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We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of l[per thousand] on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which [delta][sup.56]Fe of magnetite > silicate [greater than or equal to] carbonate. The intermineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature-dependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record.

Abstract: The Wyoming Province is a distinctive Archean craton in the northwestern United States that can be subdivided into three subprovinces, namely, from oldest to youngest, the Montana metasedimentary province, [...]

Sr isotope data on groundwater samples from coal and overlying sandstone aquifers in the eastern Powder River Basin, Wyoming, demonstrate that the Sr isotope ratio effectively identifies groundwater from different aquifers where major ion geochemistry and O and H stable isotope data fail. Groundwaters from sandstone aquifers have a uniform [sup.87]Sr/[sup.86]Sr ratio of 0.7126-0.7127. Waters from coal seams vary from [sup.87]Sr/[sup.86]Sr ratio = 0.7127 near the recharge area to 0.7151 farther into the basin. The distinct Sr isotope signatures of sandstone and coal aquifers may reflect different sources of Sr in these two rock types: Sr in sandstones is held primarily in carbonate cement, whereas coals contain more radiogenic Sr in organic matter. The Sr isotope ratio is useful in identifying wells that contain mixed waters, whether due to well construction or to incomplete aquifer isolation. Measurement and continued monitoring of the Sr isotope ratio in groundwaters should provide a powerful tool for characterizing the impact of the burgeoning coal-bed methane industry on the hydrology of the Powder River Basin. Keywords: coal, coal-bed methane, isotopes, groundwater, Wyoming.

It is accepted that continental growth takes place both in intraplate and convergent margin settings, but the relative importance of crustal growth in these two tectonic environments is the subject of ongoing debate. In this study we suggest that magmatic underplating in continental interiors results in significant increases in continental volume. We maintain that A-type, or anorogenic, granites are derived from young, underplated mafic crust. Magma sources for A-type granites typically are difficult to identify due to the lack of isotopic contrast between mantle and crustal sources. However, the northern portion of the Mesoproterozoic Sherman batholith, southeastern Wyoming, intrudes Archean gneiss. Isotopic data from these granites preclude derivation from felsic crust, and instead require the involvement of a mantle or mantle-like isotopic reservoir. The data are analogous to those for eruptive equivalents of A-type granites, the fayalite rhyolites of Yellowstone, which also ascended through Archean felsic crust but carry little Archean isotopic signature. Anorogenic granites thus may represent a middle to upper crustal record of magmatic underplating at depth. Keywords: A-type granites, isotopes, Proterozoic, Wyoming province, crustal origin.