TY  - GEN
N2  - Noble gases in meteorites contain a ?cosmogenic? component which is due to their exposure to cosmic rays. Cosmogenic noble gases (like radionuclides) are generally produced during the time a meteoroid travels from its parent body to encounter with earth and define its cosmic ray exposure (CRE) age. However, cosmogenic noble gases may also be produced at an earlier stage (?pre-irradiation?). Noble gases in chondrules, in particular, may provide information about the energetic particle environment and the lifetime of chondrules as free-floating objects in the early solar system in case they were pre-irradiated in the solar nebula and about surface processes on asteroids in the case of a pre-irradiation in the parent body regolith. Therefore, in this study noble gases were determined to search for possible pre-irradiation effects in chondrules and matrix samples from nine carbonaceous chondrites, including the CR2 chondrites Renazzo, El Djouf 001 and NWA 852, the CR3 chondrites MET 00426 and QUE 99177, the CV3 chondrites Allende, Vigarano and Acfer 082 as well as ungrouped Acfer 094.
After the separation of chondrules from the bulk meteorites using a freeze-thaw technique, abundances of important target elements were determined using instrumental neutron activation analysis (INAA). Noble gas measurements (He, Ne, Ar) were performed on the same sample material, which eliminates problems of sample inhomogeneity which were a problem in other earlier studies. CRE ages were calculated with 21Nec production rates calculated following Eugster (1988). In addition, mineralogical analyses of chondrules using a scanning electron microscope (SEM) were performed, and 40K-40Ar ages of chondrules and matrix samples were determined. In this study so-called ?nominal? CRE ages with the shielding parameter (22Ne/21Ne)c = 1.11 were used. Although this ?nominal? CRE ages are not accurate, this approach is most reliable for the comparison of the ages of different samples from the same location because uncertainties involved in the shielding correction were avoided. Ages were calculated using cosmogenic 21Ne. T21 CRE ages are most reliable because He is easily affected by gas loss and Ar is mostly influenced by trapped components in primitive meteorites. CRE ages for Ne with ?normal? shielding are indicated as T21-1.11 CRE ages.
Strong evidence for pre-irradiation effects was found in all chondrules from El Djouf 001 and NWA 852, which have elevated T21-1.11 CRE ages relative to the matrix. While the age difference of El Djouf 001 chondrules ranges between 0.4 and 1.9 Ma relative to the matrix, the two analyzed NWA 852 chondrules have nominal CRE ages that are 27.1 and 32.9 Ma longer than the respective matrix age, which is comparable with values for Murchison (CM2) chondrules reported in the literature. However, only 20% of Murchison chondrules show pre-irradiation effects (Roth et al. 2011, Huber et al. 2012). The differences concerning the percentage of pre-irradiation and duration of pre-irradiation may suggest different processes in the respective parent body regoliths. However, while El Djouf 001 is clearly different, the situation for NWA 852 may be similar to Murchison based on the large T21-1.11 CRE differences. Although pre-irradiation was observed in all chondrules from NWA 852, no reliable conclusion can be drawn concerning the percentage of pre-irradiated chondrules because only a small number (2) was analyzed.
Some of the chondrules from MET 00426, QUE 99177, and Acfer 082 show apparent pre-irradiation effects since they have elevated T21-1.11 CRE ages relative to the matrix. Remarkably, however, since the MET 00426 and QUE 99177 matrices have elevated T21-1.11 CRE ages compared to at least one chondrule, not only chondrules, but also the matrix from MET 00426 and QUE 99177 appear to have been pre-irradiated. The indication for pre-irradiation of matrix material is an important new finding of this work in terms of analyzing pre-irradiation effects in primitive meteorites.
Chondrules from Allende and Vigarano show no evidence for pre-irradiation since their T21-1.11 CRE ages are indistinguishable from the matrix. In the case of Allende chondrules this is in accordance with the results from previous studies.
For Renazzo and Acfer 094 no reliable conclusion concerning pre-irradiation can be reached, because only a small number of chondrules were analyzed, which show T21-1.11 CRE ages indistinguishable from the corresponding matrix.
General reasons why a pre-irradiation of chondrules (as a whole) are difficult to discern include i) no discernibility of pre-irradiation effects in meteorites with long CRE ages, ii) the dependency of the production rate of cosmogenic noble gas nuclides on chemistry and shielding, iii) lower CRE ages of the matrix due to recoil losses, iv) noble gas loss from the bulk meteorite/matrix due to thermal metamorphism or the lower retentivity for cosmogenic gases, and vi) the superimposition on pre-irradiation records in chondrules or in the matrix by records from single grains or clasts. In addition, pre-irradiation effects in chondrules might not be expected based on the following arguments: i) the impossibility of pre-irradiation of chondrules in the early solar system due to high dust densities, ii) the limited lifetime of small objects in the inner solar system, and iii) the impossibility of pre-irradiation of chondrules on parent body surfaces (regolith). All these general difficulties and arguments could be neutralized by methodical approaches (e.g., selection of meteorites with i) short CRE ages and iv) limited thermal metamorphism; ii) use of T21-1.11 ?nominal? CRE ages without shielding correction) and/or theoretical considerations (e.g., i) redistribution of chondrules to the outer edges of the protoplanetary disk with lower dust densities, where an irradiation should have been possible; iii) possibility of size sorting in a granular medium due to the Brazil-nut effect). 
On the other hand, the observations clearly show that chondrules were pre-irradiated. These observations include i) elevated nominal CRE ages of some chondrules relative to the matrix in previous studies as well as in this study, which cannot be explained by different abundances of target elements or shielding; this pre-irradiation of chondrules is recorded by an excess of 21Nec; ii) different CRE ages of chondrules within a single meteorite, which suggests either different irradiation environments or identical irradiation and subsequent gas loss affecting some chondrules. In addition, iii) locally different regions of chondrule formation and different irradiation histories can explain these observations.
Overall, pre-irradiation of all chondrules from NWA 852 and El Djouf 001 as well as some chondrules from MET 00426, QUE 99177, and Acfer 082 is indicated. In addition, the matrices of MET 00426 and QUE 99177 show also evidence for pre-irradiation.
This study, like previous studies analyzing pre-irradiation effects in chondrules, is limited since the setting of pre-irradiation cannot be determined with certainty. Parameters like the shielding parameter (22Ne/21Ne)c, which may help to decide where the chondrules were pre-irradiated (in the regolith on the meteorite parent body vs. solar nebula setting), are not unequivocal. In case of NWA 852 and El Djouf 001 with dominant solar wind components in the matrices pre-irradiation in the parent body regolith is indicated. The considerably longer pre-irradiation and in most cases higher concentration of trapped (solar) gases of NWA 852 chondrules compared to El Djouf 001 chondrules may imply that NWA 852 chondrules resided in a near-surface position (up to 1 or 2 m depth) on the CR meteorite parent body, whereas El Djouf 001 chondrules were shielded from cosmic radiation in much deeper regions and spent less time in a near-surface position. On the other hand, the highly varying T21-1.11 CRE ages of MET 00426 and QUE 99177 chondrules may imply pre-irradiation as free-floating objects in the solar nebula. Especially for QUE 99177, which lacks solar wind gases, a pre-irradiation in the solar nebula appears highly likely.

UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/17459/
A1  - Beyersdorf-Kuis, Uta
ID  - heidok17459
TI  - Pre-irradiation of chondrules in the Early Solar System
Y1  - 2014///
AV  - public
CY  - Heidelberg
ER  -