Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine,USA |
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| Authors: | Douglas Rumble III John M Ferry Thomas C Hoering |
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| Institution: | (1) Geophysical Laboratory, 2801 Upton Street, N.W., 20008 Washington, DC, USA;(2) Department of Earth and Planetary Sciences, The Johns Hopkins University, 21218 Baltimore, MD, USA |
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| Abstract: | Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock 
18O (SMOW) values 10.7 to 13.8 . Constituent quartz, feldspar, and muscovite have
18O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of 
Q–F (
18Oquartz–
18Ofeldspar)=2.4 and
Q–M (
18Oquartz–
18Omuscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured
Q–F and
Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic
18O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock
18O>12.1 and contain quartz and feldspar with
18O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock
18O<11.4 and contain quartz and feldspar with
18O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C,  100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO2-H2O fluids of
and
18O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of
and
18O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO2 were also enriched in 18O. Because CO2 was added to the granites during hydrothermal alteration and because fluids enriched in CO2 were enriched in
18O, some or all of the variation in
18O of altered granites may have been caused by addition of 18O to the rocks during the hydrothermal event. The source of both the CO2 and 18O could have been high-18O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in 18O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals. |
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