Stephen T. Nelson

A simple, practical methodology for routine VSMOW/SLAP normalization of water samples analyzed by continuous flow methods

Normalization of stable isotope data is important for meaningful inter-laboratory comparisons of data, especially for waters where there may be large natural variations in isotope ratios of oxygen and hydrogen. As a result, large, systematic errors may arise in continuous flow applications without correction, whereas normalization to the VSMOW/SLAP scale can facilitate inter-laboratory comparison and can be accomplished by a simple procedure in which secondary laboratory standards, carefully calibrated, are analyzed along with unknown samples. Delta values for these standards, as analyzed, are plotted against the calibrated values and a linear regression is performed. The resulting equation is applied to unknown samples to achieve the normalization. The one-sigma [1sigma] standard deviation for replicate samples by this normalization method using a Finnigan Gasbenchll should be </=0.1 per thousand. Because samples are analyzed directly against calibrated laboratory standards, this method also alleviates the requirement to carefully calibrate reference gases, to carefully control absolute temperatures for equilibration methods, or to determine H(3(+)) for deltaD(VSMOW) measurements. Copyright 2000 John Wiley & Sons, Ltd.

Inherited argon in a Pleistocene andesite lava: 40Ar/39Ar incremental-heating and laser-fusion analyses of plagioclase

By using 40 Ar/ 39 Ar incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in the xenocrysts is as much as ∼450 times older than the host lava, the age of which is independently known, and is an obstacle to dating the lava by using a whole-rock sample. The xenocrysts are impossible to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plagioclase and other phenocrysts, gives an accurate 40 Ar/ 39 Ar age for the lava. The xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from Paleozoic rocks buried under kilometers of Mesozoic and Tertiary arc rocks composing the Cordillera in central Chile. The required magma ascent velocity, on the order of kilometers/day, is extraordinarily high compared to the 10 −4 km/day minimum implied by the 226 Ra excess in continental arc lavas. These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.

Sample vial influences on the accuracy and precision of carbon and oxygen isotope ratio analysis in continuous flow mass spectrometric applications

Due to the small amounts of sample gas involved in continuous flow mass spectrometric analysis, care should be taken to evaluate the influence of sample containers on the carbon and oxygen isotope ratios of samples. Data indicate that Na-glass and borosilicate glass vials, equipped with butyl rubber septa, can cause significant changes in the isotopic composition of CO(2) gas, even where sample gases are stored within the vial for less than one day. The magnitude of these changes varies from vial to vial. Given the leverage that contamination can potentially exert on small gas samples, each researcher should carefully evaluate the effect of sample vials in order to eliminate unknown and unwanted changes in the composition of samples. Copyright 2000 John Wiley & Sons, Ltd.

Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO2 gas and contain signifi cant quantities of CO2 (~34.6 mmol/kg), predominantly as H2CO3 and HCO3 –. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault. Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confining unit, the gypsiferous silty Seligman Member of the Kaibab Formation. Mass-balance modeling suggests that the majority of CO2 discharge is the product of the quantitative dissolution of CO2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO2 is relatively low because Pah Tempe waters are grossly undersaturated in CO2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO2 is dominated by a crustal component complemented by minor mantle contributions.

Interactions between mantle‐derived magmas and mafic crust, Henry Mountains, Utah

The Henry Mountains represent one of three major igneous centers of the Colorado Plateau interior during mid-Tertiary (25–30 Ma) time. These intrusions occur ∼1000 km from the paleotrench and are unrelated to subduction in a classical sense, despite many compositional similarities to orogenic magmatic rocks. Most of the intrusive volume (95%) consists of plagioclase-hornblende porphyry of intermediate composition, with minor syenite making up the remainder, and both suites appear to have cooled rapidly as evidenced by the fine-grained texture of their groundmass. Although many elemental variations can be explained by fractional crystallization, Sr, Nd, Pb, and O isotope systematics require open system interaction of mantle derived magmas and Proterozoic amphibolite crust. The plagioclase-hornblende porphyry evolved via assimilation fractional crystallization (AFC) in deep-crustal magma chambers. Syenite porphyry evolved in two distinct stages. First, AFC produced a shallow, zoned, subintrusive magma chamber containing Ne-normative syenite magma. Second, as magma was withdrawn, batches of Ne-normative magma mixed with tonalitic melts produced by fusion of amphibolite country rock to produce Q-normative syenite porphyry. Trace element abundance patterns suggest that both plagioclase-homblende and syenite porphyry were derived from the same mantle source. A higher degree of partial melting may explain the silica-oversaturated character and lower trace element abundances in the plagioclase-hornblende porphyry relative to the syenite porphyry. Trace-element systematics also indicate that high large ion lithophile (LILE) to high field strength element (HFSE) ratios, typical of subduction-derived magmas, are characteristic of the source and have not been imposed solely by crustal contamination. Therefore, the porphyries of the Henry Mountains appear to be related to contemporaneous voluminous regional magmatism of the western United States as part of a large-scale igneous system with arc-like affinities. The relatively minor volume of igneous rocks implies that the Colorado Plateau acted as a structural barrier to the ascent of magma to high crustal levels.

Basement complexes in the Wasatch fault, Utah, provide new limits on crustal accretion

New and reinterpreted isotopic data for crystalline rocks ex- posed in the Wasatch Range require a reevaluation of Precambrian crustal boundaries in Utah. Crystalline rocks of the Santaquin Com- plex underwent metamorphism prior to ca. 1670 Ma, consistent with Sr and Nd isotope data. Mafic to intermediate rocks have major element, trace element, and isotope ratios indicative of derivation in an arc accreted to the Archean craton in Proterozoic time, requiring the crustal suture to be north of the Santaquin Complex. Farther north, the Farmington Canyon Complex has been considered Ar- chean based on published Nd model ages and discordant U/Pb zir- con ages. However, Nd model ages and zircons could be inherited from sedimentary protoliths. U/Pb and electron microprobe ages of monazite have a mode at 1650 to 1700 Ma, concordant with the Santaquin Complex, and lack inheritance. We propose that the Farmington Canyon Complex was first cratonized from Archean- derived sediments in the Proterozoic, requiring a crustal suture to be north of it as well. Accretion ages of arc terranes in southeastern Wyoming are ;60-100 m.y. older than in Utah. Thus, a serious reevaluation of basement architecture in Utah is needed and a pre- viously unrecognized temporal complexity of accretion is indicated.

Paleohydrologic record of spring deposits in and around Pleistocene pluvial Lake Tecopa, southeastern California

Tufa (spring) deposits in the Tecopa basin, California, reflect the response of arid groundwater regimes to wet climate episodes. Two types of tufa are represented, informally defined as (1) an easily disaggregated, fine-grained mixture of calcite and quartz (friable tufa) in the southwest Tecopa Valley, and (2) hard, vuggy micrite, laminated carbonate, and carbonate-cemented sands and gravels (indurated tufa) along the eastern margin of Lake Tecopa. High δ18OVSMOW (Vienna standard mean ocean water) water values, field relations, and the texture of friable tufa suggest rapid nucleation of calcite as subaqueous, fault- controlled groundwater discharge mixed with high-pH, hypersaline lake water. Variations between δ18OVSMOW and δ13CPDB (Peedee belemnite) values relative to other closed basin lakes such as the Great Salt Lake and Lake Lahontan suggest similarities in climatic and hydrologic settings. Indurated tufa, also fault controlled, formed mounds and associated feeder systems as well as stratabound carbonate-cemented ledges. Both deposits represent discharge of deeply circulated, high total dissolved solids, and high p CO2 regional groundwater with kinetic enrichments of as much as several per mil for δ18OVSMOW values. Field relations show that indurated tufa represents episodic discharge, and U-series ages imply that discharge was correlated with cold, wet climate episodes. In response to both the breaching of the Tecopa basin and a modern arid climate, most discharge has changed from fault-controlled locations near basin margins to topographic lows of the Amargosa River drainage at elevations 30–130 m lower. Because of episodic climate change, spring flows may have relocated from basin margin to basin center multiple times.

A reassessment of Mojavia and a new Cheyenne Belt alignment in the eastern Great Basin

A separate basement terrane, Mojavia or the Mojave province with characteristic 2.0–2.3 Ga model ages, has been proposed to underlie much of the western U.S. Its existence, posited on patterns of Pb and Nd isotope data, has been propagated in the literature for more than two decades. New and compiled U/Pb geochronology shows there is no direct evidence of >2.0 Ga juvenile basement rocks exposed within Mojavia of the eastern Great Basin. Pb and Nd model ages, by contrast, vary from Archean to Neoproterozoic with large variations exhibited, commonly within small regions. Archean ages are concentrated northward, suggesting the influence of sediment shed southward from the Wyoming province onto Paleoproterozoic basement terranes. In places, including the Farmington Canyon complex, sediment has been tectonically reprocessed and now is preserved as high-grade metamorphic rock in accretionary melanges. There is no strong evidence that Mojavian basement exists outside of the Mojave Desert region proper. A statistical evaluation of common Pb and Sr isotopes in Phanerozoic igneous rocks shows distinct differences north and south of one proposed boundary between Mojavia and the Yavapai province. However, close examination suggests that variation in these parameters could be produced by the influence of Archean material shed from the Wyoming province rather than representing a distinct difference in the age and isotopic character of the basement. Statistical discrimination of candidate terranes and terrane boundaries may be valuable for their recognition, but such differences alone do not prove their existence. The northern boundary of Mojavia with the Wyoming province in the eastern Great Basin has been given the same name, the Cheyenne Belt, as the exposed suture be tween Paleoproterozoic and Archean basement in southern Wyoming. We reassess the location of this boundary in the Great Basin. New age controls on key basement outcrops in the Uinta Mountains and Farmington Canyon complex that were previously considered Archean indicate that these rocks are Paleoproterozoic in age (∼1.7 Ga). Thus, the Cheyenne Belt has traditionally been placed too far south; it must lie in a poorly defined location north of these localities.

Tertiary tectonic history of the southern Andes: The subvolcanic sequence to the Tatara–San Pedro volcanic complex, lat 36°S

The Tatara–San Pedro volcanic complex, a Quaternary to Holocene composite volcano in central Chile, is underlain by late Miocene (6 Ma) plutons and Tertiary metavolcanic rocks. The plutons were intruded into greenschist facies metavolcanic rocks at a depth of about 4–5 km. Together, these rocks reveal important information about the development of continental arcs. In some cases, apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons, indicating that there was (1) pervasive reheating of metavolcanic rocks or (2) that metavolcanic rocks were locally derived and intruded by their own magma chambers (plutons). We have estimated uplift or denudation rates of nearly 1 mm/yr since late Miocene time. Normal faulting currently dominates the brittle behavior of the upper crust; therefore, uplift is probably related to magmatic addition to the crust rather than to compression and shortening. Although plutonic and metavolcanic rocks display expected arc geochemical affinities, the isotopic similarity of metavolcanic and plutonic rocks to Quaternary volcanic rocks is strong; this makes it difficult to use subvolcanic rocks as upper crustal contaminants to evaluate the igneous evolution of the Quaternary deposits. Most plutonic, metavolcanic, and Quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust. Some samples, however, exhibit sparse but unmistakable evidence ( 87 Sr/ 86 Sr and 40 Ar/ 39 Ar isotopic data) for the presence of older, probably Precambrian through Triassic radiogenic crust beneath the volcano. Crustal structure in this arc can be described by a three-layer model in which a central section of Precambrian through Triassic rock is underplated by juvenile material, is intruded by juvenile material, or has juvenile material transported through it to be emplaced at or near the surface. Continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism.

The role of interbasin groundwater transfers in geologically complex terranes, demonstrated by the Great Basin in the western United States

In the Great Basin, USA, bedrock interbasin flow is conceptualized as the mechanism by which large groundwater fluxes flow through multiple basins and intervening mountains. Interbasin flow is propounded based on: (1) water budget imbalances, (2) potential differences between basins, (3) stable isotope evidence, and (4) modeling studies. However, water budgets are too imprecise to discern interbasin transfers and potential differences may exist with or without interbasin fluxes. Potentiometric maps are dependent on conceptual underpinnings, leading to possible false inferences regarding interbasin transfers. Isotopic evidence is prone to non-unique interpretation and may be confounded by the effects of climate change. Structural and stratigraphic considerations in a geologically complex region like the Great Basin should produce compartmentalization, where increasing aquifer size increases the odds of segmentation along a given flow path. Initial conceptual hypotheses should explain flow with local recharge and short flow paths. Where bedrock interbasin flow is suspected, it is most likely controlled by diversion of water into the damage zones of normal faults, where fault cores act as barriers. Large-scale bedrock interbasin flow where fluxes must transect multiple basins, ranges, and faults at high angles should be the conceptual model of last resort.RésuméDans le Grand Bassin, Etats-Unis, l’écoulement interbassin en domaine de socle constitue un mécanisme par lequel des flux importants d’eaux souterraines s’écoulent à travers plusieurs bassins et seuils associés. Le mécanisme d’écoulement interbassin proposé est basé sur: (1) les déséquilibres de bilans, (2) les différences de charge hydraulique entre bassins, 3) les preuves apportées par les isotopes stables, et (4) des études de modélisation. Cependant, les bilans de nappe sont trop peu précis pour discerner les transferts interbassins, et des différences de charge peuvent exister, avec ou sans transferts interbassins. Les cartes piézométriques dépendent de concepts sous-jacents, conduisant à des déductions erronées quant aux transferts interbassins. L’expertise isotopique est sujette à interprétations multiples et faussée par les effets du changement climatique. Les considérations structurales et stratigraphiques dans une région géologiquement complexe telle celle du Grand Bassin devraient conduire à une compartimentation, où l’augmentation de la taille d’un aquifère augmente les particularités suivant un axe d’écoulement donné. Les hypothèses conceptuelles initiales devraient expliquer un écoulement avec une recharge locale et des chemins d’écoulements courts. Là où l’on suppose un flux interbassin à travers le socle il est très probablement contrôlé par l’écoulement de l’eau à travers les zones altérées de failles normales, dont les plans jouent le rôle de barrières. L’écoulement interbassins à grande échelle à travers le socle, où les flux doivent traverser des bassins multiples, des seuils et des failles à fort pendage, devrait être le modèle conceptuel de dernier recours.ResumenEn la Great Basin, EEUU, el flujo intercuenca en el basamento está conceptualizado como el mecanismo por el cual grandes flujos de agua subterránea fluyen a través de múltiples cuencas y de las montañas interpuestas. Se propuso el flujo intercuenca basado en: (1) desequilibrios en el balance de agua, (2) diferencias potenciales entre cuencas, (3) evidencias de isótopos estables, y (4) estudios de modelación. Sin embargo, los balances de agua son demasiados imprecisos para discernir las transferencias intercuencas y pueden existir diferencias de potencial con o sin flujos intercuencas. Los mapas potenciométricos dependen de fundamentos conceptuales, que conducen a a posibles inferencias falsas en relación a las transferencias intercuencas. La evidencia isotópica es propensa a una interpretación que no es única y puede ser confundida por los efectos del cambio climático. Las consideraciones estructurales y estratigráficas en una región geológicamente compleja como la Great Basin deben producir una compartimentación, donde el tamaño creciente del acuífero incrementa las posibilidades de segmentación a lo larga de una trayectoria de flujo dada. Las hipótesis conceptuales iniciales las deben explicar el flujo de con la recarga local y las trayectorias cortas de flujo. Se sospecha que la existencia del flujo intercuencas en el basamento está muy probablemente controlado por el desvío dentro de las zonas de daño de las fallas normales, donde los núcleos de las falla actúan como barreras. El flujo intercuenca en el basamento a gran escala donde los flujos deben atravesar múltiples cuencas, cordilleras y fallas de alto ángulo deben ser el modelo conceptual del último recurso.摘要在美国大盆地,基岩跨流域水流机理概念化为大的地下水通量流经多个盆地和介于中间的山脉。跨流域水流基于下列原因而提出:(1)水量收支不平衡,(2)流域之间的势差,(3)稳定同位素证据,(4)建模研究。然而,水量收支很不精确以至于不能识别流域间的转移,有流域间通量或没有流域间通量,都可能存在势差。静水压面图取决于概念基础,导致跨流域转可能移虚假的推断。同位素证据倾向于非唯一解译,可能被气候变化影响所混淆。地质复杂地区如大盆地中构造及地层上的考量应该能够对此划分,在此,增大的含水层规模增加了沿水流流径的分割的可能性。最初概念假设可以解释具有当地补给及短流径的水流。在 怀疑有基岩跨流域水流的地方,水流很可能受水转移进入正常断层损伤带的控制,在正常断层,断层核心充当屏障。通量很可能高角度横切多个盆地、山脉和 断层的大型基岩跨流域水流应当是不得已的概念模型。ResumoNa Grande Bacia, nos EUA, concetualiza-se o escoamento no bedrock entre bacias hidrográficas, como o mecanismo pelo qual grandes fluxos subterrâneos atravessam múltiplas bacias e montanhas intercaladas. A suposição da existência de fluxo subterrâneo entre bacias baseia-se em: (1) desequilíbrios no balanço hídrico, (2) diferenças no potencial hidráulico entre bacias, (3) evidências de isótopos estáveis, e (4) estudos de modelação. No entanto, os balanços hídricos são demasiado imprecisos para diferenciar possíveis transferências entre bacias e, relativamente às diferenças de potencial hidráulico, estas podem existir independentemente de haver ou não fluxo entre bacias. Os mapas potenciométricos dependem de pressupostos concetuais, levando a possíveis falsas inferências relativamente às transferências entre bacias. As evidências isotópicas são sujeitas à interpretação não-exclusiva e podem ser confundidas pelos efeitos das alterações climáticas. Numa região geologicamente complexa, como a da Grande Bacia, as considerações estruturais e estratigráficas deverão induzir uma compartimentalização, onde, à medida que aumenta a dimensão de um aquífero, crescem as hipóteses de segmentação ao longo de um determinado caminho de fluxo. As suposições concetuais iniciais devem procurar explicar o escoamento com base em recarga local e caminhos de fluxo curtos. Onde se suspeita de fluxo subterrâneo entre bacias através do bedrock, ele será muito provavelmente controlado pelo desvio de água em zonas afetadas por falhas normais, onde os núcleos atuam como barreiras. O fluxo subterrâneo inter-bacias de grande escala através do bedrock deve atravessar várias bacias, cadeias montanhosas e falhas com ângulos elevados e deverá ser considerado o modelo concetual de último recurso.