The degree to which chemical heterogeneity in the mantle affects the chemistry of lavas erupted on the spreading centers of the globe-encircling mid-ocean ridge is a topic of hot debate in the marine geology and geophysics community. Up until now, most work has focused on characterizing and trying to understand geochemical trends in axial lavas. This project focuses on off-axis and transform lavas from the East Pacific Rise which, due to their low magma recharge rates, should provide better indicators of undiluted mantle chemical differences. Project goals are to examine the extent to which these lavas exhibit compositions inherited directly from the mantle or from mixing during melt aggregation. Magmatic volatiles such as H2O, CO2, S, F, and Cl will be analyzed in about 200 geochemically well-characterized lavas from two different areas near the Rise. These data will be combined with geophysical data to determine the evolution of the magmas and whether it can be determined how much of the mantle composition of origin they represent. Broader impacts of the work include support of an institution in an EPSCoR state, international (Japan and France) and inter-institutional collaboration, and undergraduate training.
Establishing the budget and distribution of volatiles in the Earth's upper mantle is a fundamental step in our understanding of how the Earth evolves. Volatiles (H,C,F,S,Cl) provide important constraints on models of mantle melting and degassing (and their impact on Eart's climate), mantle convection, mantle heterogeneity and crustal recycling. Over the period of this proposal we have geochemically characterize (major, trace and volatile elements) 500 individual submarine glasses from seamount/intra-transform basalts from the EPR and Galapagos Archipelago. The new data first allowed us to determine the extent of shallow level processes such as the degree of degassing and the amount contamination by interaction with hydrothermally altered oceanic crust. Once the extent of degassing and contamination was accounted for , we determined the volatile contents of the primitive melts and their mantle sources beneath the East-Pacific-Rise and the Galapagos Archipelago. During the period of this proposal three graduate students have been working on the project to obtain their Ph.D. Kei Shimizu, Mary Peterson, Diane Wetzel. Kei Shimizu has focused on the analysis for the EPR, Mary Peterson on the Galapagos Archipelago and Diane Wetzel on empirical evidence for Sulfur degassing. Three undergraduates, Harrison Lisabeth, Andrea Weber and Tom Weinreich worked on the project during summer research opportunities at Brown University and Harrison Lisabeth (now graduate students at the University of Maryland) presented his senior thesis working on the project During the period of this proposal PI Saal and a new Research Scientist (Dr. Soumen Mallick) have set up a new state-of-the-art clean lab and mass spectrometer lab (funded by NSF-MRI). We set up a NEPTUNE MC-ICPMS a Quadrupole X-Series II and a 193 Excimer laser system for the measurement of isotopes ratios and trace elements contents in geological samples.
