Characterization of Hypersolidus and Subsolidus Fabrics in the Lower Oceanic Crust, Ocean Drilling Program Hole 735B, Southwest Indian Ridge

We use microstructural observations and EBSD data from troctolite and gabbro samples recovered from shear zones in Ocean Drilling Program (ODP) Hole 735B to show how fabric development and strain are accommodated during the transition from hypersolidus (melt present) to subsolidus (fully crystalline) flow in mid-ocean ridge fault systems. We examine a suite of 3 samples that we interpret as being representative of the transition between hypersolidus and subsolidus flow based on petrographic observations; in hypersolidus fabrics, olivine shows evidence for crystal plasticity whereas plagioclase and clinopyroxene do not, and in subsolidus fabrics, plagioclase shows the most evidence for crystal plasticity relative to olivine and clinopyroxene.

We use EBSD analyses to acquire microstructural, lattice preferred orientation (LPO), and misorientation data from plagioclase, olivine, and clinopyroxene in each sample. In the hypersolidus end member fabric, seemingly undeformed plagioclase laths define a shape fabric LPO where {010} is parallel to foliation and <100> axes are parallel to lineation, but clustering of low-angle (<10º) misorientation axes about [001] suggests the onset of the high-temperature slip system {010}<100>. Olivine and clinopyroxene grains are not abundant enough to define a LPO, but misorientation data suggest the activity of the {001}<100> slip system in olivine and a lack of slip system activity in clinopyroxene. Average internal grain misorientations show that olivine and clinopyroxene are the most and least strained minerals, respectively. In the subsolidus end member fabric, recrystallized plagioclase and olivine grains both define LPOs suggestive of the {010}<100> slip system, which is also supported by clustering of low-angle misorientation axes about [001]. Clinopyroxene grains define a LPO indicative of the {010}<001> slip system, but low-angle misorientation axes are scattered. Average internal grain misorientations show that plagioclase and olivine are equally strained, but clinopyroxene is least strained. Our data suggest that petrographic observations and LPO data alone do not fully reveal details of fabric development, and that misorientation analysis is needed to interpret the onset of crystal plasticity and relative strain in minerals.