Mantle-derived CO_2 in Hot Springs of the Rehai Geothermal Field,Tengchong,China

Gas concentrations and isotopic compositions of He and CO2 were determined on free gas samples from ten hot springs of the Rehai geothermal field, Tengchong, China. The results showed that hot-spring CO2 gas, together with He,was derived mainly from the mantle, indicating the accumulation of mantle-derived volatiles beneath the survey area. The δ^13C values of CO2, higher than those of the typical mantle-derived carbon and the isotopic composition of hot-spring-free CO2 in unequilibrium with dissolved CO2, are recognized only in the Rehai geothermal field, suggesting that there seems to be a still-degassing magmatic intrusion at depths, which provides mantle-derived volatiles to the hydrothermal system above. The accumulation of those volatiles has probably played an important role in triggering earthquakes in this region.In addition, the isotopic characteristics of He and C also indicate that the magmatic intrusion seems to have been derived from the MORB source, and could be contaminated by crustal materials during its upwelling through the continental crust.     

Carbon and Noble Gas Isotopes in the Tengchong Volcanic Geothermal Area, Yunnan, Southwestern China

Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo-Eurasian suture zone. All samples contain a resolvable component of mantle-derived 3He. Occurrence of mantle-derived 3He coincides with surface volcanism. However, 3He occurs over a larger geographic areathan do surface volcanics. δ13C values for CO2 and CH4 vary from -33.4‰ to 1.6 ‰ and from -52.8‰ to -2.8‰, respectively. He and C isotope systematics indicate that CO2 and CH4 in the CO2-rich gases originated predominantly from magmatic component mixed with crustal CO2 produced from carbonate. However, breakdown of organic matter and near-surface processes accounts for the CH4 and CO2 in N2-rich gases. 3He/4He ratio distribution pattern suggests that mantle-derived He and heat sources of high-temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO2-rich gases with the highest 3He/4He ratio (5.2 Ra) may be representative of the     

Dynamics of the Galapagos hotspot from helium isotope geochemistry

We have measured the isotopes of He, Sr, Nd and Pb in a number of lava flows from the Galapagos Archipelago; the main goal is to use magmatic helium as a tracer of plume influence in the western volcanoes. Because the Galapagos lava flows are so well preserved, it is also possible to measure surface exposure ages using in situ cosmic-ray-produced ³He. The exposure ages range from <0.1 to 580 Ka, are consistent with other chronological constraints, and provide the first direct dating of these lava flows. The new age data demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compositions simultaneously for the last ∼10 Ka. The magmatic ³He/⁴He ratios range from 6.9 to 27 times atmospheric (R_a), with the highest values found on the islands of Isabela (16.8 R_a for Vulcan Sierra Negra) and Fernandina (23 to 27 R_a). Values from Santa Cruz are close to typical mid-ocean ridge basalt values (MORB, of ∼9 R_a) and Pinta has a ³He/⁴He ratio lower than MORB (6.9 R_a). These data confirm that the plume is centered beneath Fernandina which is the most active volcano in the archipelago and is at the leading edge of plate motion. The data are consistent with previous isotopic studies, confirming extensive contributions from depleted asthenospheric or lithospheric mantle sources, especially to the central islands. The most striking aspect of the helium isotopic data is that the ³He/⁴He ratios decrease systematically in all directions from Fernandina. This spatial variability is assumed to reflect the contribution of the purest plume component to Fernandina magmatism, and shows that helium is a sensitive indicator of plume influence. The highest ³He/⁴He ratios are found in volcanoes with lowest Na₂O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between ³He/⁴He and Nb/La, suggesting that the Galapagos plume source is characterized by high concentrations of Nb (and Ta). The major and trace element correlations demonstrate that helium is controlled by silicate melting and source variations rather than degassing and/or metasomatic processes. Although lavas with radiogenic isotopic compositions tend to have higher ³He/⁴He, the island-wide isotopic variability cannot be explained by simple two components mixing alone. The preferred model to explain the isotopic data includes a heterogeneous plume, centered at Fernandina, which undergoes polybaric melting, and spatial divergence and mixing with asthenospheric material at shallower depths. The unique regional pattern of the helium isotopic data suggests that helium is extracted more efficiently than other elements during the early stages of melting in the ascending plume.     

He,Sr and Nd isotopic variations in lavas from the juan fernandez archipelago,SE pacific

Helium, Sr, and Nd isotopic ratios and major and trace element compositions have been measured on a suite of lavas from the intra-plate volcanos of the Juan Fernandez Archipelago, Chile. Lavas from the islands of Mas Afuera and Mas a Tierra and from Monte Alpha and Friday seamounts have Sr and Nd isotopic ratios lying on the low¹⁴³Nd/¹⁴⁴Nd side of the mantle array (⁸⁷Sr/⁸⁶Sr:0.7034–0.7037;¹⁴³Nd/¹⁴⁴Nd:0.51281–0.51289). The homogeneity of these tracers suggests the involvement of a restricted range of mantle source compositions throughout Juan Fernandez volcanism. In marked contrast is the large range in³He/⁴He, from 7.8 to 18.0 R_A. A bimodal³He/⁴He distribution on Mas a Tierra is associated with two distinct volcanic lineages;³He/⁴He ratios of 14.5–18.0 R_A (n=15) occur in alkalic and tholeiitic shield basalts, whereas post-shield basanites range from 11.2 to 13.6 R_A (n=12). Elemental and isotopic systematics demonstrate a transition from an enriched (Loihi-like) plume source in the shield lavas to a more MORB-like source in the post-shield volcanics. The transition between these sources is much more pronounced in³He/⁴He than in the other isotopic tracers. The predominantly tholeiitic basalts of Mas Afuera have exceptionally uniform isotopic and elemental characteristics; Sr and Nd ratios are similar to those of Mas a Tierra, but³He/⁴He ratios are lower and more uniform at 8.3±0.5 R_A (n=17). The dramatic distinction between Mas Afuera and Mas a Tierra helium is surprising given the great similarity between the two islands in other geochemical characteristics. Both the Mas Afuera and Mas a Tierra results demonstrate that helium records systematic processes not readily apparent from other isotopic or elemental indicators. Neither magma chamber degassing nor local metasomatic events are likely to be responsible. We suggest that the observed variations may be attributed to mixing of plume and asthenospheric sources in which the plume component is characterized by a heterogeneous distribution of volatiles or has suffered extraction of small degree partial melts prior to mixing.     

Geochemistry of gas emanations: A case study of the Réunion Hot Spot,Indian Ocean

The results of analysis of natural emanations in Réunion Island show a clear magmatic origin for CO₂ and He, while N₂ and Ar are predominantly derived from the atmosphere. The distribution of magmatic gases in the Piton des Neiges massif fits the local volcanotectonic context well and suggests that the areas concerned are still subject to volcanic activity at depth. A simple method is proposed for correcting gas concentration and isotope composition for water degassing. In doing so, the isotope and elemental (C, He) composition of gases is homogeneous for the two volcanoes. The isotope ratio of He (12.5 ± 0.5R/Ra) in the present discharges is in agreement with the results of previous studies on rocks of various ages from the two volcanoes. The isotope ratio of C(δ¹³C= −5 ‰ to −4 ‰ vs PDB) and the C/³ He ratio (∼4 × 10⁹) are similar to those found in other Hot Spot volcanic systems such as Kilauea (Hawaii) and Hengill (Iceland). These similarities suggest comparable volatile history for the respective mantle sources, the main differences being in the relative proportions of radiogenic ⁴He. In detail, Hot Spots appear enriched in C having a light composition with respect to MORB, possibly due to the addition of a C-rich (e.g. subducted) component, in addition to a relatively undegassed, ³He-rich, component.     

Temporal association of arc–continent collision,progressive magma contamination in arc volcanism and formation of gold-rich massive sulphide deposits on Wetar Island (Banda arc)

Whole-rock ⁸⁷Sr/⁸⁶Sr and δ¹⁸O analyses of volcanic rocks and ³He/⁴He analyses of sulphides and sulphates from mineralized rocks on Wetar, Indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted Australian craton to the south. These new data support the idea of progressive source contamination with precisely dated events showing that Wetar Island hosts the most extreme examples of crustal assimilation in the region. The increased continental contamination occurs during the Pliocene (Zanclian to Piacenzian) during distinct magmatic events between 5 and 4 Ma, and at 2.4 Ma when ⁸⁷Sr/⁸⁶Sr ratios in unaltered lavas, with whole-rock δ¹⁸O values between 5.7 and 9.6‰, increase from 0.707484 to extreme radiogenic values of 0.711656.The earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on Wetar. Samples from this yield radiogenic ³He/⁴He ratios between 0.5 and 1.4 R/R_A, similar to the data from volcanic rocks on nearby Romang. The later magmatic event coincides with the arrival of the Australian Continental Margin at the subduction zone along the Banda arc. Progressive incorporation of continental-sourced components into the source region below the Wetar Island edifice coincides with the formation of gold-rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile.     

Pb, Sr and Nd isotope study of two co-existing magmas in the Nízke Tatry Mountains, Western Carpathians (Slovakia)

Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley – Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite and the Raztocna diorite. The ⁸⁷Sr/⁸⁶Sr₍₀₎ ratios vary between 0.7075 and 0.7118, the ε Nd₍₀₎ values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and a lower crustal source. Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material.     

Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N₂/Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hemipelagic sediment on the down-going plate. Using a mantle endmember with δ¹⁵N = −5‰ and N₂/He = 100 and a subducted sediment component with δ¹⁵N = +7‰ and N₂/He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcán Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO₂/N_{2 exc.} ratios (N_{2 exc.} is the N₂ abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 × 10⁸ mol N/a from ³He flux, 6.9 × 10⁸ mol/a from SO₂ flux, and 2.1 × 10⁸ and 1.3 × 10⁹ mol/a from CO₂ fluxes calculated from ³He and SO₂, respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hemipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism.     

Hydrothermal He and CO2 at Wudalianchi intra-plate volcano,NE China

Chemical and isotopic compositions have been measured for CO₂-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed ³He/⁴He ratios (2–3 R_A) and δ¹³C values of CO₂ (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO₂/³He ratios are in wide range of (0.4–97 × 10⁹). Based on the apparent mixing trend in a ³He/⁴He and δ¹³C of CO₂ diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has ³He/⁴He, δ¹³C and CO₂/³He value of ∼3.2 R_A, ∼−4.6‰ and ∼6 × 10¹⁰, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO₂ from the adjacent Changbaishan volcanic area.     

稀有气体同位素对岩浆侵入方向的制约:以夏日哈木镍铜硫化物矿床为例

我国东昆仑造山带新发现的夏日哈木镍铜硫化物矿床是造山带环境产出全球镍资源最大的岩浆镍铜矿床。含矿岩体不同类型岩石中橄榄石和辉石的He、Ne和Ar同位素组成表明:~3He/~4He(0.39~0.03Ra)和~(40)Ar/~(36)Ar比值(292.0~316.9)较低,~(20)Ne/~(22)Ne和~(21)Ne/~(22)Ne沿放射性成因Ne及大陆地壳线分布,表明岩浆起源演化过程中有大陆地壳组分和大气饱和流体在橄榄石结晶前加入。He和Ar同位素混合模型计算表明岩浆中有7.8%再循环洋壳组分和87.7%大气饱和流体的加入,再循环洋壳可能带入了大气及地壳组分。~3He/~4He和~(40)Ar/~(36)Ar比值的自西向东系统性降低,以及微量元素、成矿元素和稀有气体同位素的空间协同变化特征表明地壳物质的逐步加入,即岩浆可能自西向东方向侵入,侵位过程中地壳流体的加入促使硫饱和及硫化物的熔离成矿。     

H,O, Sr,Nd, and Pb isotope geochemistry of the Latir volcanic field and cogenetic intrusions,New Mexico,and relations between evolution of a continental magmatic center and modifications of the lithosphere

Over 200 H, O, Sr, Nd, and Pb isotope analyses, in addition to geologic and petrologic constraints, document the magmatic evolution of the 28.5–19 Ma Latir volcanic field and associated intrusive rocks, which includes multiple stages of crustal assimilation, magma mixing, protracted crystallization, and open- and closed-system evolution in the upper crust. In contrast to data from younger volcanic centers in northern New Mexico, relatively low and restricted primary ¹⁸O values (+6.4 to +7.4) rule out assimilation of supracrustal rocks enriched in ¹⁸O. Initial ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.708), ¹⁸O values (-2 to-7), and ²⁰⁶Pb/²⁰⁴Pb ratios (17.5 to 18.4) of metaluminous precaldera volcanic rocks and postcaldera plutonic rocks suggest that most Latir rocks were generated by fractional crystallization of substantial volumes of mantle-derived basaltic magma that had near-chondritic Nd isotope ratios, accompanied by assimilation of crustal material in two main stages: 1) assimilation of non-radiogenic lower crust, followed by 2) assimilation of middle and upper crust by inter-mediate-composition magmas that had been contaminated during the first stage. Magmatic evolution in the upper crust peaked with eruption of the peralkaline Amalia Tuff (26 Ma), which evolved from metaluminous parental magmas. A third stage of late, roofward assimilation of Proterozoic rocks in the Amalia Tuff magma is indicated by trends in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios from 0.7057 to 0.7098 and 19.5 to 18.8, respectively, toward the top of the pre-eruptive magma chamber. Highly evolved postcaldera plutons are generally fine grained and are zoned in initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, varying from 0.705 to 0.709 and 17.8 to 18.6, respectively. In contrast, the coarser-grained Cabresto Lake (25 Ma) and Rio Hondo (21 Ma) plutons have relatively homogeneous initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of approximately 0.7053 and 17.94 and 17.55, respectively. ¹⁸O values for all the postcaldera plutons overlap those of the precaldera rocks and Amalia Tuff, except for those for two late-stage rhyolite dikes associated with the Rio Hondo pluton that have ¹⁸O values of-8.6 and-9.5; these dikes are the only Latir rocks which may be largely crustal melts.Chemical and isotopic data from the Latir field suggest that large fluxes of mantle-derived basaltic magma are necessary for developing and sustaining large-volume volcanic centers. Development of a detailed model suggests that 6–15 km of new crust may have been added beneath the volcanic center; such an addition may result in significant changes in the chemical and Sr and Nd isotopic compositions of the crust, although Pb isotope ratios will remain relatively unchanged. If accompanied by assimilation, crystallization of pooled basaltic magma near the MOHO may produce substantial cumulates beneath the MOHO that generate large changes in the isotopic composition of the upper mantle. The Latir field may be similar to other large-volume, long-lived intracratonal volcanic fields that fundamentally owe their origins to extensive injection of basaltic magma into the lower parts of their magmatic systems. Such fields may overlie areas of significant crustal growth and hybridization.     

水文地球化学方法在赣地横径地区地热水成因分析中的应用

本文根据从横径地区获得的温泉水、气体和同位素化学成分资料，系统地研究了该区地热水的成因。水、气体、同位素地球化学研究表明，横径地区热水的形成与岩浆和火山活动无关；地热水主要起源于大气降水，其中气体组分起源于大气、地壳和地幔的混合；地热水处在中低温的地热储中；地热水的补给区位于乌泥障地区，其补给区可能的高程为337．5－743．6m。     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

He,Ne, Ar,and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < ³He/⁴He < 8.6 Rc/Ra) and carbon (−20 < δ¹³C_PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ¹³C_PDB and ³He/CO₂ ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO₂ en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The ³He/⁴He signature of summit fumaroles, thought to reflect the ³He/⁴He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the ³He/⁴He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature (³He/⁴He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower ³He/⁴He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low ³He/⁴He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low ³He/⁴He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments.The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.     

Noble gas isotopic compositions and water/gas chemistry of soda springs from the islands of Bioko,São Tomé and Annobon,along with Cameroon Volcanic Line,West Africa

Chemical and isotopic compositions are reported for water, and CO₂ and noble gases in groundwater and soda springs from Bioko, Principé, São Tomé and Annobon, all islands located in the off-shore part of the Cameroon Volcanic Line in West Africa. The soda spring waters are of Ca–Mg–HCO₃ type, with δD and δ¹⁸O values that range from −20 to −8‰ and −5.4 to −2.7‰ respectively, indicative of a meteoric origin. CO₂ is the main gas species in the springs. δ¹³C–CO₂ values vary from −2.8 to −5.0‰, overlapping the observed mantle C range (−3 to −8‰). CO₂/³He ratios (3–9×10⁹) suggest that most C (∼90%) in the samples is derived from the mantle. Neon has atmospheric isotopic compositions, while Ar is slightly enriched in radiogenic ⁴⁰Ar. ³He/⁴He ratios (3.0 to 10.1×10⁻⁶ or 2.1 to 7.2R_a, where R_a is the atmospheric ratio of 1.4×10⁻⁶) are much higher than those for typical crustal fluids (∼10⁻⁸) but lower than those expected for fluids derived from ‘high-³He/⁴He’ hotspots like Loihi and Iceland. This precludes significant contributions of such fluids in the source regions of the gases, and by inference, in the magmatism of these oceanic islands. Alternatively, approximately 90% of the He in São Tomé gases is inferred to be derived from a source similar to the MORB source. The ³He/⁴He ratio for the Bioko gas (6.6×10⁻⁶) may be derived from a source with a higher time integrated (U+Th)/³He ratio than the MORB source.     

Fluid sources for the La Guitarra epithermal deposit (Temascaltepec district, Mexico): Volatile and helium isotope analyses in fluid inclusions

The La Guitarra deposit (Temascaltepec district, South-Central Mexico), belongs to the low/intermediate sulfidation epithermal type, has a polymetallic character although it is currently being mined for Ag and Au. The mineralization shows a polyphasic character and formed through several stages and sub-stages (named I, IIA, IIB, IIC, IID, and III). The previous structural, mineralogical, fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study. The N₂/Ar overall range obtained from analytical runs on fluid inclusion volatiles, by means of Quadrupole Mass Spectrometry (QMS), is 0 to 2526, and it ranges 0 to 2526 for stage I, 0 to 1264 for stage IIA, 0 to 1369 for stage IIB, 11 to 2401 for stage IIC, 19 to 324 for stage IID, and 0 to 2526 for stage III. These values, combined with the CO₂/CH₄ ratios, and N₂-He-Ar and N₂-CH₄-Ar relationships, suggest the occurrence of fluids from magmatic, crustal, and shallow meteoric sources in the forming epithermal vein deposit. The helium isotope analyses, obtained by means of Noble Gas Mass Spectrometry, display R/Ra average values between 0.5 and 2, pointing to the occurrence of mantle-derived helium that was relatively diluted or “contaminated” by crustal helium. These volatile analyses, when correlated with the stable isotope data from previous works and He isotope data, show the same distribution of data concerning sources for mineralizing fluids, especially those corresponding to magmatic and crustal sources. Thus, the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other.The three main sources for mineralizing fluids (magmatic, and both deep and shallow meteoric fluids) are accountable at any scale, from stages of mineralization down to specific mineral associations. The volatile and helium isotope data obtained in this paper suggest that the precious metal-bearing mineral associations formed after hydrothermal pulses of predominantly oxidized magmatic fluids, and thus it is likely that precious metals were carried by fluids with such origin. Minerals from base-metal sulfide associations record both crustal and magmatic sources for mineralizing fluids, thus suggesting that base metals could be derived from deep leaching of crustal rocks. At the La Guitarra epithermal deposit there is no evidence for an evolution of mineralizing fluids towards any dominant source. Rather than that, volatile analyses in fluid inclusions suggest that this deposit formed as a pulsing hydrothermal system where each pulse or set of pulses accounts for different compositions of mineralizing fluids.The positive correlation between the relative content of magmatic fluids (high N₂/Ar ratios) and H₂S suggests that the necessary sulfur to carry mostly gold as bisulfide complexes came essentially from magmatic sources. Chlorine necessary to carry silver and base metals was found to be abundant in inclusion fluids and although there is no evidence about its source, it is plausible that it may come from magmatic sources as well.     

Evidence for Multi-stage Magmatic Evolution during the past 60 kyr at Campi Flegrei (Italy) Deduced from Sr, Nd and Pb Isotope Data

The Campi Flegrei caldera, an active volcanic field in the Campanianprovince, Italy, is a nested structure generated by the CampanianIgnimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 kaBP) eruptions. Since at least 60 ka BP Campi Flegrei has producedmagmas with variable chemical and Sr isotopic compositions.⁸⁷Sr/⁸⁶Sr ratios increase through time from 0·7068 to0·7086, with the highest ratios detected in the least-evolvedshoshonitic products. The origin of this progressive Sr isotopicvariability has been investigated using new Sr, Nd and Pb isotopicdata for volcanic rocks and entrained xenoliths. The data obtainedare combined and discussed with previous geochemical and Srisotope data and used to suggest a multi-stage evolution forthe magmatic system, mainly involving deeper and shallower crustalmagma storage reservoirs. The deeper reservoir is proposed tobe a magma chamber periodically refilled by primitive maficmagmas which subsequently undergo contamination by crustal material.The assimilated crustal material is represented by xenolithsrecovered in the shoshonitic pyroclastic products. Magma batchesoriginating from the deeper reservoir migrated towards the surfaceand fed a shallower complex magmatic system. The deeper chamberwas tapped during the eruption of least evolved magmas by regionalfault systems. In addition to crystal–liquid fractionation,open-system processes occurred in the shallower system. KEY WORDS: Campi Flegrei; crustal contamination; xenoliths     

Crust–mantle interaction triggered by oblique subduction of the Pacific plate: geochronological,geochemical, and Hf isotopic evidence from the Early Cretaceous volcanic rocks of Zhejiang Province,southeast China

Large-scale volcanism in the late Mesozoic was a prominent geological event in southeast China. The late Mesozoic volcanic sequences, named the Moshishan Group, are exposed in Zhejiang Province and are predominantly felsic in composition with subordinate mafic magma and rare andesites. To understand the late Mesozoic tectonic evolution of southeast China, we present zircon U–Pb dating, major and trace element analyses, and Hf isotopic compositions from felsic volcanic rocks of the Moshishan Group. Zircon U–Pb dating shows that the Moshishan Group formed between 145 and 129 Ma. The ε_Hf(t) of the analysed zircons ranges from ?16.58 to +6.89, and the T_DM2 age ranges from 753 to 2238 Ma with a major peak at ca. 1870 Ma. Hf isotopic compositions of zircons in Early Cretaceous volcanic rocks are more radiogenic than that of the metamorphic basement rocks, indicating a juvenile component in these magmas. Major element concentrations show that the volcanic rocks mainly belong to the high-K calc-alkaline series. Both zircon saturation temperatures and the ε_Hf(t) values of zircons gradually increased with the evolution of the magma. Trace element data indicates that neither magmatic differentiation of mantle-derived magma nor mixing of magmas from different sources were the predominant magmagenetic processes. Earlier studies suggest that contemporaneous underplating contributed to the heat source that induced crustal melting and to the material origin that inconsistently mixed with the local crustal melts. Magmatic underplating is likely to have occurred because of the southwestward subduction of the Pacific plate with episodic slab rollback. The data obtained in this study suggest that the crust–mantle interaction under the influence of slab rollback played a progressive role in the formation of Early Cretaceous felsic volcanic rocks in southeast China.     

长白山天池火山区深部流体成分及其稳定同位素组成

天池火山区深部流体成分和稳定同位素组成特征指示,该区地下有相对独立的四个含水层。2.2km以上为冷水层,2.2-3.4km、3.5-3.9km、4km以下分别为上部、中部和深部热水层。现代水热活动均伴随强烈的深源气体释放,碳和氦同位素比值揭示,这些气体属幔源气体,其中仅混入少量大气,壳源物质混染不明显。大规模幔源气体释放主要集中在天池火山湖周围,这表明该区地壳浅部可能存在一定规模的热的幔源岩浆体。据碳同位素地质测温估算,该岩浆体的热变质带顶部距地面约5km.气体动态变化显示该岩浆体目前处在不稳定期,值得引起重视。     

Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe

In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO₂-rich (>99 vol.%) and have δ¹³C values of −1.95 and −4.29‰, respectively. With distance from the center of CO₂ degassing (Bublak) the δ¹³C values decrease, most likely due to physico-chemical fractionation of CO₂ between gaseous and aqueous phases rather than to admixture of organic/biogenic CO₂. The δ¹⁵N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by ³He/⁴He ratios decreasing from 5.9 R_a in the center (Bublak) to 0.8 R_a in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest ³He/⁴He ratio (5.9 R_a) is accompanied by the highest ⁴⁰Ar/³⁶Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ¹⁵N value of ≈−4.5 ± 1.0‰ represents the first free gas δ¹⁵N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the ³He/⁴He ratios, the δ¹⁵N values are equal for ESCM and MORB, respectively.