Mantle Heterogeneity and Melting and Magma Transport Beneath the Ultraslow-Spreading Gakkel Ridge: Evidence from Volatiles and Trace Elements

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Intellectual Merit: Because its spreading rate varies progressively along its length and it has no major offsets or obliquity, the ultraslow-spreading Gakkel Ridge is an ideal laboratory for testing hypotheses that relate spreading rate to mantle melting and the formation of crust. The Arctic Mid-Ocean Ridge Expedition (AMORE 2001), the first comprehensive cruise to sample Gakkel Ridge, succeeded well beyond expectations, and produced the first high-resolution, well-navigated bathymetric chart of Gakkel Ridge. The Principal Investigators recovered basement samples from more than 200 sites (only 60 were planned) and discovered abundant hydrothermal activity. The new bathymetric map and samples show a striking boundary where the ridge changes from being volcanically robust west of 3degreesE to virtually magmatic (peridotitic) to the east. Volcanic activity resumes even farther to the east, suggesting that spreading rate changes are not responsible for the variations in volcanism. Basalt geochemistry has revealed a remarkable geochemical boundary that coincides closely with the ridge's morphologic boundary. West of 3degreesE, basalts have high H2O /Cesium (Ce) and Barium (Ba)/ Niobium (Nb) compared to other ridges, and Indian Ocean isotopic signatures. To the east, H2O /Ce and Ba/Nb are typical of other ridges, and isotopic ratios resemble Pacific basalts. To test hypotheses about the origin of this geochemical province, the Principal Investigator will define its geochemistry in terms of key elements and volatiles that can be compared to geochemical reservoirs. He will also define its geographic extent in relation to the tectonic history of this region and look for evidence that the geochemical signal arises from a discrete component in the mantle by examining melt inclusions and crystal zoning of barium in basalts and peridotites. Geochemical data for Gakkel Ridge is compared with other mid-ocean ridges, especially Southwest Indian Ridge (SWIR), to determine the extent to which high Ba and H2O can be attributed to ultraslow spreading rate as opposed to Arctic tectonic history. The second part of the study addresses the structure of the uppermost lithosphere and crust. He will examine the pathways by which magmas flow eventually to build the crust of the Arctic Ocean by determining CO2 abundances in basalt glasses, their vesicles and in melt inclusions. These data will be used to describe the gas exsolution to test hypotheses about the depths from which magmas ascend, and the paths that they have taken to reach parts of the ridge axis that are distant from volcanic centers. In the third aspect of the study, he will examine the extent to which the enrichment of H2O has influenced the depth and extent of melting on Gakkel Ridge. The H2O data generated for basalts will be integrated with major element data. Models will be constructed that take into account the influence of H2O. In particular, the Principal Investigator will test hypotheses that H2O has played a major role in creating the large variations in volcanic intensity along Gakkel Ridge, and that ridges respond more acutely to such changes at ultraslow spreading rates. Throughout all aspects of the study, he will integrate constraints from geophysics and make comparisons with other ridges, especially SWIR. Broader Impacts of the Proposed study: Undergraduate students will play an important role in analyzing the data. At least two students will be involved in this project, and will derive undergraduate thesis projects from it. A large part of the project will constitute the Masters thesis project for a graduate student. The Principal Investigator will continue giving illustrated presentations about Gakkel Ridge to primary and secondary school classes, scout groups, and civic groups.