HR: 0800h
AN: V11B-0592    [Abstracts]
TI: Sulfur-rich Apatites in Silicic, Calc-alkaline Magmas: Inherited or not?
AU: * Broderick, C A
EM: cindyb@pdx.edu
AF: Portland State University, Department of Geology, Portland, OR 97201, United States
AU: Streck, M J
EM: streckm@pdx.edu
AF: Portland State University, Department of Geology, Portland, OR 97201, United States
AU: Halter, W E
EM: halter@erdw.ethz.ch
AF: ETH, Istope Geochemistry & Mineral Resources, Department of Earth Sciences, Zurich, 8092, Switzerland
AB: We investigated apatite from the Fish Canyon Tuff and from tuffs and lavas of the San Luis Caldera complex, Colorado to determine the origin of sulfur-rich (>~0.7 wt% SO₃) apatite in silicic, calc-alkaline magmas. Apatites have variable sulfur concentrations varying between ¡Ü 0.2 wt% to 2.0 wt% SO₃, with the majority of the apatites having 0.3-0.5 wt% SO₃ . Sulfur-rich apatites occur in all units ranging in composition from andesite to rhyolite. Sulfur contents of such apatites exceed values for apatite of experimental studies in which apatite with the highest SO₃ content of 0.6-0.7 wt% grew from rhyolitic melts containing 400-700 ppm sulfur (Parat and Holtz, 2005). One possible explanation for the occurrence of sulfur-rich apatites in silicic magmas is that they were inherited from mafic magmas that can dissolve more sulfur. We performed laser-ablation ICP-MS analyses on apatite with a range in sulfur concentrations from selected units to correlate variations observed in sulfur with variations in trace elements (e.g REE) to find evidence for melt compositional changes during crystallization of sulfur-rich vs. sulfur-poor apatites. Apatite among units indicates characteristic compositional changes but within single units, apatite tends to form tight compositional clusters in parameters like Eu/Eu*, La/Yb and Sr contents, while REE concentrations may vary by a factor of two. Exceptions are a few distinct apatites. The REE signatures of these apatites suggest little to no variation in melt composition, thus providing little evidence that sulfur-rich and sulfur-poor apatites grew from melts that strongly varied in their composition. To the contrary, Sr contents of apatite suggest all apatites grew in melts of rhyodacitic to rhyolitic composition. Both points argue against inheritance of sulfur-rich apatites from mafic magmas. This conclusion requires that silicic melts had either higher sulfur contents than were achieved in experimental studies or alternative mechanisms to explain upper end of the sulfur range in apatite. One such alternative explanation may involve some sort of interaction of sulfur-rich fluids (which could be largely derived from an underplated, degassing mafic magma) with crystallizing apatite. Parat, F., and Holtz, F., (2005), CMP 150: 643-651
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting