Robert E. Criss

An 18O/16O and D/H study of Tertiary hydrothermal systems in the southern half of the Idaho batholith

During Eocene time, 37 to 49 m.y. ago, a series of large hydrothermal systems was developed around a group of epizonal granite plutons in the Idaho batholith. These systems involved deep and extensive circulation of fluids derived from low-δ 18 O (∼−16) and low-δD (∼−120) meteoric waters. Water-rock interactions occurred at temperatures of 150 to 400 °C, lowering the 18 O/ 16 O and D/H ratios in the surrounding Mesozoic rocks (tonalite, granodiorite, and granite), such that the feldspar δ 18 O and biotite δD values became as low as −8.2 and −176, respectively. These values contrast markedly with the primary isotopic compositions of +9.3 ± 1.5 and −70 ± 5, respectively. Widespread propylitization of the Mesozoic plutonic rocks accompanied these isotopic exchange effects. Systematic mapping shows that anomalous δD and δ 18 O values occur over more than 15,000 km 2 , indicating the extensive lateral dimensions of the ancient circulating systems. The former zones of intense hydrothermal activity are marked by low- 18 O zones, which were mapped in the vicinity of the margins of several Eocene plutons (for example, at Rocky Bar) and also within a giant (5- to 20-km wide, 60- to 40-km diam) ring zone that surrounds the Sawtooth Mountains. The latter anomaly is coincident with the high-permeability ring fracture zone of an Eocene caldera system. Most of the ore deposits in the southern half of the Idaho batholith are epithermal and mesothermal Au-Ag veins that are located near the periphery of the low- 18 O zones (that is, near the outermost δ 18 O = 8 isopleth). This association links these deposits with the Tertiary hydrothermal activity and has great potential as an exploration tool in the heavily forested region. Evidence is presented that the Eocene ground-water circulation pattern was affected over large lateral distances (25 to 50 km) and great depths (5 to 7 km). These conclusions, together with the indications that large amounts of water (> 7,000 km 3 ) were involved in some systems and that the circulation patterns probably are related to caldera ring structures, may be of particular importance in geothermal exploration and exploitation of analogous modern systems. For example, the “fossil” hydrothermal activity mapped in the Idaho batholith may be characteristic of deep-level fluid circulation in geothermal systems such as Yellowstone National Park, Wyoming. In such regions, the major zones of hydrothermal activity seem to be principally associated with either (1) the caldera ring zones or (2) the central plutons (resurgent domes).

Oxygen isotope exchange kinetics of mineral pairs in closed and open systems: Applications to problems of hydrothermal alteration of igneous rocks and Precambrian iron formations

The systematics of stable-isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass-balance considerations, both in closed and in open systems. On mineral-pair δ^(18)O plots, samples from terranes that have exchanged with large amounts of fluid typically map out steep positively-sloped non-equilibrium arrays. Analytical models are derived to explain these effects; these models allow for different exchange rates between the various minerals and the external fluids, as well as different fluid fluxes. The steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories. These isochrons have initially-steep near-vertical positive slopes that rotate toward a 45° equilibrium slope as the exchange process proceeds to completion. The actual data-point array is thus analogous to the hand of an “isotopic clock” that measures the duration of the hydrothermal episode. The dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter (u/k) determines the shape of each individual exchange trajectory. In a fluid-buffered system (u/k ≫ 1), the solutions to the equations: (1) are independent of the mole fractions of the solid phases; (2) correspond to Taylors open-system water/rock equation; and (3) yield straight-line isochrons that have slopes that approach 1/f, where f is the fraction reacted of the more sluggishly exchanging mineral. The isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models, thereby simplifying the application of theory to actual hydrothermal systems in nature. In all of the models an order of magnitude of time (in units of kt) separates steep non-equilibrium arrays (e.g., slope ≈ 10) from arrays approaching an equilibrium slope of unity on a δ-δ diagram. Because we know the approximate lifetimes of many hydrothermal systems from geologic and heat-balance constraints, we can utilize the ^(18)O/^(16)O data on natural mineral assemblages to calculate the kinetic rate constants (ks) and the effective diffusion constants (Ds) for mineral-H_2O exchange: these calculated values (k_(qtz) ≈ 10^(−14), k_(feld) ≈ 10^(−13)–10^(−12)) agree with experimental determinations of such constants. In nature, once the driving force or energy source for the external infiltrating fluid phase is removed, the disequilibrium mineral-pair arrays will either: (1) remain “frozen” in their existing state, if the temperatures are low enough, or (2) re-equilibrate along specific closed-system exchange vectors determined solely by the temperature path and the mineral modal proportions. Thus, modal mineralogical information is a particularly important parameter in both the open- and closed-system scenarios, and should in general always be reported in stable-isotopic studies of mineral assemblages. These concepts are applied to an analysis of ^(18)O/^(16)O systematics of gabbros (Plagioclase-clinopyroxene and plagioclase-amphibole pairs), granitic plutons (quartz-feldspar pairs), and Precambrian siliceous iron formations (quartz-magnetite pairs). In all these examples, striking regularities are observed on δ-δ and δ-Δ plots, but we point out that δ-δ plots have many advantages over their equivalent δ-Δ diagrams, as the latter are more susceptible to misinterpretation. Using the equations developed in this study, these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation. In particular, we present a new interpretation indicating that Precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide (magnetite precursor?) has exchanged much faster with low-temperature (< 400°C) fluids than has the relatively inert quartz.

Kinetic theory of oxygen isotopic exchange between minerals and water

Kinetic and mass conservation equations are used to describe oxygen isotopic exchange between minerals and water in “closed” and open hydrothermal systems. In cases where n coexisting mineral phases having different reaction rates are present, the exchange process is described by a system of n + 1 simultaneous differential equations consisting of n pseudo first-order rate equations and a conservation of mass equation. The simultaneous solutions to these equations generate curved exchange trajectories on δ-δ plots. Families of such trajectories generated under conditions allowing for different fluid mole fractions, different fluid isotopic compositions, or different fluid flow rates are connected by positive-sloped isochronous lines. These isochrons reproduce the effects observed in hydrothermally exchanged mineral pairs including 1) steep positive slopes, 2) common reversals in the measured fractionation factors (δ), and 3) measured fractionations that are highly variable over short distances where no thermal gradient can be geologically demonstrated.

Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho

Regional variations in initial 87Sr/86Sr ratios (ri) of Mesozoic plutons in central Idaho locate the edge of Precambrian continental crust at the boundary between the late Paleozoic-Mesozoic accreted terranes and Precambrian sialic crust in western Idaho. The ri values increase abruptly but continuously from less than 0.704 in the accreted terranes to greater than 0.708 across a narrow, 5 to 15 km zone, characterized by elongate, lens-shaped, highly deformed plutons and schistose metasedimentary and metavolcanic units. The chemical and petrologic character of the plutons changes concomitantly from ocean-arc-type, diorite-tonalite-trondhjemite units to a weakly peraluminous, calcic to calcalkalic tonalite-granodiorite-granite suite (the Idaho batholith). Plutons in both suites yield Late Cretaceous ages, but Permian through Early Cretaceous bodies are confined to the accreted terranes and early Tertiary intrusions are restricted to areas underlain by Precambrian crust. The two major terranes were juxtaposed between 75 and 130 m.y. ago, probably between 80 and 95 m.y.Oxygen and strontium isotopic ratios and Rb and Sr concentrations of the plutonic rocks document a significant upper-crustal contribution to the magmas that intrude Precambrian crust. Magmas intruding the arc terranes were derived from the upper mantle/subducted oceanic lithosphere and may have been modified by anatexis of earlier island-arc volcanic and sedimentary units.Plutons near the edge of Precambrian sialic crust represent simple mixtures of the Precambrian wall-rocks with melts derived from the upper mantle or subducted oceanic lithosphere with ri of 0.7035. Rb/Sr varies linearly with ri, producing “pseudoisochrons” with apparent “ages” close to the age of the wall rocks. Measured δ18O values of the wall rocks are less than those required for the assimilated end-member by Sr-O covariation in the plutons, however, indicating that wall-rock δ18O was reduced significantly by exchange with circulating fluids. Metasedimentary rocks of the Belt Supergroup are similarly affected near the batholith, documenting a systematic depletion in 18O as much as 50 km from the margin of the batholith.Plutons of the Bitterroot lobe of the Idaho batholith are remote from the accreted terranes and represent mixtures of Precambrian wall-rocks with melts dominated by continental lower crust (ri>0.708) rather than mantle. “Pseudoisochrons” resulting from these data are actually mixing lines that yield apparent “ages” less than the true age of the wall rocks and meaningless “ri”. Assimilation/ fractional-crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region.

Flood enhancement through flood control

Flood stages for constant discharge have increased 2–4 m over the past century at numerous locations in the Mississippi River basin. However, no increases are observed on rivers such as the Meramec and the upper Missouri, which have been spared extensive river engineering projects. Flood-stage increases on the middle Mississippi River and lower Missouri River are mostly attributable to channelization.

NaCaCl relations in basinal fluids

Abstract A new mathematical transformation of Na, Ca, and Cl concentrations in numerous basinal fluids around the world produces a linear slope of unity between the mill iequivalencies of Na and Ca cations. The transformation entails a simple milliequivalent comparison between the excess Ca and the Na deficit relative to seawater reference ratios. The relevant parameters are: Ca excess = [ Ca means − ( Ca/Cl ) sw Cl means ] 2 40.08 , Na deficit = [( Na/Cl ) sw Cl means − Na means ] 1 22.99 , where the concentrations (in mg/L) of the ions measured (meas) in a sample are referred to those in seawater (sw), and the numerical constants convert the results to meq/L. For >800 samples from numerous fluid reservoirs, with Cl concentrations that range from approximately 1–300 g/L and host lithologies from carbonates to granites, a highly correlated regression termed the Basinal Fluid Line (BFL) is found: Caexcess = 0.967 (Nadeficit) + 140.3R = 0.981. The unit slope of the BFL indicates a net cation exchange ratio of 2 Na for 1 Ca. The excess-deficit parameters show no correlation to Mg or K. If a single predominating reaction is presumed to control the BFL, only albitization of plagioclase by 2 Na for 1 Ca exchange is plausible. The BFL offers no support for a predominating reaction involving the 1:1 exchange of Na for Ca that has also been proposed for albitization reactions, nor for the hypothesis that dolomitization produces the elevated Ca contents of basinal fluids. The BFL may incorporate the effects of other water-rock reactions provided that they involve a net exchange of 2 Na for 1 Ca in sedimentary basins. The small y-intercept of 140.3 of the BFL is generally consistent with an origination of the brines from seawater, which would plot at the origin of an excess-deficit graph. However, for regressions derived for fluids from individual basins, the y-intercepts increase with increasing salinity of their fluids, consistent with model predictions for dissolution of halite into either a seawater or freshwater parent, followed by 2 Na for 1 Ca exchange. Because the hydrosphere is dominated by seawater and the upper crust by feldspar minerals, the BFL arguably represents the overall product of cation exchange of high salinity fluids in deep continental environments.

ISOTOPE HYDROLOGY OF VOLUMINOUS COLD SPRINGS IN FRACTURED ROCK FROM AN ACTIVE VOLCANIC REGION, NORTHEASTERN CALIFORNIA

The more than 1550 km2 (600 mi2) Hat Creek Basin in northeastern California is host to several first magnitude cold springs that emanate from Quaternary basaltic rocks with individual discharge rates ranging from 1.7 to 8.5 m3 s−1 (60–300 ft3 s−1). Stable isotope (δ18O, δD, δ13C) and 14C measurements of surface and groundwater samples were used to identify recharge areas, and to evaluate aquifer residence times and flow paths. Recharge locations were constrained from the regional decrement in meteoric water δ18O values as a function of elevation, determined to be −0.23‰ per 100 m for small springs and creek waters collected along the western Cascade slope of this region. In general, the large-volume springs are lower in (δ18O than surrounding meteoric waters, and are inferred to originate in high-elevation, high-precipitation regions up to 50 km away from their discharge points. Large spring 14C abundances range from 99 to 41 % modern carbon (pmc), and most show evidence of interaction with three distinct carbon isotope reservoirs. These reservoirs are tentatively identified as (1) soil CO2 gas equilibrated under open system conditions with groundwater in the recharge zone [δ13CDIC ≈ −18‰, 14C > 100 pmc], (2) dissolved carbon equilibrated with atmospheric CO2 gas [δ13CDIC ≈ +1‰, 14C > 100 pmc], and (3) dissolved carbon derived from volcanic CO2 gas emissions [δ13CDIC≈0‰, 14C=0 pmc]. Many regional waters show a decrease in 14C abundance with increasing δ13C values, a pattern indicative of interaction with dead carbon originating from volcanic CO2 gas. Several lines of evidence suggest that actual groundwater residence times are too short (⩽ 200 years) to apply radiocarbon dating corrections. In particular, water temperatures measured at springs show that deep groundwater circulation does not occur, which implies an insufficient aquifer volume to account for both the high discharge rates and long residence times suggested by 14C apparent ages. The large springs also exhibit rapid decreases in flow during periods of drought that suggests a high level of aquifer interconnectivity to the recharge area. The estimated amount of volcanic CO2 dissolved in surface and groundwater originating from the Lassen highlands is consistent with the conversion of approximately 10% of the geothermal CO2 flux into dissolved inorganic carbon.

Geochemistry of tectonically expelled fluids from the northern Coast ranges, Rumsey Hills, California, USA

Tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed Upper Cretaceous rock along the eastern margin of the California Coast ranges. The fluids emerge as near-neutral, perennial sodium chloride springs at high elevations with flow rates as high as 10 L per min. Higher spring discharges are more common around the exposure of a west-vergent fault propagation fold axis. Spring waters range from ~1000 to 27,000 mg/L TDS. The least saline water (δ18O = −7.5‰) closely represents local meteoric water that mixes with saline fluid (δ18O = +5.3‰) and forms a slope of ~3.5 on a δD vs. δ18O plot. A Na (125 to 8000 mg/L) vs. Cl (150 to 17,000 mg/L) plot shows a linear dilution trend that extends close to, but below, the values for modern seawater. Calcium (75–3000 mg/L) is considerably enriched relative to seawater and forms a nonlinear trend with chloride. In detail, the “Na deficit,” defined by the difference between the measured Na content and the Na concentration on a hypothetical seawater dilution line, is approximately balanced by the Ca excess, similarly defined by the seawater dilution line. This relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths. Simultaneous B and 18O enrichment of the fluids, accompanied by deuterium depletion, further suggest that the seawater modification is influenced by clay diagenesis. Bicarbonate and SiO2 concentrations show an inverse correlation with Cl, with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures. Some freshwater springs with near-meteoric stable isotope values may represent mixing of young groundwater from perched aquifers, but in many cases, the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids, and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out. Basinal fluids elsewhere commonly show dilution trends with local meteoric water, and in the case of the Rumsey Hills, some of the dilute saline waters may indicate deep penetration of meteoric water (> 1 km) in the Pleistocene before the latest tectonic uplift. Geothermometry of the spring waters (maximum ~90°C) suggest an origin from as deep as 4.0 km. This depth is consistent with the depth of the core of a fault propagation anticline below the surface of the Rumsey Hills developed by active internal deformation of an east-tapering wedge beneath the southwestern Sacramento Valley. Active tectonic compression causes near-lithostatic fluid pressures in the shallow subsurface below the Rumsey Hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths.

Isotopic imaging of surface water/groundwater interactions, Sacramento Valley, California

Groundwater isotope data across the Sacramento Valley, California establish two types of groundwater mining: (1) overdraft of ancient groundwater with limited recharge by surface waters, producing cones of depression; (2) ancient groundwater withdrawal followed by rapid recharge of irrigation water, reducing groundwater quality. The first type occurs in the Sacramento metropolitan area, where meteoric runoff is unnaturally high and 40 years of pumping have depressed water levels to 25 m below sea-level, inducing recharge from losing reaches of the Sacramento and American rivers. Lateral migration rates are quantified by the binary mixing between river water (δ18O = −10.8) and natural groundwater (δ18O = −7.0). The second type of mining occurs in agricultural regions to the west, where 14C ages indicate that irrigation waters constitute more than 80% of modern recharge. This recharge has several characteristics of evaporated irrigation water, including: (1) high δ18O values (to > −6.0) that define closed contour patterns; (2) elevated NO3 concentrations (to 100 ppm); (3) low 14C ages of less than 500 years. Stable isotope contours, augmented by 14C data, provide dynamic recharge patterns in this profoundly disturbed, giant alluvial aquifer. On a large scale (> 100 km2), the lateral permeabilities of alluvial aquifers are essentially isotropic, whereas on a smaller scale (< 25 km2), anisotropy is evident and isotope values can be geographically complex and seasonally transient. Groundwater flow patterns implied by the isotope data can differ substantially from steady-state models based on head measurements.

Stable isotope imaging of a dynamic groundwater system in the southwestern Sacramento Valley, California, USA

Abstract A dynamic image of a shallow (45–160 m below the surface) aquifer in a 25km2 area in the southwestern Sacramento Valley, California has been obtained by 18 O 16 O and D/H determinations of groundwater from municipal wells. The regular summertime drawdown of the water table is strongly correlated with municipal-wide increases in the δ18O values, particularly in wells with shallow perforation levels, thus demonstrating that the high 18O water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates. These 1.5‰ enrichments in the δ18O values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m. The increased δ18O values correlate with increased deviation from the meteoric water line and with increased nitrate levels, indicating that the high 18O groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen-fertilized, flood-irrigated cropland. Isotopic gradients provide a new method for qualitatively determining subsurface permeability. Zones of low isotopic gradients, interpreted as high 18O plumes, probably follow coarse-textured deposits of ancient stream channels. The 18O gradients are primarily controlled by subsurface permeability, although additional influences may include: (1) the heterogeneous distribution of the high 18O groundwater; (2) isotopic contributions from deeper groundwater; (3) variable perforation depths of the wells; (4) local differences in the water table level. Deeper, sodium bicarbonate groundwaters (300–650 m below the surface) have lower δ18O values (to −9.4 ‰) than the more shallow magnesium bicarbonate groundwaters, suggesting derivation from 18O depleted meteoric groundwaters of the Sierra Nevada. Minor mixing of the deeper groundwater into the municipal discharge occurs, even though previous workers considered the deeper groundwater to be confined.