This grant supports continued innovation and application of techniques of He isotope geochemistry to address Earth science problems. The grant will be used to purchase a new He isotope mass spectrometer capable of making precise and accurate He isotope abundance measurements even on very small quantities of helium. The instrument replaces a decades-old predecessor and requires no modification to the existing hardware for extracting gases from rocks and minerals for analysis. This instrument will be used in support of many Earth science subdisciplines and will be the main tool used by multiple undergraduate and graduate students and postdoctoral investigators in their studies.
The two isotopes of He, 3He and 4He, provide a wealth of information on the origin and history of terrestrial rocks. 4He in rocks is almost exclusively derived from α-decay of the elements U and Th. Accumulation of 4He in minerals allows a unique form of chronometry. Because He diffuses out of minerals at elevated temperatures characteristic of burial to a few km depth in the crust, the accumulated 4He in minerals can be used to determine when rocks were brought to the Earth's surface by erosion or faulting. Such data provide important insights to key geologic processes such as the timing and causes of mountain building. 3He in rocks and minerals can come from more unusual sources. For example, 3He is produced when cosmic rays interact with the elements that make up minerals in the uppermost ~ 2 meters of the crust. This too acts as a chronometer, but a chronometer that can be used to measure erosion rates and also to establish when rocks first became exposed at the surface. The latter technique is one of the central tools in establishing the glaciation history and Milankovitch-driven climate change over the last few tens of thousands of years. In marine sediments, 3He is derived from the accumulation of cosmic dust. By measuring 3He concentrations in sequences of marine sediments it is possible to establish sedimentation rates (another key indicator of time) and, in some cases, to identify major cosmic-dust producing events such as the rare destructive collision of many-km-sized asteroids.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
