DIVERSE ORIGINS OF FLUID IN MAGMATIC INCLUSIONS AT BINGHAM (UTAH, USA), BUTTE (MONTANA, USA), ST-AUSTELL (CORNWALL, UK), AND ASCENSION-ISLAND (MID-ATLANTIC, UK), INDICATED BY LASER MICROPROBE ANALYSIS OF CL, K, BR, I, BA PLUS TE, U, AR, KR, AND XE
Jj. Irwin et E. Roedder, DIVERSE ORIGINS OF FLUID IN MAGMATIC INCLUSIONS AT BINGHAM (UTAH, USA), BUTTE (MONTANA, USA), ST-AUSTELL (CORNWALL, UK), AND ASCENSION-ISLAND (MID-ATLANTIC, UK), INDICATED BY LASER MICROPROBE ANALYSIS OF CL, K, BR, I, BA PLUS TE, U, AR, KR, AND XE, Geochimica et cosmochimica acta, 59(2), 1995, pp. 295-312
Saline fluid inclusions (FI) trapped at close to the temperature of fi
nal solidification of a granitic melt occur in rocks from Bingham, Uta
h, Ascension Island, mid-Atlantic Ocean, and St. Austell, Cornwall. Sl
ightly lower temperature FI occur at Butte, Montana. Argon, Kr, and Xe
were extracted from FI by laser microprobe decrepitation of minute po
rtions of the samples after neutron irradiation (along with synthetic
FI of known composition), and measured in a low blank, high sensitivit
y, pulse-counting mass spectrometer. Results enable measurement of Cl,
K, Br, and I simultaneously with Ar-36, Ar-40, Kr-84, and Xe-129, in
approximately 10(-7) cc of fluid, the contents of a single spherical i
nclusion approximately 57 mum in diameter. Average K/Cl in Bingham and
St. Austell FI are approximately 0.25 and approximately 0.15, respect
ively, broadly consistent with the composition of fluids equilibrated
with rocks at temperatures close to the eutectic in the granite system
(400-600-degrees-C and 0.5-2 Kb). Within-sample variations in K/Cl ar
e significant and may be a result of exsolution of fluids from magmas
over a range of temperatures and/or pressures. Halogen ratios are conf
ined to a narrow range, with I/Cl and Br/Cl in Bingham, Ascension, and
some Butte FI approximately 1 - 8 x 10(-5) and 1 - 3 x 10(-3), respec
tively, probably evidence of a common source of salinity, presumably p
art of the Earth's mantle. A component of salinity derived from contin
ental crust may be indicated by higher I/Cl and lower Br/Cl in St. Aus
tell FI. Radiogenic Ar-40 produced in situ from K in FI after trapping
is usually insignificant. Ar-40e is defined as Ar-40 in excess of the
amounts attributable to atmospheric gases and that produced by decay
of K in FI. Variations in Ar-40e/Cl in Bingham, Ascension, and St. Aus
tell FI are greater than can be explained by just different Cl concent
rations in FI, (typically between approximately 5 x 10(-7) and 8 x 10(
-6)), probably because Ar and Cl have been fractionated within these s
ystems by outgassing of a magma in the interval between exsolution of
different FI generations. Concentrations of Ar and Kr-84 in most Bingh
am, Ascension, and St. Austell FI are approximately 2 orders of magnit
ude greater than in most lavas and granites, with Kr-84/Ar-36 ranging
between approximately 0.015 and approximately 0.08, grossly consistent
with the composition of fluids exsolved from magmas. Several distinct
fluid types are present at Butte: (1) rare ''magmatic'' FI having hal
ogen and noble gas abundances similar to Bingham and Ascension and (2)
abundant FI having Ar-36 and Kr-84 concentrations similar to air-satu
rated fresh waters, with slightly lower Br/Cl than FI at Bingham and A
scension (approximately 5 - 10 x 10(-4)) and relatively high Ar-40e/Cl
(approximately 1 x 10(-5) and 2.8 x 10(-5). Mixing prior to trapping
between fluid derived from magmas and meteoric water that acquired Ar-
40e plus some halogens by interaction with country rocks is the most l
ikely origin of most Butte FI.