The 1783 Lakagigar eruption in Iceland: geochemistry,CO2 and sulfur degassing

About 12.3 km³ of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe–Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo₈₆ to Fo₇₅ olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe–Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H₂O and 430 to 510 ppm for CO₂. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe–Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO^* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO₂ flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO₂-induced vesicularity of the magma.     

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

New volatile data (CO₂, H₂O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO₂ and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ¹³C values of dissolved CO₂ or coupled CO₂-Ar variations. This suggests that δ¹³C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H₂O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO₂ with δ¹³C of −6‰, 0.1 to 0.35 wt.% H₂O, ³He/⁴He ∼11 R_A, and CO₂/³He of ∼2 × 10⁹. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the ⁴⁰Ar/³⁶Ar may be low. The ³He/⁴He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R_A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of ³He/⁴He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.     

The origin of rare gases on Earth: The noble gas ‘subduction barrier’ revisited

The origin of the Earth's atmosphere can be constrained by the study of noble gases in oceanic basalts. If it is clear that the mantle is degassed and formed part of the present atmosphere, it has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, altering the primordial signature of the solid Earth. This subduction process has been suggested on the basis of the measurements of light xenon isotopes in CO₂ well gases. Moreover, the fact that the ³⁸Ar/³⁶Ar ratio is atmospheric in all oceanic basalts, even for uncontaminated samples (e.g. with high ²⁰Ne/²²Ne), may also suggest that a massive subduction of atmospheric argon occurred, if the primitive Earth had a solar-like ³⁸Ar/³⁶Ar. This also implies that the atmosphere suffered a massive gas loss accompanied by mass fractionation (e.g. hydrodynamic escape) after mantle degassing or that a late veneer with an atmospheric composition occurred. Such a hypothesis is explored for rare gases, by developing a model in which degassing and subduction of atmospheric noble gases started ∼4.4 Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used (e.g. ³⁸Ar/³⁶Ar and ⁴⁰Ar/³⁶Ar; ¹²⁴Xe/¹³⁰Xe and ¹²⁹Xe/¹³⁰Xe) to constrain the subduction flux and the degassing parameters. It is shown that subduction and massive contamination of the entire mantle is possible, but implies that the ⁴⁰Ar/³⁶Ar and the ¹²⁹Xe/¹³⁰Xe ratios were higher in the past than today, which is not observed in Archean samples. It also implies that the sediments and the altered oceanic crust did not loose their noble gases during subduction or that the contaminated mantle wedge is mixed by the convective mantle. Moreover, such a model has to apply to the oceanic island source, since this later shows the same signature of argon and xenon non-radiogenic isotopic ratios. A scenario where the isotopic compositions of the argon and xenon were settled before or during accretion is therefore preferred to the subduction.     

Argon isotope fractionation induced by stepwise heating

Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks, stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g., atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by ⁴⁰Ar-³⁹Ar stepheating plateaus. However, diffusion theory-based model calculations predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence, adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The degree of fractionation, as monitored by ³⁸Ar/³⁶Ar and ⁴⁰Ar/³⁶Ar ratios, very well agrees with theoretical predictions assuming an inverse square root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence, subatmospheric ⁴⁰Ar/³⁶Ar ratios obtained for argon extracted at low temperatures may not represent paleoatmospheric argon. Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear difficult to obtain, as shungites are multicomponent systems.In ⁴⁰Ar-³⁹Ar stepwise heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and 0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for hydrothermal gas exhalations, or for carbonaceous carrier phases such as “Q” in meteorites that have been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.     

Subduction of solar-type noble gases from extraterrestrial dust: constraints from high-pressure low-temperature metamorphic deep-sea sediments

Solar-type helium (He) and neon (Ne) in the Earths mantle were suggested to be the result of solar-wind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earths mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure–low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of ⁴He, ²¹Ne and ⁴⁰Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earths mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.     

Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence

Sarda and Graham (1990) proposed that in mid-ocean ridge basalts (MORBs), degassing occurs through equilibrium vesiculation followed by various extents of vesicle loss. This model predicts that in a bulk sample of MORB glass with vesicles, the rare gases represent a binary mixture between a vesicle component and a component dissolved in the melt. As vesiculation is expected to produce very different rare gas concentrations and elemental ratios in gas and melt, binary mixing systematics should be recorded in the MORB rare gas abundance data. Indeed, a large range of ⁴He/⁴⁰Ar∗ ratios was known to exist, but these binary mixing systematics remained elusive because helium was used as a proxy for rare gas abundance because helium is not affected by air addition. Here we show that using Ar instead of He, the ⁴He/⁴⁰Ar∗ ratio is higher where the Ar concentration is lower, as expected from simple binary mixing systematics.Taking advantage of the growing Ne database, we further show that the predicted binary mixing is recorded by the He-Ar and He-Ne couples, provided He concentration is not used to trace vesicle abundance. This is because a significant part of helium remains in the melt due to its higher solubility. In contrast, Ar or Ne concentrations, which can both be corrected for air addition, clearly trace vesicles and yield binary mixing patterns that hold for ridges worldwide. The model of vesiculation and vesicle loss thereby finds geochemical support in the rare gas abundance data.The He-Ne-Ar concentration data is best explained by assuming the ratio of helium to neon or argon solubility is about 5 to 15 times higher than values measured in 1 bar laboratory experiments, due to higher He and lower Ne and Ar solubilities. We propose that this is a pressure effect, and vesiculation mainly occurs during magma ascent in the mantle after melting.     

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.     

Ar/Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by ⁴⁰Ar/³⁹Ar techniques. Previous ⁴⁰Ar/³⁹Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, ⁴⁰Ar/³⁹Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana.Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated ⁴⁰Ar/³⁹Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages.The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve.It is argued that the majority of older ⁴⁰Ar/³⁹Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all ⁴⁰Ar/³⁹Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the ⁴⁰Ar/³⁹Ar dating technique. However, if the first model is valid, then the older ⁴⁰Ar/³⁹Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.     

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe Ar/Ar investigation

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, ⁴⁰Ar/³⁹Ar ratios and K₂O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb ⁴⁰Ar) than shock veins and matrix minerals (0.3-3 ppb ⁴⁰Ar). The ⁴⁰Ar/³⁶Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic ³⁶Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (⁴⁰Ar/³⁶Ar: 480-490) in matrix minerals. Average apparent ⁴⁰Ar/³⁹Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These ⁴⁰Ar/³⁹Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped ⁴⁰Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic ³⁶Ar and ³⁸Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.     

Noble gas study of HIMU and EM ocean island basalts in the Polynesian region

Noble gas isotopes of HIMU and EM ocean island basalts from the Cook-Austral and Society Islands were investigated to constrain their origins. Separated olivine and clinopyroxene (cpx) phenocrysts were used for noble gas analyses. Since samples are relatively old, obtained from the oceanic area and showing chemical zoning in cpx phenocrysts, several tests on sample preparation and gas extraction methods were performed. First, by comparing heating and crushing methods, it has been confirmed that the crushing method is suitable to obtain inherent magmatic noble gases without radiogenic and cosmogenic components which were yielded after eruption, especially for He and Ne analyses. Second, noble gas compositions in the core and the rim of cpx phenocrysts were measured to evaluate the zoning effect on noble gases. The result has been that noble gas concentrations and He and Ne isotope ratios are different between them. The enrichment of noble gases in the rim compared to the core is probably due to fractional crystallization. Difference of He and Ne isotope ratios is explained by cosmogenic effect, and isotope ratios of the trapped component seem to be similar between the rim and the core. Third, leaching test reveals no systematic differences in noble gas compositions between leached and unleached samples.³He/⁴He ratios of HIMU samples in the Cook-Austral Islands are uniform irrespective of phenocryst type (olivine and cpx) and age of samples (10–18 Ma), and lower (average 6.8 R_A) than those of the Pacific MORB. On the other hand, ³He/⁴He of EM samples in the Cook-Austral Islands are similar to MORB values. EM samples in the Society Islands show rather higher ³He/⁴He than MORB. Ne, Kr and Xe isotope ratios are almost atmospheric within analytical uncertainties. ⁴⁰Ar/³⁶Ar are not so high as those of MORB. Anomalous noble gas abundance pattern such as He and Ne depletion and Kr and Xe enrichment relative to atmospheric abundances was observed. Furthermore, Ne/Ar and Kr/Ar show correlation with some trace elemental ratios like La/Yb.Lower ³He/⁴He of HIMU than MORB values requires relatively high time-integrated (U + Th)/³He for the HIMU source, which suggests that the HIMU source was produced from recycled materials which had been once located near the Earth’s surface. Moreover, extreme noble gas abundance pattern and strong correlation of Ne/Ar and Kr/Ar with La/Yb indicate that the HIMU endmember is highly depleted in light noble gases and enriched in heavy noble gases. Such feature is not common to mantle materials and is rather similar to the noble gas abundance patterns of the old oceanic crust and sediment, which supports the model that the HIMU source originates from subducted oceanic crust and/or sediment.If the HIMU source corresponds to the oceanic crust which subducted at 1–2 Ga as suggested by Pb isotope studies, however, the characteristic ³He/⁴He of HIMU (6.8 R_A) would be too high because radiogenic ⁴He produced by U and Th decay should dramatically decrease ³He/⁴He. To overcome this problem, the He open system model is introduced which includes the effects of ⁴He production and diffusion between the HIMU source material and the surrounding mantle. This model favors that the HIMU source resides in the upper mantle, rather than in the lower mantle. Furthermore, this model predicts the thickness of the HIMU source to be in the order of 1 km.In contrast to low and uniform ³He/⁴He character of HIMU, ³He/⁴He of EM are rather variable. Entrainment of upper mantle material and/or a less-degassed component are required to explain the observed ³He/⁴He of EM in the Polynesian area. Participation of the less-degassed component would be related to the “superplume” below the Polynesian region.     

40Ar-39Ar stepheating studies of clay concentrates from Irish orebodies

⁴⁰Ar-³⁹Ar stepheating analyses of clay concentrates from the Gortdrum and Tynagh orebodies (Ireland) previously dated by conventional K-Ar, indicate major losses of ³⁹Ar (32–45%) and rad. ⁴⁰Ar (25–35%) during the irradiation. The proportion of rad. ⁴⁰Ar loss, unlike that of ³⁹Ar, increases with J-value. The difference between ³⁹Ar and rad. ⁴⁰Ar proportion losses is related to the mineralogy and grain intimacy. These also affect the stepwise release patterns—the Gortdrum concentrates yield age spectra very consistent with ³⁹Ar recoil predictions, whereas the Tynagh concentrates in which the grains are intimately intergrown, show no clear evidence for ³⁹Ar recoil depletion in the K-rich phases. The difference is resolvable if illite argon release is not a simple volume diffusion type process under vacuum conditions.     

Solubility controlled noble gas fractionation during magmatic degassing: Implications for noble gas compositions of primary melts of OIB and MORB

Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the ⁴He/⁴⁰Ar* ratio (where ⁴⁰Ar* is ⁴⁰Ar corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-Ar fractionation for a given fraction of gas loss depends on the ratio of the solubilities, S_He/S_Ar, which is sensitive to (among other things) the CO₂ and H₂O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that ⁴He/⁴⁰Ar* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial ⁴⁰Ar of ≈1.5 × 10⁻⁵ ccSTP/g, i.e., similar to that of the “popping rock.” However, OIB glasses generally have lower ⁴⁰Ar* concentration for a given ⁴He/⁴⁰Ar*. While this would appear to require lower ⁴⁰Ar* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce S_He/S_Ar (relative to MORBs) during degassing. By modeling S_He/S_Ar in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the ⁴He/⁴⁰Ar* ratio. We show that undegassed ⁴⁰Ar concentrations of OIB magmas were probably similar to those of MORBs.     

Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The ³He/⁴He ratios of 8.22 and 8.51 R_atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R_atm), whereas a mean ratio of 8.06±0.35 R_atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of ²⁰Ne and ²¹Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess ¹²⁹Xe is observed in the sample which also shows ²⁰Ne and ²¹Ne excesses. Observed δ¹³C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO₂/³He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.     

Helium and argon isotopes in spinel lherzolite from Damaping, northern Zhangjiakou

Spinel lherzolite found in Damaping, northern Zhangjiakou, Hebei Province occurs as xenoliths in the Hannuoba basalts that consist of alkali basalt and tholeiite. Spinel lherzolites contain 50%–70% olivine (Fo: 90%), 10%–20% clinopyroxene (predominantly Di), 10%–30% orthopyroxene (predominantly En), and less than 5% spinel.³He/⁴He and⁴⁰Ar/³⁸Ar ratios in the olivine are 7.56×10⁻⁷ and 299.1, respectively.³He/⁴He and⁴⁰Ar/³⁸Ar ratios in the orthopyroxene (enstatite) are 9.1×10⁻⁷ and 307, respectively. Olivine grains are fractured irregularly, and pyroxene grains characterized by well developed cleavages, which would have resulted from explosion during the rapid eruption of lava from the deep interior to the surface. The lower isotope ratios of helium and argon may indicate that the spinel lherzolite xenoliths were derived from the strongly degassed and depleted upper mantle, and that the mantle is inhomogeneous.³He losses to some extent might affect the helium isotope ratios. The project was financially supported by the National Natural Science Foundation of China (No. 49273185).     

Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China： Evidence from Laser Microprobe ^40Ar/^39Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province (LIP), SW China was investigated for laser microprobe 40Ar/39Ar dating. There are two 40Ar/39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger 40Ar/39Ar ages for some basalts with low-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The 40Ar/39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration (about 3 Ma) of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.     

Noble gases in ultramafic xenoliths from San Carlos,Arizona

This work reports the results of noble gas (Ne, Ar, Kr, Xe) analyses of accidental mantle xenoliths from San Carlos, Arizona. Except for the addition of radiogenic ⁴⁰Ar and mass fractionation effects, the isotopic structures of these gases are indistinguishable from atmospheric composition. The absence of ¹²⁹Xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths. The dominant influence on the noble gas abundances in the San Carlos xenoliths appears to have been diffusive gas loss, which may have occurred in a mantle metamorphic event or during contact with the host basanite magma. Evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals; the proportion of each gas in the inclusions increases with increasing atomic weight of the gas, possibly reflecting solubility effects. The noble gases are present in greater concentration in pyroxenes than in olivine, similar to the behavior of other incompatible elements.     

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. ⁴He/³He ratios in He released by fusion vary from pure radiogenic values ∼10⁸ down to 6 × 10⁴. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of ³He, up to 4 × 10⁻⁹ cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low ⁴He/³He = 3 × 10⁴, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 10⁴, indicating the contribution of ³He-rich plume component. Magnetites are principal carriers of this component. Trapped ³He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic ⁴He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of ⁴He sites degraded by U, Th decay.Similar ⁴He/³He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source.The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional ²⁰Ne/²²Ne versus ²¹Ne/²²Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). ²⁰Ne/²²Ne ratios (up to 12.1) correlate well with ⁴⁰Ar/³⁶Ar ones, allowing to infer a source ⁴⁰Ar/³⁶Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (³He/²²Ne)_PRIM versus (⁴He/²¹Ne)_RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (⁴He/²¹Ne)_RAD ≅ 2 × 10⁷ is translated via this array into (³He/²²Ne)_PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source.The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.     

40Ar39Ar studies of Precambrian cherts; an unsuccessful attempt to measure the time evolution of the atmospheric 40Ar/36Ar ratio

KAr isochron techniques can provide, in principle, an experimental reconstruction of the time evolution of the atmospheric ⁴⁰Ar/³⁶Ar ratio if minerals can be found which contain samples of argon from the ancient atmosphere and which have had a simple geologic history. Authigenic sedimentary minerals with low potassium content appear to be the best candidates. An experimental reconstruction of the evolution of the atmospheric ⁴⁰Ar/³⁶Ar ratio will serve as a test of various models for the chemical and thermal evolution of the Earth.⁴⁰Ar³⁹Ar studies of five chert samples from the Swaziland sequence and the Bulawayan and Gunflint Formations indicate that lower Precambrian cherts do not contain appreciable samples of the ancient atmospheric argon and have experienced complicated geologic histories. The chert sample from the Kromberg Formation contains excess ⁴⁰Ar. The other four samples yield age spectra which are complicated but which are interpretable in terms of geologically reasonable ages.The lack of evidence for argon loss in the chert data suggests that some cherts may prove to be datable sedimentary minerals.     

He,Ne, Ar stepwise crushing data on basalt glasses from different segments of Bouvet Triple Junction

Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.     

Composition and sources of volatiles and noble gases in fluid inclusions in pyroxenites and carbonatites of the Seblyavr Massif,Kola Peninsula

Carbonatites and related pyroxenites from the Seblyavr alkaline-ultramafic massif were analyzed for isotopic composition and concentrations of carbon (in carbon dioxide), nitrogen, and noble gases using the stepwise crushing technique. The C isotopic composition in crushing steps of calcite from the carbonatite varies from ?6.6 to ?15.0‰ (PDB) with average values from ?8.5 to ?10.5‰, which is lower than the mantle range for \(\delta ^{13} C_{(CO_2 )} \) (from ?3 to ?5‰) and can likely be explained by long-term isotopic exchange between the carbon of CO₂ in inclusions and their host Ca carbonate. The ⁴⁰Ar/³⁶Ar ratios in the crushing extractions of the calcites vary from the atmospheric value of 296 to 3200. Diopside from the pyroxenite has these ratios as high as 26000–33000 (such high values for pyroxenite in the Kola alkaline-ultramafic province have been obtained for the first time), which corresponds to the values obtained for MORB chilled glasses. Nitrogen in the samples is isotopically heavy, δ¹⁵N from +1 to +2 on average, which is consistent with earlier data on carbonatite massifs in the Kola alkaline province (Dauphas and Marty, 1999) and carbonatites of the Guli Massif (Buikin et al., 2011). The N₂ content in the crushing extractions is correlated with the 36Ar concentration, which is an indicator of atmospheric contamination and suggests the dominance of the crustal N component in the samples, likely as a result of subduction or penetration of the ancient meteoric water into the magma chamber or a metasomatic source. The variations in the isotopic and elemental composition of the gas components between crushing steps suggest that the investigated samples contain inclusions of at least two populations.