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An Integrated Petrological and Geochemical Approach to Understanding Magmatism Along the East Pacific Rise

Zerda, Christina Louise
http://rave.ohiolink.edu/etdc/view?acc_num=osu1471827298

Abstract Details

2016, Master of Science, Ohio State University, Earth Sciences.
The 65,000 km long Mid-Ocean Ridge system marks the site where oceanic crust is created by the intrusion and eruption of magma formed by decompression melting in the mantle. Newly formed oceanic lithosphere spreads at rates from ~ 10 to 200 mm yr-1, and there is a direct relationship between spreading rate and the pressure of partial crystallization of magma. The East Pacific Rise (EPR) is a fast-spreading ridge that extends from ~ 55°S into the Gulf of California with ~ 20 axial discontinuities. The segment from 5°N to 14°N contains the Siqueiros and Clipperton transforms as well as 5 overlapping spreading centers. These axial discontinuities have been associated with relatively high pressures of partial crystallization for erupted magmas by some workers, who also suggest that magma crystallization beneath the EPR began in the upper mantle. In order to test this proposal, new pressures of partial crystallization of magmas from the EPR from 5°N to 14°N have been calculated using the method of Kelley & Barton, (2008). Pressures were calculated for 5384 MORB glasses from two different datasets. One dataset is from the online Petrological Database (PetDB) and the other is from an internally-consistent compilation of glass analyses by Gale et al., (2013). No difference in calculated pressures was found for the two datasets, with both yielding an average pressure of ~ 218 MPa. After correcting for the water column and filtering to exclude results for samples that did not form by cotectic crystallization, the final average pressure of the ridge was calculated to be 198 ± 3.26 MPa, which is equivalent to a depth of 6.95 ± 0.115 km. This corresponds to a depth very close to the base of the crust. The average pressures of partial crystallization for the Siqueiros (232 ± 25.4 MPa) and Clipperton (199 ± 21.3 MPa) transforms are virtually identical to those calculated for normal ridge segments. This leads to the conclusion that high pressures do not necessarily correlate with axial discontinuities along this portion of the EPR. About 13% of the samples do yield high pressures (P = 300 MPa) of crystallization. Careful filtration of these results using regression methods to define the MgO contents of glasses lying along the liquid + ol + plag + cpx cotectic led to the removal of some, but not all, high-pressure results. Some of the high-pressure glasses have anomalous chemical compositions (elevated K2O and Al2O3, low CaO) that appears to reflect assimilation of crustal material, but it is important to note that magma-crust interaction does not explain all of the high pressure glasses present along the ridge. High-pressure glasses from 11°47’N, for example, have compositions that provide no evidence for magma-crust interaction. The results for these glasses support the suggestion that crystallization of magmas erupted along the EPR began at sub-crustal depths. It is concluded that detailed petrological and geochemical studies of glasses from selected localities are necessary to better constrain the origin of high pressure glasses.
Michael Barton (Advisor)
Thomas Darrah (Committee Member)
Matthew Saltzman (Committee Member)
89 p.
Geology; Petrology
MORB; East Pacific Rise

Recommended Citations

Citations

  • Zerda, C. L. (2016). An Integrated Petrological and Geochemical Approach to Understanding Magmatism Along the East Pacific Rise [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1471827298

    APA Style (7th edition)

  • Zerda, Christina. An Integrated Petrological and Geochemical Approach to Understanding Magmatism Along the East Pacific Rise. 2016. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1471827298.

    MLA Style (8th edition)

  • Zerda, Christina. "An Integrated Petrological and Geochemical Approach to Understanding Magmatism Along the East Pacific Rise." Master's thesis, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1471827298

    Chicago Manual of Style (17th edition)

osu1471827298
558
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This open access ETD is published by The Ohio State University and OhioLINK.