HR: 10:20h
AN: V32C-01 INVITED    [Abstracts]
TI: Geochemistry of Apatite in Climactic and Pre-Climactic Tephra from Mt. Mazama, Crater Lake, Oregon
AU: * Mandeville, C W
EM: cmandy@amnh.org
AF: American Museum of Natural History, Central Park West at 79th St., New York, NY 10024, United States
AU: Langstaff, M
EM: mrdth@uchicago.edu
AF: University of Chicago, 5734 S Ellis Ave., Chicago, IL 60637, United States
AB: Apatite is a common accessory mineral in arc volcanic rocks that potentially records information about the dissolved volatile (S,Cl,F,OH) and trace-element concentrations (Sr, Ba, REEs) of the melt from which it crystallized. In a previous study of apatite from arc and convergent margin volcanic rocks, Peng et al. (1997) reported 0.63 wt.% SO₃ in Mazama apatite grains with a corresponding SrO content of 0.18 wt.%, comprising some of the highest SO₃ and SrO values in their data. Our electron microprobe study of apatite in climactic and pre-climactic Mazama tephra was done in order to assess possible correlation of apatite SO₃ with Sr content of low-Sr and high-Sr recharge magmas identified based on whole-rock and matrix glass data (Bacon and Druitt, 1988) and Sr content of plagioclase (Druitt and Bacon 1989). Samples chosen represent all magmatic components erupted during the ca. 7700 year before present climactic eruption and precursor Llao Rock and Cleetwood eruptions. We compare the S, Cl, and F content of Mazama apatites with recent experimental data for S, Cl, and F partitioning between apatite and melt and with dissolved volatiles previously measured in melt inclusions from corresponding or similar Mazama samples. Our electron microprobe data confirm the presence of rare Mazama apatites with up to 0.78 wt.% SO₃ and 0.12 wt.% SrO in Llao Rock, Cleetwood, and climactic scoria and pumice samples. However, high SO₃ and SrO apatites are not restricted to high-Sr scoria hosts, but have been observed in low-Sr scoria, in Llao Rock rhyodacitic pumices and in Cleetwood rhyodacitic pumices, thus indicating significant magma mixing prior to the Llao Rock, Cleetwood and climactic eruptions. Most apatite SO₃ and SrO data falls within the 0.06 to 0.36 wt.% and 0.04 to 0.12 wt.% range, respectively. Experimental data on SO₃ partitioning between apatite and melt and maximum sulfur contents of 300 to 350 ppm measured in climactic and Cleetwood rhyodacitic melt inclusions predict that most Mazama apatites in Cleetwood and climactic pumices should contain 0.4 wt.% or more SO₃. Most apatites in Cleetwood and climactic rhyodacites contain less than 0.20 wt.% SO₃ indicating crystallization from rhyodacitic melt that exsolved a sulfur-rich vapor prior to eruption that drastically reduced melt sulfur concentration. Apatites with SO₃ greater than 0.60 wt.% most probably were derived from more mafic oxidized magmas with dissolved sulfur contents of 2000 ppm or more that have been measured in high-Sr andesitic scoria melt inclusions. Mole fraction fluorapatite in Mazama apatites ranges from 0.20 to 0.96 and based on comparison to recent experimental data predicts melt F concentrations of 200 to 1800 ppm that agrees with measured F in melt inclusions of 200 to 1300 ppm. Mole fraction chlorapatite ranges from 0.11 to 0.19 and based on recent experimental data predicts Cl concentrations in rhyodacitic melt of 0.3 to 0.4 wt.% in reasonable agreement with Cl concentrations in melt inclusions ranging from 0.18 to 0.39 wt.%. References Bacon C.R., and Druitt T.H. (1988) Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon. Contrib. Mineral. Petrol. 98:224-256. Druitt T.H., and Bacon C.R. (1989) Petrology of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon. Contrib. Mineral. Petrol. 101:245-259. Peng G., Luhr J.F. and McGee J.J. (1997) Factors controlling sulfur concentrations in volcanic apatite. Am. Mineral. 82:1210-1224.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting