Hydrogen Isotope Geochemistry of the Mantle: Constraints from Back Arc Basin Basalts and Mantle Xenoliths

Abstract

A new technique for measuring the hydrogen isotopic compositions of small quantities of hydrous and nominally anhydrous silicate materials was developed. The hydrogen isotopic composition of mantle derived basalts and nominally anhydrous minerals were measured to examine the systematics of the hydrogen isotope geochemistry of the mantle and to better constrain the mantle water cycle. Back arc basin basalts (BABB) from the Mariana Trough are variably enriched in deuterium relative to mid ocean ridge basalts (MORBs). The δD values of 15 Mariana Trough BABB samples range from -74 ‰ to -34 ‰. High δD values in BABB lavas are associated with the trace element signature of H2O rich fluids derived from the altered oceanic crust that is subducting into the mantle. We calculate a mixing model to constrain the isotopic composition and trace element chemistry of the fluid infiltrating the back arc lava source region. The δD value of the infiltrating fluid is close to -30 ‰. When the composition of the infiltrating fluid is compared to a forward calculation of the hydrogen isotopic evolution of altered oceanic crust it indicates that hydrous fluids in the back arc source region originate from the hydrated lithospheric mantle section of altered oceanic crust. Forward model calculations predict that hydrogen deeply subducted into the mantle may have an isotopic composition that is highly fractionated from the initial composition of altered oceanic crust. Pyroxenes from mantle xenoliths are deuterium depleted relative to all mantle materials, with δD values ~50 ‰ lower than average MORB. Using continuous flow mass spectrometry we analyzed the hydrogen isotopic composition of clinopyroxene and orthopyroxene from mantle xenoliths. Our results confirm the previous observations of very low δD values for xenoliths from continental interiors but found δD values similar to MORB in Central American arc xenoliths. The relationship between δD and water content observed in pyroxenes suggests that most mantle xenoliths have undergone extensive degassing (40% – 60%) and therefore the hydrogen content of upper mantle minerals may be higher than previously thought.