samoan volcanoes, Ta'u, Malumalu and Vailulu'u. Most samples are visibly sulfide saturated, so have likely lost some S during fractionation. Cl/K ratios (0.04 -0.15) extend to higher values than pristine MORBs, but are suspected to be partly due to source differences since Cl/K roughly varies as a function of 87 Sr/ 86 Sr. There are no resolvable differences in the relative enrichment of F among sources, and compatibility of F during mantle melting is established to be nearly identical to Nd. Shallow degassing has affected CO 2 in all samples, and H 2 O only in the most shallowly erupted samples from Vailulu'u. Absolute water contents are high for Samoa (0.63 -1.50 wt%), but relative enrichment of water compared to equally incompatible trace elements (Ce, La) is low and falls entirely below normal MORB values. H 2 O/Ce (58 -157) and H 2 O/La (120 -350) correlate inversely with 87 Sr/ 86 Sr compositions (0.7045 -0.7089). This leads us to believe that, because of very fast diffusion of hydrogen in mantle minerals, recycled lithospheric material with high initial water and trace element content will lose water to the drier ambient mantle during storage within the inner Earth. The net result is the counter-intuitive appearance of greater dehydration with greater mantle enrichment. We expect that subducted slabs will experience a two-stage dehydration history, first within subduction zones and then in the ambient mantle during long-term convective mixing.

Tracking halogens through the subduction cycle

“…4B; Stroncik and Haase, 2004;Workman et al, 2006); this contrasts with the relative enrichment of halogens seen in BABB (Fig. 4B).…”

“…The ocean island basalt (OIB) glasses contain 280-1500 ppm Cl; this is fairly typical of basaltic melts sourced from enriched mantle (EM) reservoirs (e.g., Workman et al, 2006). The BABB glasses contain 420-4200 ppm Cl, with the highest concentrations in differentiated samples with low MgO (Fig.…”

“…Much of this heterogeneity is attributed to the presence of recycled crustal materials introduced by subduction of oceanic lithosphere (e.g., Hofmann, 2003;Workman et al, 2006;Jackson et al, 2007). However, the extent to which strongly incompatible, fl uid-mobile, volatile elements dissolved in seawater are incorporated into hydrous mineral phases within subducting slabs, and transferred into the deeper mantle beyond the zones of arc magma genesis, remains poorly constrained (Dixon et al, 2002;Holland and Ballentine, 2006;Sumino et al, 2010;Kendrick et al, 2011b).…”

Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume‐Ridge Interaction

“…The more refractory DC matrix is treated as peridotite with 100 ppm water (i.e., C sw0 = 100 ppm) and the more fusible EC veins are treated as hydrous peridotite with 500 ppm water (i.e., C sw0 = 500 ppm). The bulk water content is thus 140 ppm, which is intermediate between estimated contents of the source of normal mid-ocean ridge basalt (NMORB) and the mantle component FOZO [Hirth and Kohlstedt, 1996;Workman et al, 2006]. One effect of water is on viscosity in Model 2.…”

Cook Island artifact geochemistry demonstrates spatial and temporal extent of pre-European interarchipelago voyaging in East Polynesia

“…Three complete adzes (2009-399, -401, and -402) originated from 'Upolu island, Samoa, suggested by the match in trace element data to published compositions for two basaltic volcanics (ref. 29, samples U14 and U10). For reasons outlined above, the match deteriorates somewhat for the most and least incompatible elements.…”