Edward F. Duke

Are discontinuous chondrite-normalized REE patterns in pegmatitic granite systems the results of monazite fractionation?

Abstract All 14 stable rare earth elements (REEs) in biotite-muscovite granite and tourmaline-rich granite of the Harney Peak Granite, Black Hills, South Dakota, USA, have been analyzed using inductively coupled plasma mass spectrometry. Chrondrite-normalized REE patterns of the tourmaline-rich granites are discontinuous between Nd and Sm. The discontinuity was modeled successfully by the fractional crystallization of monazite from a biotite-muscovite granite initial composition. This explanation may also apply to the development of such discontinuous patterns in other highly evolved rocks.

Near infrared spectra of muscovite, Tschermak substitution, and metamorphic reaction progress: implications for remote sensing

Near infrared (NIR) spectra of Precambrian metagraywacke in the Black Hills, South Dakota, demonstrate that reflectance spectroscopy can be used to monitor progressive changes in mineral chemistry as a function of metamorphic grade. The wavelength of a combination Al-O-H absorption band in muscovite, measured using both laboratory and field-portable NIR spectrometers, shifts from 2217 nm in the biotite zone to 2199 nm in the sillimanite + K-feldspar zone. The band shift corresponds to an increase in the Al vi content of muscovite, determined by electron microprobe, and is thus a monitor of Al 2 Si -1 (Fe,Mg) -1 (Tschermak) exchange. Spectroscopic measurements such as these are useful in the case of aluminum-deficient rocks, which lack metamorphic index minerals or appropriate assemblages for thermobarometric studies, and in low-grade rocks (subgarnet zone), which lack quantitative indicators of metamorphic grade and are too fine grained for petrographic or microprobe studies. More important, spectroscopic detection of mineral-chemical variations in metamorphic rocks provides petrologists with a tool to recover information on metamorphic reaction histories from high-spectral-resolution aircraft or satellite remote sensing data.

Magnetic field enhanced thermal conductivity in heat transfer nanofluids containing Ni coated single wall carbon nanotubes

In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a ...

Hydrothermal graphite in New Hampshire: Evidence of carbon mobility during regional metamorphism

Graphite precipitated from hydrothermal fluids pervades the sillimanite-grade metasedimentary and plutonic rocks of New Hampshire. Hydrothermal graphite occurs as microscopic veinlets, halos of spherulites around shear zones, and metre-thick veins. Carbon isotope analyses of the graphite range from −25‰ to −9‰ δ13C (PDB), intermediate between the two biogenic crustal reservoirs of reduced organic matter and carbonate. It is proposed that carbon was mobilized from sediments as CO2 and CH4 during metamorphic devolatilization reactions. The carbon-bearing species were transported in aqueous fluids through hydraulic fractures. Graphite precipitated when aqueous fluids with different CO2/CH4 ratios were mixed in fractures.

Textural and isotopic variations in graphite from plutonic rocks, South-Central New Hampshire

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 δ13C 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 δ13C 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 δ13C values. Values of δ13C vary by up to 3.4‰ within veins, with samples taken only 3 cm apart differing by 2.0‰ These variations in δ13C 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).

Fluid inclusion and carbon isotope studies of quartz-graphite veins, Black Hills, South Dakota, and Ruby Range, Montana☆

Abstract Fluid inclusions and graphite are intimately associated in quartz veins that cut high grade metamorphic rocks in the Black Hills, South Dakota, and at the Crystal Graphite Mine in the southwestern Ruby Range, Montana. Measured fluid inclusion compositions and volumetric properties were compared with calculated compositions of graphite-saturated fluids and with estimates of metamorphic P-T conditions and carbon isotope ratios of graphite were measured to evaluate possible sources of carbon in veinforming fluids. Fluid inclusions from the two areas contrast markedly in their reliability as recorders of metamorphic fluid compositions and metamorphic conditions. The δ 13 C of graphite associated with the veins indicates that the source of carbon was also different in the two areas. In the Black Hills veins, fluid inclusions are dominantly H 2 OCO 2 mixtures with 24–96 mol% CO 2 and a maximum of ∼5 mol% N 2 and ∼ 13 mol% CH 4 . Isochores for the highest density inclusions pass near estimated peak metamorphic conditions (550°–600°C, 4.5–6.5 kbar) and fluid inclusion compositions are compatible with thermodynamic predictions for fluids in equilibrium with graphite in the stated P - T range at geologically reasonable ƒ O 2 .Graphite in a 12-cm wall-rock alteration zone adjacent to one of the veins has uniform δ 13 C of −20.8 ± 0.2%., indicating that carbon in the vein-forming fluid was derived largely from reduced organic carbon. In the Ruby Range, peak metamorphic conditions were higher— ∼750°-850°C, 5–8 kbar. In contrast to the Black Hills veins, fluid inclusions are almost all CO 2 CH 4 mixtures (with unknown N 2 content). Many contain > 20 equivalent mol% CH 4 and mixed H 2 OCO 2 inclusions were observed in only one sample. Inclusions in one vein have ∼ 84–97 mol% CH 4 . Virtually all inclusion compositions are incompatible with computed graphite equilibria and inclusion isochores likewise do not pass through estimated metamorphic conditions. The density and composition of most, if not all, inclusions have been modified subsequent to original trapping, possibly through H 2 O loss. The range of δ 13 C values of vein graphites (−5.8 to −8.6%.) is nearly indistinguishable from values for graphite in dolomitic marble near the veins (−4.8 to −7.1%.). Carbon was probably mobilized through devolatilization reactions in the marble and precipitated as 13 C-rich graphite in the veins at fairly constant temperature and from fluid of fairly constant composition.

Near infrared spectra of white mica in the Belt Supergroup and implications for metamorphism

Abstract The wavelength of the white mica (illite and muscovite) Al-OH absorption band near 2200 nm in 1036 samples from the Belt Supergroup and associated metasedimentary rocks was determined with a portable visible and near infrared reflectance spectrometer. The Al-OH wavelength decreases from 2225 nm in sub-biotite-zone samples to 2194 nm in sillimanite-zone samples; this decrease corresponds with an increase in total Al content of white mica from ~2.0 to ~2.8 atoms per 11 O atoms. These observations indicate that: (1) the frequency of the Al-OH vibration is controlled by the aluminoceladonite exchange reaction [IVSi+VI(Mg,Fe2+) = IVAl+VIAl], and (2) the reaction proceeds toward more Al-rich composition with increasing metamorphic grade. In these circumstances, Al-OH wavelength provides an indirect monitor of compositional variation and metamorphic grade. Metamorphic grade in most of the study area is in the biotite zone or lower, yet Al-OH wavelengths in low-grade rocks define systematic patterns that correlate with depth of burial and later structural displacements. In higher grade areas, wavelengths generally decrease from the garnet isograd through the sillimanite zone; however, anomalies occur locally, and it is not clear whether these result from differences in bulk composition and mineral assemblage or whether they point to actual metamorphic or structural discontinuities. These findings indicate that reflectance spectroscopy can yield valuable information on metamorphic intensity in rocks containing white mica, particularly in low-grade sequences where conventional indicators of metamorphic grade are lacking. Furthermore, this information can be obtained with field-portable spectrometers and the potential exists to obtain comparable results from airborne and spaceborne imaging spectrometers

Contrasting scales of element mobility in metamorphic rocks near Harney Peak Granite, Black Hills, South Dakota

Outcrop-scale and regional dispersion patterns of rare elements help establish origins of fluid components, mechanisms of fluid flow, and the scale of element mobility in metamorphic rocks near the Harney Peak Granite. Enrichment of granite-derived elements (Sn, B, W, Li, Cu, F, P) adjacent to a quartz vein 3.5 km north of the granite indicates that fracture-controlled flow was one mechanism for movement of magmatic fluids. Graphitization of wall rocks and CO2-CH4 fluid inclusions in vein quartz show that C was also transported by the vein-forming fluid. Mixing of H2O-rich magmatic fluid and CO2-CH4–bearing metamorphic fluid within a concealed intrusive below the vein is proposed, based on the observation of late-stage graphite in several exposed pegmatite bodies in the area. Other wall-rock alteration includes small relative gains of Rb, K, and Al toward the vein, and relative losses of Na, Sr, Ca, Si, Fe, Mg, Mn, and possibly Ti. Changes in P, K, Al, and Si, and reequilibration of whole-rock δ18O, are confined to a 50-cm-wide zone next to the vein. Additions of C, Sn, W, and Cu, and depletion of Ca, Na, and Sr reach 100–150 cm from the vein. Boron reaches a maximum 25–150 cm from the vein and drops to background levels between 150 and 250 cm. In contrast to these elements, Li is three times the regional background level throughout the outcrop (at least 600 cm from any known veins), and F and Rb remain at elevated levels. Comparing these results with a regional geochemical profile away from the Harney Peak Granite and published results on dispersion around rare-element pegmatites documents the extent of element redistribution on a broader scale. For example, Rb and F can be affected up to ∼100 m from local fluid sources, whereas Li appears to record interaction with granite-derived fluids in rocks that are 500 m or more from map-scale fluid sources. Thus regional dispersion patterns of Li and other rare elements provide a minimum estimate of the volume of mid-crustal rock that interacted with granite-derived fluids in the Harney Peak area.

Mineral distribution in contact-metamorphosed siliceous dolomite at Ubehebe Peak, California, based on airborne imaging spectrometer data

Abstract Visible and near-infrared reflectance spectra of minerals in the CaO-MgO-SiO2-H2O-CO2 system and airborne imaging spectrometer data with a spatial resolution of 3.6 m were used to map the distribution of dolomite, calcite, tremolite, serpentine, brucite, and related minerals in the Ubehebe Peak contact aureole. The results are entirely consistent with previously mapped isograds, but the high-spatial-resolution imagery offers a more accurate representation of the complex distribution of metamorphic minerals, and serves to highlight critical relationships between mineralogy and controlling factors such as stratigraphy, structure, and fluid infiltration. Rocks in the aureole are predominantly quartz-poor dolomite and limestone, and these are classified as pure dolomite or calcite, accordingly. Tremolite and serpentine (after forsterite) are largely restricted to stratigraphic units that contain siliceous dolomite, quartzite, or chert. In one area, however, tremolite and serpentine delineate a zone of retrograde hydration attributed to fluid flow along a fault. Wall rocks within 200 m of the contact and inliers of marble in the pluton contain brucite (after periclase), which documents sites of focused infiltration of H2O-rich fluid. In addition, the imagery reveals local metasomatic zones, marked by grossular-rich skarn and epidote alteration of the adjacent quartz monzonite, as well as large-scale patterns of bleaching, interpreted to define the maximum extent of fluid flow. The degree of bleaching and the intensity of plastic deformation are closely coupled; both effects increase abruptly at the forsterite isograd, which appears to represent a fundamental discontinuity with respect to chemical, physical, and mechanical processes in the aureole

Mass transfer during wall-rock alteration: An example from a quartz-graphite vein, Black Hills, South Dakota

Abstract Mass transfer and fluid-rock interaction have been evaluated along two sample traverses in low-sillimanite grade quartz-mica schist adjacent to a synmetamorphic quartz-graphite vein in the southern Black Hills, South Dakota. In an ~ 17 cm halo between apparently unaltered schist and the vein contact is an outer zone of cryptic alteration and three inner zones of visible alteration. The cryptic zone consists of the original prograde metamorphic mineral assemblage (quartz + biotite ± muscovite + plagioclase + microcline) plus anomalously high amounts of tourmaline. The outermost visible zone contains abundant graphite. The second visible zone is defined by intensive bleaching of the schist. The innermost visible zone, immediately adjacent to the vein, is tourmaline + quartz + plagioclase + limonite + graphite. The vein is composed almost entirely of quartz, but also contains trace amounts of graphite. Mass balance calculations indicate that Al was essentially inert. The predominant chemical changes during wall-rock alteration were addition of B and C from the vein-forming fluid along with loss of K from the wall rocks, corresponding to precipitation of tourmaline and graphite, and the progressive destruction of microcline, biotite, and muscovite toward the vein. In addition, the elements V, Cr, Cu, Zn, Pb, As, Sb, W, and Au were introduced into the country rock, whereas Si, Rb, Ba, and Cs were removed. On the basis of a constant Al reference frame, calculations indicate a net volume loss of 21–34% within one centimeter of the vein with little or no volume loss further from the vein. Fluid-rock interaction modeling suggests that between one and four equivalent masses of fluid interacted chemically with the most altered mineral assemblages. In addition, greater than one equivalent mass of reactive fluid penetrated to distances of at least 5 cm from the vein contact.