Stable isotope geochemistry

This review is intended principally to be a reasonably comprehensive bibliography. It covers papers by workers in stable isotope geochemistry in the United States that have been published or presented at national meetings since the last quadrennial report. The titles of the papers in the bibliography are in most cases indicative of the general content, so only the highlights are described below

Investigation of the influence of a step change in surface roughness on turbulent heat transfer

The use is studied of smooth heat flux gages on the otherwise very rough SSME fuel pump turbine blades. To gain insights into behavior of such installations, fluid mechanics and heat transfer data were collected and are reported for a turbulent boundary layer over a surface with a step change from a rough surface to a smooth surface. The first 0.9 m length of the flat plate test surface was roughened with 1.27 mm hemispheres in a staggered, uniform array spaced 2 base diameters apart. The remaining 1.5 m length was smooth. The effect of the alignment of the smooth surface with respect to the rough surface was also studied by conducting experiments with the smooth surface aligned with the bases or alternatively with the crests of the roughness elements. Stanton number distributions, skin friction distributions, and boundary layer profiles of temperature and velocity are reported and are compared to previous data for both all rough and all smooth wall cases. The experiments show that the step change from rough to smooth has a dramatic effect on the convective heat transfer. It is concluded that use of smooth heat flux gages on otherwise rough surfaces could cause large errors

Oxygen 18/oxygen 16 and D/H studies of plutonic granitic and metamorphic rocks across the Cordilleran Batholiths of southern British Columbia

Hydrogen and oxygen isotope ratios of 500 samples, mainly from granitic plutons, were measured along a 700-km, E-W traverse across the “accreted terranes” of southern British Columbia (latitudes 49°–52°N). Despite the geological complexity and range of intrusive ages (Late Triassic to Tertiary) and although there are “steps” in the isotopic values at some geologic boundaries (e.g., across the Strait of Georgia), two clear patterns emerge: (1) The ^(18)O/^(16)O and D/H ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in D and ^(18)O. Enormous areas were affected by the hydrothermal processes, but the most intense alteration is localized along major north trending lineaments (e.g., Okanagan Lake). (2) Independent of the hydrothermal effects, the primary δ^(18)O values of the granitic rocks also change systematically eastward, from +7.0 to +8.5 in Vancouver Island, reaching a minimum of +5.5 to +7.0 in the western and central Coast Plutonic Complex, then increasing progressively from the eastern Coast Batholith to the Okanagan Batholith, and attaining a maximum of +10.0 to +12.0 in the Nelson Batholith. Two groups of samples are unique in their high δD values. The first group is represented by two geographically isolated batholiths (Guichon and Thuya) that were not affected by the Tertiary meteoric-hydrothermal systems and that have therefore preserved their Early Jurassic to Triassic K/Ar ages. The second group is represented by the Jurassic plutons of Vancouver Island; there, the hydrothermal fluids were both D-rich and ^(18)O-rich (δ^(18)O > 0), as evidenced by the fact that feldspars in the altered granites are enriched in ^(18)O relative to coexisting quartz. Both “anomalies” can be explained if these terranes were located closer to the equator and/or in a maritime environment at the time of intrusive and hydrothermal activity, in agreement with available paleomagnetic data. Excluding these anomalous areas, two distinct ages of meteoric-hydrothermal activity can be identified, Cretaceous in the west and early to mid-Tertiary in the east. The isotopic trends in the rocks are similar to the present-day patterns of meteoric waters in the region, with one primary difference: the paleowaters are enriched in D by about 20%, compatible with a northward translation of these terranes, a climatic change, or both. The similarities of the patterns suggest a topography similar to that of the present day (a mountain chain along the coast) during the early Tertiary. The whole-rock δ^(18)O values of the granitic rocks (determined by extrapolation of the trends of δ^(18)O quartz versus δ^(18)O feldspar, to correct for hydrothermal alteration effects) suggest that the main part of the Coast Plutonic Complex formed from magmas similar to those in oceanic island arcs, derived from basaltic crust or from an upper mantle source. Toward the east, an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas. It may be significant that right at the edge of the North American craton there is a drop in the δ^(18)O of the plutons from +11 down to +9.5, suggesting involvement of old granitic crust instead of sedimentary rocks. These new data from southern British Columbia confirm and amplify previous studies that have documented: (1) the existence of widespread and pervasive hydrothermal effects throughout the North American Cordillera, and (2) regular west-to-east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these Mesozoic and Cenozoic granitic magmas were derived

Oxygen isotope studies on the origin of tektites

O^(18)/O^(16) ratios of thirteen tektites from Czechoslovakia, Libya, Texas, Indochina, Philippine Islands, Australia, Java, and Peru have been determined by the fluorine-extraction technique. All but one of these lie in relatively restricted range of O^(18)/O^(16) ratio, with δ = 9.6 to 10.4 per mil relative to the ocean-water standard. Tektites are therefore 0.5 to 1.5 per mil heavier than average granitic igneous rocks or about the same as average igneous quartz. The sample from Macusan, Peru (δ = 12.0), is anomalous and is probably not a true tektite. Six analyzed shales are isotopically heavier than tektites and show a much wider range in values (δ = 14.2, 14.8, 15.3, 15.5, 17.6, 18.2). Several detrital sedimentary rocks previously analyzed by Silverman and the present authors have δ values of 10.2 to 15.5; analyzed metasedimentary rocks have δ values of 12.1 to 15.7 per mil. Therefore, tektites are not sedimentary or metasedimentary material fused by lightning or by impact of an astronomical body with the earth. Chemically, tektites are unlike terrestrial igneous rocks, and their oxygen isotopic composition is unlike that of sedimentary or metasedimentary rocks. They therefore may be extraterrestrial objects

Correlations between O^(18)/O^(16) ratios and chemical compositions of tektites

The limits of oxygen isotope variation in tektites have been determined by analyzing 33 tektites that cover the widest possible spectrum of chemical compositions. δO^(18) values were obtained for 7 australites (8.9 to 11.0), 4 javaites (9.4 to 10.0), 3 indochinites (9.3 to 10.6), 5 philippinites (9.8 to 10.6), 8 moldavites (10.7 to 11.8), and 6 Ivory Coast tektites (13.0 to 13.6), as well as 2 samples of Darwin glass (14.4 to 14.9). Various tektite groupings based on chemical composition and geographic occurrence all show a systematic increase in O^(18) with decreasing SiO_2 content, similar to that previously found for the bediasites. These systematic correlations must arise either by (1) vapor fractionation of tektite material during impact melting or (2) mixing of a SiO_2-rich igneous component and a low-SiO_2 component formed at a much lower temperature; perhaps the mixing occurred during weathering at the earth's surface or during hydrothermal alteration of silicic igneous rocks either on the earth or the moon

An integrated chemical and stable-isotope model of the origin of Midocean Ridge Hot Spring Systems

Chemical and isotopic changes accompanying seawater-basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs, incorporating provision for addition and subtraction of a wide-range of reactant and product minerals, as well as cation and oxygen and hydrogen isotopic exchange equilibria. The models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100° to 350°C, with an overall water-rock ratio of about 0.5 being constrained by an assumed δ^(18)O_(H_2O) at 350°C of +2.0 per mil (H. Craig, personal communication, 1984). This is a realistic model because: (1) the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust; (2) the water-rock ratio is high (>50) at low temperatures and low (<0.5) at high temperatures; and (3) it allows for back-reaction of earlier-formed minerals during the course of reaction progress. The results closely match the major-element chemistry (Von Damm et al., 1985) and isotopic compositions (Craig et al., 1980) of the hydrothermal solutions presently emanating from vents at 21°N on the East Pacific Rise. The calculated solution chemistry, for example, correctly predicts complete loss of Mg and SO_4 and substantial increases in Si and Fe; however, discrepancies exist in the predicted pH (5.5 versus 3.5 measured) and state of saturation of the solution with respect to greenschist-facies minerals. The calculated δD_(H_2O) is +2.6 per mil, in excellent agreement with analytical determinations. The calculated chemical, mineralogic, and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges (Humphris and Thompson, 1978; Stakes and O'Neil, 1982), as well as with data from ophiolites (Gregory and Taylor, 1981). Predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite-epidote-chlorite-tremolite (greenschist) assemblage at 350°C. The models demand that the major portion of the water-rock interaction occur at temperatures of 300°–350°C. Interaction at temperatures below approximately 250°C results in negative δ^(18)_O_(H_2O) shifts, contrary to the observed positive δ^(18)O values of the fluids exiting at midocean ridge vents. Hydrogen isotope fractionation curves by Suzuoki and Epstein (1976), Lambert and Epstein (1980), and Liu and Epstein (1984), among others, are compatible with the model, and require δD_(H_2O) to increase at all temperatures as a result of seawater-basalt interaction

Low-^(18)O meteoric-hydrothermal signatures in groundmass and fiamme of fossil fumaroles of the 0.76 Ma Bishop Tuff, California

A 150-m thick δ^(18)O profile of whole-rock (WR) samples from an area of fossil fumaroles in the Bishop tuff at Owens River Gorge shows a low-^(18)O horizon sandwiched between an essentially unwelded upper portion and a densely-welded lower part displaying magmatic ^(18)O/^(16)O (see figure). Shallow-dipping columnar joints (sdcj) are correlative with the low-^(18)O horizon, the base of which is extremely sharp (~3 per mil per meter)

The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study

Geological, petrological, and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100×20 km area of the northern Samail ophiolite in Oman. Most of these samples are from three detailed profiles through the pillow lavas, sheeted dikes, and layered gabbros of this laterally heterogeneous fragment of Cretaceous oceanic crust, down to and across the petrologic Moho. The profiles encompass a range of petrologic and tectonic styles, and each profile exhibits distinctive ^(18)/^(16)O variations compared to one another and to mid-ocean ridge basalts, as a result of pervasive seawater-hydrothermal interaction that varied in intensity along strike in the ophiolite. In general, ^(18)O depletions are observed in the layered gabbros and ^(18)O enrichments in most of the sheeted dikes and pillow lavas, similar to results previously observed in the southern part of the ophiolite (Ibra area), where ^(18)O depletions within the gabbroic section are quantitatively balanced by ^(18)O enrichments in the shallower parts of the oceanic crust. The Wadi Hilti profile, selected as an example of relatively intact crust, differs from Ibra in having more uniform and slightly higher δ^(18)O in the gabbros (+5.4 to +6.3), as well as in containing more hydrous alteration minerals (amphibole, epidote, chlorite, and prehnite). The profiles in the Wadi Kanut-Shafan and Wadi Rajmi sections are much more complex and reveal the impact of off-axis intrusions and deep crustal shearing. Plagiogranite-wehrlite intrusions in the Shafan-Kanut area superimposed a local hydrothermal aureole on the ophiolite, evident in dikes highly depleted in ^(18)O, quartz-sulfide veins, abundant epidote, thullite, and chlorite in shallower rocks, and low-temperature hydrous alteration of deeper gabbroic rocks; the latter produced an overall increase in whole rock δ^(18)O (+6.2 to +6.9). Such late stage intrusions are found throughout the northern half of the Samail ophiolite. The Wadi Rajmi area, which is a possible fossil transform or propagating rift, represents the most complex of the three profiles; it also contains the most abundant highly deformed and hydrothermally altered rocks, together with the deepest and largest zone of ^(18)O depletion yet found in any ophiolite (locally δ^(18)O < +2.0). Conduits for large volumes of high-temperature hydrothermal fluids were provided by fractures now occupied by low-^(18)O gabbro pegmatites and low-^(18)O dikes. Material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk δ^(18)O averages (+5.9 to +6.3) that are, within sampling error, almost identical to the average MORB basalt value of about +5.8, if both vertical and lateral crustal heterogeneities are integrated into a three-dimensional model. This supports and amplifies the conclusion of earlier workers that the δ^(18)O of seawater is buffered and controlled by hydrothermal interaction with oceanic crust, as long as the cumulative effects (both spatial and temporal) of all seafloor magmatic/hydrothermal processes are considered. The very slight out-of-balance enrichment of the integrated crustal average δ^(18)O compared to MORB may be explained by the ubiquitous mineralogical and isotopic evidence for a late, low-temperature alteration event in the basal gabbros; these effects are prominent in the vicinity of the petrologic Moho and may indicate exchange with low-temperature aqueous fluids during or after detachment of the obducted slab

Oxygen isotope equilibrium between muscovite and water

Oxygen isotopes have been equilibrated between muscovite and aqueous alkali chloride solution and between paragonite and alkali chloride solution in the temperature range of 400°–650°C at 1 and 1.5 kb fluid pressure. Isotopic equilibrium was inferred from the fact that compatible fractionation factors were obtained using 3 different chemical reactions to produce the mica: (1) muscovite or paragonite was prepared by reacting natural kaolinite with 2–3 molal KCl or NaCl solutions; (2) muscovite was crystallized in pure water from a gel; and (3) synthetic paragonite was reacted with 2–3 molal KCl solution, producing muscovite by an alkali ion exchange reaction. The 1 M modification of the mica was made in every experiment. In several cases the extent of oxygen exchange was traced by running companion equilibrations in solutions of unusually low O^(18)/O^(16) ratio. No isotopic fractionation was discernible between muscovite and paragonite in the temperature range studied. Per mille fractionations between muscovite and water are given by the expression 10^3 In α = 2.38(10^6T^(−2)) − 3.89. These data can be combined with the results of other laboratory equilibration studies to establish a set of calibrated oxygen isotope geothermometers. Analogous to the alkali feldspar systems previously reported, the direct relationship between cation and oxygen isotope exchange suggests that some type of solution-redeposition mechanism operated during muscovite-paragonite transformations in aqueous solutions. Also, the extensive oxygen isotope exchange (with the solution) that accompanies the formation of muscovite from kaolinite implies a breakdown of the kaolinite structure. This notion does not concur with hypotheses based on rate studies and X-ray measurements that the unaltered kaolinite structure is partially inherited by the mica

Interview with Hugh P. Taylor

An interview in eight sessions in the summer of 2002 with Hugh P. Taylor, Robert P. Sharp Professor of Geology, emeritus, in the Division of Geological and Planetary Sciences. In this wide-ranging interview, Dr. Taylor recalls his upbringing in Arizona and New Mexico, where his father was an agent for the Santa Fe Railroad; his move to Southern California; and his undergraduate education at Caltech. After receiving his BS at Caltech in geochemistry in 1954 (he was one of the first two geochemistry majors to graduate from the institute), and a master's degree at Harvard, he returned to Caltech for his PhD, working on oxygen-isotope ratios with geochemist Samuel Epstein. He recalls their refinement of the separation technique and his application of Oxygen-18/Oxygen-16 ratios to the study of magmatic intrusions, especially Iceland's Skaergaard intrusion--studies that led to a new understanding of hydrothermal convection and the effects of meteoric groundwater (essentially, rainwater) on basaltic intrusions. He recalls Caltech's move into geochemistry in the early 1950s under the chairmanship of Robert P. Sharp, the advent of plate tectonics in the mid-1960s, the lunar program at Caltech, and his friendship with astronaut/geologist Harrison "Jack" Schmitt. Further recollections include the accomplishments of Gerald J. Wasserburg's laboratory in analyzing the lunar material; Wasserburg's feud with colleague Leon T. Silver; Silver's reluctance to publish; Taylor's collaboration with Silver on isotopic analysis of the Peninsula Ranges Batholith; Taylor's collecting trip to the Skaergaard intrusion; his work with Robert Coleman of the United States Geological Survey on the Red Sea Rift Zone; his work with Bruno Turi on igneous rocks in Italy; and the discoveries made by several of his outstanding graduate students and postdocs. The latter part of the interview amounts to a history of Caltech geology, as he describes the evolution of the division from a classical, field-oriented geology department to a first-rank division incorporating geophysics, geochemistry, and planetary sciences. Along the way, Taylor gives his assessment of the various strengths and weaknesses of the division's chairmen: Robert P. Sharp, Clarence Allen, Eugene Shoemaker, Barclay Kamb, Peter Wyllie, Gerald Wasserburg, Peter M. Goldreich, David J. Stevenson, and Edward M. Stolper