Apparent decoupling of the He and Ne isotope systematics of the Icelandic mantle: The role of He depletion, melt mixing, degassing fractionation and air interaction

We present new He-Ne data for geothermal fluids and He-Ne-Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios (³He/⁴He) encompassing typical MORB-like ratios through values as high as 36.8 R_A (where R_A = air ³He/⁴He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic ²¹Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by ²⁰Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, ‘solar’ mantle component. However, binary mixing between these two endmembers can account for the He-Ne isotope characteristics of the basalts only if the ³He/²²Ne ratio of the primordial mantle endmember is lower than in the MORB component. Indeed, the helium to neon elemental ratios (⁴He/²¹Ne∗ and ³He/²²Ne_s where ²¹Ne∗ = nucleogenic ²¹Ne and ²²Ne_s = ‘solar’-derived ²²Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth’s mantle, as observed previously for other OIB samples. Helium may be depleted relative to neon in high-³He/⁴He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher ⁴He/⁴⁰Ar∗ (⁴⁰Ar∗ = radiogenic Ar) values for a given ⁴He/²¹Ne∗ value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He-Ne-Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high-³He/⁴He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He-Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive ‘solar-like’ characteristics in central and southern Iceland.     

Helium isotope characteristics of Andean geothermal fluids and lavas

The first comprehensive helium isotope survey of the Andes is reported here. We have sampled geothermal fluids and phyric lava flows from the Southern (svz) and Central (cvz) Volcanic Zones, the volcanically active Pun˜a region and the Precordillera, Salta Basin, Longitudinal Valley and the aseismic region between the two volcanic zones. Although the active areas are characterised by significant differences in crustal age and thickness, the svz, cvz and Pun˜a are characterised by a wide and overlapping range in ³He/⁴He ratios (for fluids and phenocrysts) from predominantly radiogenic values to close to the MORB ratio. The measured ranges in ³He/⁴He ratios (R) (reported normalised to the air ³He/⁴He—R_A) are: svz (0.18 < R/R_A< 6.9); cvz (0.82 < R/R_A< 6.0); and Pun˜a (1.8 < R/R_A< 5.4). Modification of magmatic ³He/⁴He ratios by water/rock interactions (fluids) or post-eruptive grow-in of radiogenic ⁴He or preferential diffusive loss of ³He (phenocrysts) is considered unlikely; this means that the wide range reflects the helium isotope characteristics of magma bodies in the Andean crust. The mechanism controlling the ³He/⁴He ratios appears to be a mixing between mantle (MORB-like) helium and a radiogenic helium component derived from radioactive decay within the magma (magma aging) and/or interaction with ⁴He-rich country rock: a process expected to be influenced by pre-eruptive degassing of the mantle component. Assimilation of lower crust is also capable of modifying ³He/⁴He ratios, albeit to a much lesser extent. However, it is possible that the highest measured values in each zone were established by the addition of lower crustal radiogenic helium to MORB helium. In this case, the higher ‘base level’ ratios of the svz would reflect the younger crustal structure of this region. In contrast to helium, there is no overlap in the Sr or Pb isotope characteristics of lavas from the active zones: in all areas, therefore, ³He/⁴He ratios appear to vary independently of Sr and Pb isotope variations. This decoupling between the lithophile tracers and helium reflects the different processes controlling their isotopic characteristics: crust-mantle interactions, alone, for Sr and Pb but for helium the effects of pre-eruptive degassing and possibly magma aging are possibly superimposed. The presence of mantle helium in the Pun˜a region, and to a lesser extent in the Salta Basin, gives an across-arc perspective to the helium isotope distribution and shows mantle melting to occur significantly to the east of the active arc: this is most probably a consequence of lithospheric delamination. The Precordillera hot spring water has the only pure radiogenic helium signal of the entire sample suite and thus marks the western limit of asthenospheric mantle under the Andes.     

The occurrence of selected human pharmaceutical compounds in UK estuaries

This report describes a scoping study conducted in order to establish whether pharmaceutical compounds may be present in UK estuaries. Surface water samples collected from five UK estuaries were analysed for the presence of 14 pharmaceutical compounds selected from the priority lists of the UK Environment Agency and the Oslo and Paris Commission (OSPAR). The pharmaceutical compounds/metabolites clofibric acid, clotrimazole, dextropropoxyphene, diclofenac, ibuprofen, mefenamic acid, propranolol, tamoxifen and trimethoprim were detected at measurable concentrations in the samples collected. The concentrations of erythromycin, lofepramine, paracetamol, sulfamethoxazole and acetyl-sulfamethoxazole were all below the limits of detection of the methods used (between 4 and 20 ng l(-1)). The anti-fungal agent clotrimazole was the most frequently detected at a maximal concentration of 22 ng l(-1) and a median concentration of 7 ng l(-1). The analgesic compound ibuprofen was detected at a maximal concentration of approximately 930 ng l(-1) and a median concentration of 48 ng l(-1), whilst the other pharmaceutical compounds were detected between the limits of detection of the method used and 570 ng l(-1).     

Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador

2 and approximately 85% SO₂ of the total sulfur gas. Relative amounts of He, Ar, and N₂ show a distinct hot-spot signature ( ). The δ¹³C–CO₂ is approximately −3.6‰ and δ³⁴S_T is approximately +3.3‰. The δD/δ¹⁸O of fumarole H₂O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from ³H analyses is ≤40 years. Fumarolic activity at Alcedo is controlled by a caldera-margin fault containing at least seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993–1994 produce approximately 15 m³/s of steam, yet discharge temperatures are ≤97°C. Water content of the total gas is 95–97 mol.%, noncondensible gas is 92–98 mol.% CO₂, and sulfur gas is dominated by H₂S. Relative amounts of He, Ar, and N₂ show extensive mixing between hot spot and air or air-saturated meteoric water components but the average . The δ¹³C–CO₂ is approximately −3.5‰ and δ³⁴S_T is approximately −0.8‰. The δD/δ¹⁸O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3–5‰ in ¹⁸O compared with local meteoric water. ³H indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry indicates a reservoir temperature of 260–320°C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource. Received: 19 April 1999 / Accepted: 23 October 1999     

Potential for growing Arabica coffee in the extreme south of Brazil in a warmer world

Agriculture appears to be one of the human activities most vulnerable to climatic changes due to its large dependence on environmental conditions. However, the diversity of Brazilian environmental conditions could be of great advantage to adapting this sector to new climatic conditions, which should be assessed as in this study on shifting Arabica coffee cultivation to the extreme south of the country. The methodology applied is the same the one used to define climatic risks in current productive regions of Brazil and their vulnerability to climatic change predicted by IPCC reports. The basic climatic parameters applied were frost probability and annual average temperature, since annual water deficit did not prove to be a restricting factor for Arabica coffee cultivation in the study area. The climatic conditions suitable for coffee production are: annual average temperature between 18??C and 22??C, annual water deficit less than 100?mm and frost probability (risk of lowest annual temperature less than 1??C) less than 25%. An area is said to have ??low climatic risks?? for coffee production when these three climatic conditions are met. Current climatic conditions were used and simulations of four temperature increases between 1??C and 4??C were also performed. The results indicated a substantial increase in the size of low climatic risks areas for the production of Arabica coffee in the extreme south of Brazil, mainly for mean temperature increases of 3??C in the study area in relation to present conditions. Increases of 2??C and 4??C were also favorable, but not as good as those obtained for 3??C. It should be underscored that areas with low climatic risks will be able to be found mainly in the extreme south of the study region, the border with Uruguay and North of Argentina.     

Transport and storage of bed material in a gravel‐bed channel during episodes of aggradation and degradation: a field and flume study

The dynamics of sediment transport capacity in gravel‐bed rivers is critical to understanding the formation and preservation of fluvial landforms and formulating sediment‐routing models in drainage systems. We examine transport‐storage relations during cycles of aggradation and degradation by augmenting observations of three events of channel aggradation and degradation in Cuneo Creek, a steep (3%) gravel‐bed channel in northern California, with measurements from a series of flume runs modeling those events. An armored, single‐thread channel was formed before feed rates were increased in each aggradation run. Output rates increased as the channel became finer and later widened, steepened, and braided. After feed rates were cut, output rates remained high or increased in early stages of degradation as the incising channel remained fine‐grained, and later decreased as armoring intensified. If equilibrium was not reached before sediment feed rate was cut, then a rapid transition from a braided channel to a single‐thread channel caused output rates for a given storage volume to be higher during degradation than during aggradation. Variations in channel morphology, and surface bed texture during runs that modeled the three cycles of aggradation and degradation were similar to those observed in Cuneo Creek and provide confidence in interpretations of the history of change: Cuneo Creek aggraded rapidly as it widened, shallowed, and braided, then degraded rapidly before armoring stabilized the channel. Such morphology‐driven changes in transport capacity may explain the formation of flood terraces in proximal channels. Transport‐storage relations can be expected to vary between aggradation and degradation and be influenced by channel conditions at the onset of changes in sediment supply. Published in 2011. This article is a US Government work and is in the public domain in the USA.     

The IAU 2009 system of astronomical constants: the report of the IAU working group on numerical standards for Fundamental Astronomy

In the 2006?C2009 triennium, the International Astronomical Union (IAU) Working Group on Numerical Standards for Fundamental Astronomy determined a list of Current Best Estimates (CBEs). The IAU 2009 Resolution B2 adopted these CBEs as the IAU (2009) System of Astronomical Constants. Additional work continues to define the process of updating the CBEs and creating a standard electronic document.     

Helium–carbon relationships in geothermal fluids of western Anatolia,Turkey

We investigate the helium, carbon and oxygen–hydrogen isotopic systematics and CO₂/³He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). ³He/⁴He ratios of the samples (R) normalized to the atmospheric ³He/⁴He ratio (R_A = 1.39 × 10^? 6) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R / R_A values are generally found in areas of significant heat potential (K?z?ldere and Tuzla fields). CO₂/³He ratios of the samples, ranging from 1.6 × 10⁹ to 2.3 × 10¹⁴, display significant variation and are mostly higher than values typical of an upper mantle source (2 × 10⁹). The δ¹³C (CO₂) and δ¹³C (CH₄) values of all fluids vary from ? 8.04 to + 0.35‰ and ? 25.80 to ? 23.92‰ (vs. PDB), respectively. Stable isotope values (δ¹⁸O–δD) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface.Evaluation of He–CO₂ abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO₂/³He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He–C systematics: however, crustal inputs dominate the CO₂ inventory. Considering that limestone is the main source of carbon (～ 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of ³He/enthalpy ratios (0.000032 to 0.19 × 10^? 12 cm³ STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface.     

Crustal volatile release at Merapi volcano; the 2006 earthquake and eruption events

High‐temperature gas in volcanic island arcs is widely considered to originate predominantly from the mantle wedge and from subducted sediments of the down‐going slab. Over the decade (1994–2005) prior to the 2006 eruption of Merapi volcano, summit fumarole CO₂ gas δ¹³C ratios are relatively constant at ?4.1 ± 0.3‰. In contrast, CO₂ samples taken during the 2006 eruption and after the May 26th 2006 Yogyakarta earthquake (M6.4) show a dramatic increase in carbon isotope ratios to ?2.4 ± 0.2‰. Directly following the earthquake (hypocentre depth 10–15 km), a 3–5‐fold increase in eruptive intensity was observed. The elevated carbon isotope gas data and the mid‐crustal depth of the earthquake source are consistent with crustal volatile components having been added during the 2006 events, most probably by the thick local limestone basement beneath Merapi. This ‘extra’ crustal gas likely played an important role in modifying the 2006 eruptive behaviour at Merapi and it appears that crustal volatiles are able to intensify and maintain eruptions independently of traditional magmatic recharge and fractionation processes.