HR: 0800h
AN: T41A-1165    [Abstracts]
TI: Magmatism in the Siqueiros Transform: Major and Trace Element Evidence for Mixing and Multiple Sources.
AU: * Hays, M R
EM: mrcooper@ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611-2120 United States
AU: Perfit, M R
EM: perfit@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, Gainesville, FL 32611-2120 United States
AU: Fornari, D J
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Ridley, W
AF: U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 United States
AB: The Siqueiros transform is a fast slipping transform fault (63 km Ma-1 half rate) that offsets the Northern East Pacific Rise (NEPR) to the west by 138 km. Alvin submersible dive observations made in 1992 and Sea MARC II sonar data collected in 1987 identified three intra-transform spreading centers that exhibit organized spreading and recent volcanism. Deep portions (3000-4000 m) of some of the active strike-slip offsets within the transform domain were found to contain evidence of recent volcanic eruptions in the form of small constructs of pillow basalts. The intra-transform spreading centers (Fornari et al., 1987) are believed to result from plate motion changes over the last few million years that has caused extension within the transform (Pockalny et al., 1997). The spreading centers may have begun as leaky transforms evolved into organized spreading centers with continued extension. Compared to the adjacent EPR, Siqueiros transform basalts are typically more primitive and porphyritic. The transform domain contains a diverse group of MORB including enriched (E), normal (N), and depleted (D) varieties. Samples from similar morphotectonic locations within the transform domain (e.g. transform splays versus spreading centers) generally have similar geochemical characteristics. D-MORB from the strike-slip faults, especially the A-B fault, differ from the spreading center basalts in that they are more primitive and depleted in incompatible elements. Major and trace element models indicate that the majority of the N-MORB samples from the spreading centers can be explained by low-pressure fractional crystallization of parents similar to primitive samples found within the deep transform offsets within Siqueiros. However, the compositional variability among the spreading centers requires more than one parental composition to explain the entire transform suite. Mixing of magmas is evident in major element variations, disequlibrium phenocryst compositions, over enrichments in some incompatible elements, and the range in REE patterns. Much of the major and trace element variability can be explained by mixing of relatively primitive melt bodies with highly fractionated magmas. Such mixing is believed to occur in melt lenses or sills where highly evolved interstitial melt exists. Significant differences in REE patterns indicate that the D-MORB and N-MORB cannot be produced from similar sources. REE abundances typical of N-MORBs can be produced by mixing D-MORB with approximately 4-6% of E-MORB. Variability in Na8.0 and Fe8.0, coupled with trace element and isotopic data, suggest mixing of melts from variable depths and extents of melting and from sources with different compositions. The more fractionated nature of samples from the intra-transform spreading centers and the eruption of only N-MORB at those sites suggest that more crystal fractionation and mixing of source compositions occur beneath the spreading centers. The more primitive and variable REE patterns of basalts from the strike-slip faults suggest that magmas erupted there without extensive low-pressure fractionation. The overall distribution of samples suggests that melt lenses capable of mixing different parental compositions exist beneath the more well developed spreading centers.
DE: 4825 Geochemistry
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting