Mantle Petrology and Tectonics at the Gakkel Ridge

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

ABSTRACT Dick OPP-0327591 The Principal Investigator will study the petrology and tectonic setting of Arctic Ocean abyssal peridotites from the Gakkel Ridge, a previously inaccessible geologic end-member for sea floor spreading. It provides a window into the Earth's interior which critical for geochemical mapping. Due to ultraslow spreading rates, the Gakkel is tectonically distinct from other ridges with no transform faults along its 1800 km length, with thin highly variable crust, and abundant mantle exposures. The Arctic Mid-Ocean Ridge Expedition (AMORE) was the first attempt to sample this ridge and the expedition succeeded beyond all expectations. There were 200 successful dredges. From 3degreesE to 87degreesE, the Ridge proved to consist of local volcanic centers linked by sparsely magmatic segments and peridotites were dredged at 36 rift valley walls and floor locations. The Principal Investigator will analyze peridotite modal mineralogy and trace and major elements of about ~150-200 samples. He will also study the tectonic setting and emplacement using dredge results and the new high-resolution Seabeam data. This research will constrain the relationship between peridotites and basalts, the nature of mantle melting at its lower limit, and the composition of the Arctic mantle. This will be a unique opportunity to assess models for mantle melting and the controls on mid-ocean ridge basalt composition. Because peridotites representing the lowest melting temperatures ocean ridges, they can also be used to direct constrain models for the primary composition and variability of the MORB source. Broader Impacts: The Principal Investigator will continue use various outreach efforts to publicize the expedition and the importance of Arctic research to the general public and scientific community. He has already made a number of presentations at scientific meetings and held press group interviews. He made presentations to a number of prominent congressional staff and senators. There have been three comprehensive studies of abyssal peridotites that show that these samples have much more complicated histories that this proposal implies. Refertilization, melt impregnation, high-pressure melt-rock interactions, "excess olivine" etc. have all been identified in abyssal peridotites. These processes are also widely recognized in xenoliths and massif-type peridotites, but the PI of this proposal wants to interpret these GR data in a vacuum. This field is rapidly advancing and the PI needs to become current. In this proposal the PI states that approximately 2/3 of abyssal peridotites 'are like simple residues of melting'. This statement is perhaps contradicted by studies that show that most abyssal peridotites are enriched in Na and Ti above what is expected based on fractional melting (Elthon, JGR, 1992; Asimow, EPSL 1999; Baker and Beckett, EPSL, 1999). It could be that most abyssal peridotites are not particularly like simple residues of melting, and this proposal doesn't suggest that the PI plans to consider this issue. Moreover, in the PIs compendium of mineral composition and modal data from abyssal peridotites (Dick, 1989; in Magmatism in the Ocean Basins), the Mg and Fe contents of minerals are noted to be potentially inaccurate in a footnote from the author. Finally, it seems that Ni in olivine data published by the PI is perhaps systematically low, relative to studies by other workers (in Dick, 1989, olivine in most samples are listed as having NiO between 0.22 and 0.28 wt% and this range is definitely low relative to results on abyssal peridotites published by most other workers). I am somewhat concerned that the PI sends samples to be analyzed by someone else at MIT, rather than personally overseeing the work. Objective 3 makes the case that the Gakkel ridge allows one to look at AP away from fracture zones-so we should be able to test the idea that there are more dunites within magmatic segments. Surely the PI has an idea about the answer to this right now simply by looking at the rocks collected in hand sample. Indeed, an annoying statement is that there is 'a growing body of data, LARGELY UNPUBLISHED, that dunite is common in rift valley walls.' I have to suspect that much of these data are the PIs-why not published-why aren't the data at least put in some assessable form rather than this statement.