Helen M. Lang

Coexisting clinopyroxene/spinel and amphibole/spinel symplectites in metatroctolites from the Buck Creek ultramafic body, North Carolina Blue Ridge

Abstract Buck Creek metatroctolites preserve evidence of three stages in the geologic history of the region: a relict igneous stage; a high-pressure, granulite-facies metamorphic stage; and a hydrous alteration stage. During high-pressure granulite-facies metamorphism, adjacent olivine and plagioclase reacted to produce complex coronitic textures. Relict olivine is surrounded by radial, columnar orthopyroxene, and plagioclase is rimmed by fine-grained symplectite composed of separate grains of clinopyroxene/ spinel, tschermakitic amphibole/spinel symplectite and minor clinopyroxene/sapphirine symplectite. Examination of textural relationships in back-scattered electron images and X-ray element maps of the symplectites demonstrates that amphibole/spinel and clinopyroxene/spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks. Clinopyroxene/sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples. A successful model for the main corona-forming reaction in sample BC7, 1.00 Pl + 1.72 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.68 Spl + 0.79 Opx, results only if amphibole is included. The model predicts relative volumes of product clinopyroxene, amphibole and spinel that are generally consistent with the proportions in imaged symplectites. Proportions of clinopyroxene- and amphibole-hosted symplectite appear to have been governed by local mass-balance of Na and Al in plagioclase and/or availability of H2O. The most successful model for formation of the inner clinopyroxene/sapphirine corona, 1.00 Pl + 1.47 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.30 Spr (10 ox.) + 0.57 Opx, also includes amphibole, which is present in association with clinopyroxene/sapphirine symplectite. Sufficient H2O for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high P-T conditions.

Quantitative determination of metamorphic reaction history: mass balance relations between groundmass and mineral inclusion assemblages in metamorphic rocks

Qualitative and quantitative information about metamorphic reaction history and PT paths may be obtained from mineral inclusions in garnet by comparing the mineralogy, distribution, and compositions of paragenetically-related inclusions with minerals in the groundmass assemblage. Using the algebraic technique of singular value decomposition (SVD), we document mass balance relations between inclusion and groundmass assemblages in metapelitic rocks from two metamorphic terranes that experienced different peak metamorphic conditions, and whose transition from inclusion to groundmass assemblage records different PT path segments relative to peak conditions. We calculate mass balances relating an inclusion assemblage consisting in part of armored relics of chloritoid to groundmass mineral assemblages in a kyanite-staurolite mica schist from the Solitude Range, British Columbia, and an inclusion assemblage of kyanite, staurolite, and rutile to groundmass minerals in a sillimanite-cordierite gneiss from the Skagit Gneiss, North Cascade Range, Washington. Mass balances for each rock are consistent with reaction histories inferred from petrographic observations. In the Solitude Range schist, the results of mass balance calculations are consistent with the growth of staurolite and garnet at the expense of chloritoid during prograde metamorphism and suggest that chlorite, although not preserved as an inclusion, was involved in initial staurolite growth. In the Skagit sillimanite gneiss, mass balance relations exist between the inclusion suite, which formed during high pressure metamorphism, and the associated groundmass assemblage, which equilibrated at high temperature but much lower pressure. Mass balance does not exist between the groundmass of the Skagit sillimanite gneiss and the groundmass of a nearby kyanite-staurolite schist that has been proposed as a possible lower-grade equivalent of the sillimanite-bearing rocks. These results indicate that, although compositional modification and selective preservation of minerals must be taken into account, mineral inclusion suites may nevertheless preserve enough compositional information to allow reconstruction of complete or nearly complete pre-existing assemblages. This information may not be retrievable from any other source if no lower-grade equivalents of the rocks of interest are exposed.

Pressure-temperature-reaction history of metapelitic rocks from the Maryland Piedmont on the basis of correlated garnet zoning and plagioclase-inclusion composition

Abstract Regional structural interpretation, thermobarometry, and Gibbs-method calculations were combined to determine an approximate P- T path for kyanite-grade metapelitic rocks of the Baltimore Gneiss terrane. Garnet growth along the estimated P-T path was modeled using the computer program DiffGibbs. Models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite-bearing sample from the area. Modifications in the P-T path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model-garnet profiles. In the most successful model, garnet growth began at 550 °C and 6500 bars in the assemblage chlorite + biotite + garnet + muscovite + plagioclase + quartz with increasing temperature and with decreasing and then constant pressure. At 570 °C and 5724 bars, staurolite was added to the assemblage and some garnet was consumed. In the model, garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure. The region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615 °C at 6474 bars. After kyanite was added to the assemblage, grossular increased to the constant rim value only if temperature decreased and pressure increased. Cooling and decompression affected garnet zoning only very near the rim. Measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model. This study demonstrates that complex garnet zoning can be modeled successfully, even when the reaction history includes episodes of garnet consumption, over a path that is consistent with geologic evidence. Details of garnet zoning provide surprisingly tight constraints on the P-T path.

A case study approach to teaching environmental mineralogy

Recent student interest in environmental geology encouraged me to design a new course entitled Environmental Mineralogy as a way to teach advanced topics of applied mineralogy. Rather than try to cover all topics that might be included in Environmental Mineralogy, I concentrated on a few illustrative subjects and investigated, with the students, a case study related to each topic. The first topic, silicosis, was illustrated by the 1930–31 Hawks Nest incident in which many workers died from acute silicosis contracted while excavating a five-kilometer tunnel through sandstone. The second topic, asbestos-related disease, was illustrated by the two-week delay in opening New York City schools in 1993 because of fear that children were in danger from asbestos in their schools. The third topic concerned pollution from metal mining, and the case study was the Summitville gold mine in southwestern Colorado. The fourth topic was minerals and mineraloids proposed for isolating radioactive waste at the Yucca Mountai...