Geochemical and Strontium Isotope Characterization of Produced Waters from Marcellus Shale Natural Gas Extraction

Extraction of natural gas by hydraulic fracturing of the Middle Devonian Marcellus Shale, a major gas-bearing unit in the Appalachian Basin, results in significant quantities of produced water containing high total dissolved solids (TDS). We carried out a strontium (Sr) isotope investigation to determine the utility of Sr isotopes in identifying and quantifying the interaction of Marcellus Formation produced waters with other waters in the Appalachian Basin in the event of an accidental release, and to provide information about the source of the dissolved solids. Strontium isotopic ratios of Marcellus produced waters collected over a geographic range of ∼375 km from southwestern to northeastern Pennsylvania define a relatively narrow set of values (εSr SW = +13.8 to +41.6, where εSr SW is the deviation of the 87Sr/86Sr ratio from that of seawater in parts per 104); this isotopic range falls above that of Middle Devonian seawater, and is distinct from most western Pennsylvania acid mine drainage and Upper Devonian Venango Group oil and gas brines. The uniformity of the isotope ratios suggests a basin-wide source of dissolved solids with a component that is more radiogenic than seawater. Mixing models indicate that Sr isotope ratios can be used to sensitively differentiate between Marcellus Formation produced water and other potential sources of TDS into ground or surface waters

Stable Isotopes of Carbon as an Invaluable Stratigraphic Tool: An Example from the Cambrian of the Northern Appalachians, USA

This study illustrates the usefulness of carbon isotopes in stratigraphic interpretations of poorly fossiliferous strata beyond the resolution possible by biostratigraphy. The Upper Cambrian strata of the lower Gorge Formation in northwestern Vermont have been interpreted as highly condensed continental-slope deposits with a hiatus bracketed by the trilobite fauna of the Dunderbergia zone (Steptoean) and the Saukia zone (upper Sunwaptan). This interpretation was based on information from two thin fossiliferous intervals, the occurrence of one of them being unconfirmed in a recent study. Carbon isotopes provide a means for testing this interpretation because marine carbonate rocks deposited during the Steptoean Age of the Late Cambrian record a large, global, positive carbon isotope excursion (δ13C values of up to +5‰ relative to the Vienna Peedee belemnite standard). If the proposed age for the lowermost Gorge Formation is correct, these strata should record the start of the excursion, and the hiatus should produce an abrupt termination in the record of the excursion near maximum values. The determined δ13C values (-0.57% to +0.39%) indicate that the excursion is not recorded at this locality. The results question the age for these strata and suggest that the hiatus is greater than previously recognized, encompassing most of the Steptoean. This major sedimentary hiatus lends supporting evidence for a significant eustatic or pan-Laurentian sea-level event during the Steptoean perturbation in the global cycling of carbon

Geochemistry and chronology of the bunburra rockhole ungrouped achondrite

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca-phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The 26Al-26Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of 26Al, but a slight excess in the radiogenic 26Mg in a bulk sample allows the determination of a model 26Al-26Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The 207Pb-206Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole-rock isochron provides a re-equilibration age of ~4.1 Ga, most likely caused by impact heating.The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula

Non-basaltic asteroidal magmatism during the earliest stages of solar system evolution: A view from Antarctic achondrites Graves Nunatak 06128 and 06129

The recently recovered paired Antarctic achondrites Graves Nunatak 06128 and 06129 (GRA) are meteorites that represent unique high-temperature asteroidal processes that are identified in only a few other meteorites. The GRA meteorites contain high abundances of sodic plagioclase, relatively Fe-rich pyroxenes and olivine, abundant phosphates, and low temperature alteration. They represent products of very early planetesimal melting (4565.9 ± 0.3 Ma) of an unsampled geochemical reservoir from an asteroid that has characteristics similar to the brachinite parent body. The magmatism represented by these meteorites is contrary to the commonly held belief that the earliest stages of melting on all planetary bodies during the first 2-30 Ma of solar system history were fundamentally basaltic in nature. These sodic plagioclase-rich rocks represent a series of early asteroidal high-temperature processes: (stage 1) melting and partial extraction of a low-temperature Fe-Ni-S melt, (stage 2) small degrees of disequilibrium partial melting of a sodium- or alkali-rich chondritic parent body with additional incorporation of Fe-Ni-S melt that was not fully extracted during stage 1, (stage 3) volatile-enhanced rapid extraction and emplacement of the Na-rich, high-normative plagioclase melt, (stage 4) final emplacement and accumulation of plagioclase and phosphates, (stage 5) subsolidus reequilibration of lithology between 962 and 600 °C at an fO2 of IW to IW + 1.1, and (stage 6) replacement of merrillite and pyroxene by Cl-apatite resulting from the interaction between magmatic minerals and a Cl-rich fluid/residuum melt. The subsolidus events started as early as 4561.1 Ma and may have continued for upwards of 144 million years. The existence of assemblages similar to GRA on several other planetary bodies with different geochemical characteristics (ureilite, winonaites, IAB irons) implies that this type of early asteroidal melting was not rare. Whereas, eucrites and angrites represent extensive melting of a parent body with low concentrations of moderately-volatile elements, GRA represents low-degrees of melting of a parent body with chondritic abundances of moderately volatile elements. The interpretation of the low-temperature mineral assemblage is somewhat ambiguous. Textural features suggest multiple episodes of alteration. The earliest stage follows the interaction of magmatic assemblages with a Cl-rich fluid. The last episode of alteration appears to cross-cut the fusion crust and earlier stages of alteration. Stable isotopic measurements of the alteration can be interpreted as indicating that an extraterrestrial volatile component was preserved in GRA