Overprinting Deformations in Mantle Rocks, Dun Mountain, New Zealand

Dun Mountain is the northern-most ultramafic massif of the Dun Mountain Ophiolite Belt, South Island, New Zealand, and is the type-locale of dunite. The dunites that are preserved at this location provide an opportunity for studying olivine rheology over a range of naturally-occurring mantle temperatures, pressures, stresses, and strain rates. Field observations include mesoscale dunite and harzburgite compositional bands ranging from 1 cm to 1 m in scale and oriented at 038?, 40? SE. Spinel lineations are oriented at 004?, 40? and lie approximately in the plane of the compositional bands. Thin sections oriented perpendicular to the compositional bands and parallel to the spinel lineations show olivine bi- and tri- modal grain size distributions and olivine exhibits undulose extinction, subgrains, and a shape-preferred orientation. Olivine lattice preferred orientation indicates ?pencil glide? or that the (010)[100] and (001)[100] slip systems were active. In harzburgite, straight and aligned phase boundaries occur between olivine and orthopyroxene grains and microscale olivine and orthopyroxene compositional bands. Olivine microstructures suggest deformation was accommodated by dislocation creep and, in harzburgite, by dislocation creep and phase boundary sliding. Olivine grain size and grain size distributions in dunite and harzburgite are similar, suggesting that phase boundary sliding in harzburgite occurred due to the presence of orthopyroxene, and not due to a finer olivine grain size.

Grain size analyses of olivine indicate grain size distributions containing three populations of grain size with average chord lengths of 5, 1.8, and 0.4 mm. A stress piezometer empirically fit to the experimentally derived relationship between stress and dynamically recrystallized grain size suggests stresses of 2, 5 and 15 MPa, respectively, for these chord lengths. Two-pyroxene thermometry suggests that the intermediate grain size population deformed at 1100 ?C, while the finer grain size population deformed at lower temperatures, perhaps as high as 950 ?C. Mineral equilibria in plagioclase-bearing spinel peridotite yields pressures of 550 to 600 MPa at these temperatures. Deformation mechanism maps based on olivine flow laws at 950 and 1100 ?C both suggest strain rates of 10^(-13) s^(-1). Olivine flow law and stress piezometry indicate deformations at a constant strain rate while stresses increased in response to decreasing temperature conditions.