Four- and five-phase peridotites from a continental rift system: evidence for upper mantle uplift and cooling at the Ross Sea margin (Antarctica)

Abstract

Upper mantle plagioclase+spinel- and spinel-peridotite xenoliths occur in basanitic and tephritic lavas of the 2.7 my to Recent Mt. Melbourne Volcanic Field (Antarctica). This field belongs to the Cenozoic McMurdo volcanic group which is located between the deep western trough of the Ross Sea rift system and the uplifted rift shoulder of the Transantarctic Mountains. Our samples cover the transition zone between rift and shoulder. We examined texture and composition of plagioclase+spinel and normal spinel peridotites and determined temperatures and pressures of formation using the internally consistent Ca-ol/cpx and 2px-thermobarometer of Köhler and Brey (1990) and Brey and Köhler (1990). Distinct calcium distribution patterns in olivines correspond to three different petrographic textures: type ELZ have equigranular textures, and low calcium concentrations of 60 ppm in the olivine cores which are strongly zoned to 200 ppm in their rims. Type PLH are protogranular to porphyroclastic and have low and homogeneous calcium contents in the range of 120 to 200 ppm. Type EHH peridotites are equigranular and have olivines with high and homogeneous calcium values of 467–485 ppm. The application of the 2 px-thermometer give rim temperatures of 800 to 860 °C for Type ELZ, 900 to 1080 °C for type PLH and 1030 to 1050 °C for type EHH. Pressures of 13 to 17 kb calculated with the Ca-ol/epx-barometer for EHH peridotites are consistent with the Ross Rift geotherm. For the other two types, this barometer yields unreasonable high pressures exceeding 30 kb for both, plagioclase-bearing and ‘normal’ spinel-peridotites. This indicates disequilibrium and continued calcium-loss from the olivines during cooling below the closure temperature for the 2 px-thermometer. Inversion of the Ca-in-olivine-barometer into a thermometer and application to core compositions of ELZ olivines (60 ppm) suggests that cooling occurred to temperatures of ca. 580 °C. Based on petrographical and geothermobarometric results and diffusion arguments, a four stage model is developed for the evolution of the upper mantle beneath the margin of the Ross Rift: (1) adiabatic uplift into the plagioclase/spinel-peridotile field; (2) subsequent cooling below the blocking temperature (800 °C) of the 2 px-thermometer to about 600 °C as indicated by low Ca in olivine cores; followed by (3) reheating to 760 °C as suggested by zonation to high calcium concentrations of up to 200 ppm in olivine rims. Calcium concentrations of up to 800 ppm were measured in one ELZ-olivine in the outermost rim (10 μm) reflecting (4) a last heating event during transport and ascent in the basanitic host magma. Timing for the latter two stages has been roughly calculated from Ca-diffusivities in olivine. A minimum duration of 1200 to a few million years is indicated for stage 3 and 90 h to 22–23 days for stage 4, respectively. This timing of events correlates to increased mantle temperatures for the duration of magmatic activity of the Mt. Melbourne Volcanic Field and the short-term transport in host magmas. Our results also indicate that anomalous shallow mantle exists at the transition from the Ross Rift into the uplifted Transantarctic Mountains.