Crystallographic Preferred Orientations and Seismic Properties of troctolitic rocks from fast-spread lower ocean crust (IODP Expedition 345 at Hess Deep

Abstract:

IODP Expedition 345 (Dec 2012 - Feb 2013) recovered the first significant sections of modally layered gabbroic rocks from fast-spread lower ocean crust exposed at the Hess Deep Rift (Gillis et al., 2014, doi:10.1038/nature12778). Olivine gabbro and troctolite are the dominant plutonic rock types recovered, with minor gabbro and gabbronorite. Magmatic foliation is moderate to strong in intervals with simple modal layering but weak to absent in troctolitic intervals, and typically absent in intervals with heterogeneous textures and/or diffuse banding.

We present crystallographic preferred orientations (CPO) of primary igneous phases (plagioclase, olivine, cpx) in troctolitic samples, measured using the Electron-Backscattered Electron Diffraction (EBSD) technique at Géosciences Montpellier. The samples are divided into 12 coarse-grained troctolites, 3 fine-grained troctolites, 9 clinopyroxene oikocryst-bearing troctolites, 1 skeletal olivine-bearing troctolite, and 3 olivine gabbros. Plagioclase CPO are typical magmatic fabrics, dominantly axial-B, or intermediate between axial-B and type P (BA-index ranges from 0.1 to 0.6; Satsukawa et al., 2013, doi:10.5194/se-4-511-2013). BA increases when the [100] point maximum, indicating the mineral lineation, becomes better defined. The coarse-grained troctolite samples, from the troctolite series in Holes U1415 J and P, generally display too large grains for the CPO to be statistically meaningful; fabrics appear poorly defined at the scale of the thin sections. Large olivine grains commonly display subgrains resulting from crystal-plastic deformation. In the other troctolitic samples, the plagioclase fabrics are generally well defined, and vary in intensity (J-index) from 1.8 to 7. The olivine gabbros are from banded intervals in Hole U1415P; they display weak, oblate plagioclase CPO (J = 1.5 to 2, BA < 0.2).

CPO are used to model seismic properties of the samples. The fast direction of elastic P-wave propagation in plagioclase is [010]. Consequently, the effect of plagioclase in foliated troctolites (i.e., fast direction normal to foliation) tends to be opposite to that of olivine and cpx (i.e., fast direction parallel to lineation), resulting in a weak anisotropy, with a fast-propagation direction that depends on the modal composition of the rock.