Jim D. Gleason

Stable isotope composition of waters in southeastern California 1. Modern precipitation

Over a 7-year period from April 1982 to April 1989, integrated samples of rain and snow were collected at 32 sites by oil-sealed storage gage stations in (and adjoining) the southeast California desert; station elevations ranged from −65 m to 2280 m, and the collection network covered an area measuring about 400 km in each dimension. Deuterium (δD) analysis of 406 samples shows that the average δD of summer precipitation was −56 per mil (‰) whereas winter values averaged −78‰, averaged annual values were close to −69‰ because most of the area is in a winter-dominated precipitation regime. We found no correlation between wetness or dryness of a season and the δD of its precipitation. The δ18O versus δD plots show that rain samples define a line of slope 6.5, less than the 8 of the Meteoric Water Line, whereas snow samples define a line of slope 9.2. These differences in slope are the result of isotopic fractionation which occurred during evaporation of raindrops but not during sublimation of snow. Trajectory plots of 68 of the major storm events show that all of the winter storms originated in the Pacific, and passed over high mountains before reaching our collection stations. However, 21 of the 30 summer storms had trajectories that originated either over the Gulf of Mexico or the subtropical Pacific and traveled either west or north to reach our stations, without traversing high mountains. The difference in δD between winter and summer precipitation is due to different air flow patterns during those seasons.

Meteoric Water in Magmas

Oxygen isotope analyses of sanidine phenocrysts from rhyolitic sequences in Nevada, Colorado, and the Yellowstone Plateau volcanic field show that δ18O decreased in these magmas as a function of time. This decrease in δ18O may have been caused by isotopic exchange between the magma and groundwater low in 18O. For the Yellowstone Plateau rhyolites, 7000 cubic kilometers of magma could decrease in δ18O by 2 per mil in 600,000 years by reacting with water equivalent to 3 millimeters of precipitation per year, which is only 0.3 percent of the present annual precipitation in this region. The possibility of reaction between large magmatic bodies and meteoric water at liquidus temperatures has major implications in the possible differentiation history of the magma and in the generation of ore deposits.

Recognition of Petroleum-bearing Traps by Unusual Isotopic Compositions of Carbonate-cemented Surface Rocks

Unusual carbon and oxygen isotopic compositions characterize outcropping dolomitic sandstone of Permian age over the Davenport oil field, Oklahoma. The δC 13 PDB ranges from −5.1 to −11.3/mil and the δO 18 SMOW ranges from +28.8 to +48.8/mil. Areal distribution of both isotopic ranges is systematic and closely reflects the distribution of petroleum in the producing interval at a depth of 1,000 m (3,300 ft). The 20/mil positive oxygen range in these surface rocks suggests evaporative fractionation of ground water controlled by the near-surface expansion of vertically migrating dry natural gas from the underlying deposit. The rather light, though unexceptional, carbon values, unlike those reported for other hydrocarbon-derived carbonate rocks, indicate only partial chemical or biochemical oxidation of the gas, suggesting further that the leakage rate was relatively rapid. At the Cement oil field, Oklahoma, similar relations in surface rocks occur, but the carbon isotopic values are exceptionally light (−5.5 to −39.2/ml relative to PDB), indicating significant near-surface oxidation of slowly seeping hydrocarbons of low molecular weight. The occurrence of carbonate rocks derived from the chemical oxidation or bacterial conversion of hydrocarbons is well known (Thode and others, 1953; S. R. Silverman and others, unpub. data; Russell and others, 1967; Hathaway and Degens, 1968; Mamchur, 1969; Davis and Kirkland, 1970). Such carbonates typically have δC 13 PDB values more negative than about −25/mil. Similarly, unusual isotopic signatures have been documented in carbonate-cemented sandstone and calcitized gypsum cropping out over the Cement oil field in the southeastern part of the Anadarko basin of Oklahoma (Donovan, 1974). There, the rock cements display carbon isotopic ratios that are increasingly light and oxygen ratios that are increasingly heavy toward the crest of the anticlinal structure. These trends are limited areally and are systematic in their patterns, reflecting the subsurface distribution of petroleum. Both liquid and gaseous hydrocarbons have leaked from the Cement reservoirs; much of the hydrocarbon was oxidized and the carbon incorporated into the interstitial carbonate cements of the overlying rocks. Rapid vertical leakage of gas, expansive under depressurization, resulted in preferential evaporation of H 2 O 16 , compared with H 2 O 18 from near-surface ground waters. This evapo-escape to the atmosphere. Rayleigh distillation proceeds if escape rates cause evaporation of ground water at rates faster than the replacement or recharge rate of meteoric water. Rocks whose isotopic compositions reflect an intermediate mix of these contrasting processes have not yet been found but presumably exist. The existence of these kinds of isotopically anomalous carbonate cements would prove useful in petroleum exploration independently of other kinds of geochemical indicators.

Isotopic evidence of Holocene climatic change in the San Juan Mountains, Colorado

Abstract The δD of cellulose from 14C-dated wood, collected in the San Juan Mountains of southwestern Colorado, decreased by about 45‰ from 9600 to 3100 yr B.P. and an additional 25‰ to the present. The wood samples are from trees that grew above present-day tree line and reflect a time of warmer average summer temperatures. These changes in δD are interpreted to indicate a major change during the Holocene in the sources of moisture, in the seasonality of precipitation, or in both.

Stable isotope composition of waters in southeastern California: 2. Groundwaters and their relation to modern precipitation

We compare isotopic data on modern groundwater recharge, estimated from a 7-year collection of precipitation samples, with the isotopic compositions of 82 samples from wells and perennial springs collected in southeastern California. Over half the samples represent recharge that is considerably more depleted in deuterium (δD) than the lightest (winter) precipitation δD values now precipitating in nearby areas. Wells in the vicinity of irrigation canals fed from the Colorado River contain water whose δD resembles that of the irrigation canals, and wells adjacent to the Mojave River are recharged by Mojave River water. The remainder of the samples from the 17 hydrologic units discussed in this paper include one or more whose δD values are depleted in deuterium by at least 10‰ compared to recharge estimated from modern winter precipitation. A plot of δ18O versus δD for 56 wells and 22 springs shows substantial scatter (R2=0.66) and defines a slope of 4.9, which is lower than the slope of the Meteoric Water Line. This slope and the scatter of the data suggest these diverse waters were evaporated to different degrees before recharge, and they may also have exchanged oxygen with solid mineral phases. We interpret the fact that many groundwaters in this region have lower δD values than modem precipitation to indicate that recharge of many of the groundwater basins occurred during an earlier period, probably the late Pleistocene, when winters were colder, evaporation rates were lower, and precipitation was quantitatively greater and isotopically lighter.

Water, hydrogen, deuterium, carbon, carbon-13, and oxygen-18 content of selected lunar material.

The water content of the breccia is 150 to 455 ppm, with a δD from—580 to —870 per mil. Hydrogen gas content is 40 to 53 ppm with a δD of —830 to —970 per mil. The CO2 is 290 to 418 ppm with δ 13C = + 2.3 to + 5.1 per mil and δ18O = 14.2 to 19.1 per mil. Non-CO2 carbon is 22 to 100 ppm, δ13C = —6.4 to —23.2 per mil. Lunar dust is 810 ppm H2O (D = 80 ppm) and 188 ppm total carbon(δ13C = —17.6 per mil). The 18O analyses of whole rocks range from 5.8 to 6.2 per mil. The temperature of crystallization of type B rocks is 1100� to 1300�C, based on the oxygen isotope fractionation between coexisting plagioclase and ilmenite.

Oxygen isotopes of some trondhjemites, siliceous gneisses, and associated mafic rocks

Abstract Analyses of oxygen isotopes in whole-rock samples of 58 Precambrian and Phanerozoic trondhjemites and siliceous gneisses and of 28 cogenetic mafic to intermediate rocks from North America, Fennoscandia, and southern Africa give the following results: 1. (1) 47 trondhjemites, tonalites, and mostly Archean acidic gneisses that apparently are not isotopically disturbed show an overage δ 15O of +7.3‰ and a range of 5.2–8.9‰; 11 other samples are slightly to moderately disturbed and show higher values; and 2. (2) the mafic rocks show a wide range of δ-values, from about 0–9‰ but the undisturbed ones give an average δ 18O of 5.2‰ . The δ 18O values of the trondhjemitic intrusives and siliceous gneisses of similar composition are lower than those of most granitic rocks and support models for derivation of these rocks from basaltic parents. This approach, however, cannot be used to determine if individual bodies formed by differentiation or by partial melting.

Water and Carbon in Rusty Lunar Rock 66095

Lunar rock 66095 contains a hydrated iron oxide and has an unusual amount of water for a lunar rock (140 to 750 parts per million), 90 percent of which is released below 690�C. The δof water released at these low temperatures varies from -75 to -140 per mil relative to standard mean ocean water (SMOW). The small amount of water released between 690� and 1300�C has a δ of about -175 �25 per mil SMOW. These δ values are not unusual for terrestrial water. The δ18O of water extracted from 110� to 400�C has a value of +5� I per mil SMOW, similar to the value for lunar silicates from rock 66095 and different from the value of -4 to -22 per mil found for samples of terrestrial rust including samples of rusted meteoritic iron. The amount of carbon varies from 11 to 59 parts per million with a δ13C from -20 to -30 per mil relative to Pee Dee belemnite. Only very small amounts of reduced species (such as hydrogen, carbon monoxide, and methane) were found, in contrast to the analyses of other lunar rocks. Although it is possible that most of the water in the iron oxide (goethite) may be terrestrial in origin or may have exchanged with terrestrial water during sample return and handling, evidence presented herein suggests that this did not happen and that some lunar water may have a δD that is indistinguishable from that of terrestrial water.

Modification of ?D values in eastern Nevada granitoid rocks spatially related to thrust faults

Stable isotope data have been determined for 13 Mesozoic and Tertiary plutons in eastern Nevada and nearby Utah. In the southern Snake Range of eastern Nevada, where relations are best exposed and have been most intensively studied, δD, δ18O, and apparent K-Ar ages depend on proximity to the Snake Range decollement. Where stresses resulting from late movement on the decollement have caused cataclasis of Oligocene (37 Ma) granitoid rock, δ18O, δD, and K-Ar age values as low as −2.5‰, −155‰, and 18 Ma, respectively, have been determined. Where there has been no cataclasis, δ18O values of Jurassic, Cretaceous, and Oligocene granitoid rocks are apparently unaffected, but both δD values and K-Ar ages have been modified for distances of tens of meters below the decollement.Results similar to those in the southern Snake Range have been observed in other eastern Nevada granitoid rocks spatially related to regional thrust faults, as in the Kern Mountains, the Toana Range, and the northern Egan Range. In each of these areas cataclasis or deformation of granitoid rocks has resulted in lowered δ18O, δD, and K-Ar age values. Where there has been no cataclasis or deformation, δ18O values are unaffected, but both δD and K-Ar age values have been lowered by stresses resulting from postcrystallization movement along overlying thrust faults.Many of the plutons discussed have not been deeply eroded, and spatially related thrust faults crop out. Where thrust faults are not in evidence and the granitoid rocks give δD values lower than about −130‰ along with spuriously low K-Ar age results, modification of the δD and K-Ar age values may have been caused by stresses related to late movement along an overlying (now eroded) thrust fault.

Modeling of ancient climate from deuterium content of water in volcanic glass

Abstract The explosive nature of the eruptions that produced rhyolitic tephras resulted in the ash being distributed over large areas. This ash, within a few thousand years after deposition, incorporated relatively large amounts of environmental water (up to 3.5%) into the glass structure. This hydration water is shown to retain its original deuterium concentration through time, and because the deuterium content of precipitation has been used for climate characterization, the hydration water, which is related to ancient precipitation, can be used to investigate ancient climates. Based on the analysis of ash samples dated at 13,700, 11,200 and 8500 BP, the climate in the states of Washington and Montana may have been about 3–6°C cooler at the end of the Pleistocene or Early Holocene than the present. We observe no change in the deuterium concentration of surface waters, and hence climate, in that region post-8500 BP.