Douglas E. Crowe

Micro-analysis of sulfur-isotope ratios and zonation by laser microprobe

The University of Wisconsin laser microprobe/mass spectrometer system combines high spatial resolution with precise, accurate, and rapid analysis of sulfur-isotope ratios. Spot sizes of 100–200 μm are routine and much higher spatial resolution may be possible. Analytical precision ranges from 1σ = ±0.15%. (pyrite, pyrrhotite, sphalerite) to 1σ = ±0.43%.. (galena, chalcopyrite). Comparison of laser microprobe and conventional combustion analyses indicates that laser δ34S values are approximately equal to or lower than combustion values for these minerals, and an empirical correction of −0.1‰ (sphalerite) to +1.4%. (pyrrhotite) is required. Variations in δ18O of SO2, laser power density, and burn duration affect the magnitude of this correction; thus, consistent technique is critical. Natural samples were analyzed by laser microprobe to evaluate the extent of sulfur-isotope ratio zonation. Coexisting sulfide mineral pairs from the Rua Cove Mine, Alaska, show that isotopic equilibrium exists in the stockwork feeder zone, while disequilibrium characterizes coexisting pairs from the overlying, more rapidly cooled massive sulfide blanket. Conventional sulfur isotope extraction data on these samples are not valid due to the fine-grained, intergrown nature of the sulfide phases, which precludes satisfactory mineral separation. Isotopically homogeneous sphalerite and pyrite crystals were found in samples from the amphibolite facies Balmat Mine, while isotopically heterogeneous pyrite crystals were found in a sample from the upper greenschist facies Sullivan Mine. Sulfur-isotope zonation up to 2.9%. in 200 μm was found in banded sphalerite blacksmoker chimney material from the Bayda Mine (Oman). Retrograde sulfides from the Marcy Anorthosite Massif vary by 4.2%. over <1 cm, and finely disseminated sulfides from an amphibolite-granulite facies transition zone sample, southern India, are in disequilibrium across the 25 cm facies transition. These results show that sulfur isotope homogeneity may be attained in the cores of annealed high-grade metamorphic minerals while lower temperature, retrograde, or hydrothermally deposited minerals are often isotopically zoned due to temporal variations in fluid chemistry and temperature.

Controls on Geochemical Expression of Subaerial Exposure in Ordovician Limestones from the Nashville Dome, Tennessee, U.S.A.

ABSTRACT Stable-isotope and trace-element data from 301 samples in six stratigraphic sections largely support previous inferences of subaerial exposure at third-order sequence boundaries in Mohawkian and Cincinnatian limestones of the Nashville Dome. They also reveal the presence of previously unrecognized surfaces of subaerial exposure. For example, evidence of subaerial exposure at the tops of parasequences suggests that at least some of the latter are better interpreted as high-frequency sequences. Clusters of surfaces of subaerial exposure near some previously interpreted sequence boundaries suggest that the latter may represent sequence boundary zones. Some surfaces of subaerial exposure are at neither recognized sequence boundaries nor parasequence boundaries and may represent missed beats in which sea-level fluctuations are preserved geochemically but not recognized on stratigraphic criteria. Recognition of these unexpected surfaces of subaerial exposure may in part depend on the close spacing of samples used in this study. Different sequence boundaries exhibit different degrees of geochemical alteration, and more extensive alteration appears to characterize sequence boundaries of greater temporal significance. Overprinting of exposure surfaces is controlled by accommodation rate, in that meteoric alteration (as suggested by low C isotope composition and Sr concentration) is seemingly pervasive in an interval of slow accommodation but not in an interval of rapid accommodation. 18O values show little relationship to exposure surfaces in these strata, so that Sr-13C plots are more useful than 18O-13C plots in recognizing meteoric diagenesis in aggregated data. In this data set, 13C excursions interpreted to result from pedogenic processes are at most 2.4 ‰ and extend at most only 1.5 meters below exposure surfaces, less than the extent of such signatures in younger carbonates. Such minima are best developed in paleo-lowland and paleo-coastal settings, and they are essentially unrecognizable in paleo-inland settings where more section was removed by lowstand and transgressive erosion.

Oxygen isotope and trace element zoning in hydrothermal garnets: Windows into large-scale fluid-flow behavior

Single garnet crystals from a large, multicyclic hydrothermal system in Far East Russia are strongly growth zoned. Microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents, reflecting evolution of the hydrothermal system from magmatic- to meteoric-dominated conditions, and repetitive influx of magmatic fluids. Large intracrystalline variations in oxygen isotope composition (>12‰ δ 18 O) delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system, and suggest that fluid-flow regimes differed between central and distal portions of the system. These results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex, large-scale fluid-rock systems.

Muscovite-garnet granites in the Mojave Desert: Relation to crustal structure of the Cretaceous arc

A previously undocumented belt of Upper Cretaceous, primary muscoviteand garnet-bearing granites is exposed within the Mesozoic batholith of the central and western Mojave Desert of California. This granite belt occurs more than 140 km west of the better known Cordilleran interior belt of primary muscovite-bearing granitoids, and the central and western Mojave granites are derived from much different sources, possibly Pelona-Orocopia-Rand schists or similar rocks. Anatexis of Pelona-Orocopia-Rand schists to produce these granites would require emplacement of the schists prior to the Late Cretaceous–early Tertiary, the most widely accepted time of schist emplacement. The granite belt may also correlate with similar rocks in the southernmost Sierra Nevada and eastern Salinia. The distinctive isotopic and petrologic character of the granite belt reflects differences in crustal structure between the Mojave–southern Sierra Nevada and the central Sierra Nevada portions of the Cretaceous continental margin batholith. Possible correlation of this belt with rocks in eastern Salinia also places further limits on the pre-early Tertiary position of Salinia.

Molluscan radiocarbon as a proxy for El Niño–related upwelling variation in Peru

Sequential measurements of molluscan radiocarbon are demonstrated to be an effective proxy of seasonal and El Niño–related upwelling variation in coastal Peru. A Trachycardium procerum valve from southern Peru was measured through ontogeny for radiocarbon via accelerator mass spectrometry (AMS) as well as δC and δO. A specimen collected in 1984 near Casma, Peru (~9.30°S) grew before and during the 1982–1983 El Niño/southern oscillation warm event. Shell morphology recorded El Niño warming as a shallow growth break with subsequent realignment of aragonite crystal microstructure. The presence of this growth pattern indicates that shell material was precipitated during the warm event and that each AMS sample could be independently identifi ed to represent a defi ned period in the El Niño/southern oscillation cycle. Samples taken from portions of the shell precipitated prior to the El Niño warm event (before the diagnostic growth break) had a mean value of 99.8 percent modern carbon (pMC), with a maximum seasonal range of 2.1 pMC. During warming, as indicated by a negative excursion in δO and the growth break, there was an abrupt increase to 107.9 pMC. Aragonite precipitated near the margin of the valve, after the El Niño/southern oscillation event concluded, had radiocarbon values approaching those present before the growth break. We attribute this radiocarbon distribution to variations in vertical mixing of surface and deeper upwelled water of greater ventilation age. As an El Niño event begins and the thermocline deepens, less deep water reaches the surface. Thus, radiocarbon values in shell precipitated during El Niño appear younger (more positive) relative to non–El Niño periods, which represent periods of more intense upwelling. The results from the modern specimen validate the use of molluscan radiocarbon as a proxy of upwelling conditions related to El Niño/southern oscillation and suggest the utility of similar analysis of more ancient valves in both oceanography and paleoclimatology.

Preservation of original hydrothermal δ34S values in greenschist to upper amphibolite volcanogenic massive sulfide deposits

Laser microprobe analysis of sulfide mineral pairs from several volcanogenic massive sulfide (VMS) stockwork feeder zones shows that greenschist to upper amphibolite facies metamorphism will reset sulfur isotope thermometers if the mineral grains are in direct grain-to-grain contact. However, discrete monomineralic sulfide grains that are completely encased in quartz matrix and that are not in direct grain-to-grain contact with one another, will preserve their original hydrothermal sulfur isotope compositions, because exchange during metamorphism is inhibited by the enclosing quartz. Calculated temperatures from three separate VMS systems range from 170 to 424 °C for sulfide pairs that are not in direct grain-to-grain contact, and are consistent with independently determined temperature estimates. Sulfide pairs that are in direct contact yield unreasonably low temperatures, indicative of reequilibration during metamorphism and subsequent slow cooling.

Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge, eastern Pacific

Abstract The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown BEar seamount chain and the axial rift of the ridge. Several large (to 12 m) silica+sulfide+sulfate hydrothermal spires reflect high-temperature (to 300°C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation inδ 34 S, both within individual conduits and between conduits. The maximum intra-conduitδ 34 S variation (on wurtzite) is 5‰ and the maximum inter-conduit variation (on sphalerite) is 7.4‰ Inter-conduitδ 34 S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generallyδ 34 S mineral decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduitδ 34 S variation has not been documented at this scale (5‰ within 1 mm), and the potential mechanisms responsible for this variation include: (1) within-spire seawater-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter theδ 34 S fluid . Mixing of hydrothermal fluid with seawater cannot produce a range ofδ 34 S mineral -values of this magnitude (5‰), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases ofδ 34 S fluid . The fluids will ulti precipitate 34 S-enriched sulfide phases, although this excursion inδ 34 S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced andδ 34 S-values will correspondingly decrease. This process may explain the major (to +5‰) isotope excursions that occur within individual conduits on a very small (