Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine, USA

Abstract

Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock δ ¹⁸O (SMOW) values 10.7 to 13.8‰. Constituent quartz, feldspar, and muscovite have δ ¹⁸O in the range 12.4 to 15.2‰, 10.0 to 13.2‰, and 11.1 to 12.0‰, respectively. Mean values of Δ _Q−F (δ ¹⁸O_quartz−δ ¹⁸O_feldspar)=2.4 and Δ _Q−M (δ ¹⁸O_quartz−δ ¹⁸O_muscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured Δ _Q−F and Δ _Q−M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic δ ¹⁸O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have δD (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 δ ¹⁸O>12.1‰ and contain quartz and feldspar with δ ¹⁸O>13.8‰ and >11.4‰, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock δ ¹⁸O<11.4‰ and contain quartz and feldspar with δ 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 ${\text{X}}_{{\text{CO}}_{\text{2}} } = 0.03 - 0.13$ and δ ¹⁸O=10.8 to 12.2‰ while samples with assemblages (b), (c), or (d) coexisted with fluids of ${\text{X}}_{{\text{CO}}_{\text{2}} } \leqslant 0.03$ and δ 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 δ ¹⁸O, some or all of the variation in δ 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.