Christopher J. Eastoe

HIGH-PRECISION MEASUREMENT OF CHLORINE STABLE ISOTOPE RATIOS

We present an analysis procedure that allows stable isotopes of chlorine to be analyzed with precision sufficient for geological and hydrological studies. The total analytical precision is ≤±0.09%., and the present known range of chloride in the surface and near-surface environment is 3.5‰. As Cl− is essentially nonreactive in natural aquatic environments, it is a conservative tracer and its δ37Cl is also conservative. Thus, the δ37Cl parameter is valuable for quantitative evaluation of mixing of different sources of chloride in brines and aquifers.

Stable chlorine isotopes in the Palo Duro Basin, Texas: Evidence for preservation of Permian evaporite brines

Experimental evaporation of seawater yields brines with δ37Cl from 0.0‰ (initial) to −0.9‰. In the Palo Duro Basin, brines with δ37Cl values overlapping the −0.1 to 0.4‰ range of halite evaporite can be generated by a set of processes including dissolution of halite in meteoric water. Such brines occur above and below an evaporite aquitard. Brines with δ37Cl values of −0.4 to −1.0‰ in the evaporite aquitard and in a deep brine aquifer cannot be generated by dissolution of halite. Considered with Br and Cl content, δD and noble gas content, the δ37Cl data indicate that such brines originated as evapoconcentrated seawater. High Br evaporite brine formed directly from seawater on the eastern side of the basin, whereas low Br evaporite brines on the western side formed after an influx of meteoric water at the time of evaporite formation. There has not been detectable vertical flow of meteoric water across the evaporite aquitard. Strata beneath the evaporite contain compartments that have been isolated geochemically since the Permian.

Stable chlorine isotopes in hydrothermal processes

Abstract Values of δ37Cl can now be routinely measured with an analytical precision of ±0.2%., as reported herein on fluid inclusion leachates from the Elmwood-Gordonsville Mississippi Valley-type deposit and formation waters from its host formation; basin brines from the Gulf Coast and Palo Duro Basins; and fluid inclusion leachates and biotites from the Bingham, Silver Bell and Panguna porphyry copper deposits. Isotopic measurements are not affected by reaction yield in the range 85–100% or by generation of free I2 during sample preparation. In the Elmwood-Gordonsville deposit, five successive paragenetic stages of ore and gangue (sphalerite, fluorite, calcite 2, barite and calcite 3) each yielded multiple δ37Cl measurements. Ranges of δ37Cl vary in alternating fashion between stages with more negative values (sphalerite, calcites 2 and 3) and those with more positive values (fluorite, barite). An interface between two brine systems with δ 37 Cl and >0.0%. appears to have moved back and forth across the zone of mineralization. Brines from the Gulf Coast and Palo Duro basins are apparently bimodal in δ37Cl with modes near −0.9%. and 0.0%., consistent with the variation in δ37Cl observed in fluid inclusion leachates from Elmwood-Gordonsville. Biotite in the Bingham deposit has δ37Cl values up to 1.7%. and chlorine isotopes that are fractionated with respect to salt-rich hydrothermal fluid. Hydrothermal fluids in all three porphyry copper deposits studied have a δ37Cl range of −0.1 to 0.4%. with the one exception of a −0.5%. value at Bingham. Loss of low -δ 37 Cl chloride in vapor may balance chloride with a net δ 37 Cl > 0 in the porphyry deposits.

Stable chlorine isotopes in halite and brine from the Gulf Coast Basin: brine genesis and evolution

Stable chlorine isotope data, used in conjunction with other geochemical parameters, are useful in determining the origin of solute in formation waters with salinity different from that of seawater. Jurassic salt in the Gulf Coast Basin has δ37Cl ranges of −0.5‰ to 0.3‰ (bedded) and 0.0‰ to 0.5‰ (diapiric). The values for bedded salt are consistent with a δ37Cl value of 0.0‰ for Jurassic seawater chloride, as in modern oceans. The slightly higher values for diapiric salt are possibly caused by incongruent solution of halite. Formation waters have a δ37Cl range of −1.9‰ to 0.7‰. Waters with δ37Cl −0.2‰. Low-salinity water from geopressured aquifers appears to have resulted from shale dewatering. Low δ37Cl values in such water are related to low mCl/mNa (a parameter indicating dewatering) but show no general relationship to Cl/Br. They are consistent with diffusion of chloride from allochthonous brine. Diffusion can generate domains of water with negative δ37Cl on a scale of hundreds of meters in 104–106 years, and may also generate positive-δ37Cl chloride in residual source brine.

Isotopic composition of carbonates in the SNC meteorites, Allan Hills 84001 and Zagami

We have measured the 13 C/ 12 C and 14 C/ 12 C ratios in CO 2 released by acid etching of the meteorites Zagami and Allan Hills 84001. We use the 14 C as a label to identify extraterrestrial carbonate phases, as they will have a low 14 C/ 12 C ratio (< - 4% modern terrestrial ratio) compared to recent terrestrial material. The new studies on Allan Hills 84001 confirm previous conclusions that the Fe, Mg-rich carbonate grains in this meteorite contain carbon with δ 13 C as high as +45 ‰. In contrast, the carbon released from Zagami is depleted in 13 C with δ 13 C as low as -20‰. We conclude that the isotopic composition of the carbon as carbonate released from acid etching of Zagami is different from the carbonates observed in both Allan Hills 84001 and Nakhla. With the assumption that all of these meteorites sample the surface of Mars, we propose that the Zagami carbonate samples a different carbon reservoir on this planet, such as a magmatic source. With this interpretation, the high δ 13 C values of carbonate observed in Allan Hills 84001 and Nakhla can be ascribed to a fractionated source compared with the originally light carbon. A likely origin for this 13 C-enriched component is an isotopically heavy Martian atmosphere; however, given the possibility of biological activity involving Allan Hills carbonates, we cannot exclude this as a source of the isotopic fractionation.

Geochemical Quantification of Semiarid Mountain Recharge

Analysis of a typical semiarid mountain system recharge (MSR) setting demonstrates that geochemical tracers help resolve the location, rate, and seasonality of recharge as well as ground water flowpaths and residence times. MSR is defined as the recharge at the mountain front that dominates many semiarid basins plus the often-overlooked recharge through the mountain block that may be a significant ground water resource; thus, geochemical measurements that integrate signals from all flowpaths are advantageous. Ground water fluxes determined from carbon-14 ((14)C) age gradients imply MSR rates between 2 x 10(6) and 9 x 10(6) m(3)/year in the Upper San Pedro Basin, Arizona, USA. This estimated range is within an order of magnitude of, but lower than, prior independent estimates. Stable isotopic signatures indicate that MSR has a 65% +/- 25% contribution from winter precipitation and a 35% +/- 25% contribution from summer precipitation. Chloride and stable isotope results confirm that transpiration is the dominant component of evapotranspiration (ET) in the basin with typical loss of more than 90% of precipitation-less runoff to ET. Such geochemical constraints can be used to further refine hydrogeologic models in similar high-elevation relief basins and can provide practical first estimates of MSR rates for basins lacking extensive prior hydrogeologic measurements.

A secondary isotopic reference material of chlorine from selected seawater

Abstract A secondary isotopic reference material of chlorine as NaCl collected from a sea water, located at 4°18′N, 161°08′E and collected on October 22, 1994, was prepared using ion exchange technique. The δ37Cl of an NaCl product named ISL 354 was measured to be −0.39±0.05‰ (2σ) related to NIST SRM 975, an isotopic standard reference material with an absolute 37Cl/35Cl ratio of 0.31977. No contamination or isotopic fractionation was observed due to a complete recovery of chlorine and the avoidance of chemical reagents during preparation of ISL 354 from selected seawater. The isotopic composition of chlorine in ISL 354 does not change on long-term storage. So ISL 354 can be used as a secondary isotopic reference material of chlorine.

Preliminary evidence for fractionation of stable chlorine isotopes in ore-forming hydrothermal systems

Chloride from fluid inclusions in hydrothermal minerals is found to have variable and distinctive {delta}{sup 37}Cl values spanning the range -1.1 0/{per thousand} to +0.8 {per thousand}. In Mississippi Valley-type deposits of Tennessee, brines of high (>0{per thousand}) and low (near -1{per thousand}) {delta}{sup 37}Cl are present. High {delta}{sup 37}Cl brines may be saline formation waters, but low {delta}{sup 37}Cl brines remain unexplained. In porphyry copper deposits, both high {delta}{sup 37}Cl (0.8{per thousand}, 0.3{per thousand}) and low {delta}{sup 37}Cl (-1.1{per thousand}, 0.7{per thousand}) hypersaline brines of probable magmatic origin occur. High-salinity magmatic brines with low {delta}{sup 37}Cl values contrast isotopically with high {delta}{sup 37}Cl, less concentrated brines responsible for quartz-sericite-pyrite assemblages.

Chlorine stable isotope distribution of Michigan Basin formation waters

Abstract The Cl isotope ratio, mass37Cl/35Cl, was measured on 22 formation waters from Mississippian, Devonian, Silurian, Ordovician and Cambrian strata in the Michigan Basin. Because of its resistance to fractionation, the ratio was used to find evidence of mixing of formation waters within the Michigan Basin and between the Canadian Shield and the basin. The δ37Cl composition of waters decreased from +0.05 to −0.55 (per mil difference from SMOC, precision of 0.16) with age of the strata among Devonian, Silurian, Ordovician and Cambrian samples from the basin margin. Mississippian samples from the middle of the basin were isotopically heaviest at +0.1. Ordovician samples, also from mid-basin, were isotopically lightest at −1.2. On plots of δ37 Cl vs Cl/Br and δ37 Cl vs87Sr/86Sr samples at the basin margin trend toward enrichment in35Cl and87Sr and increasing Cl/Br suggesting interformational mixing of the waters. On a δ37 Cl Cl/Br plot, three samples not on this trend and tending toward high Cl/Br may reflect evaporite dissolution. Canadian Shield Waters were plotted with Michigan Basin waters on the graphs of δ37 Cl vs Ca/Cl and δ37 Cl vs K/Cl. On both plots data fall along linear trends of35Cl depletion with Ca/Cl increase and with K/Cl decrease. Ordovician waters from the middle of the basin and shield waters are end members on the plots. The results suggest that despite water-rock interactions, δ37 Cl data may be useful in studies of mixing relations in formation waters.

Secular variation of Delta C-14 during the Medieval Solar Maximum: A progress report

The Earth is within the Contemporaneous Solar Maximum (CSM), analogous to the Medieval Solar Maximum (MSM). If this analogy is valid, solar activity will continue to increase well into the 21st century. We have completed 75 single-ring and 10 double-ring measurements from AD 1065 to AD 1150 to obtain information about solar activity during this postulated analog to solar activity during the MSM. Delta (super 14) C decreases steadily during the period AD 1065 to AD 1150 but with cyclical oscillations around the decreasing trend. These oscillations can be successfully modeled by four cycles. These four frequencies are 1/52 yr (super -1) , 1/22 yr (super -1) , 1/11 yr (super -1) , and 1/5.5 yr, i.e., the 4th harmonic of the Suess cycle, the Hale and Schwabe cycles and the 2nd harmonic of the Schwabe cycle.