Induced Thermoluminescence Properties of Forsterite and Implications for the History of Primitive Solar System Materials

Introduction: As a primary refractory condensate, forsterite represents an important constituent of many primitive solar system materials such as nebular dust, dust associated with galaxies, and cometary dust [1-11]. Preliminary studies on micrometeorites and the finegrained matrix of Semarkona, a primitive ordinary chondrite, indicate that forsterite is an important component of these materials [12, 13]. We have measured the TL properties of a suite of terrestrial forsterites in order to better understand the TL properties of extraterrestrial materials in which this mineral is thought to be present. Samples and Experimental Procedures: Seven samples were supplied by the Smithsonian Institution and a commercial source representing both igneous and metamorphic formation environments. Our TL procedures are those used for many years to study extraterrestrial samples. The TL measurements were performed with a modified Daybreak Nuclear and Medical systems apparatus. An EMI 9635QB photomultiplier tube with heat filters and photon-counting electronics produced a signal-to-noise ratio in excess of 10 for the samples in the present study. Two or three separate fragments from each forsterite sample were crushed with mortar and pestle to ~100 μm so the powder would pour and not clump. Each sample was drained of any natural TL by heating briefly to ~500°C, and then the TL signal induced by a five-minute exposure to a 200 mCi 90 Sr beta radiation source was measured. The induced TL measurements were repeated three times. As with previous studies, we measured the maximum light produced (which is normalized to 4 mg of the H3.9 ordinary chondrite Dhajala and termed “TL sensitivity”), the temperature at which TL production is a maximum (the “peak temperature”), and the temperature range of the full-width at half-maximum of the TL peak (the “peak width”). Monitoring of black body curves enabled us to ensure the stability of the equipment. Results: Our TL data are given in Table 1. Three of the forsterites from igneous regions have TL sensitivities similar to those of the metamorphic regions, but the San Carlos forsterite is a factor of two higher. The unknown Arizona sample shows some heterogeneity, but most of the data are in the lower range. Figure 1 shows a plot of TL sensitivity against peak temperatures and peak widths; the data cluster fairly tightly; there is also an indication that both increase with increasing TL sensitivity. Figure 2 shows a comparison of TL peak temperatures with TL peak widths. Not only do we see a Fig. 1. TL sensitivity values compared with TL peak width (a) and TL peak temperature (b). In both cases, the forsterites representing the two formation environments (metamorphic-open symbols, volcanic-filled symbols) plot in relatively tight clusters. Table 1. TL data for seven forsterite samples†.