Bradly D. Keck

Chemical and physical studies of type 3 chondrites—IV: Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites

Abstract Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ⩽900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.

Chemical and physical studies of type 3 chondrites - VIII: Thermoluminescence and metamorphism in the CO chondrites

The thermoluminescence properties of nine CO chondrites have been measured. With the exception of Colony and Allan Hills A77307 (ALHA 77307), whose maximum induced TL emission is at approximately 350°C, CO chondrites exhibit two TL peaks, one at 124 ± 7°C (130°C peak) and one at 252 ± 7°C (250°C peak). The 130°C peak shows a 100-fold range in TL sensitivity (0.99 ± 0.21 for Isna to 0.010 ± 0.004 for Colony), and correlates with various metamorphism-related phenomena, such as silicate heterogeneity, metal composition and McSweens metamorphic subtypes. The peak at 250°C does not show these correlations and, Colony excepted, varies little throughout the class (0.3 to 0.07, Colony 0.018 ± 0.004). Mineral separation experiments, and a series of annealing experiments on Isna, suggest that the TL properties for CO chondrites reflect the presence of feldspar in two forms, (1) a form produced during metamorphism, and analogous to the dominant form of feldspar in type 3 ordinary chondrites, and (2) a primary, metamorphism-independent form, perhaps associated with the amoeboid inclusions. If this interpretation is correct, then the CO chondrites have not experienced temperatures above the order/disorder temperature for feldspar (500–600°C) and they cooled more slowly than comparable (i.e. type <3.5) type 3 ordinary chondrites. Colony and ALHA 77307 have atypical TL properties, including very low TL sensitivity, suggesting that phosphors other than feldspar are important. They have apparently experienced less metamorphism than the others, and may have also been aqueously altered.

Anomalous fading of thermoluminescence in meteorites

Abstract Anomalous fading in meteorites of diverse origin and history has been studied. The meteorites included the four known members of the shergottite class, Shergotty, Allan Hills A77005, Zagami and Elephant Moraine A79001, the type 3 ordinary chondrite Dhajala and the howardite Kapoeta. After irradiation and 15 days storage at room temperature, considerable fading of the TL of the Kapoeta meteorite and two of the shergottites (Shergotty and 77005) was observed, while the TL of the other shergottites and Dhajala remained constant. Mineral separation experiments and energy-dispersive X-ray analyses indicate that the phosphor responsible for the TL is located in the low-density feldspar rich fraction in the Dhajala meteorite and the shergottites and this is probably also true of Kapoeta. Despite the diverse history of the samples and major differences in composition, it is only the samples in which feldspar is thought to be present in the low-temperature form which display anomalous fading. It is argued that the present data are most consistent with the mechanism for anomalous fading involving overlap between the wavefunctions for the excited and ground state electrons. Fortunately, most meteorites contain feldspar in the high-temperature form and large scale studies of their natural TL should not be hampered by anomalous fading.

Chemical and physical studies of type 3 chondrites, VII. Annealing studies of the Dhajala H3.8 chondrite and the thermal history of chondrules and chondrites

Abstract Samples of the Dhajala H3.8 chondrite have been annealed for 10 hours at 600, 700, 800, 900 and 1000°C and at 1000°C for 1, 2, 20 and 100 h and their thermoluminescence (TL) properties measured. The TL sensitivity decreased by a factor of 2 after annealing at , but at higher temperatures fell by an order of magnitude. An abrupt increase in the temperature of the TL peak from 172 ± 9°Cto231 ± 8°C and a steady increase in the width of the peak from 169 ± 7°Cto212 ± 5°C were caused by the annealing treatment. The TL phosphor in Dhajala is thought to be feldspar predominantly in the high-temperature (disordered) form, but the present data indicate that a contribution from the low-temperature form is also present and that this low-temperature component is converted to the high form by the annealing treatment. The low-temperature feldspar is located in a few of the chondrules ( ∼ 20% of those separated from the meteorite) which are also noteworthy for having high TL sensitivities. These chondrules must have suffered greater crystallization of their mesostasis than the other chondrules, and equilibrated to lower temperatures. It is argued that, for compositional reasons, their mesostasis constituted less of a barrier to diffusion and therefore equilibration. Presumably the post-metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization, and the width and temperature of the TL peaks for petrologic types 3.5–3.9 are somehow related to cooling rate. Based on TL, there is no indication of a correlation between petrologic type and cooling rate for types 3.5–3.9; this is not consistent with a simple, single internally heated meteorite parent body.

Thermoluminescence and the shock and reheating history of meteorites. II - Annealing studies of the Kernouve meteorite

Abstract Samples of the unshocked, equilibrated chondrite, Kernouve (H6), have been annealed for 1–100 hours at 500–1200°C, their thermoluminescence sensitivity measured and Na, K, Mn, Ca and Sc determined by instrumental neutron activation analysis. The TL sensitivity decreased with temperature until by 1000°C it had fallen by 40%. The process responsible has an activation energy of ~8 kcal/mole and probably involves diffusion. Samples annealed 1000–1200°C had TL sensitivities 10 −2 times the unannealed values, most of the decrease occurring ~1100°C. This process has an activation energy of ~100 kcal/mole and is probably related to the melting of the TL phosphor, feldspar, with some decomposition and loss of Cs, Na and K. Meteorites whose petrography indicates healing > 1100°C by natural shock heating events (shock facies d-f). have TL sensitivities similar to samples annealed > 1100°C. Our own and literature compositional data indicate that TL is more stable to annealing than Ag, In, Tl, Bi, Zn and Te and less stable than Na, K, Mn, Ca, Se and Co, while the TL decrease resembles very closely the pattern of Cs loss on annealing.