Noble gases and meteorites

Abstract— Noble gases repeatedly have served to widen the scope of meteorite research. During the first half century of such measurements, the emphasis was on the determination of U, Th/He-gas retention ages of iron meteorites, which is the most unsuitable class of meteorites for such studies. With the realization that the He in these meteorites results from the interaction of cosmic rays with meteoritic matter, meteorites became to be used as “the poor man's space probe” that yielded information on the constancy in time and space of the cosmic radiation. Another widening of scope came with the discovery of extremely high noble gas contents in the outermost layers of the individual grains that make up stony meteorites. These gases are of solar origin; they have been implanted as low-energy solar wind (SW) or as solar energetic particles (SEP) into the grains before their compaction. Presently they offer the only opportunity to precisely measure the isotopic composition of solar matter and to learn about potential changes of the Sun in time. Stony meteorites of the “carbonaceous” variety contain “stardust” that carries the undiluted nucleosynthesis products of individual stars that yield incredibly detailed information concerning the parameters that prevailed during the synthesis.     

Implanted radiogenic and other noble gases in crustal diamonds from Northern Kazakhstan

Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of ⁴He in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that ⁴He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of ⁴He(5.6 × 10⁻⁴ cm³ STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of ²³⁸U accompanies the radiogenic ⁴He; the ¹³⁶Xe_f/⁴He ratio of (2.5 ± 0.3) × 10⁻⁹ agrees well with the production ratio of 2.3 × 10⁻⁹ expected in a reservoir where Th/U 3.3. Radiogenic ⁴⁰Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation.

³He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10⁻¹² cm³ STP/g, with a ³He/⁴He ratio of 1 × 10⁻⁸. Excess ²¹Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.     

On unfractionated solar noble gases in the H3–6 meteorite Acfer 111

Abstract Solar noble gases He, Ne, Ar and Kr implanted in the H3–6 meteorite regolith breccia Acfer 111 agree in their elemental composition with that in present-day solar wind and, except for a 25% deficit of ⁴He, also with adopted solar abundances. The presence of such unfractionated solar gases makes Acfer 111 unique (until now). Closed system stepped etching releases noble gases that can be explained as mixtures of two distinct types of He, Ne, and Kr of isotopic compositions as they have been derived previously from meteorites and lunar samples that contain heavily fractionated solar gases. Since the same putative end members, ascribed to the solar wind (SW) and supra-thermal solar energetic particles (SEP), are also present in Acfer 111, we argue that these end members represent two truly independent components. We discount the possibility that one isotopic composition derived from the other by diffusion of the gases within, or upon their release from, their host phases. The isotopic signatures of noble gases in Acfer 111 agree with those in a lunar ilmenite of young antiquity ?100 Ma) but are in disagreement with the noble gases in lunar ilmenite 79035 of 1–2 Ga antiquity. Systematic changes are discussed of the nuclide abundance ratios as etching proceeds; they are ascribed to differences in trapping efficiency and in penetration depth of the different noble gas ion species upon their implantation.     

Al-correlated26Mg excess in a large Ca-Al-rich inclusion of the Leoville meteorite

Minerals from a large (～1.5 cm diameter) Type B inclusion of the C3V-carbonaceous chondrite Leoville have been analyzed mass spectrometrically for their Mg isotopic composition.²⁵Mg/²⁴Mg ratios were found to be indistinguishable in all cases from those of terrestrial Mg while in anorthites²⁶Mg is overabundant by up to 15%. The excess²⁶Mg(=²⁶Mg*) is linearly correlated with the²⁷Al/²⁴Mg ratio, with the constant of proportionality²⁶Mg*/²⁷Al=(5.9±0.3)×10^?5. This is in agreement with values reported for Allende and another Type B inclusion from Leoville; it is at variance with the twenty times higher ratio found in Leoville hibonites.     

NOBLE GASES AND THE CLASSIFICATION OF BRACHINA

Noble-gas systematics show that Brachino is not a member of the SNC-group of meteorites. The whole-rock K-Ar gas retention age is (3.11 ± 0.07) AE as compared to the 1.3 AE solidification ages of SNCs; the content of radiogenic¹²⁹ Xe* of (3.47 ±. 15) × 10^?10 cm³ STP/g is about two orders of magnitude higher, and the¹²⁹ Xe/¹³² Xe ratio (11.0), the ratio of radiogenic¹²⁹ Xe* to fissiogenic¹³⁶ Xe_f (300), and the ratio³⁶ Ar/¹³² Xe in the trapped gases are about one order of magnitude higher than observed for SNCs. The same evidence argues strongly against any simple genetic relationship with eucrites. The noble-gas abundance pattern resembles closely that in silicate inclusions from the iron meteorites Campo del Cielo and Udei Station. Abundances of cosmic-ray produced³ He and²¹ Ne (5.7 and. 99 × 10^?8 cm³ STP/g, resp.) indicate an exposure age of ～ 2.4 Ma. Irradiation conditions appear to have been perfectly normal except for an unaccountably low content of spallogenic ³⁸Ar. Losses by diffusion of radiogenic⁴ He are severe; they must have occurred at or before the onset of the exposure of the meteoroid to the cosmic radiation.     

Comment on “The nature and origin of ureilites” by J. L. Berkley et al.

The record of primordial noble gases in ureilites imposes serious constraints on models of ureilite petrogenesis. It is argued that the inhomogeneous distribution between different mineral phases and the low ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio in particular can only be satisfied with difficulties in the model proposed by Berkley et al.