Development of major element proxies for magmatic H2O content in oceanic basalts.

Analysis of H 2 O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H 2 O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H 2 O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H 2 O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H 2 O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H 2 O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al 2 O 3 /FeO* (7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H 2 O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H 2 O contents. H 2 O (7.0) positively correlates with Al 2 O 3 /FeO* (7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H 2 O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al 2 O 3 /FeO* (7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H 2 O (7.0) = 1.109 Al 2 O 3 / FeO ∗ (7.0) − 1.111. This model enables the estimation of magmatic H 2 O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages. • An empirical model is developed to estimate magmatic water contents. • Liquid lines of descent demonstrate a covariation of Al 2 O 3 /FeO* (7.0) and H 2 O (7.0). • H 2 O suppressing plagioclase crystallization is the dominant cause of covariation. [ABSTRACT FROM AUTHOR]