The concept of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. CRE ages have implications for several interrelated questions. From how many different parent bodies do meteorites come? How well do meteorites represent the population of their source region? How many distinct collisions on each parent body have created the known meteorites of each type? How often do parent bodies collide? How big and how energetic were the collisions that produced meteoroids? What factors control the CRE age of a meteorite and how do meteoroid orbits evolve through time? This chapter touches on these questions as it examines the data. In recent years, CRE ages also have become important for interpreting small variations in the abundances of stable isotopes.

Cosmic-Ray Exposure Ages of Meteorites

1.13 – Cosmic-Ray Exposure Ages of Meteorites

The Zaklodzie meteorite was found in September 1998, about 40 km west of Zamośc, in southeast Poland. Macroscopic and microscopic observations (in transmitted and reflected light), microprobe analyses, cathodoluminescence images, and X-ray diffraction data show that the meteorite is composed of clino- and orthoenstatite, two generations of feldspars, relict olivine (forsterite), a polymorph of SiO2 (apparently cristobalite), and opaque minerals: Fe-Ni alloy (kamacite and taenite), troilite, schreibersite, graphite, and sulfide (Mg, Mn, Fe)S, which is probably keilite. The texture is fine- to inequigranular of cumulate type, locally intergranular. The MgS-FeS thermometer indicates that the sulfides crystallized at ~580-600 oC. Thus, the Zaklodzie meteorite formed by the nearly complete melting of an enstatite chondrite protolith, probably at ~4.4 Ga; the process was likely caused by the decay of the 26Al nuclide in the planetesimal interior. The second stage of its evolution, which could have happened at ~2.1 Ga, involved partial re-melting of most fusible components, probably due to collision with another body. The structure, composition, and origin of the meteorite and its relation to the parent rock indicate that Zaklodzie may represent a primitive enstatite achondrite.

/pdf/the-zaklodzie-enstatite-meteorite-mineralogy-petrology-2hbtey89zx.pdf

The Zakłodzie enstatite meteorite: Mineralogy, petrology, origin, and classification

This thesis investigates the processes and environments of formation of the anomalous enstatite achondrites NWA 4301, Zakłodzie, and NWA 8173 to determine how they relate to other enstatite meteorites (chondrites, impact melt rocks, aubrites, and so-called “primitive enstatite achondrites”). Observations of Zakłodzie and NWA 4301 showed that both meteorites may share a common parent body, but have experienced different metamorphic conditions. Different zonation patterns in plagioclase, silica polymorphs and associations, and sulfide assemblages indicate that Zakłodzie experienced higher metamorphic temperatures and cooled faster than NWA 4301, therefore NWA 4301 was buried deeper in the parent body relative to Zakłodzie. In NWA 8173, mineral assemblages and textures indicate that it formed at high temperatures, and underwent subsolidus annealing. Detailed mineral, chemical and structural classification of fluorophlogopite in NWA 8173 was made, to determine the role of halogens in formation mechanisms of enstatite meteorites. Overall findings showed that incipient partial melting and annealing following impact melting can each form the anomalous enstatite meteorites. However, a model is proposed where halogens can act as a fluxing agent in the melt as represented by NWA 8173 and can cause a thermally metamorphosed rock such as Zakłodzie and NWA 4301 to melt, fractionally crystallize and ultimately differentiate.

Enstatite achondrites as indicators of processes and environments in the early solar system

We measured concentrations and isotopic ratios of noble gases in enstatite (E) chondrites Allan Hills (ALH) 85119 and MacAlpine Hills (MAC) 88136. These two meteorites contain solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated heliocentric distances, parent body exposure ages, and space exposure ages of the two meteorites. The parent body exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH 85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136. Derived heliocentric distances indicate the locations of parent bodies in the past when constituents of the meteorites were exposed to the Sun. From the mineralogy and chemistry of E chondrites, it is believed that E chondrites formed in regions within 1.4 AU from the Sun. The heliocentric distances of the two E chondrite parent bodies are not different from the formation regions of E chondrites. This may imply that heliocentric distances of E chondrites have been relatively constant from their formation stage to the stage of exposure to the solar wind.

/pdf/cosmic-ray-exposure-age-and-heliocentric-distance-of-the-4o24dqx5v4.pdf

Cosmic-ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

Introduction: In two recent studies, the noble gas records of 57 enstatite chondrites (E-chondrites) are presented [1,2]. Here, we report the rare gas content of one new (SAH00225) and three so far undetermined Echondrites recovered in Antarctica (ALHA81260, MAC88180, MAC88184) (tab. 1). Both meteorites from the Mac Alpine Hills are EL3s and have been considered to be paired with MAC88136 [3,4]. In accordance with those findings, all the three of them show the same mineralogical type [5] and also reveal clear Cr excesses [6]. However, another work concluded on the basis of textural differences that they are not paired [7]. An additional well-established parameter to prove pairing is the noble gas record of meteorites. The noble gas concentrations and isotopic ratios of MAC88136 have been reported in [1,2]. Here, we present the rare gas content of MAC88180 and 88184. The other two E-chondrites investigated in this study are ALHA81260 and SAH00225, both EL6s. Additionally, we determined the noble gas record of Zaklodzie, an unusual enstatite meteorite that was found in Poland recently [8]. We will compare it with our rare gas results for Itqiy to be presented at this conference as well [9]. Itqiy is also classified as an unusual enstatite meteorite and was recovered from the Sahara in 2000 [10]. Results and discussion: MAC88180+88184. MAC88180 clearly exhibits solar noble gases and all isotopic ratios closely resemble those of MAC88136. Its Ne cosmic-ray exposure age (T21) is 2.8 Ma, compared to 3.7 Ma of MAC88136. MAC88184 reveals a similarly low T21 of 2.7 Ma. The noble gases of this meteorite, however, are not dominated by the solar component. He and Ar are basically radiogenic or trapped whereas the isotopic composition of Ne plots on a mixing line of the cosmogenic and solar signature. ALHA81260 and SAH00225. These two meteorites contain no solar rare gases but show a subsolar component. T21 are 30.6 Ma and 34.6 Ma, respectively. According to [2], the noble gas content of Echondrites leads to two different patterns of trapped heavy noble gases depending on the petrologic type (1). All E-chondrites of type 4 to 6 contain a mixture of trapped planetary rare gases (Q) and a subsolar component (average elemental ratios for EL6s are Ar/Xe = 990 ± 58 and Ar/Kr = 368 ± 16). In contrast to that, E3-chondrites, if solar gas free, show a mixture of Q and a so-called sub-Q signature with lower elemental ratios (Ar/Xe = 23.1 ± 4.1 and Ar/Kr = 34 ± 17). Trapped solar noble gases only occur in type 3 E-chondrites. These data are supported by the new results with a subsolar component in ALHA81260 and SAH00225, a solar signature in MAC88180 and a sub-Q pattern in MAC88184, which is mainly solar gas free. Zaklodzie. Unlike Itqiy, Zaklodzie exhibits radiogenic He and Ar as well as essentially Q-type trapped noble gases. Its T21 of about 77 Ma as well as its Xe concentration are considerably higher than the corresponding values of Itqiy. The only similarity between Zaklodzie and Itqiy is the cosmogenic Ne signature.

New Noble Gas Data of Four Enstatite Chondrites and Zaklodzie

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New Noble Gas Data of Four Enstatite Chondrites and Zaklodzie