Bleaching rate of optically stimulated luminescene signal of granite, sandstone and the significance for rock surface dating
CUI Furong;LIU Jinfeng;State Key Laboratory of Earthquake Dynamics, Institute of Geology,China Earthquake Administration;School of Earth Sciences and Resources, China University of Geosciences;
Background, aim, and scope Optically stimulated luminescence(OSL) signal has been successfully used to determine the exposure and burial age of rock surfaces and to discriminate the history rocks experienced. OSL dating of sediments determines the time elapsed since the last sunlight exposure event of the mineral(quartz or feldspar), i.e. the depositional time. Recently, this approach is used to measure luminescence signal with depth into rock surfaces that exposed to light, the valuable bleaching information on the rock surfaces can directly obtain from luminescence-depth profiles. And thus, it's important to know the IRSL signal bleaching rate of different rock lithologies, and whether the IRSL signal could be completely bleached after exposing by daylight. Materials and methods We selected reddish-brown argillaceous quartz sandstone and hoary biotite-granite as samples and designed a series of rock surface bleaching experiments. The first set of experiments was that sandstone and granite rocks were exposed to sunlight(it started in July 2016) for different periods of time(0.5 h, 3 h, 6 h, 40 h, 736 h); the second set was that these signal saturation rock slices from inner part of the rock(previously unexposed to light) were bleached under both a sunlight simulator lamp(SOL2) and daylight(in July 2016) for different periods of time(0.0083 h, 0.083 h, 0.5 h, 1 h, 3 h). All samples for luminescence measurements were taken by cutting pillars from the rock block and slices with one millimeter thickness were then cut from these pillars using a water-cooled low-speed wafering saw. For all luminescence signals, we used infrared stimulated luminescence(IRSL) signal at 50℃(IR_(50)) of potassium-rich feldspar fractions from the rock. Results The results show that the IRSL signal of rocks can be quickly reset by exposing in light(sunlight and SOL2). For the rock surface experiment, it is obvious that the rock surface residual IR_(50) luminescence signal from light-colored granite are less than dark-colored sandstone for exposing same time, and the bleaching depth of granite rocks is deeper; while rock slices bleaching under daylight or SOL2, the bleaching level of granite and sandstone rock slices is same. IRSL signal could be bleached at or near zero level after exposing 0.5 h. However, for sandstone slices exposed under SOL2 for different time, it was found the residual luminescence signal intensity from the back side of slice is obviously stronger than the front side. Discussion Godfrey-Smith et al(1998) testified that the OSL signal from quartz is easier to bleach by light. The IRSL feldspar signal from rock can also be fast bleached by light in this study. The IRSL signal is bleached in certain depth into the rock surface after a given exposure time, and the longer the bleaching time is, the deeper the bleaching depth. The IR50 luminescence signal of granite rock surface can be rapidly bleached during a short time. However, the signal from the second slice(1 — 2 mm) of sandstone rock has not been completely bleached after exposing 736 h, and the first slice has extremely low signal value. By contrast with the measurement result of rock slices, it can prove that the light could not penetrate through the whole sandstone rock slice(with 1.2 mm of thickness) with exposing for a short time and the infrared light can only penetrate a certain depth of rock surface. Conclusions The light bleaching experiment results show that the bleaching rate of light-colored granite is much higher than that of the opaque dark-colored sandstone. This study also reveals that IR50 luminescence signal is just originated from a certain depth of the slice(1.2 mm thickness) and the slice is not fully penetrated by infrared light. Recommendations and perspectives Predecessors had proved that OSL dating of rock surface is a robust and reliable dating technique. It is superior to conventional sediment dating. This study is significant for determining the bleaching rate of different lithological rocks and for selecting proper dating samples, pretreatment and measurement in practical application.
【Fund】： 国家自然科学基金项目(41472161);; 地震动力学国家重点实验室课题(LED2013A09);; 中央级公益性院所基本科研业务专项(IGCEA1417)~~
【CateGory Index】： P585;P597.3
【CateGory Index】： P585;P597.3