On the origins of trapped helium,neon and argon isotopic variations in meteorites—II. Carbonaceous meteorites

Data are presented from stepwise heating experiments on five carbonaceous chondrites: Alais, Ivuna, Orgueil, Cold Bokkeveld and Nogoya. The data indicate the presence of two isotopically distinct major trapped components, components A and B. Two additional components, ‘C’ and ‘E,’ were isolated on the basis of neon isotopic structure. Components B and C, found in. gas-rich meteorites, are attributed to ion implantation by the present day solar wind and solar flares respectively. Component A is interpreted as a mixture of component E and component D (Paper I), where component D is identified with the primitive solar wind and component E is assigned an extra solar system origin.     

Trapped extraterrestrial argon in meteorites

The article considers different points of view on the genesis of extraterrestrial trapped argon with a composition different from primordial/solar in meteorites and lunar rocks. An alternative hypothesis of the origin of this component is discussed.     

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).     

Helium and argon isotopes in rocks and minerals of the Lovozero Alkaline Massif

The contents and ratios of helium and argon isotopes were studied in rocks of the Lovozero Massif and related rare-metal (loparite) deposits. The gases were extracted by melting (from whole-rock and mineral samples) and crushing (mainly from fluid inclusions) methods. The wide variations in the He and Ar isotopic compositions can be explained by the fact that the trapped fluid represents a mixture of variable proportions of mantle, crustal, and atmogenic components and radiogenic in situ produced gas. The obtained gas-geochemical data reflect the complex evolution of the considered ore-magmatic system and the similar trends of melt evolution and complementary fluid phase in the magmatic chamber, in general, in three-rock (urtite-foyaite-lujavrite) units and, in each individual layers, the relative closeness of the system during magmatic crystallization and initial epimagmatic processes. It was also found that the earliest magmatic mineral was loparite and that ore units and mineralization could be partially transformed during a comparatively late postmagmatic stage. An important role of paleometeoric waters in the low-temperature mineral formation was shown.     

Uranium in the silicate inclusions of stony-iron and iron meteorites

The microdistribution of U has been studied, using fission track techniques, in eleven mesosiderites, seven pallasites and four iron meteorites with silicate inclusions. When concentrated, U is usually found in phosphates: merrillite and/or chlorapatite. As in stony meteorites, the U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also exhibit variations in the same meteorite. Uranium is found to be concentrated in merrillite (0.25 to 1.43 ppm) in all the mesosiderites except Bondoc where none was observed. No U-rich phase was identified in six of the seven pallasites. In the seventh, Marjalahti, there are merrillite grains with concentrations ranging from 0.06 to 0.14 ppm. Where observed, the phosphates from silicate inclusions in the irons appear to have U concentrations similar to the mesosiderites.     

Chemical fractionation in iron meteorites and its interpretation

Published analyses of trace and minor elements in iron meteorites have been compiled and the distributions interpreted with the chemical groups defined by Wasson. When each element is plotted against Ni on log scales, groups are often clearly resolved with all the members of a group falling within the limits of sampling and analytical error on a straight line. The lines for groups IIIa,b and IVa are generally parallel with IIa,b plotting on a steeper gradient. In contrast to Ga and Qe, many elements show variations within a group which may approach that shown by all the iron meteorites. Group I members have a fairly uniform concentration of elements which are severely fractionated in the other major groups. There are also fewer correlations of elements in group I.     

Helium and argon isotopes in lower crustal xenoliths from the belomorian mobile belt

The isotopic systematics of noble gases (He and Ar) were studied in Neoarchean and Paleoproterozoic lower crustal xenoliths from the Belomorian mobile belt. The xenolith suite is dominated by garnet granulites (Grt + Cpx + Pl ± Opx ± Qtz ± Kfs ± Phl ± Hbl) and two-pyroxene or garnet pyroxenites (Cpx + Pl ± Grt ± Opx ± Hbl ± Qtz). The xenoliths and the host Devonian ultramafic lamprophyres forming diatremes and explosion dikes contain fluid with similar He and Ar isotopic compositions. It was found that the fluid was trapped by the rocks and xenolith minerals approximately simultaneously with the formation of the lamprophyres. This conclusion is based on the identical K-Ar ages of the majority of xenoliths and ultramafic lamprophyres. When the xenoliths were transported toward the surface by high-temperature ultrabasic melts, the noble gases occurring in them were partly (Ar) or completely (He) lost. The melts were contaminated by meteoric waters during their emplacement in the upper crust, which resulted in that the Ar isotopic composition of lamprophyres approached the composition of atmospheric Ar. The fluid phase that was liberated during melt crystallization severely affected the xenoliths, diminishing the difference between the isotope compositions of He and Ar in the xenoliths and ultramafic lamprophyres. The He isotope composition includes an admixture of mantle ³He, which is suggested by the high measured ³He concentrations, exceeding the calculated values, and high ³He/³⁶Ar ratios in the xenoliths and their host lamprophyres. The fraction of the mantle component in the fluid trapped by the xenoliths was estimated as ～20%.     

Helium and neon isotope geochemistry of some ground waters from the Canadian Precambrian Shield

Ground waters in a Precambrian granitic batholith at the Whiteshell Nuclear Research Establishment (WNRE) in Pinawa, Manitoba contain between 5 × 10^?5 and 10^?1 cc STP/g_H2O of radiogenic helium-4 but have relatively uniform ³He/⁴He ratios of between 0.6 × 10^?8 and 2.3 × 10³. The highest helium samples also contain radiogenic ^21,22Ne produced by (α,n) or (n,α) reactions with other isotopes. As much as 1.8 × 10^?9ccSTP/g_H2O of excess ²¹Ne and 3.8 × 10^?9ccSTP/gH₂O of excess ²²Ne have been measured. Helium and ²¹Ne ages of these ground waters, calculated on the basis of known crustal production rates of ⁴He and ²¹Ne, are unreasonably high (up to 2 × 10⁵ years) and incompatible with the ¹⁴C ages and other isotopic and hydrogeologic data. Uranium enrichment in the flow porosity of the granite may dominate ⁴He and ^21,22Ne production in this granite and mask the contributions from more typical U and Th concentrations in the rock matrix.At the Chalk River Nuclear Laboratories in Ontario helium concentrations in ground waters in a Precambrian monzonitic gneiss range from 1.5 × 10^?7 to 8.7 × 10^?4ccSTP/g_H2O with the ³He/⁴He ratios ranging from 2.0 × 10^?3 to 1.5 × 10^?7. The highest helium concentrations may be attributable to the presence of a thick uraniferous pegmatite vein and yield helium ages more than two orders of magnitude higher than the ¹⁴C ages. Application of He age dating equations to ground waters from Precambrian granitic rocks requires knowledge of the nature of uranium and thorium enrichment in the subsurface in order to select appropriate values for porosity and uranium and thorium concentration in the rock.     

He,Ne and Ar isotopes in inclusions of some iron meteorites

He³, He⁴, Ne²¹ and Ar³⁸ contents were determined in 18 metal, troilite, sehreibersite and graphite inclusions of 9 iron meteorites, by total outgassing and stepwise heating. The $\text{He}{}^4\text{He}{}^3$ ratio in metal phase ranges from 3.85 to 4.65, but in non-metallic samples, from 6.70 to 30.5. The results for cosmogenic isotopes of helium, neon and argon disagree appreciably with data on accelerator-irradiated targets. It should be noted, however, that some inclusions have lost considerable amounts of gas by diffusion.Uranium contents of 22 troilite and sehreibersite samples were determined by the fission track technique. The average uranium content of troilite is 0.4-0.7 ppb. Excess He⁴ of unknown origin was observed in troilite inclusions. If one assumes that the excess He⁴ was produced by uranium decay in situ, then the apparent U-He⁴ age is at least 5.9 × 10⁹ yr.     

Pallasites—metal composition,classification and relationships with iron meteorites

Concentrations of Au, As, Co, Ga, Ge, Ir, Ni and W were determined in the metal of 28 different pallasites plus 6 which are probably paired, to help elucidate their origin. Most divide into two clusters:

     

16.

Mineralogy and petrology of silicate inclusions in iron meteorites     

T. E. Bunch  Klaus Keil  E. Olsen 《Contributions to Mineralogy and Petrology》1970,25(4):297-340

Silicate inclusions in 17 iron meteorites have been analyzed by the electron microprobe and classified, according to their phase assemblages, compositions, and textures, into three major types: Odessa, Copiapo, and Weekeroo Station, and three miscellaneous types: Enon, Kendall County, and Netschaëvo. Phase compositions in both Odessa- and Copiapo-type inclusions are very similar, but the two types are different in texture and constituent phases. Weekeroo Station-type inclusions are very different in every respect from other inclusions.For Odessa- and Copiapo-type inclusions, the distribution coefficients of Fe²⁺ and Mg in coexisting orthopyroxene and clinopyroxene indicate equilibration temperatures of 1,000° C, and the Ca/(Ca+Mg) ratios indicate temperatures of 900° C to 1,000° C. Equilibration temperatures determined for chromite-olivine pairs have a higher range of 1,154° C to 1,335° C. Minor element distributions among coexisting ferromagnesian silicates in these inclusions follow consistent patterns and are constant for any given sample, suggesting equilibrium assemblages. Major and minor element distributions for Weekeroo Station inclusions are anomalous, indicating nonequilibrium.Compositional data, the fragmentary shapes of many inclusions, the highly differentiated characteristic of two types of inclusions, the apparent disequilibrium between kamacite in inclusions and kamacite of the iron host, and the relict chondrules found in Netschaëvo suggest that many of the inclusions did not form cogenetically with the iron host, but represent pre-existing stony material that was taken up by an iron melt, probably not in the core of the parent body (or bodies).     

17.

Nitrogen and xenon in acid residues of iron meteorites     

S.V.S. Murty  P.S. Goel  D.Vu. Minh  Yu.A. Shukolyukov 《Geochimica et cosmochimica acta》1983,47(6):1061-1068

Total nitrogen, measured by neutron activation analysis, is highly enriched in residues from iron meteorites obtained by dissolution of the metal in dilute H₂SO₄, relative to the bulk value. On the average, the residues, representing 3% mass, contain 22% of total N. Group IA has more dissolved N than IIIA. Lithium and Ir show a distribution pattern parallel to N. Total Xe has been measured in several residues and its isotopic composition is, similar to atmospheric Xe for mass numbers 131 to 136 but not for ¹²⁴Xe and ¹²⁶Xe which are strongly depleted in the non-magnetic residues. It is suggested that iron meteorites have trapped in their micro-inclusions, some pre-solar nebular matter which is isotopically heterogeneous.     

18.

Metallographic cooling rates of the IIIAB iron meteorites     

Jijin Yang  Joseph I. Goldstein 《Geochimica et cosmochimica acta》2006,70(12):3197-3215

An improved computer simulation program has been developed and used to re-measure the metallographic cooling rates of the IIIAB irons, the largest iron meteorite chemical group. The formation of this chemical group is attributed to fractional crystallization of a single molten metallic core during solidification. Group IIIAB irons cooling rates vary by a factor of 6 from 56 to 338 °C/My. The cooling rate variation for each meteorite is much smaller than in previous studies and the uncertainty in the measured cooling rate for each meteorite is greatly reduced. The lack of correction for the orientation of the kamacite-taenite interface in the cooling rate measurement of a given meteorite in previous studies not only leads to large cooling rate variations but also to inaccurate and low cooling rates. The cooling rate variation with Ni content in the IIIAB chemical group measured in this study is attributable, in part, to the variation in nucleation temperature of the Widmanstatten pattern with Ni content and nucleation mechanism. However, the factor of 6 variation in cooling rate of the IIIAB irons is hard to explain unless the IIIAB asteroidal core was exposed or partially exposed in the temperature range in which the Widmanstatten pattern formed. Measurements of the size of the island phase in the cloudy zone of the taenite phase and Re-Os data from the IIIAB irons and the pallasites make it hard to reconcile the idea that pallasites are located at the boundary of the IIIAB asteroid core.     

19.

The isotopic composition of ‘graphitic’ carbon from iron meteorites and some remarks on the troilitic sulfur of iron meteorites     

Peter Deines  Frans E. Wickman 《Geochimica et cosmochimica acta》1973,37(5):1295-1319

     

20.

东天山黄土坡铜锌矿床成矿流体的氦-氩同位素示踪          下载免费PDF全文

柴凤梅  许强奋王  雯  王海培 《地质科学》2017,(4):1263-1281

黄土坡铜锌矿床是东天山卡拉塔格地区一中型海相火山岩型块状硫化物（VHMS）矿床,赋存于早志留世海相火山-沉积岩系中。矿体产于酸性火山角砾岩与凝灰岩之间,主矿体呈厚大的扁豆状,矿石主要呈块状、浸染状、条带状和网脉状构造。本文报道了黄土坡铜锌矿床成矿流体的氦和氩同位素组成,黄铁矿流体包裹体的³He 含量低,介于0.039×10^-12 cm³ STP/g～0.64×10^-12 cm³ STP/g,n（³He）/n（⁴He）值为 0.11～0.94 Ra,介于壳源He与幔源He之间。⁴⁰Ar 含量变化于 3.49×10^-8cm³ STP/g～34.25×10^-8 cm³ STP/g 之间,n（⁴⁰Ar）/n（³⁶Ar）介于301.40～425.98 之间。成矿流体中有幔源 He（1.71%～10.97%）和放射性成因 Ar（⁴⁰Ar*含量为1.96%～30.63%）。结合矿床包裹体及 H-O-S 同位素特征,认为幔源流体和富放射性成因He 和Ar 的海水参与了成矿。     

	No.	Ni (%)	Ga (μg/g)	Ge (μg/g)	Au (μg/g)	Fa (mole %)
Main group	19	7.8–11.7	16–26	29–65	1.7–3.0	11–13
Eagle Station trio	3	14–16	4.5–6	75–120	0.8–1.0	19–20

Mineralogy and petrology of silicate inclusions in iron meteorites

Silicate inclusions in 17 iron meteorites have been analyzed by the electron microprobe and classified, according to their phase assemblages, compositions, and textures, into three major types: Odessa, Copiapo, and Weekeroo Station, and three miscellaneous types: Enon, Kendall County, and Netschaëvo. Phase compositions in both Odessa- and Copiapo-type inclusions are very similar, but the two types are different in texture and constituent phases. Weekeroo Station-type inclusions are very different in every respect from other inclusions.For Odessa- and Copiapo-type inclusions, the distribution coefficients of Fe²⁺ and Mg in coexisting orthopyroxene and clinopyroxene indicate equilibration temperatures of 1,000° C, and the Ca/(Ca+Mg) ratios indicate temperatures of 900° C to 1,000° C. Equilibration temperatures determined for chromite-olivine pairs have a higher range of 1,154° C to 1,335° C. Minor element distributions among coexisting ferromagnesian silicates in these inclusions follow consistent patterns and are constant for any given sample, suggesting equilibrium assemblages. Major and minor element distributions for Weekeroo Station inclusions are anomalous, indicating nonequilibrium.Compositional data, the fragmentary shapes of many inclusions, the highly differentiated characteristic of two types of inclusions, the apparent disequilibrium between kamacite in inclusions and kamacite of the iron host, and the relict chondrules found in Netschaëvo suggest that many of the inclusions did not form cogenetically with the iron host, but represent pre-existing stony material that was taken up by an iron melt, probably not in the core of the parent body (or bodies).     

Nitrogen and xenon in acid residues of iron meteorites

Total nitrogen, measured by neutron activation analysis, is highly enriched in residues from iron meteorites obtained by dissolution of the metal in dilute H₂SO₄, relative to the bulk value. On the average, the residues, representing 3% mass, contain 22% of total N. Group IA has more dissolved N than IIIA. Lithium and Ir show a distribution pattern parallel to N. Total Xe has been measured in several residues and its isotopic composition is, similar to atmospheric Xe for mass numbers 131 to 136 but not for ¹²⁴Xe and ¹²⁶Xe which are strongly depleted in the non-magnetic residues. It is suggested that iron meteorites have trapped in their micro-inclusions, some pre-solar nebular matter which is isotopically heterogeneous.     

Metallographic cooling rates of the IIIAB iron meteorites

An improved computer simulation program has been developed and used to re-measure the metallographic cooling rates of the IIIAB irons, the largest iron meteorite chemical group. The formation of this chemical group is attributed to fractional crystallization of a single molten metallic core during solidification. Group IIIAB irons cooling rates vary by a factor of 6 from 56 to 338 °C/My. The cooling rate variation for each meteorite is much smaller than in previous studies and the uncertainty in the measured cooling rate for each meteorite is greatly reduced. The lack of correction for the orientation of the kamacite-taenite interface in the cooling rate measurement of a given meteorite in previous studies not only leads to large cooling rate variations but also to inaccurate and low cooling rates. The cooling rate variation with Ni content in the IIIAB chemical group measured in this study is attributable, in part, to the variation in nucleation temperature of the Widmanstatten pattern with Ni content and nucleation mechanism. However, the factor of 6 variation in cooling rate of the IIIAB irons is hard to explain unless the IIIAB asteroidal core was exposed or partially exposed in the temperature range in which the Widmanstatten pattern formed. Measurements of the size of the island phase in the cloudy zone of the taenite phase and Re-Os data from the IIIAB irons and the pallasites make it hard to reconcile the idea that pallasites are located at the boundary of the IIIAB asteroid core.     

东天山黄土坡铜锌矿床成矿流体的氦-氩同位素示踪

黄土坡铜锌矿床是东天山卡拉塔格地区一中型海相火山岩型块状硫化物（VHMS）矿床,赋存于早志留世海相火山-沉积岩系中。矿体产于酸性火山角砾岩与凝灰岩之间,主矿体呈厚大的扁豆状,矿石主要呈块状、浸染状、条带状和网脉状构造。本文报道了黄土坡铜锌矿床成矿流体的氦和氩同位素组成,黄铁矿流体包裹体的³He 含量低,介于0.039×10^-12 cm³ STP/g～0.64×10^-12 cm³ STP/g,n（³He）/n（⁴He）值为 0.11～0.94 Ra,介于壳源He与幔源He之间。⁴⁰Ar 含量变化于 3.49×10^-8cm³ STP/g～34.25×10^-8 cm³ STP/g 之间,n（⁴⁰Ar）/n（³⁶Ar）介于301.40～425.98 之间。成矿流体中有幔源 He（1.71%～10.97%）和放射性成因 Ar（⁴⁰Ar*含量为1.96%～30.63%）。结合矿床包裹体及 H-O-S 同位素特征,认为幔源流体和富放射性成因He 和Ar 的海水参与了成矿。