HR: 14:40h
AN: DI43A-05    [Abstracts]
TI: Volatiles in Basalts From Intra-Transform Spreading Centers: Implications for Melt Migration Models
AU: * Nagle, A N
EM: Ashley_Nagle@brown.edu
AF: Brown University Department of Geological Sciences, 324 Brook St, Providence, RI 02912, United States
AU: Pickle, R C
EM: Robert_Pickle@brown.edu
AF: Brown University Department of Geological Sciences, 324 Brook St, Providence, RI 02912, United States
AU: Saal, A E
EM: Alberto_Saal@brown.edu
AF: Brown University Department of Geological Sciences, 324 Brook St, Providence, RI 02912, United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, DC 20015, United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University Department of Geological Sciences, 324 Brook St, Providence, RI 02912, United States
AB: Volatile elements significantly affect partial mantle melting and their abundance and spatial distribution within the source region provides important constraints on models of melt migration and mantle heterogeneity. An advantage of studying basalts from intra-transform spreading centers (ITSCs) is that high eruption pressures in oceanic transform faults inhibit the degassing of most volatiles, sometimes including CO₂. ITSCs also provide well-defined locations of melt delivery and crustal formation because there is no high-level, along-axis melt transport from other parts of the spreading system. In addition, magmatism in fracture zones is probably not supported by long-lived magma chambers. Thus lavas from ITSCs experience less mixing and fractionation than axial MORBs and their compositions provide insight into models of melt migration beneath mid-ocean ridges. The Quebrada/Discovery/Gofar (QDG) fracture zone system offsets the fast-spreading East Pacific Rise (3- 5°S) by approximately 400km and is composed of 7 active ITSCs ranging in length from 5 to 70km. Forty- seven dredges of young intra-transform basalts were collected from this area and analyzed for volatiles, major and trace elements. Incompatible trace element ratios (e.g., Th/La) of QDG lavas range from ultra-depleted to fairly enriched compositions. The level of enrichment correlates well with ratios of volatiles to similarly incompatible refractory elements and with indicators of depth of melt segregation (e.g., Sm/Yb). Overall, the chemical variation of these basalts is greater than that previously found in fracture zones and is similar to the compositional range defined by EPR seamounts (5-15°N). Melt migration models at MORs can be tested by comparing predicted model lava compositions at ITSCs with dredged MORBs. A 3D model detailing mantle temperatures and pressures can be derived by assuming plate- driven passive flow and using measured QDG plate geometries and spreading rates. In conjunction with the pHMELTS model of thermodynamic equilibrium (Asimow et al., 2004), it is possible to construct a 3D array of melt volumes and compositions per cubic km. A simple dynamic pressure gradient model was created from solid flow paths to guide melt from its origin to an ITSC. Melt was traced to its point of eruption, summed, and compositionally contrasted with dredged MORBs. In this model, the deepest melts from beneath the small ITSCs are extracted to nearby large ridge segments, predicting a more depleted lava composition in the transforms. The trace element compositions of QDG lavas are well matched by this simple pressure gradient model using both a depleted and enriched dry mantle source. We will further test this model by comparing the volatile and trace element compositions of the dredged MORBs with the predicted model compositions when water is present in source region.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1065 Major and trace element geochemistry
DE: 3039 Oceanic transform and fracture zone processes
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting