David B. Wenner

Using H- and O-isotopic data for estimating the relative contributions of rainy and dry season precipitation to groundwater : example from Cheju Island, Korea

Abstract A comparison of deuterium excess or d -values of precipitation and groundwater at Cheju Island, Korea, indicates that, unlike in many temperate climates, precipitation during the whole year contributes to groundwater recharge. This in turn suggests that evapotranspiration effects are minimal, consistent with the fact that the island contains highly permeable volcanic rocks overlain by thin soils. This hypothesis is contrary to current water budget models that ascribe a significant role to evapotranspiration processes. Three coastal springs, and several streams and ponds throughout the island have enriched δ 18 O and δ D values that plot off the local meteoric water line and have relatively high Cl concentrations. This relationship is suggestive of seawater mixing for the coastal springs and evaporation for the streams and ponds. This database thus provides a useful means for evaluation of groundwater resources of the island.

Possible fossil H2O liquid-ice interfaces in the Martian crust

Abstract Throughout the northern equatorial region of Mars, extensive areas have been uniformly stripped, roughly to a constant depth. These terrains vary widely in their relative ages. A model is described here to explain this phenomenon as reflecting the vertical distribution of H 2 O liquid and ice in the crust. Under present conditions the Martian equatorial regions are stratified in terms of the stability of water ice and liquid water. This arises because the temperature of the upper 1 or 2 km is below the melting point of ice and liquid is stable only at greater depth. It is suggested here that during planetary outgassing earlier in Martian history H 2 O was injected into the upper few kilometers of the crust by subsurface and surface volcanic eruption and lateral migration of the liquid and vapor. As a result, a discontinuity in the physical state of materials developed in the Martian crust coincident with the depth of H 2 O liquid-ice phase boundary. Material above the boundary remained pristine; material below underwent diagenetic alteration and cementation. Subsequently, sections of the ice-laden zone were erosionally stripped by processes including eolian deflation, gravitational slump and collapse, and fluvial transport due to geothermal heating and melting of the ice. The youngest plains which display this uniform stripping may provide a minimum stratigraphic age for the major period of outgassing of the planet. Viking results suggest that the total amount of H 2 O outgassed is less than half that required to fill the ice layer, hence any residual liquid eventually found itself in the upper permafrost zone or stored in the polar regions. Erosion stopped at the old liquid-ice interface due to increased resistance of subjacent material and/or because melting of ice was required to mobilize the debris. Water ice may remain in uneroded regions, the overburden of debris preventing its escape to the atmosphere. Numerous morphological examples shown in Viking and Mariner 9 images suggest interaction of impact, volcanic, and gravitational processes with the ice-laden layer. Finally, volcanic eruptions into ice produces a highly oxidized friable amorphous rock, palagonite. Based on spectral reflectance properties, these materials may provide the best analog to Martian surface materials. They are easily eroded, providing vast amounts of eolian debris, and have been suggested (Toulmin et al. , 1977) as possible source rocks for the materials observed at the Viking landing sites.

D/H and O18/O16 studies of serpentinization of ultramaflc rocks☆

Abstract Oxygen and hydrogen isotope analyses have been obtained on serpentine and related metasomatic talc and ‘blackwall’ minerals from the following ultramaflc-rock types: 1. (1) alpine associations in unmetamorphosed or low-grade metamorphic terranes; 2. (2) alpine associations from medium- to high-grade regionally metamorphosed areas; 3. (3) magmatic ultramafic intrusions such as the layered stratiform complexes, the concentrically-zoned Alaskan-type bodies, the high-temperature Mt. Albert alpine intrusion, and the kimberlite body at Moses Rock, Utah; 4. (4) dikes and sills of Precambrian shield areas; and 5. (5) deweylites, which are serpentine-like mineraloids of probable near-surface weathering origin. The δD values of the lizardite-chrysotile serpentines from all of the ultramafic rock types exhibit a total range from −59 to −205 per mil and, except for the Precambrian samples, most show an exceptionally good correlation with geographic position and latitude of the sample locality. The lizardite-chrysotile samples show a progressive change in δD that parallels the present-day variation in δD of meteoritic waters across the North American continent: Caribbean (−59 to −71); Central America (−78 to -97); California (−85 to −108); Oregon (−107 to −149); Washington (−127 to −142); S.E. Alaska ( − 127 to −140); southern British Columbia ( − 150 to −163); northern British Columbia ( −162 to −205). The δO18 values of lizardite-chrysotile range from +8.7 to −5-0 per mil and are more correlative with the δO18 of the country rocks than with latitude. These relationships, combined with model estimates of the isotopic compositions of waters involved in such serpentinization, suggest that much of this lizardite-chrysotile serpentinization probably formed by waters of meteoric-hydrothermal origin at relatively shallow levels in the Earths crust. Inasmuch as sedimentary formation waters typically contain a major meteoric-water component, they are also possible agents of this type of serpentinization. In contrast to lizardite-chrysotile, the pure antigorites have very restricted ranges of δD (−39 to δ-66) and δO18 ( +4.7 to +8.7) similar to values in metamorphic chlorites. This indicates that the antigorites generally have formed during regional metamorphism in the presence of non-meteoric waters; similar relationships are also observed for all metasomatic talc and ‘blackwall’ minerals. The deweylites are consistently higher in δO18 than all true serpentines, compatible with a very low temperature of formation (~15–30°C) and serpentine-H2O fractionation factors of 1.0185 ( O 18 O 16 ) and 0.946, (D/H). The observed isotopic values of all serpentines, combined with our experiments on isotopic exchange rates in the serpentine-H2O system, suggest that most samples examined in this work probably have largely preserved the isotopic compositions they acquired at the time of their formation.

Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri

Isotopic analyses were made on whole-rock samples and minerals from 25 granites, 21 rhyolites, and 10 basaltic dikes and sills from the l,500-m.y.-old St. Francois Mountains terrane. The δD of chlorite is relatively uniform at −44 to −65, but the whole-rock and feldspar δ18O values systematically increase from +7 in the northeast to +14 in the southwest, correlating with an increasing intensity of “brick-red” alteration of the K-feldspars. Coexisting quartz and feldspar are typically not in isotopic equilibrium, except in the northeast. The coarser grained (>1 mm) quartz (δ18O = 8.8 to 10.6) is isotopically similar to “normal” igneous quartz, but the δ18O of the finer grained quartz correlates with the whole-rock δ18O pattern. Although the St. Francois terrane is geologically similar to many low-18O Tertiary volcanic-plutonic complexes that have interacted with meteoric-hydrothermal fluids at high temperatures, only a single locality (Stono Mountain) contains low-18O quartz and feldspar (δ18O quartz = 4.6 to 5.4). Even at this locality, the feldspars were later enriched in 18O by the same alteration event that affected the rest of the region. This second stage of hydrothermal activity apparently involved aqueous fluids having δ18O ≈ 0 to −6 and δD = 0 to −25. The temperature of alteration may have been as low as 50° to 100°C in the southwest (upper part of the volcanic section). This event affected some of the basaltic dikes and sills, but others were not altered and must have been intruded afterward. Rb-Sr and K-Ar age data suggest that the regional alteration occurred as late as 1,100 to 1,200 m.y. ago, long after the age of primary igneous activity. The hydrothermal fluids apparently originated as meteoric surface waters, indicating that during late Precambrian time such waters were isotopically similar to present-day meteoric waters in warm climates; this interpretation implies that the late Precambrian oceans must have been isotopically similar to present-day ocean.

Isotopic studies of the late Archean plutonic rocks of the Wind River Range, Wyoming

Isotopic studies of the Rb-Sr and U-Th-Pb systems in whole-rock samples and the U-Pb systematics for zircons document the existence of two late Arehean intrusive events in the Wind River Range. All of the systems examined indicate an age of ∼2,630 ± 20 m.y. for the Louis Lake batholith. Apparent ages for the Bears Ears pluton range from 2,504 ± 40 m.y. to 2,575 ± 50 m.y. The scatter in apparent ages for the Bears Ears pluton does not appear to be primarily the result of disturbance by postintrusive events, but it may be explained by an isotopically inhomogenous magma at the time of intrusion. Data for a few samples indicate that the Wind River Range was affected locally by a postmagmatic hydrothermal event that was approximately Tertiary in age. This event lowered δ 18 O values and disturbed parent-daughter relationships in most of the isotopic systems investigated, but it was recent enough that there is no demonstrable effect in the Pb-Pb system. The Bears Ears pluton has some chemical and petrologic features that are similar to those reported for the granites in the Granite Mountains to the east. These granites are spatially associated with low-temperature uranium deposits of Tertiary age and have been shown to have lost large amounts of uranium during the early to middle Tertiary. U-Pb systematics indicate, however, that the low to moderate uranium contents and highly variable Th/U values noted for the Bears Ears pluton are best interpreted as being primary features. If uranium was lost after magma generation, the loss most likely occurred at the time of intrusion. Such a loss could account for uraniferous Precambrian pegmatites southwest of the main part of the Range. The two intrusive units apparently were derived from different protoliths that were formed during early to middle Archean. Initial isotopic ratios and petrochemistry for the Louis Lake batholith are consistent with an early Archean trondhjemitic to tonalitic source. The protolith for the Bears Ears pluton must have been more evolved and somewhat younger. Inconsistencies as to the degree of evolution of this protolith, as inferred from isotopic and trace-element data, suggest that the protolith may have been subjected to high-grade meta-morphism that caused loss of Rb and U prior to generation of the magma.

Hydrogen, oxygen and carbon isotopic evidence for the origin of rodingites in serpentinized ultramafic rocks☆

Abstract D H , 18 O 16 O and 13 C 12 C analyses were made of 14 whole rock and 28 mineral samples of rodingites associated dominantly with lizardite-chrysotile serpentinites from the West Coast of the U.S.A., New Zealand, and the Northern Appalachian Mtns. The δD values of the rodingite minerals are in three groupings: 5 monomineralic veins of pectolite, −281 to −429; 8 monomineralic veins of xonotlite, −112 to −135; all other minerals, including hydrogarnet, idocrase, prehnite, actinolite, nephrite, and chlorite, −34 to −80. Most calcites in rodingites have δ18O (+9.3 to +14.4) and (δ13C (−6.7 to +0.9) values similar to calcites in other Franciscan rocks, but distinct from the very low temperature calcite veins in serpentinites. The D H data, combined with δ18O values of xonotlite (+5.7 to +10.9) and pectolite (+8.9 to +12.4) suggest formation from meteoric-type waters at low temperatures; the D H depletion of pectolite, however, is anomalous. Rodingite whole rock values range from δ18O = +4.1 to +11.5 and δD = −50 to −86; one sample containing minor amounts of lizardite-chrysotile serpentinite has δD = −92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in δ18O (+4.1 to +8.6). Such a wide range in δ18O is consistent with the idea that most rodingites form over a relatively broad range of hydrothermal temperatures. Hydrogen isotopic data for most rodingite minerals (except xonotlite and pectolite) and for whole rocks are suggestive of non-meteoric waters. These D H data overlap those observed for veins of hydrous minerals found in Franciscan igneous rocks studied by Margaritz and Taylor (1976, Geochim. Cosmochim. Acta 40, 215–234), possibly suggesting evolved D-enriched, connate type metamorphic waters generated during high P, low T Franciscan-type metamorphism at temperatures (250–500°C) comparable to estimates based on mineral stabilities. Such an interpretation is supported by the 18 O 16 O and 13 C 12 C data for calcite in rodingites. The isotope data appear to contradict some of the conclusions derived from geologic and petrologic studies that indicate concomitant metasomatism and serpentinization of their presently observed host rock. These data appear most consistent with the interpretation that most rodingite minerals, with the exception of late-stage veins of xonotlite and possibly pectolite, may involve metasomatism in association with antigorite serpentinization of ultramafic rock. Subsequent upward tectonic transport in many instances may result in incorporation of the rodingites into their presently observed lizarditechrysotile host rock during or subsequent to pervasive shallow level serpentinization by meteoric waters.

Oxygen isotopic compositions of the late orogenic granites in the Southern Piedmont of the Appalachian Mountains, U.S.A., and their relationship to subcrustal structures and lithologies

Abstract Data were acquired from 143 whole rock samples from 20 late orogenic, post-metamorphic, Hercynian-age granitic plutons from the Piedmont of the southern Appalachians, principally in Georgia and South Carolina. These plutons exhibit a regional gradient in oxygen isotopic compositions in which the granites confined to the Inner Piedmont to the northwest are 18 O-enriched (11.4 to 7.9) whereas those toward the southeast within the Charlotte-Slate and portions of the Kiokee belts have distinctly lower 18 O/ 16 O compositions (8.2 to 5.5); one body that lies along the southeastern edge of the Piedmont in South Carolina, however, appears to be anomalously 18 O-enriched (8.9). Most plutons display 18 O/ 16 O variations of 18 O-enriched plutons occupy areas characterized by negative anomalies, whereas low- 18 O bodies are invariably restricted to regions of positive anomalies; (2) 87 Sr/ 86 Sr data, where granites with δ 18 O values 18 O-enriched plutons (>9‰) have ratios >0.710; (3) contrasting chemical and accessory mineral compositions, in which many 18 O-depleted granites have a number of I-type characteristics, whereas several of the most 18 O-enriched plutons exhibit a number of S-type features. It can be inferred from these data that the 18 O-enriched granites were formed from continental crustal protoliths that underlie much of the Inner Piedmont and portions of the Kiokee belt, whereas the low- 18 O plutons were derived from more mafic sources beneath the Charlotte-Slate and portions of the Kiokee belt. The overall correspondence between the regional 18 O/ 16 O patterns exhibited by the granites and gravity data implies that these grantes may be essentially rooted to their protoliths, in turn suggesting that the large-scale translational movement recently proposed for the Southern Piedmont may have occurred prior to intrusion of these granites ∼320 m.y. ago.

Correlation of magnetitic susceptibility with18O/16O data in late orogenic granites of the southern Appalachian Piedmont

Initial magnetic susceptibility (generally indicative of magnetite content) has been determined for 445 samples from 17 granites located in the southern Appalachian Piedmont of Georgia and South Carolina. These values have been correlated with whole rock δ18O data from the same plutons, yielding a pronounced inverse relationship. It has previously been shown for the southern Piedmont that low oxygen isotopic (18O-enriched) values usually occur in S-type granites (Wenner [1], this issue). It follows, then, that I-type granites are characterized by high susceptibilities (χ > 1 × 10−4 G/Oe), and S-type granites by low susceptibilities (χ < 1 × 10−4 G/Oe). An interesting result of this work has been the observation that some S-type granites exhibit good within-site clusters of remanent magnetic directions while I-type granites generally do not.

Using 18O/16O data to examine the mixing of water masses in floodplain wetlands

Oxygen isotopic data were used to assess how far waters from the Savannah River, a major river in the Southeastern United States, backed up and inundated the wetlands along a small Coastal Plain tributary during a flood. This approach worked because the water of this tributary, Unper Three Runs, had an oxygen isotopic composition (δ18O = −4.9 ‰) distinct from Savannah River water (δ18O = −3.2 ‰).Two sample surveys were taken from the mouth of Upper Three Runs to 2 km upstream. Waters were isotopically uniform along the length of the sample transect when both river and tributary water levels were below bankfull. Visual estimates of turbidity taken when both the river and tributary overflowed their channels and inundated adjacent wetlands indicated that Savannah River water extended about 300 meters up the tributary channel. In contrast, the isotopic data indicated that a mixing zone of river and tributary waters extended about 1100 meters upstream. Although this mixing zone was documented only in the channel of Upper Three Runs, it probably extended into adjacent parts of the riparian wetland, potentially affecting ecological processes due to the differing water quality. This study was conducted in a single river-tributary system and the approach is probably applicable in other large rivers.Corresponding Editor: R. Sharitz

Oxygen isotopic study of the nature and provenance of large quartz and chert clasts in gold-bearing conglomerates of South Africa

Large quartzite and chert clasts in the Archean gold-bearing reefs of South Africa have distinctive δ18O values that fall within a restricted range, 8.8‰ to 13.1‰. These values are thought to reflect the sources of the clasts and are not the result of oxygen isotopic reequilibration during postdepositional processes. Although the clasts are isotopically similar to several quartzite and chert source rocks, most potential provenance materials examined appear to be isotopically homogeneous on a centimetre scale, whereas many clasts are not. This difference indicates that several proposed sources may not be important provenance sites, as previously thought.