Abstract
Experiments from 640 to 680 °C, 200 MPa H2O at␣f O2 ≈ NNO, employing a natural␣F-rich␣vitrophyric rhyolite from Spor Mountain, Utah, assessed the effect of variable Mg′ [100Mg/(Mg + Mn + Fe)] on the partitioning of fluorine and chlorine between biotite (Bt) and melt. Over this temperature interval, Bt ( ± fluorite, ± quartz) is the sole liquidus phase. Partition coefficients for fluorine between biotite and glass (DF Bt/melt) show a strong dependence on the Mg′ of Bt.␣DF Bt/melt varies from ˜ 1.5 to 7.2 over the range of Mg′ from 21 to 76. A strong linear correlation between␣DF Bt/melt␣and Mg′ has a slope of 9.4 and extrapolates through the origin (i.e., DF Bt/melt ≈ 0 at Mg′ = 0, an annite-siderophyllite solid solution in these experiments). DCl Bt/melt values ( ˜ 1 to 6) in the same experiments vary inversely with Mg′. The Al-content of biotite does not vary with the aluminum saturation index (ASI = molar Al2O3/Σ alkali and alkaline earth oxides) of melt, but two exchange mechanisms involving Al appear to operate in these micas: (1) Alvi + Aliv ? Siiv + Mgiv, and Mgiv + 2Aliv? 2Siiv + □iv. The effects of other components such as Li or other intensive parameters including f O2 have yet to be evaluated␣systematically. At comparable Mg′ of Bt, however, the Spor Mountain rhyolite yields higher DF Bt/melt values than an Li-rich, strongly peraluminous melt previously investigated. The results indicate that the Mg′ of Bt exerts the principal control on halogen partitioning, with ASI and T as second-order variables. The experimental partition coefficients compare well with other experimental results but not with most volcanic rocks. Magmatic Bt from most rhyolites records higher DF Bt/melt due to reequilibration with degassed (H2O-depleted) magma and perhaps with F2O−1 exchange that may accompany oxidation ([Fe3+O] [Fe2+OH]−1). This behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near Morococala, Bolivia: Bt is sharply zoned with F-rich rims, but Bt(core)-melt inclusion pairs fall on our experimental curve for DF Bt/melt. These experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks. For plutonic rocks, the actual F-content of melt, not a relative activity ratio involving HF species, can be reasonably estimated if the mica has not undergone subsolidus reequilibration. This information is potentially useful for some shallow-level Ca-poor magmas that are thought to be rich in F (e.g., A- and S-type granites) but do not conserve F well as rocks.
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Received: 11 April 1996 / Accepted: 14 August 1996
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Icenhower, J., London, D. Partitioning of fluorine and chlorine between biotite and granitic melt: experimental calibration at 200 MPa H2O. Contrib Mineral Petrol 127, 17–29 (1997). https://doi.org/10.1007/s004100050262
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DOI: https://doi.org/10.1007/s004100050262