Thermoluminescence study of ordinary chondrites by TL spatial distribution readout system

The thermoluminescence (TL) image reading technique by the TL spatial distribution readout system is improved 1) to obtain a quantitative glow curve in any part of the TL image, 2) to get fine structure of a TL image and 3) to heat a sample to a higher temperature. This technique is applied to measure the natural and artificial TL glow curves of chondrules in ordinary chondrites, ALH-77294 (H5) and ALH-77216 (L3.8). The fluctuation in the natural LT/HT (region) ratios (LT(region); photons counted in a low temperature region, HT (region); in a high temperature region) of the equilibrated chondrite ALH-77294 is small though that in the unequilibrated chondrite ALH-77216 is large. The equivalent doses of ALH-77294 and ALH-77216 can be estimated from the correlation between natural LT (region) and artificial LT (region) to be about 240krad and 16krad respectively, and are consistent with isotopic ages

Thermoluminescence characteristics and chemical compositions of mesostases in ordinary chondrites

Induced thermoluminescence (TL) images of ordinary chondrites, ALH-77214 (L3.4-3.5), Y-74191 (L3.6), ALH-77216 (L3.8) and ALH-78043 (L6), were measured by the TL spatial distribution readout system combined with a microscope and TL characteristics [peak temperature and peak width] of mesostases were analyzed. Their chemical compositions were also analyzed by an electron probe X-ray microanalyzer. We found that; (1) The mesostasis was responsible for much of the TL in the ordinary chondrites, (2) A mesostasis of normative anorthite compositions showed low peak temperature (∿90℃) and narrow width (∿65℃), while a mesostasis of normative albite compositions showed high peak temperature (∿125℃) and wide width (∿100℃), (3) A main phosphor in a low petrologic grade chondrite 3.5 was a high albite mesostasis, (4) Some chondrules in the same fragments of the type 3 chondrites showed no or weak TL emission and these mesostases had high normative albite. These facts suggest that in type 3 ordinary chondrites; (1) The post-accretional metamorphism cannot account for the coexistence of high albite mesostases with TL emission and no emission and a high anorthite mesostasis with TL emission, (2) Low petrologic grade chondrites 3.5 have a large population of slowly cooled chondrules

Red thermoluminescence of enstatite from the Chainpur meteorite

For most ordinary chondrites feldspar is mainly responsible for thermoluminescence [TL], but in type 3 ordinary chondrites, especially those which are most primitive, other minerals are important. We observed red TL with a ∿660nm spectral peak in an ordinary chondrite, Chainpur (LL3.4). The mineral responsible for the red TL was identified as iron-free enstatite. Spatial distribution of TL and cathodoluminescence [CL] for the same specimen was also investigated, and it was found that the red TL areas corresponded to the high-sensitivity areas of red CL

Thermoluminescence of chondrules in primitive ordinary chondrites,Semarkona and Bishunpur

The spatial distribution of the induced thermoluminescence (TL) and TL glow curves of the primitive ordinary chondrites, Semarkona (LL3.0) and Bishunpur (LL3.1), were investigated over a wide range of wavelengths using a TL spatial distribution readout system. Although bulk samples of Semarkona and Bishunpur have very low TL sensitivity, individual chondrules show a wide variety of induced TL intensity and glow curve shape. Chondrules with anorthite-normative mesostases have especially high induced TL intensity, and their TL is produced at wavelengths >480nm, compared with <480nm for the sensitivity range of the usual TL measuring systems. Some of the metamorphism-dependent TL sensitivity of type 3 ordinary chondrite therefore results in changes in the spectrum of the light produced

Silica phase as a thermoluminescence phosphor in ALH-77214 (L3.4)chondrite

The induced thermoluminescence (TL) images for a slice sample of ALH-77214 (L3.4) have been measured to examine TL phosphors in type 3 ordinary chondrites with low TL sensitivity, using a TL spatial distribution readout system combined with microscope. The chemical compositions of TL phosphors were analyzed by X-ray microanalyzer. The TL emissions due to silica phases have been newly observed in two pyroxene chondrules with silica inclusions (SiO_2=97-99wt%) and one porphyritic olivine chondrule with very silica-rich mesostasis (SiO_2=84wt%), in addition to those due to feldspar crystals formed in chondrule mesostases enriched in plagioclase component. The glow curves of the silica phases (the peaks around 240-280℃) are quite different from those of the usual chondrule mesostases (the peaks around 80-120℃). We have tried to utilize glow curves of induced TL for five silica phases (hydrothermal quartz, volcanic quartz, tridymite, cristobalite and silica glass). From similarities of the shape of glow curve, the silica phase was tentatively identified to be cristobalite. Utilizing silica phases as a common TL phosphor, we can make a comparative study of TL characteristics among chondritic meteorites