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Meital Field Raz
PhD student in the Department of Earth and Environmental Sciences at Ben-Gurion University of the Negev
My name is Meital Sade Raz, a PhD student in the Department of Earth and Environmental Sciences at Ben Gurion University of the Negev. As part of my doctoral dissertation under the supervision of Dr. Itai Habib, I characterize an innovative method for dating carbonate minerals.
We examine and characterize an innovative approach to dating carbonate minerals in general, and carbonate cave sediments in particular using the uranium-thorium / helium method. This method is not limited to a certain age range like the currently accepted uranium / thorium method and is expected to extend the time frame of the paleo-climatic record based on cave sedimentation. Carbonate cave sediments have a clear advantage in characterizing the potential of carbonates as a geochronometer since their investment environment allows the preservation of carbonate sediments without loss of uranium and thorium. In addition, the temperature in the caves is relatively constant, a fact that makes it possible to better understand the diffusion processes of helium in carbonates. To the best of our knowledge this is the first attempt to date carbonate cave sediments by this method.
Radiogenic helium is a product of alpha decay in the decay chain of uranium and thorium. Cave sediments may accumulate enough helium to measure but helium may also escape from the crystal in diffuse processes depending on the temperature and size of the diffusion space. As part of the study, we examine and quantify the method's capabilities by dating sediments from 7 caves with average temperatures of 0 to 20 degrees located from Siberia in the north to the Negev in the south and comparing the ages obtained in our laboratory to ages obtained by other dating methods. In addition, we conduct diffusion experiments that include graded heating at different temperatures in order to determine the kinetic parameters that affect the rate of diffusion and to examine the size of the diffusion spaces and the rate of helium loss at different temperatures. Further characterization of the size of the diffusion space is performed by examining the microstructure of the sample using images obtained under a light microscope and EBSD analyzes performed using an electron microscope.
The uranium-thorium / helium ages obtained in our laboratory are the same as the independent ages for carbonate sediments from caves where the temperature is between 1-7 degrees Celsius. These results indicate potential for full conservation of helium in carbonates from cold regions for geological time periods. Sample ages from caves where the temperature is 18-20 degrees are lower than the independent ages and indicate a partial loss of helium at these temperatures. The results of diffusion experiments together with the characterization of the crystalline structure allow prediction of helium loss as a function of cave temperature and crystal size. The results of the study significantly improve our understanding of the conditions under which radiogenic helium is preserved and indicate that dating carbonates using the uranium-thorium helium method is possible. In addition, because the temperature at which helium begins to escape from the crystal is low (about 60 degrees for argonite, and about 40 degrees for calcite) we are able to use this method also to date thermal events that caused partial loss of helium in the sample, such as tectonic processes or significant changes In Earth's climate (paleo temperature). Until now, this ability has been reserved mainly for trending rocks containing minerals rich in uranium and thorium such as apatite. My research shows that this method can also be used for dating carbonate sedimentary rocks and defines the conditions for this.