Ｌｕｍｉｎｅｓｃｅｎｃｅ ｄａｔｉｎｇ ｏｆ Ｑｕａｔｅｒｎａｒｙ ｖｏｌｃａｎｉｃ ｒｏｃｋｓ ｕｓｉｎｇ ｏｌｉｖｉｎｅ. P-H4. TAKADA Masashi (Nara Women ’ s University, Japan : [email protected] ）.
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Ｌｕｍｉｎｅｓｃｅｎｃｅ ｄａｔｉｎｇ ｏｆ ＱｕａｔｅｒｎａｒｙＬｕｍｉｎｅｓｃｅｎｃｅ ｄａｔｉｎｇ ｏｆ Ｑｕａｔｅｒｎａｒｙ
ｖｏｌｃａｎｉｃ ｒｏｃｋｓ ｕｓｉｎｇ ｏｌｉｖｉｎｅ
TAKADA Masashi(Nara Women’s University, Japan : [email protected]）
1. INTRODUCTIONIn Luminescence dating, the principal minerals mostly used are quartz and feldspar. However, tephra and lava originated from mafic magma seldom contain quartz and volcanic feldspar often shows anomalous fading (Aitken, 1985). It practically inhibits luminescence dating.
Olivine ([Mg,Fe]2SiO4) is one of the common minerals included in igneous rocks such as basalt, diabase and gabbro which seldom contain quartz. Olivines vary in chemical composition from forsterite (Fo100; Mg2SiO4) to fayalite (Fo0; Fe2SiO4), there being complete diadochy between Mg2+ and Fe2+ in the structure (Deer et. al., 1966). Though Koike et al. (2002) clarified some luminescence properties of forsterite as interstellar matter within the low temperature environment, little had been known about ones of natural olivine from geological sites. Therefore Takada et al. (2006) preliminarily studied on TL and OSL behavior of olivine relative to dosimetry and Cenozoic dating.
In this presentation, the author reports the practical application of luminescence from natural olivine to Quaternary dating of volcanic rocks.
Table 1. Relationship between depth (μｍ）of removed outer layer and residual volume/weight (%) of an olivine crystal
2. TL MEASUREMENTTL measurements were made on an automated RISO TL/OSL-DA15 reader mounted with an integral 90Sr-90Y beta source. Photon counting used a Thorn-EMI 9235 QA photomultiplier tube coupled to HA-30 and BG7-59 detection filters (290-480 nm transmission) for TL. Optical stimulation were performed using blue LEDs (475±45 nm, power of 15 mW/cm to the sample).
Fig. 1 Relationship between etching time (minutes) and residual weight (%) of olivine specimens from Arizona, U. S. A.
1)Immersion in 10% hydrofluoric acid
3. SAMPLE PREPARATION Firstly I used specimens of pure olivine crystals from Arizona, U.S.A. to check etching condition (Figures 1 and Table 1 ) and internal dose. Secondly I tried to extract olivine crystals from volcanic rocks in the Hakkoda-san district, northern Japan (Figures ２, 4 and Tables 2, 3). The procedures of the sample preparation and TL measurements are shown in Figures ３ and 5.
4. RESULTS AND DISCISSIONS Figure 1and Table 1suggest immersion in 10% hydrofluoric acid for about 30 minutes allows etching of the outer layer of the olivine grains to a depth sufficient for the cores remaining to have a negligible component of alpha particle dosage. Alpha counting of the olivine specimen suggests the internal dose from olivine is negligible. Modified SAR protocol for TL measurements of olivine extracted from Ohsegawa pyroclastic flow deposits give TL ages but they are disperse and underestimated (Figs. 5, 6, 7 and Table 4). The reasons are unknown at present but further studies on sensitivity change and fading behaviour of olivine are needed.
Fig. 3. Sample preparation
Fig. ２. Location of volcanicsamples
Fig. 4. Extracted olivine grains from Ohsegawa pyroclastic deposits
Table２. Samples in this study
Fig. 5. Modified SAR protocol for TL measurement
Table 4. TL ages of Ohsegawa pyroclastic flow deposits.
Fig. 7. Regenerated growth curve of olivine extracted from Ohsegawa pyroclastic rocks.
・ Aitken, M.J., 1985. Thermoluminescence dating. Academic Press, 359pp.
・ Deer, W.A., Howie, R.A. and Zussman, T., 1966. An introduction to the rock forming minerals. Longman, 528pp.
・ Koike, K., Nakagawa, M., Koike, C., Okada, M. and Chihara, H., 2002. Thermoluminescence of Simulated Interstellar Matter after Gamma-ray Irradiation. Astronomy & Astrophysics, 390, 1133-1139.
・ Takada, M., Tani, A. and Shimada, A., 2006. Preliminary study of the application of natural olivine in Cenozoic dating. Radiation Measurements, 41, 982-986.