Paper No. 10
Presentation Time: 10:45 AM
THE SULFIDE CHALLENGE: MODELING CHALCOPHILE ELEMENTS IN INTERMEDIATE ARC MAGMAS
MENGASON, Michael J., Division of Mathematics, Science and Engineering, Northern Virginia Community College, 8333 Little River Turnpike, Annandale, VA 22003, PICCOLI, Philip M., Department of Geology, University of Maryland, College Park, College Park, MD 20742 and CANDELA, Philip A., Department of Geology - LMDR, University of Maryland, College Park, MD 20742, mmengason@nvcc.edu
Andesites and rhyolites from volcanic arcs are products of processes acting throughout the crust. Chalcophile metals such as Cu, Ag, and Au are incompatible or weakly compatible in silicate and oxide minerals produced by—or forming residues that result from—these processes. And yet, the Cu concentration decreases as SiO
2 increases in volcanic rocks from a range of continental and island arcs. Experiments were performed to simulate magmas of intermediate (andesitic) composition at 1000
°C and 150 MPa, with
fO
2 equal to the Co-CoO or Ni-NiO oxygen fugacity buffers and with sulfur fugacity ranging from -1.8 to -0.2 (log) bar. The partition coefficients (D
sulfide/melt) for Cu are consistent with compatible behavior in sulfides, and range from 1320±220 (1σ SDOM) for pyrrhotite (
po) and 7800±1400 for a Cu-Fe-S liquid (
CFSL)
. Sulfides are likely a major host of chalcophile metals in arc magmas. However, modeling the effects of the addition or removal of sulfides from the magma is complicated by variations in partition coefficients among phases. In this study, D
s/m (Ag) are 90±19 for po and 6800±1300 for CFSL, and D
s/m (Au) are 500±87 and 84000±19000, respectively. The stable assemblages in the Cu-Fe-S system change as the magma cools from temperatures relevant to basaltic melts to those relevant to rhyolitic melts with sulfide liquids giving way to sulfide minerals. Addition or removal of sulfides from the magma along the liquid line of descent will complexly affect the chalcophile metal composition of the silicate melt.
In these experiments, the minimum concentration of Cu in the silicate melt needed to saturate with CFSL or CFSL + po as opposed to po alone averaged 32±13 ppm (1σ). The ratios of Cu, Fe, and S in the silicate melt as compared to the CFSL and po suggest that on cooling, po is produced by a reaction at the expense of the CFSL, rather than by co-precipitation along a cotectic boundary. The removal of CFSL will therefore drive the system toward, and into, po stability and accelerate the transition between sulfides. This strongly links changes in concentrations of Ag and Au to changes in the concentration of Cu in arc magmas given the magnitude of differences in partition coefficients and further underscores the importance of considering the composition of the sulfide phase(s) present throughout the evolution of arc magmas.
