Concentrations Of 40Ar Research Articles

Probing the Rock Mass Fraction and Transport Efficiency inside Uranus Using 40Ar Measurements

The bulk of Uranus consists of a rock–ice core, but the relative proportions of rock and ice are unknown. Radioactive decay of potassium in the silicates produces 40Ar. If transport of argon from the core to the gaseous envelope is efficient, a measurement of 40Ar in the envelope will provide a direct constraint on the rock mass present (assuming a chondritic rock composition). The expected 40Ar concentrations in this case would be readily detectable by a mass spectrometer carried by a future atmospheric probe. For a given envelope concentration there is a trade-off between the rock mass present and the transport efficiency; this degeneracy could be overcome by making independent determinations of the rock mass (e.g., by gravity and seismology). Primordial 40Ar is a potential confounding factor, especially if Ar/H2 is significantly enhanced above solar or if degassing of radiogenic 40Ar were inefficient. Unfortunately, the primordial 40Ar/36Ar ratio is very uncertain; better constraints on this ratio through measurement or theory would be very helpful. Pollution of the envelope by silicates is another confounding factor but can be overcome by a measurement of the alkali metals in the envelope.

Periods of Extremely High Rates of the Production of Cosmogenic 14C and Temperature Variation According to Data on the 40Ar Content in the Ice Layers for the Last 4000 Years

The possible application of temperature reconstructions based on measurement of the 40Ar concentration in ice layers of the Greenland ice sheet to study past solar activity with radiocarbon data is considered. The rate of 14С production and the heliosphere modulation potential from the 2nd millennium B.C. to the middle of the 18th century A.D. is reconstructed with allowance for the Earth’s climate variation. The temperature reconstruction based on the 40Ar content in the annual ice layers of Greenland revealed two global minima of solar activity with duration of ~200 years during the cold Sub-Atlantic period, which lasted on the Earth from 900 B.C. to 300 B.C. This suggests that solar-activity variation can influence climate change.

Reappraisal of the GL‐O Reference Material for K‐Ar Dating: New Insight from Microanalysis, Single‐Grain and Milligram Ar Measurements

The homogeneity and Ar‐dating suitability of the GL‐O reference material were re‐evaluated to determine whether this material is sufficiently homogeneous to be suitable for the calibration of modern high sensitivity instruments. Based on new micro‐analyses and noble gas determinations, our contribution reveals several kinds of inhomogeneity at the grain scale: disparity in the glauconitisation among and within the pellets, variable occurrence of a phosphatic component within pellets (1% m/m on average), and rare occurrences of calcite and detrital grains. Measurements on test portions of ≤ 1 mg reflect such heterogeneity with variability in 40Ar* content that exceeds analytical uncertainty, including a few highly anomalous values. The lesser evolved glauconite population yielded 40Ar* contents ~ 15% lower than the value of 24.8 nl g−1 recommended by Odin et al. (1982, Numerical dating in stratigraphy. Wiley (Chichester, UK), 123–148). But the measured concentrations of 40Ar* converge towards the aforementioned value as test portion mass increased to > 3 mg. A few rare 3 mg experiments still yielded 40Ar* contents lower than the recommended value (down to 24.0 nl g−1), and we recommend using more conservative minimum masses of 5–10 mg. A further purification step for GL‐O or the intercalibration of its powder version could be considered to diminish the size of the test portions and the intensity of the measured signals.

Helium and Argon Isotopes in the Fe-Mn Polymetallic Crusts and Nodules from the South China Sea: Constraints on Their Genetic Sources and Origins

In this study, the He and Ar isotope compositions were measured for the Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS), using the high temperature bulk melting method and noble gases isotope mass spectrometry. The He and Ar of the SCS crusts/nodules exist mainly in the Fe-Mn mineral crystal lattice and terrigenous clastic mineral particles. The results show that the 3He concentrations and R/RA values of the SCS crusts are generally higher than those of the SCS nodules, while 4He and 40Ar concentrations of the SCS crusts are lower than those of the SCS nodules. Comparison with the Pacific crusts and nodules, the SCS Fe-Mn crusts/nodules have lower 3He concentrations and 3He/4He ratios (R/RA, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the 40Ar/36Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low 3He/4He ratios and high 40Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic 4He and 40Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic 3He, 4He and 40Ar produced by decay of U, Th and K have no effect on their isotope compositions. Thus, the SCS crusts/nodules inherited the noble gases characteristics of their sources. Helium and Ar isotope compositions in the SCS Fe-Mn crusts and nodules reflect the product of an equilibrium mixture between air-saturated seawater and radiogenic components during their growth, while the partial 3He excess in some SCS samples may represent a little mantle-derived origin. The different He and Ar isotope compositions of the Fe-Mn crusts and nodules between the South China Sea and the Pacific Ocean are due to their different sources and genetic processes. The characteristics of He and Ar isotope compositions in the SCS polymetallic crusts and nodules are similar to the properties of hydrogenetic Fe-Mn oxide/hydroxide precipitates, which reflects mainly the product of an equilibrium mixture between air-saturated seawater and radiogenic components.

Origin of the ore-forming fluids and metals of the Hetai goldfield in Guangdong Province of South China: Constraints from C-H-O-S-Pb-He-Ar isotopes

The Hetai goldfield, located in the southern segment of the Qinzhou Bay-Hangzhou Bay Juncture Orogenic Belt (QHJB), is one of the largest gold mining districts in South China. Although the goldfield shares some similarities in ore geology and geochemistry to orogenic gold deposits, there has been controversy on the origin of the ore-forming fluids and metals. An integrated analysis on C-H-O-S-Pb-He-Ar isotopes has been carried out to trace the origins of the ore fluids and metals, thereby to evaluate the genesis of the Hetai goldfield. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions in sulfides from the Hetai ores exhibit a range of 0.27–0.62Ra and 932.4–6324.2, with the 4He and 40Ar concentrations varying from 1.99 to 96.80 (10−7cm3 STP/g) and 0.51–7.75 (10−7cm3 STP/g), respectively. This suggests a dominant crust-derived fluid mixed with significant mantle-derived fluids for the Hetai gold mineralization. The calculated δ18OH2O-VSMOW and measured δDH2O-VSMOW values of fluid inclusions in gold-bearing quartz range from 0.5 to 10.4‰ and from −83.1 to −52.4‰, respectively. The δ13CPDB and δ18OPDB of calcite range from −3.53 to −1.91‰ and −21.21 to −19.67‰, respectively, suggesting a deep-seated source of the C-bearing portion of the fluid. The δ34SCDT values of sulfides intergrown with native gold range from −4.0 to −1.3‰, with an average of −2.8‰, indicating that the majority of the sulfur came from granitic magma by partial melting of the host rocks, but likely with minor input from the mantle. The Pb isotopic components of sulfides from the Hetai ores have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.044–19.404, 15.568–15.793, and 38.426–39.597, respectively, indicating a main upper continental crust source of ore metals, but probably with minor contribution from the mantle. Linked to the ore geology and regional tectonomagmatic events, the integration of the isotopic compositions obtained in this study with previously published fluid inclusion and geochronologic data suggests that the ore fluids and metals may have undergone two stages of evolution, i.e. from the Indosinian metamorphic water-dominated fluids mixed with minor magmatic waters, to the Yanshanian magmatic water-dominated fluids mixed with mantle-derived fluids and meteoric waters.

Noble gases, nitrogen and cosmic ray exposure age of the Sulagiri chondrite

The Sulagiri meteorite fell in India on 12 September 2008, LL6 chondrite class is the largest among all the Indian meteorites. Isotopic compositions of noble gases (He, Ne, Ar, Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed. Low cosmogenic (22Ne/21Ne)c ratio is consistent with irradiation in a large body. Cosmogenic noble gases indicate that Sulagiri has a 4π cosmic-ray exposure (CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of LL chondrites. Radiogenic 4He and 40Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and 4.56 Ga, indicating the loss of He from the meteorite. Xenon and krypton are mixture of Q and spallogenic components.

Origin of the ore-forming fluids and metals of the Bangpu porphyry Mo–Cu deposit of Tibet, China: Constraints from He–Ar, H–O, S and Pb isotopes

The Bangpu porphyry Mo–Cu deposit is a representative Mo-dominated deposit besides the Sharang porphyry Mo deposit in the Gangdese metallogenic belt. The Mo–Cu mineralization has a close relationship with the monzogranite porphyry and diorite porphyrite. We identify three stages during the ore formation: a pre-ore stage, a main-ore stage with Mo–Cu deposited dominantly, and a post-ore stage. In this study, He–Ar, H–O, S and Pb isotopic compositions of the Bangpu deposit were determined. Based on these determinations, integrated isotope geochemistry studies were performed to constrain the possible sources of the ore-forming fluids and metals. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions exhibit a range of 0.12209–0.36370Ra and 275.6–346.1, respectively. The 4He and 40Ar concentrations vary from 1.51 to 3.57 (10−7cm3STPg−1) and 0.49 to 9.31 (10−7cm3STPg−1), respectively. He–Ar isotopic compositions suggest dominantly crustal-derived fluid with minor amount of meteoric water in the main ore stage. The δ18Ofluid and δDfluid values vary from −1.3‰ to 3.9‰ and −140.5‰ to −73.7‰, respectively, indicating that magma fluids mixed with meteoric water. The average δ34S value of the sulfides (0.3‰) in the main-ore stage is close to the ore-forming porphyries, indicating a magmatic source. The lead isotopic components of ore sulfides exhibit restricted ranges with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.450–18.728, 15.602–5.672, and 38.715–39.211, respectively and μ values in the range of and 9.46–9.58, indicating ore-forming metals of primarily an upper crust source with a small amount of mantle materials. Compared to the Bangpu deposit, the ore metals derived from mantle are even greater in the Jiama and Qulong deposits, which leads to Cu being the dominant mineralization in the Jiama and Qulong deposit.

Retrogression of eclogite-facies shear zones by short-lived fluid infiltration during the Caledonian orogeny, Lofoten islands, Norway

Abstract Norwegian eclogite-facies shear zones in the Lofoten islands served as major pathways for short-lived pulses of both 40 Ar-rich and 40 Ar-poor hot fluids during eclogitization and retrogression events related to the Caledonian orogeny. The open system for Ar affected most of the minerals leading to old 40 Ar/ 39 Ar ages, particularly in trioctahedral micas. 40 Ar concentration in the fluids appears to decrease during the amphibolite-facies retrogression and Ca-rich amphiboles yield 40 Ar/ 39 Ar ages of c. 415 Ma (Scandian event). Late- and post-Caledonian 40 Ar/ 39 Ar muscovite and K-feldspar ages and Rb/Sr biotite ages coincide with multiple extensional events, fluid infiltration and thermal activity during the final exhumation of the crustal rocks, potentially reflecting major tectonic episodes such as rifting of Pangaea and seafloor spreading between Europe and North America. Supplementary material: Sample coordinates from all the rocks, analytical methods, complete 40 Ar/ 39 Ar step-heating descriptions and electron microprobe results are available at http://www.geolsoc.org.uk/SUP18687 .

The signature of devolatisation: Extraneous 40Ar systematics in high-pressure metamorphic rocks

The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are is common in micas that have equilibrated under (ultra-)high pressure ((U)HP) conditions: metasediments from six Phanerozoic (U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲50% in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ∼700% older despite lower K2O concentrations. A model is developed that calculates Are age fractions as a function of variable mica–fluid KD, bulk K2O and porosity under closed system conditions. Measured Are concentrations in mafic eclogites are reproduced only when porosities are ≲10-4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10-2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.

Source characteristics and fluid evolution of the Beiyingxigou Pb–Zn–Ag deposit, central North China Craton: An integrated stable isotope investigation

The Beiyingxigou quartz vein and alteration type Pb–Zn–Ag deposit forms part of the metallogenic region southwest of the Mapeng batholith in the Central North China Craton. The quartz vein-type polymetallic deposit is associated with intermediate-felsic intrusives emplaced with Mesoarchean metamorphic basement. The mineralization is dominantly controlled by N–S and NW–SE trending faults. We identify four distinct stages of mineralization associated with the hydrothermal activity: pyrite phyllic stage, quartz–pyrite stage, quartz–polymetallic sulfide stage, and quartz–carbonate stage. Among these, the first two stages and the quartz–polymetallic sulfide stage represent the early and main metallogenic stages, respectively. Here we present results from integrated stable isotope studies (S, Pb, He–Ar and H–O) to deduce the possible sources of the mineralizing fluids and metals. The δ34S values recorded in the sulfide minerals of the early and main metallogenic stages show ranges of +0.4‰ to +2.2‰, and −11.4‰ to −6.8‰, respectively. The data illustrate two distinct pulses of hydrothermal fluids with δ34S mean values of 1.1‰ and −8.7‰. The S isotopes of pyrite and galena, sphalerite from the early and main metallogenic stages suggest a mixed source from the Mesozoic magmatism and the Precambrian basement. We infer that the slow cooling magma at the main metallogenic stage led to a larger interaction with the country rocks and resultant contamination, as compared to the early–middle metallogenic stage. The lead isotopic composition of pyrite, galena and sphalerite from the early to main stages shows restricted ranges with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 15.720–16.275, 15.068–15.140 and 38.688–41.316, respectively. The data are all characterized by lower crustal nature, suggesting the contribution of Pb from both the Mesozoic intrusions and the basement rocks in the region. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions from the Beiyinxigou Pb–Zn–Ag deposit show a range of 0.01–0.23 Ra and 270.2–298.1, respectively. The 4He and 40Ar concentrations vary from 26.6 to 340.6 (10−7cm3STP/g) and 165.6 to 357.9 (10−7cm3STP/g), respectively, suggesting dominantly crustal contribution. The δ18OQ values of quartz and the δ18Dw values show ranges of 10.9‰ to 13.6‰ and −85.2‰ to −57.8‰, respectively. The calculated δ18OW values of the mineralizing fluids show a range of 3.7‰ to 7.5‰. Our data suggest that the hydrothermal fluids were a mixture of both magmatic and meteoric water. The results presented in this study link the Beiyinxigou Pb–Zn–Ag deposit to metallogeny associated with Mesozoic magmatism in the North China Craton.

Mars’ atmospheric 40Ar: A tracer for past crustal erosion

Noble gas 40Ar may be used as a tracer of the past evolution of volatiles in Mars’ crust, mantle and atmosphere. 40Ar is formed by the radioactive decay of 40K in the mantle and in the crust and is released from the mantle to the atmosphere due to volcanism and from the crust by erosion such as eolian and hydrothermal erosion. Furthermore, 40Ar can escape from the atmosphere into space via atmospheric escape mechanisms. The evolution of the atmospheric abundance of 40Ar thus depends on these three processes whose efficiencies vary with time.In the present study we reconsider atmospheric escape mechanism efficiencies and describe various possible scenarios of the evolution of 40Ar with a model describing the three main reservoirs of 40Ar, the mantle, crust and atmosphere. First, we show that atmospheric escape, which is stronger in the early evolution, does not significantly influence the present abundance of the atmospheric 40Ar. In the early evolution the atmospheric concentration of 40Ar is very low as the outgassing of 40Ar from the mantle occurs relatively late in the martian evolution. Thus, the atmospheric 40Ar concentration is essentially a tracer of Mars’ outgassing history and not of the escape processes. Second, using the results of the most recent published crustal formation models, the calculated present 40Ar atmospheric abundance is smaller than its observed value. This discrepancy may be explained by a significant 40Ar supply from the crust by erosion (16–30% of the 40Ar content of the upper first 10km of crust). The knowledge of the fraction of crustal 40Ar outgassed to the atmosphere is an important constraint for any future global modelling of past Mars’ hydrothermal activity aiming at better characterizing the role of subsurface aqueous alteration processes in Mars climate evolution. One of the main sources of the uncertainty of these results is the present uncertainty in the measured atmospheric 40Ar value (±20%). More precise measurements of 40Ar and 36Ar in the martian atmosphere are therefore required to better constrain the model.

A metrological approach to measuring40Ar* concentrations in K-Ar and40Ar/39Ar mineral standards

In geochronology, isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals. For dating of geological material using the K-Ar system, the simultaneous determination of 40Ar and 40K concentrations on the same aliquot is not possible. Therefore, a widely used variant, the 40Ar/39Ar technique, involves the production of 39Ar from 39K by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux, referred to as “mineral standards.” Many mineral standards still in use rely on decades-old determinations of 40Ar concentrations; resulting uncertainties, both systematic and analytical, impede the determination of higher accuracy ages using the K-Ar decay system. We discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of 40Ar and will allow for the sensitivity calibration of noble gas mass spectrometers. The design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system. A number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system.

Argon Stable Isotope Concentrations in Lunar Regolith

Argon Stable Isotope Concentrations in Lunar Regolith We performed stepwise heating experiments for determination of the two stable isotope ratios of argon fractions and total concentrations of the three stable isotopes 36Ar, 38Ar and 40Ar in lunar regolith acquired from the Apollo 11, Apollo 12 and Apollo16 missions. Also the concentration of in situ formed radiogenic 40Ar was estimated on the basis of known ages and potassium concentrations determined by isotope dilution method. The observed excess of 40Ar concentration is interpreted to be due to variable (over geological time) flux of solar energetic particles which were implanted into the material at the Moon surface.

Rb–Sr, Sm–Nd, and Ar–Ar isotopic systematics of lherzolitic shergottite Yamato 000097

Isotopic analysis of the Martian lherzolitic shergottite Yamato 000097 yields a Rb–Sr age of 147 ± 28 Ma with an initial 87Sr/ 86Sr ratio of 0.710490 ± 0.000072, a Sm–Nd age of 152 ± 13 Ma with an initial ɛ 143Nd-value of +11.7 ± 0.2, and a 39Ar– 40Ar age of ∼260 Ma. The near concordance of these ages, in combination with the Rb–Sr and Sm–Nd initial isotopic signatures, suggests that Yamato 000097 crystallized from low Rb/Sr, light rare earth element depleted source materials ∼150 Ma ago. Although the obtained 39Ar– 40Ar age is significantly higher than the Rb–Sr and Sm–Nd ages, Yamato 000097 shows little or no evidence of trapped Martian atmospheric 40Ar. The trapped 40Ar concentration of Yamato 000097 is similar to that of Zagami, suggesting that both basaltic and lherzolitic shergottites may have similarly inherited excess 40Ar from their magmas. The Rb–Sr and Sm–Nd ages, and initial 87Sr/ 86Sr and ɛ 143Nd-values of Yamato 000097 and Yamato 793605 lie on the same isotopic ingrowth curves, suggesting that they came from very similar mantle sources. Allan Hills 77005 was also probably derived from the same source, but Lewis Cliff 88516 appears to be from a distinct but similar source. Yamato 000097 represents the most recent known magmatism from its source, and is the youngest Martian meteorite for which concordant Rb–Sr and Sm–Nd ages have been determined.

39Ar‐40Ar dating of the Zagami Martian shergottite and implications for magma origin of excess 40Ar

Abstract— The Zagami shergottite experienced a complex, petrogenetic formation history (McCoy et al. 1992, 1999). Like several shergottites, Zagami contains excess 40Ar relative to its formation age. To understand the origin of this excess 40Ar, we made 39Ar‐40Ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution. Surprisingly, all these separates show similar concentrations of excess 40Ar, ˜1 × 10−6 cm3/g. We present arguments against this excess 40Ar having been introduced from the Martian atmosphere as impact glass. We also present evidence against excess 40Ar being a partially degassed residue from a basalt that actually formed ˜4 Gyr ago. We utilize our experimental data on Ar diffusion in Zagami and evidence that it was shock‐heated to only ˜70 °C, and we assume this heating occurred during an ejection from Mars ˜3 Myr ago. With these constraints, thermal considerations necessitates either that its ejected mass was impossibly large, or that its shock‐heating temperature was an order of magnitude higher than that measured. We suggest that this excess 40Ar was inherited from the Zagami magma, and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old, K‐rich crustal material. Similar concentrations of excess 40Ar in the analyzed separates imply that this magma maintained a relatively constant 40Ar concentration throughout its crystallization. This likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released. These concepts have implications for excess 40Ar in other shergottites.

Intercalibration of 40Ar– 39Ar age standards NL-25, HB3gr hornblende, GA1550, SB-3, HD-B1 biotite and BMus/2 muscovite

40Ar– 39Ar dating requires precise and accurate age data of standard fluence monitors. Besides primary calibration of K and 40Ar concentrations (i.e. the ages are calculated with precise K–Ar dating), intercalibration of the different monitors in use provides important cross-checks of the validity of standard ages and, implicitly, reliability and uncertainty of chronological studies that rely on these standards. Here we report intercalibration of 5 primary international 40Ar– 39Ar standards and one internal laboratory monitor. A major goal of this study was to check the ages of the NL-25 hornblende standard and of the muscovite laboratory standard BMus/2 against the international standards HD-B1, GA1550, SB-3 and HB3gr. In replicate measurements of three irradiation series, 40Ar– 39Ar ages for the fluence monitors were found which agree within 1σ-error limits with the reported age values of 2660 ± 9 Ma for NL-25, Hb3gr: 1073.6 ± 4.6 Ma, BMus/2: 328.5 ± 1.1 Ma, SB-3: 162.9 ± 0.8, GA1550: 98.79 ± 0.54 and 24.21 ± 0.32 for HD-B1. For the hornblende NL-25 fluence monitor a new – slightly different – 40Ar– 39Ar age of 2657 ± 4 Ma is suggested. For the age of the muscovite BMus/2 laboratory standard we recommend 328.5 ± 1.1 Ma, and for biotite HD-B1 a more precise value of 24.18 ± 0.09 Ma.

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 CO 2 and H 2O 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.

Solar wind and other gases in the regoliths of the Pesyanoe parent object and the moon

We report new data from Pesyanoe‐90,1 (dark lithology) on the isotopic signature of solar wind (SW) Xe as recorded in this enstatite achondrite which represents a soil‐breccia of an asteroidal regolith. The low temperature (≤800°C) steps define the Pesyanoe‐S xenon component, which is isotopically consistent with SW Xe reported for the lunar regolith. This implies that the SW Xe isotopic signature was the same at two distinct solar system locations and, importantly, also at different times of solar irradiation. Further, we compare the calculated average solar wind “SW‐Xe” signature to Chass‐S Xe, the indigenous Xe observed in SNC (Mars) meteorites. Again, a close agreement between these compositions is observed, which implies that a mass‐dependent differential fractionation of Xe between SW‐Xe and Chass‐S Xe is >1.5%o per amu. We also observe fractionated (Pesyanoe‐F) Xe and Ar components in higher temperature steps and we document a fission component due to extinct 244Pu. Interestingly, the Pesyanoe‐F Xe component is revealed only at the highest temperatures (>1200°C). The Pesyanoe‐F gas reveals Xe isotopic signatures that are consistent with lunar solar energetic particles (SEP) data and may indicate a distinct solar energetic particle radiation as was inferred for the moon. However, we cannot rule out fractionation processes due to parent body processes. We note that ratios 36Ar/38Ar≤5 are also consistent with SEP data. Calculated abundances of the fission component correlate well with radiogenic 40Ar concentrations, revealing rather constant 244Pu/K ratios in Pesyanoe, and separates thereof, and indicate that both components were retained. We identify a nitrogen component (δ15N = 44%o) of non‐solar origin with an isotopic signature distinct from indigenous N (δ15N = −33%o). While large excesses at 128Xe and 129Xe are observed in the lunar regolith samples, these excesses in Pesyanoe are small. On the other hand, significant 126Xe isotopic excesses, comparable to relative excesses observed in lunar soils and breccias, are prominent in the intermediate temperature steps of Pesyanoe‐90,1.

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history, exposure history, and pre‐atmospheric size

Abstract— We measured the concentrations of the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of 10Be, 26Al, and 41Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the 22Ne/21Ne ratio at very high shielding. The 21Ne/10Be and 21Ne/26Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of 21Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture 36Ar in most samples are consistent with measured concentrations of neutron‐capture 36Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture 36Ar were found in those samples with an excess of 21Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced 21Ne and neutron‐capture‐produced 36Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic 4He and 40Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.

On the loss of [formula omitted] from muscovite during polymetamorphism

Recent laser studies have documented 40Ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years. A fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or, alternatively, loss during chemical reactions and deformation. New data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40Ar via lattice diffusion during overprinting events than has been inferred previously. Kyanite-muscovite assemblages in Proterozoic schist along the eastern flank of the Green Mountain massif, central Vermont Appalachians, formed during Proterozoic metamorphism at ca. 1.1 Ga and were remetamorphosed in the greenschist facies during both Taconian (ca. 465 Ma) and Acadian (ca. 390 Ma) events. The overprinting events resulted in deformation of earlier kyanite and muscovite, growth of chemically and isotopically distinct rims on earlier muscovite, and growth of neoblastic muscovite and chloritoid, with each event at inferred minimum temperatures of ca. 425°C. Laser spot analyses of the Grenville muscovite range from 988 ± 11 Ma in the centers of porphyroclasts to 397 ± 7 Ma along rims. Ages younger than 750 Ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals. Parts of early-formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40Ar; laser spot analyses along the outer 100 μm of an undeformed growth face yield ages close to the timing of Proterozoic metamorphism and cooling. The present study suggests episodic heating events are ineffective for producing measurable, diffusive argon loss along the edges of large muscovite crystals (ca. 1 mm in diameter), particularly in comparison with deformation and reaction mechanisms, and are unlikely to result in complete 40 Ar 39 Ar age resetting of such crystals. The results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work.