ALBERT

Notes: Abstract The assemblages rutile-hematite, hematite, hematite-magnetite, hematite-ilmenite-magnetite, and ilmenite-magnetite occur in sillimanite- and kyanite-grade quartzites exposed in western New Hampshire. Different assemblages are found in interlayered sedimentary beds of single outcrops. Magnetites are nearly pure Fe3O4 and contain trace amounts of Al, Si, Ti, V, Cr, Mn, and Ni. Magnetites in contact with hematites contain up to 0.4 weight % MnO, but magnetites in contact with ilmenites containing up to 2.3 weight % MnO have no detectable Mn. Ilmenite is enriched in Mn relative to coexisting hematite, and hematite is so enriched with respect to magnetite. Systematic partitioning of elements between oxide minerals and absence of crossing tie lines suggest that the minerals attained chemical equilibrium during regional metamorphism. None of the assemblages are divariant because of the presence of components in addition to FeO, Fe2O3, and TiO2; therefore, none of them constitute oxygen buffers. Nevertheless, gradients in $$\mu _{O_2 } $$ between adjacent sedimentary beds can be measured using variations of oxide mineral composition in trivariant and quadrivariant phase assemblages. Oxygen behaved as an “initial value” component or “inert” component during regional metamorphism. It is likely that the $$\mu _{O_2 } $$ gradients are due to differences in bulk composition of sedimentary beds at the time of deposition.

Notes: Abstract Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of δ 13C values determined for flake graphite in the igneous rocks (−26.5 to −13.8‰) overlaps the range for flake graphite in the wall rocks (−26.0 to −16.7‰), and spatial correlation of some δ 13C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9‰ higher to 1.4‰ lower in both igneous and metasedimentary samples. The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO2/CH4 depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of 13C between secondary graphite and the evolving fluid because 13C is preferentially concentrated into CO2 relative to CH4. Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in 13C (−15.7 to −11.6‰) relative to both the igneous and wall-rock δ 13C values. Values of δ 13C vary by up to 3.4‰ within veins, with samples taken only 3 cm apart differing by 2.0‰ These variations in δ 13C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).

Notes: Abstract To interpret correctly the isotopic composition of metmorphic rocks and minerals, the effect of nettransfer reactions must be quantitatively evaluated. Such evaluation requires a complete set of linearly independent, net-transfer reactions that fully describe the reacting system. The set of net-transfer reactions is then coupled with mass-balance equations for stable isotopes. Reaction spaces can be contoured with isopleths of °18O, °13C, and δD of minerals which allows evaluation of the effect of different reactions and bulk compositions on the stable isotopic composition of minerals and rocks. Using this approach, we examined the effect of fractionation of isotopes due to net-transfer reactions at the biotite and second-sillimanite isograds in northern New England. Our analysis shows that the shift in °13C and °18O at an isograd depends strongly upon the overall net-transfer reaction at the isograd and the bulk composition of the rock. The use of model isograd reactions to determine isotopic shifts, therefore, can lead to serious errors in the interpretation of isotopic data. At the second-sillimanite isograd °18O qtz (quartz), °18O kspar (K feldsdpar), and °18O wr (whole rock) decrease by ∼0.5, 1.0, and 0.8 per mil, respectively. Quantitative evaluation of the effect of fractionation of isotopes by net-transfer reactions shows that: (1) the relative changes in oxygen isotopes across the isograd could be caused by distillation of fluids during develatilization reactions; (2) the magnitude of the observed isotopic shifts often differs by a factor of 2 from the calculated shifts due to reaction progress alone. The difference between observed and calculated shifts is attributed to either, differences in bulk composition between individual rocks, or, to isotopic exchange between minerals after peak metamorphism. At the biotite isograd the shifts in carbon and oxygen isotope values are different from predicted shifts caused by net-transfer reactions alone. This discrepancy suggests that fluids infiltrated the rocks during the formation of the biotite isograd.

Notes: 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 18Oquartz−δ 18Ofeldspar)=2.4 and Δ Q−M (δ 18Oquartz−δ 18Omuscovite)=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.