HR: 11:35h
AN: V52A-06    [Abstracts]
TI: Experimentally Determined Emplacement Conditions of the Ultra-Depleted Komatiites of Commondale, South Africa: More wet Archean Komatiites
AU: * Barr, J A
EM: jaybarr@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Ave, Cambridge, MA 02139 United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Ave, Cambridge, MA 02139 United States
AU: Wilson, A H
EM: Wilsona@ukzn.ac.za
AF: Department of Geology and Computer Sciences, University of KwaZulu-Natal, Westville Campus, Durban, 4000 South Africa
AU: Singh, R
V52A-06 AF: Department of Geology and Computer Sciences, University of KwaZulu-Natal, Westville Campus, Durban, 4000 South Africa
AB: This study examines the emplacement conditions of the 3.33 Ga ultramafic suite from Commondale, South Africa. With a parental liquid Mg# of 0.91, Al₂O₃ wt% / TiO₂ wt% of 80, and SiO₂ content of 49.7wt%, the suite of magmas represent some of the most compositionally distinct examples of Archean komatiites yet identified (Wilson, Nature 2003, 423, 858). The well-preserved lavas contain spinifex zones, cumulate zones and well preserved chill margins. Orthopyroxene is present in both spinifex and cumulate zones; another unique characteristic of these komatiites. Phase equilibrium experiments performed under anhydrous conditions at 0.1 MPa (1 bar) indicate an olivine liquidus temperature of 1540°C. A very low-Ca pyroxene (protoenstatite) joins olivine as a crystallizing phase at 1335°C. Despite the late appearance of this initial pyroxene, the Mg# is 0.95. In the Commondale lavas, orthopyroxene is present in the cores of unaltered pyroxene grains. These natural pyroxenes are less primitive, with the average natural pyroxene having an Mg# of 0.88. The minor element compositions of the 1-atm experimental pyroxenes also do not match those from the natural samples, with Al₂O₃ being 1.00 wt% in the 1-atm pyroxenes compared to 2.85 wt% for the natural samples. Preliminary experiments under water saturated conditions at 200 MPa (2 kbar) indicate that the appearance of pyroxene is suppressed by >200°C, similar to the behavior seen in Barberton komatiite experiments (Parman, EPSL 1997, 150, 323). This serves to stabilize orthopyroxene, decrease the initial Mg#, and increase the amount of Al₂O₃ present in the equilibrium crystals, causing them to better mimic the composition of natural samples. The initial liquid composition, under water saturated conditions, would have contained >5.0 wt% H₂O. Thus, mineral chemistry supports a high H₂O content and hydrous melting origin for these Early Archean komatiites.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005