Two parameters Δ_GO²⁻ and Δ_HO²⁻ are defined as the differences between respectively the Gibbs free energies and the enthalpies of formation of an oxide and its corresponding aqueous cation. The Gibbs free energies and enthalpies of formation of phosphates from their consituent oxides are shown to be linear functions of respectively Δ_GO^2– and Δ_HO²⁻ of their constituent cations.

Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H₂O/CO₂ ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids.

Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ° C, and that the metasomatic fluid had an $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035 in the massive skarns, and ≦ 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.15.

The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was δ¹⁸O =+9.0, δD= −30 to −45. Oxygen isotope temperatures of greater than 620 ° C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 μm-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H₂O-rich C-O-H fluids ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.01) than present with grossularite-rich garnet ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035).

Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480–550 ° C), as indicated by ¹⁸O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II.

Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in ¹⁸O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite.

Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (δ¹⁸O_cale. ≃ −14.0, δD = − 107) was slightly enriched in ¹⁸O and D relative to present-day meteoric water (δ¹⁸O = 15.9, δD = − 117)

A parameter Δ_GO²⁻ is defined as the difference between the Gibbs free energy of formation from the elements of a given oxide and its corresponding aqueous cation. The Gibbs free energies of formation of compounds such as sulfates, nitrates or carbonates, from their constituent oxides, are shown to be linear functions of Δ_GO²⁻ values of their constituent cations.

The equilibrium temperatures for coexisting plagioclase and potassium feldspar pairs have been calculated for various textural varieties of feldspar from 3 post-metamorphic granites from the Georgia Piedmont; the Danburg, Siloam, and Stone Mountain plutons. Assuming an intermediate structural state for the feldspars at time of equilibration, crystallization temperatures match those expected from experimental data for quartz monzonite magmas (650 to 780° C). The variations in solidus temperature, recorded in the feldspars, may be used to estimate relative differences in depth of intrusion. Sharp reversals in plagioclase compositional trends may be caused by isothermal decreases in confining pressure associated with upward migration through the crust. In fine grained and slowly cooled intrusions, albite tends to be lost from the alkali feldspar grains, and recrystallizes as separate unzoned grains of oligoclase, thus erasing the previous thermal history. Perthite exsolution and re-equilibration within the alkali feldspar grains appears to continue down to temperatures of 400° C or so, although the zoned plagioclase does not homogenize. The recrystallization associated with changes in structural state may facilitate exsolution within alkali feldspar grains.

The chemical variations of the early mesozoic tholeiites from North-East American margin pointed out by the american authors are specified by means of multivariate statistical methods (Principal Component Analysis, Stepwise Discriminant Analysis, Multiple Regression). The highly significant correlation between chemical composition and latitude (R = 0.87) can be explained by an evolution of the initial magma from South to North. As in America, Moroccan High Atlas tholeiites are connected with the early opening of the North-Atlantic. By using these statistical methods, it appears that moroccan tholeiites are chemically related to New-Jersey — Connecticut tholeiites. This “chemical positioning” is compared to predrift reconstruction patterns.