Gabbro Fabrics from Site 894, Hess Deep: Implications for Magma Chamber Processes at the East Pacific Rise

Abstract

Rifting in Hess Deep has exposed complete sections of young oceanic lithosphere generated at the East Pacific Rise. At Ocean Drilling Program Site 894, a 150-m section of gabbroic rocks from the upper part of the plutonic section was drilled. The rocks are not compositionally layered but do have / or l-s fabrics defined by the shape-preferred orientation of idiomorphic plagioclase and other phases, including magnetite. We show here that the fabrics formed as a result of viscous magmatic flow and that, in contrast to previously drilled sections of ocean-floor gabbro, evidence for solid-state deformation is entirely absent. Restoration of the fabrics to geographical coordinates, and accounting for subsequent tectonic rotations, shows that the magmatic foliation was originally parallel to the East Pacific Rise axis, steeply dipping, and with a strong, near-vertical flow lineation.From compositional considerations the Site 894 section is thought to have crystallized at fairly high pseudostratigraphic levels within the plutonic portion of the crust, probably slightly below the level of the axial melt lens. This corresponds to a region of moderate seismic attenuation observed at the present-day East Pacific Rise axis, which is believed to represent a crystal mush zone but is otherwise poorly constrained. We discuss the possible implications of the observed fabrics for the mechanisms of melt transport through the mid-crust at fast-spreading ridges.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

“…Phase chemical studies of an ol-anorthositic gabbroic xenolith from the 1991 BBQ flow show that incomplete melt homogenization is also evident in this zone on centimeter scales [Ridley et al, 2006]. Sandwiched between the crystal mush and underlying mantle, sill-like melt bodies within the gabbroic lower crust have been (1) imaged using geophysical techniques [Garmany, 1989;Crawford et al, 1999;Crawford and Webb, 2002;Key and Constable, 2002;Canales et al, 2009], (2) exposed as plutonic sections in dismembered "tectonic windows" along the EPR [Francheteau et al, 1990;MacLeod et al, 1996;Karson et al, 2002], and (3) interpreted from observations within the lower crustal and mantle sections of the Oman ophiolite Kelemen et al, 1997].…”

“…As proposed above, more evolved residual magmas stored in the subaxial crystal mush zone following the 1991 eruption likely incrementally replenished the AMC over a decadal period (Figure 9), leading to crustal dilation, magmatic overpressurization, and eventual eruption in mid-2005. Such a filter-pressing or synkinematic differentiation model [e.g., Natland and Dick, 2001;Gao et al, 2007], where the weight of settling crystals within the mush zone results in the expulsion/squeezing of fractionated liquids into the base of the AMC, has been proposed for layered mafic intrusions [McBirney, 1995;Tegner et al, 2009], MOR [MacLeod et al, 1996;Faure and Schianoa, 2004], and lava lakes in Hawaii [Wright and Okamura, 1977]. [45] In addition to AMC overpressure, we propose that extensional forces, both local and external to the local ridge axis, facilitated the eruption.…”

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

“…Since neither high-temperature recrystallization nor mechanical twining were observed, Abelson et al [ , 2002 concluded that AMS data for the Troodos gabbros reflect the last stages of magma flow, before the solidification of the residual melt, rather than solid-state deformation. Plagioclase crystallographic fabrics of the gabbroic sections of the Oman ophiolite [Yaouancq and MacLeod, 2000], the Hess Deep [MacLeod et al, 1996], and continental layered intrusions [e.g., Gee et al, 2004] show a good correspondence to the AMS directions, further supporting the link between the orientation of gabbroic AMS and the silicate fabric. To verify this link for the Troodos gabbros, we estimate the preferred crystallographic orientation of the silicate fabric for one test specimen (T12-1) using electron backscatter diffraction (EBSD, details are given in the auxiliary material) and compare these data with the AMS data from the entire site (T12).…”

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“…Similar to the Northern Escarpment gabbroic rocks, weak to strong magmatic foliation is defined by the preferred orientation of plagioclase laths (Hékinian et al, 1993;Coogan et al, 2002a;Gillis, Mével, Allan, et al, 1993;MacLeod et al, 1996a). Reorientation of some sections of the Hole 894G core to geographical coordinates shows that the foliations are steeply dipping (mean dip = 69°) with a nearly northsouth orientation, parallel to the EPR, and that there is steeply plunging lineation (MacLeod et al, 1996a). Measurements of the anisotropy of magnetic susceptibility in samples show that there is also a magnetic fabric parallel to the plagioclase fabric .…”

“…This requires extensive hydrothermal cooling of the plutonic crust along the sides of the crystal mush zone to remove the latent heat of crystallization on-axis (Chen, 2001). Hybrid models have also been proposed, in which some crystallization occurs in the AMC and some in situ within the plutonic crust (Boudier et al, 1996;Coogan et al, 2002b;Maclennan et al, 2004). In fact, both end-member models require some portion of each process.…”