For over two decades it has been assumed that transport properties for both heat and helium are similar in the crust and that transport for both tracers is in steady-state. In addition, the presence of a terrestrial helium-heat imbalance was suggested based on the observed low mantle He/heat flux ratio at the proximity of mid-ocean ridges. Recent work, however, indicates that the driving transport mechanisms for these two tracers are of a fundamentally different nature for a high range of rock permeabilities in the crust. It was concluded that low He/heat flux ratios in a steady-state regime do not reflect a He deficit in the crust or mantle original reservoirs, but rather, the combined impact of air saturated water, advection, conduction, and diffusion. Together with other noble gas measurements and determined noble gas temperatures (NGTs), part I of the proposed work will take advantage of these recent findings to test and develop the newly created He/heat flux tracer to identify the occurrence, both in time and space, of tectonothermal events in continental regions, and in the Michigan Basin in particular. Part II will take advantage of common tools in order to address questions in relation to NGT determination in ice-covered regions. The latter is a direct measure of the temperature at which groundwater equilibrated with the atmosphere during infiltration and offer a unique opportunity to improve our understanding of past climate in continental areas as well as to assess the impact of glaciation on groundwater recharge in mid-high latitude regions. In addition to illustrating how He/heat flux ratios can be used in conjunction with NGTs to investigate the tectonothermal history of continental regions, this work will also contribute to an improved understanding of NGT determination in ice-covered regions. This work will impact knowledge in the fields of Mantle Geochemistry, Geophysics, Structural Geology, Hydrology and Paleoclimatology.
This research will be the basis of a new, exclusively research-oriented undergraduate course. Identifying problems to be addressed, collecting and analyzing data, testing hypotheses, reaching potential conclusions and writing the prototype of a scientific paper will be an integral part of this class. Of particular relevance is the participation of undergraduate non-science majors, in particular women and underrepresented groups into scientific discovery, which is expected to captivate students and ultimately draw increasing numbers into a scientific career.
??? This Career project was composed of two distinct research components in addition to an educational one. The main research component of this project (Part I) aimed at further investigating and developing the use of the helium (He)/heat flux ratios as a new indicator of past thermal and tectonic events in continental areas. It built on previous work by the Principal Investigator (PI) and collaborators which had shown that He/heat flux ratios greater than crustal production ratios could only be the result of a past thermal event of mantle origin. This work isrelevant for an improved understanding of the Earth’s formation and evolution. The second research component (Part II) was aimed at developing new inverse noble gas temperature (NGT) models based on noble gases (neon - Ne, argon - Ar, Krypton - Kr, xenon - Xe) dissolved in groundwater. It was aimed at better understanding their behavior at the interface between the water table and soil air. Noble gas concentrations are temperature dependent and have been used to reconstruct the past climate for over four decades. Noble gases are also used to obtain information about the dynamics of groundwater flow systems. This work is thus relevant to understanding past and current climate change in addition to improving groundwater resource management. The educational component consisted of the creation of a new, entirely research oriented class for non-science major undergraduates, entitled "Scientific Discovery in Earth Sciences: A Research Experience". This class provides young non-science major undergraduates with a unique opportunity to participate in scientific discovery. In this class, students choose a topic of interest among all the PI’s on-going multi-disciplinary research projects. The goal is to have these non-science major undergraduates develop an interest in sciences and hopefully to draw them into a STEM (Science, Technology, Engineering and Mathematics) field. It also provides women and minorities with the confidence they sometimes lack to engage in such scientific endeavors. In particular, this class has been very successful at attracting a majority of women and a significant number of minorities, mostly African American. Both research projects were carried out in Michigan. The first one involved several intensive field campaigns to collect groundwater and deep brine samples in the southern portion of the Michigan Basin. The second project involved two separate periods of continuous collection of noble gas samples together with the measurement of a host of physical parameters in the shallowest aquifer, the Glacial Drift Aquifer, in Ann Arbor. The Michigan Basin was chosen to carry out the first research project because the presence of a flux ratio greater than the crustal He/heat flux ratio had been previously identified in shallow groundwaters of the Michigan Basin. The second portion of the project was carried out in Ann Arbor because this is where the University of Michigan is located and geographical proximity facilitated nearly continuous sample collection. Among some of the most relevant findings of Part I are the discovery of He and Ne of mantle origin in deep brines of the Michigan Basin and the presence of a primordial, solar-like signature in this continental region where the presence of a mantle plume is highly unlikely. This contrasts to the generally accepted view that primordial He and Ne are always associated with mantle plumes and are representative of a deep, largely undegassed mantle. The atmospheric noble gas signatures of these brines also strongly support the occurrence of this mantle thermal event. In particular, the atmospheric noble gas component in these brines is extremely depleted, strongly suggesting that subsurface boiling conditions occurred. These findings support the fact that He/heat flux ratios can be used in the future to identify past thermal events of mantle origin and to provide clarification on the tectonic evolution of specific regions. With respect to Part II of this project, we have shown that atmospheric noble gases at the water table are often in excess with respect to the air saturated water at the soil air temperature leading to a bias to low NGT values. We have shown that such a bias is likely due to increased noble gas partial pressures in soil air due to O2 depletion without a corresponding build up of CO2. We have subsequently measured dissolved O2 at the water able, and soil air CO2. Our results pointed to ~10-11% depletion of these gases at the base of the unsaturated zone, confirming earlier predictions. We have also developed a new NGT model that allows for partial re-equilibration of excess air via diffusion in the gas phase.?
