Publications

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2004
Gee, JS, Meurer WP, Selkin PA, Cheadle MJ.  2004.  Quantifying three-dimensional silicate fabrics in cumulates using cumulative distribution functions. Journal of Petrology. 45:1983-2009.   10.1093/petrology/egh045   AbstractWebsite

We present a new method for quantifying three-dimensional silicate fabrics and the associated uncertainties from grain orientation data on three orthogonal sections. Our technique is applied to the orientation of crystallographic features and, hence, yields a fabric related to the lattice-preferred orientation, although the method could be applied to shape-preferred orientations or strain analysis based on passive linear markers. The orientation data for each section are represented by their cumulative distribution function, and an iterative procedure is used to find the symmetric second-rank strain tensor that will simultaneously satisfy the cumulative distribution functions observed on each section. For samples with well-developed fabrics, this technique provides a much closer match to the sectional data than do previous techniques based on eigenparameter analysis of two-dimensional orientation data. Robust uncertainty estimates are derived from a non-parametric bootstrap resampling scheme. The method is applied to two cumulates: one with a well-developed fabric and the other with a weak fabric, from the Stillwater complex, Montana. The silicate petrofabric orientations obtained for these samples compare favorably with independent direct estimates of the volume fabric from electron backscatter diffraction and magnetic techniques.

2014
Horst, AJ, Varga RJ, Gee JS, Karson JA.  2014.  Diverse magma flow directions during construction of sheeted dike complexes at fast- to superfast-spreading centers. Earth and Planetary Science Letters. 408:119-131.   10.1016/j.epsl.2014.09.022   AbstractWebsite

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources. (C) 2014 Elsevier B.V. All rights reserved.