The distribution of radiogenic heat production as a function of depth in the Sierra Nevada Batholith, California

Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows that mean heat production values first increase, then decrease, with increasing depth. Heat production is 2 μW/m³ within the 3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of 3.5 μW/m³ at 5.5 km depth, then decreases to 0.5–1 μW/m³ at 15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of 40–125 km, the batholith's root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0.14 μW/m³. This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the 35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for 70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper 30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content.     

Molybdenum in icelandic geothermal waters

Molybdenum concentrations in Icelandic geothermal waters lie in the range 1–70 ppb. Warm waters and dilute high-temperature waters which contain high concentrations of sulphide are lowest in molybdenum. No correlation is otherwise observed between molybdenum concentrations and temperature. Surface waters and cold ground waters do not contain detectable molybdenum (<1 ppb). It seems likely that leaching rate is the prime factor in limiting molybdenum levels in these waters. Within individual geothermal fields molybdenum concentrations are either approximately constant or they vary regularly across the field. This regular variation may often be correlated with variations in other solute concentrations and subsurface temperatures and is taken to indicate a control of molybdenum mobility by a temperature dependent equilibrium. The evidence suggests that the solubility of molybdenite is responsible. Molybdenite has not been found in active geothermal systems in Iceland but is known to occur in some New Zealand geothermal systems and it has been identified in hydrothermally altered Tertiary basalt formations at Reydarártindur in southeast Iceland. Boiling and mixing with cold water leads to molybdenite undersaturation and thus these processes favour leaching of molybdenum from the rock. On the other hand, conductive cooling leads to supersaturation which favours removal of molybdenum from solution.     

Germanium in Icelandic geothermal systems

Germanium concentrations in geothermal waters in Iceland lie mostly in the range 2–30 ppb. There is an overall positive relation between the germanium content of the water and its temperature. Most of the germanium occurs as Ge(OH)^?₅in solution but Ge(OH)₄ may also be present in significant amounts in saline waters when above 200°C. Evidence indicates that aqueous germanium concentrations are controlled by exchange reactions where it substitutes for silica in silicates and iron in sulphides. It is the rate of dissolution and the relative abundance of the alteration minerals which take up germanium to a variable extent that ultimately fix Ge(OH)₄ concentrations in the water. This, together with water pH, fixes total dissolved germanium. It is mostly the primary rock composition that dictates the relative abundance of the alteration minerals. Conductive cooling in upflow zones favours removal of germanium from solution. During the initial stages of boiling of rising hot water dissolution is enhanced but precipitation at later stages.Thermodynamic data of various aqueous germanium species and several minerals are summarized and dissociation constants and solubilities estimated at elevated temperatures using available predictive methods.     

Description and interpretation of the composition of fluid and alteration mineralogy in the geothermal system,at Svartsengi,Iceland

The bulk composition and mineralogy of hydrothermally altered tholeiite, along with the composition and speciation of fluid, have been determined for a well-defined alteration zone at 240°C and 110 bars at Svartsengi, Iceland. Mass balances between the geothermal fluid and altered tholeiite, relative to a seawater/fresh water mixture and unaltered tholeiite, indicate the overall reaction per 1000 cm³ is: 1325 gm plagioclase + 1228 gm pyroxene + 215 gm oxide-minerals break down to form 685 gm chlorite + 636 gm albite + 441 gm quartz + 249 gm epidote + 266 gm calcite + 201 gm oxide-minerals + 15 gm pyrite, requiring an influx of 123 gm CO₂, 10 gm H₂S and 4 gm Na₂O and a release of 57 gm SiO₂, 35 gm FeO, 21 gm CaO, 8 gm MgO and 4 gm K₂O.Principal reactions, deduced from textural evidence, include Na-Ca exchange in plagioclase, precipitation of quartz, calcite and anhydrite, and formation of chlorite and epidote by reactions between groundmass minerals and fluid.Thermodynamic analyses of authigenic minerals and downhole fluid indicate that the fluid maintains a state close to equilibrium with the secondary mineral phases chlorite, epidote, albite, quartz, calcite, prehnite, anhydrite, pyrite and magnetite, whereas remnant primary labradorite and augite are out of equilibrium with the fluid.Water/rock ratios for the system are determined under a variety of assumptions. However, the open nature of the system makes comparisons with experimental and theoretical closed system studies ambiguous.     

The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations

New data from geothermal wells in Iceland have permitted empirical calibration of the chalcedony and $\text{Na}\text{K}$ geothermometers in the range of 25–180°C and 25–250°C respectively. The temperature functions are:

$\text{t°C=}\text{1112}\text{4.91?}\text{log SiO}{}_2\text{?273.15}$

$\text{t°C=}\text{933}\text{0.993+}\text{log Na/K}\text{?273.15}$

Concentrations are expressed in ppm. These temperature functions correspond well with the chalcedony solubility data of Fournier (1973) and the thermodynamic data for low-albite/microcline/solution equilibria of Heloeson (1969).A new CO₂ geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields. Its application involves analysis of CO₂ concentrations in the fumarole steam. The temperature function which applies in the range 180?300°C is: logCO₂ = 37.43 + 73192/T- 11829· 10₃/T² + 0.18923T- 86.187·logT where T is in °K and CO₂ in moles per kg of steam.     

Distinct modes of bidecadal and multidecadal variability in a climate reconstruction of the last centuries from a South Pacific coral

The Rarotonga coral Sr/Ca time series (Linsley et al. in Science 290:1145–1148, 2000) provides a near-monthly resolved proxy record of South Pacific climate variability over the last ~300 years. Here we show that two distinct interdecadal, quasi-periodic time components with periods of ~80 and ~25 years can be identified in this time series by Singular Spectrum Analysis. Their associated spatial patterns in the global sea surface temperature (SST) field show notable differences. Whereas the multidecadal component is associated with a global SST pattern that was recently associated with solar forcing on multidecadal timescales, the bidecadal component is associated with a well known pattern of Pacific decadal to interdecadal SST variability.     

Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I–III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (La_N/Yb_N = 1.4–4.4). Isotopically, the rocks of Group I have the initial ratios ⁸⁷Sr/⁸⁶Sr_(i) = 0.709351–0.707112, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512490–0.512588 and high positive ?_Nd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values ⁸⁷Sr/⁸⁶Sr_(i) = 0.709434–0.710092, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512231–0.512210 and for ?_Nd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values ⁸⁷Sr/⁸⁶Sr_(i) = 0.710751–0.709448, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512347–0.512411 and for ?_Nd270 the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1 > Sm_N/Yb_N < 2.5; 0.9 > La_N/Sm_N < 2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu* = 0.23–0.40). The felsic rocks show the initial ratios the values: ⁸⁷Sr/⁸⁶Sr_(i) = 0.704096–0.707805, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512012–0.512021 and for ?_Nd270 the negative values from −5.27 to −5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting.     

Volcanism Pacing Slumping Gravity Flow Deposits during the Late Carboniferous in the Southern Margin of the Junggar Basin,China

Deep-water gravity depositional processes and evolution in arc systems have become topics of intense research focus in recent years. This study discusses the co-evolution of volcanism and deep-water gravity flow deposits at the southern margin of the Junggar Basin, based on petrology, geochronology and geochemical analyses. The results show that a massive collapse of unstable sediments from the slope was triggered by volcanism, resulting in the formation of slumping gravity flows. The occurrence...