J. Stephen Huebner

Conodont color and textural alteration: An index to regional metamorphism, contact metamorphism, and hydrothermal alteration

Experimental and field data are used to extend the utility of conodonts as semi-quantitative thermal indices into the regimes of regional and contact metamorphism, as well as hydrothermal alteration. An Arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color-alteration indices (CAI) above 300 °C, that is, for CAI values of 5½ through 8. Such CAI values occur in very low- to medium-grade, regionally metamorphosed, contact-metamorphosed, and hydrothermally altered rocks. The uniformity or variability of CAI values within a sample, together with conodont texture, can help to distinguish grades and environments of metamorphism, particularly in metacarbonate sequences. Induced CAI by pyrolysis in a water-methane mixture at ½ kbar results in retardation of CAI and in a disparate mixture of both low and high CAI values within each experimental sample. In this system, color-alteration processes, above a CAI of 2 to 3, seem to change from predominantly carbonization to predominantly loss of organic matter, presumably by oxidation and volatilization of oxides. These experiments approximate the type of CAI mixture characteristically found in conodonts recovered from hydrothermally altered rocks. These data indicate that CAI values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization.

Impedance spectra of hot, dry silicate minerals and rock: Qualitative interpretation of spectra

Abstract Impedance spectroscopy helps distinguish the contributions that grain interiors and grain boundaries make to electrical resistance of silicate minerals and rocks. The technique also distinguishes the low-frequency response due to the presence of instrument electrodes. We measured olivine, orthopyroxene, clinopyroxenes, and both natural and synthetic clinopyroxenite. Measurements were made at 1 bar, from 750 to 1150 °C, and over a frequency range from < 10-4 to > 106 Hz; some measurements were also made at 300-850 °C and 10-20 kbar. The grain-interior response lies at highest frequency, the sample- electrode response at low frequencies, and the grain boundary response at mid-frequencies. Grain interiors show as semicircular impedance arcs when plotted on the complex plane, and sample-electrode responses of hot single crystals and of hot dry rocks are exhibited as depressed arcs. In comparison, monofrequency measurements contain no information to identify the source of the response; at 1 kHz they detect only the resistance sum of grain interiors and grain boundaries and at low frequency (≤ 1 Hz) are likely to sense all three components. The major experimental problem is to find electrodes that make good contact with the sample and that are stable with time. The effect of pressure (10 kbar, 300-800 °C) is to diminish the resistance associated with grain boundaries and the sample-electrode interface, in the laboratory and presumably in nature. Monofrequency measurements at 1 bar may underestimate the conductivity of rocks at similar temperature but higher pressure. A network of electrical elements is presented for use in interpreting impedance spectra and conductive paths in hot or cold, wet or dry, minerals and rocks at any pressure. In dry rocks, a series network path predominates; in wet rocks, aqueous pore fluid and crystals both conduct. Finite resistance across the sample-electrode interface is evidence that electronic charge carriers are present at the surface, and presumably within, the silicate minerals and rocks measured.

Measured oxygen fugacities of the Angra dos Reis achondrite as a function of temperature

Measurements of the oxygen fugacity (ƒO2) as a function of temperature (T) were made on an interior bulk sample of the cumulate achondrite, Angra dos Reis. Data clustered between theƒO2-T relationship of the iron-wustite assemblage and 1.2 log atm units above iron-wustite. Interpretation of the data indicates that, throughout most of the cooling history of the meteorite, ƒO2 values were defined by equilibria involving iron-bearing species at values close to the ƒO2 of the assemblage iron-wustite. Measured ƒO2 data are compatible with crystallization and cooling at pressures greater than 50 bars.

Chemical fluxes and origin of a manganese carbonate-oxide-silicate deposit in bedded chert

Abstract Lens-like rhodochrosite-rich bodies within interbedded chert and shale are associated with basalt and/or graywacke in ophiolitic and orogenic zones. The Buckeye manganese mine in the Franciscan Complex of the California Coast Ranges is associated with metagraywacke. Despite blueschist-facies metamorphism, this deposit preserves the compositions and some textural features of its sedimentary protoliths. For this reason, it is a suitable deposit with which to compare more intensely altered deposits, or deposits originating in different paleoenvironments. Six Mn-rich and three Mn-poor minerals form monomineralic layers and mixtures: rhodochrosite, gageite, Mn-oxides (hausmannite, braunite), divalent Mn-silicates (caryopilite, taneyamalite), chlorite, quartz (metachert) and aegirine-augite. The Mn-rich protoliths have high Mn/Fe combined with relatively low concentrations of Ca, Al, Ti, Co, Ni, Cu, Th and REE. REE patterns of various protoliths are distinct. Rhodochrosite and gageite layers are depleted (seawater × 5 · 104) and flat, whereas patterns of metachert and the Mn-silicate-rich layers mimic the patterns of metashale and metagraywacke (seawater × 106). Hausmannite layers have flat patterns (seawater × 7 · 104) whereas braunite-rich layers are more enriched (seawater × 2 · 105) and show a distinct positive Ce anomaly. Factor analysis reveals components and fluxes attributed to sub-seafloor fluids (Ni, As, Zn, Sb, W, Mn), seawater (Mg, Au, V, Mo), detritus and veins (Ca, Ba, Sr). Silica is negatively correlated with the sub-seafloor factor. The observed variances indicate that water from the sediment column mixed with seawater, that deposition occurred near the sediment-seawater interface before mixtures of subsurface fluid and seawater homogenized, and that the system was not entirely closed during metamorphism. The variations in REE enrichment can be related to kinetics of deposition: rhodochrosite and gageite were precipitated most rapidly, and therefore were the protoliths that most effectively diluted the REE-rich background resulting from fine clastic material (derived from distal turbidites). The variation of the Ce anomaly and U/Th among diverse lithologies and the differences in Mn oxidation states are consistent with progressive dilution of reduced subsurface fluids with oxidized seawater. By this scheme, rhodochrosite, gageite and hausmannite were deposited from the most reduced fluids, braunite from intermediate mixtures, and Mn-silicates from the sub-seafloor fluids most diluted with fresh seawater. Comparison of the Buckeye with other lens-like and sheet-like deposits having high Mn/Fe and containing Mn3+ and/or Mn2+ suggests that each had three essential fluxes: a sub-seafloor source of Mn, a local source of very soluble silica and a source of relatively fresh, oxygenated water. Additional fluxes, such as clastics, appear to be more characteristic of the paleoenvironment than the three essential fluxes.

HEAT CAPACITIES AND THERMODYNAMIC PROPERTIES OF BRAUNITE (MN7SIO12) AND RHODONITE (MNSIO3)

Abstract The AI-in-hornblende barometer potentially offers a basis for estimating crystallization pressure for granitic batholiths. However, owing to the simplicity of its formulation, misuse of the barometer can occur. Many granitic intrusions are emplaced at conditions inconsistent with those of the existing experimental calibrations, including fO₂ < NNO and/or variable to high temperature. The barometer is sensitive to variations in both fO₂ and temperature: low fO₂ can cause calculated pressures to be high by a factor of two or more, and the effect of temperature is up to 2 kbar per 100 °C, depending on total Al abundance. Batholiths emplaced at elevated temperature and portions of plutons that crystallized below H2O saturation yield artificially high pressures relative to near-solidus experiments conducted at H2O saturation. Al-in-hornblende pressures within the tonalitic Mount Stuart batholith of Washington, for example, define a 1-4 kbar domal structure that is exaggerated by emplacement crystallization temperatures that range from 620 to 760 °C.Pressure correlates with temperature (r2 = 0.86), indicating that temperature-sensitive edenite exchange has greatly influenced observed pressure variations. Data for other plutons also exhibit a marked positive correlation between temperature and hornblende pressure. If corrections are not made for temperature, such large apparent pressure variations can lead to overly high estimates of pluton thickness and tilt in addition to invalid tectonic interpretations. Using experimental data at ~675 °C(Schmidt, 1992) and at ~760 °C (Johnson and Rutherford, 1989), a revised expression fur the barometer incorporating the effect of temperature is P(±0.6 kbar) = 4.76Al- 3.01 - {[T(°C) - 675]/85} × {0.530Al + 0.005294[T(°C) - 675]}. For a pluton emplaced at 100 °C above wet-solidus temperatures, this reformulation ofthe barometer lowers derived pressures by 1.3 to > 2 kbar at typical crustal pressures. For the Mount Stuart batholith, consideration of temperature yields revised pressures that are in agreement with pressures obtained from wall rocks and eliminates much of the apparent domal structure. Low- fO₂ granites have amphibole Fe/(Fe + Mg) ratios that exceed the typical 0.40-0.65 range used in most experimental and empirical calibrations. Examples from anorogenic granitic batholiths of mid-Proterozoic age yield pressures that are too high by a factor of two to three in comparison with pressures obtained from adjacent metamorphic assemblages. Hornblende in these granites not only has high Fe/(Fe + Mg) but also low ratios of Fe3+ to Fe2+. The anomalously high Al in Fe-rich, Fe3+ -poor hornblende is inferred to be the result of increased [6]Al occupancy of the M2 site not buffered by the Mg and Fe3+ abundances typical of amphiboles in calc-alkaline and other high- fO₂ plutonic rocks.

Mineralogy and geochemistry of two metamorphosed sedimentary manganese deposits, Sierra Nevada, California, USA

Abstract Laminated to massive rhodochrosite, hausmannite, and Mn-silicates from the Smith prospect and Manga-Chrome mine, Sierra Nevada, California were deposited as ocean floor sediments associated with chert and shale. The principal lithologies at Smith are chert, argillite, rhodochrosite-, hausmannite- and chlorite-rich layers, and relatively uncommon layers of jacobsite. The Manga-Chrome mine also contains layers rich in manganoan calcite and caryopilite. Tephroite, rhodonite, spessartine, and accessory alleghanyite and sonolite formed during metamorphism. Volcaniclastic components are present at Manga-Chrome as metavolcanic clasts and as Mn-poor, red, garnet- and hematite-rich layers. There is no evidence, such as relict lithologies, that Mn was introduced into Mn-poor lithologies such as chert, limestone or mudstone. Replacement of Mn-poor phases by Mn-rich phases is observed only in the groundmass of volcanic clasts that appear to have fallen into soft Mn-rich mud. Manganiferous samples from the Smith prospect and Manga-Chrome mine have high Mn Fe and low concentrations of Ni, Cu, Zn, Co, U, Th and the rare-earth elements that are similar to concentrations reported from other ancient Mn deposits found in chert-greenstone complexes and from manganiferous sediments and crusts that are forming near modern sea floor vents. The Sierra Nevada deposits formed as precipitates of Mn-rich sediments on the sea floor, probably from mixtures of circulating hydrothermal fluids and seawater. The composition of a metabasalt from the Smith prospect is consistent with those of island-arc tholeiites. Metavolcanic clasts from the Manga-Chrome mine are compositionally distinct from the Smith metabasalt and have alkaline to calc-alkaline affinities. A back-arc basin is considered to be the most likely paleoenvironment for the formation of the Mn-rich lenses at the Manga-Chrome mine and, by association, the Smith prospect. Layers of rhodochrosite, hausmannite and chert preserve the composition and some textures of the sedimentary protoliths at both Sierra Nevada deposits. Jacobsite-rich layers probably represent a Fe-rich protolith. Caryopilite and manganoan calcite represent additional protoliths at the Manga-Chrome mine. The metamorphic assemblage prehnite-chlorite-epidote-calcite in a metabasalt from the Smith prospect constrains regional metamorphic conditions to a maximum temperature of 325°C and a pressure of 2 kbar. Slightly higher temperatures are indicated by the presence of actinolite in another metabasalt. Compositions of Mn-rich minerals in Smith samples are consistent with these metamorphic conditions.

Crystallography of Some Lunar Plagioclases

Crystals of calcic bytownite from type B rocks have space group I1 with c ≈ 14 angstroms. Bytownite crystals from type A rocks are more sodic and have space group C1, c ≈ 7 angstroms. Cell parameters of eight bulk feldspar separates from crystalline rocks indicate that the range of angle gamma is about 23 times the standard error of measurement, and its value might be useful for estimation of composition. Cell parameters of seven ilmenites are close to those of pure FeTiO3.

The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415

Abstract A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg1.14Fe0.80Mn0.02Ca0.04)(Si1.97Al0.03)O6, has a unit cell in space groupPbca witha = 18.310(15)A,b = 8.904(10)A,c = 5.214(7)A, containing 2 formula units. A set of 742 counter-measured intensity data made with MoKα radiation has been used to refine the crystal structure in isotropic thermal mode toR = 0.116. Anisotropic refinement led toR = 0.092, but thermal parameters are distorted by non-random errors resulting from poor crystal texture. The resulting structure is in close agreement with that obtained by Ghose [9] for a hypersthene from Greenland. A parameterq, which gives (MgqFe1−q) for cation siteM(1) and (Mg1.14−qFeq−0.18Ca0.04) for siteM(2), was included in the least-squares analysis, yieldingq = 0.90(1). This orthopyroxene has the high degree of cation order expected of pyroxenes subjected to Apollonian metamorphism at lower than 500–600°C. No evidence exists for a subsequent thermal event of sufficient intensity to disorder the pyroxene. On the basis of previous laboratory studies of argon-release patterns of lunar plagioclase and order-disorder kinetics of terrestrial pyroxenes, we attribute the reported isotopic age (3.9–4.1 AE) to cessation of metamorphism, perhaps caused by impact excavation.