TIMS

Opportunities

The TIMS group is always looking forward to hearing from students interested in graduate student research employing petrological, geochemical, and isotopic data to examining processes in and on the Earth and the Solar System. Please contact Dr. Brandon, Dr. Alex Bentley, or Dr. Juan Carlos Silva-Tamayo if interested. Examples of current research funded by NSF, NASA, and industry are provided below.

Studies on the Earth’s Mantle



The TIMS group is interested in understanding how the mantle lithosphere chemically exchanges with passing fluids and melts. This is a long-term project and is primarily focused on samples we have obtained in the SW USA. Other locales across the globe will be examined. These investigations integrate isotopic, trace element, major element, and mineralogic characteristics to constrain the effects of metasomatism in arc and non-arc continental mantle settings. One goal of the studies is to examine the link between stabilization of continental lithosphere and growth of the overlaying continental crust. Simply, are the ages of the mantle and overlying crust the same or are they decoupled? Answering this question will help us to understand the processes that lead to continental growth over Earth history.

With Anne Peslier from Jacobs Technology at NASA-JSC, we are examining the stability of craton and off-craton mantle lithosphere by measuring hydrogen in nominally anhydrous minerals as a proxy for water contents. Peslier et al. (Nature, 2010) showed that the water content in olivine leverages the viscosity of the host peridotite. In the case of the Kaapvaal Craton, very low water contents in peridotite olivine at the base of the mantle lithosphere increases its viscosity and makes it resistant to removal via asthenospheric flow, thereby allowing this craton to be stabilized over for billions of years. Whether these low water contents in olivines are a constant in other stable cratons and their off-craton margins, is an open question.

Re-Os Chronology of Petroleum and Its Source Rock

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The Re-Os isotope system can be implemented as a precise chronometer of shale deposition in sedimentary basins and for examining the processes that led to the Os isotopic composition of seawater over Earth history. Given the paucity of other reliable chronometers for sedimentary rocks, the application of the Re-Os is isotope system is thus ideal for placing time markers in sedimentary sequences where shales are present. This opens up its application to a wide variety of problems. At present the TIMS group is focused on two problems and we expect additional locales and sedimentary sequences to be added for study in the near future.

In the first problem, with Francis Macdonald (Harvard), we are coupling the application of the Re-Os chronometer additional geological and geochemical data to sequences of Cryogenian shales from southeast Alaska and western Canada to examine climate change in this time period. Within the Neoproterozoic Era (1000 to 542 million years ago – Ma), the Cryogenian Period (850 to 635 Ma) was a time of rapid and extreme climatic, biological, and geologic change. Understanding these changes to the ocean-atmosphere system are critical given that it is also during this period that Earth’s biosphere moved away from a purely unicellular world to one with complex life. It is posited that one or more of the Cryogenian glaciations reached global coverage, resulting in a ‘Snowball Earth’ with potentially dire consequences for marine organisms. Questions remain as to how the biosphere responded and how rapid climate shifts affected biological evolution and innovation. Importantly, this is also a time when continental landmasses were undergoing dissociation of the supercontinent Rodinia. This tectonic leverage to the climate and biosphere in the Cryogenian is even more poorly understood. The goal of this research is to quantitatively merge these questions through combining Re-Os isotope chronometry with additinal gecochemical tracers in sections of sedimentary rock deposited in the oceans over this time period. Together these proxies will inform our understanding of time (rate and duration) as well as track abiological and biogeochemical changes preserved in the oceans. Results to present indicate that we can put precise time constraints on when Rodinia began to break up in this region and how it leveraged the rapid climate change. This work will be extended with additional samples and coupling to the other systems to further examine these questions.

In the second problem, we are studying the Re-Os isotope compositions of oil-bearing shale formations in Texas. We are also measuring Re and Os isotopes in the oil derived from these formations in order to answer a variety of questions. In particular we wish to understand whether the Re-Os dates to published to present, are actually related to the source rock age or if they are instead mixing lines related to open system processes during oil maturation and migration. The long-term goal is to be able to assess the source of oil and to use the Re-Os isotope system as an exploration tool. Secondary questions that can be studied from these data include the Os isotopic composition of seawater during shale deposition as well as adding to the time evolution database of Os isotopes in seawater. These data will ultimately help in constraining the processes that lead to the Os isotope composition of seawater over Earth history.

Isotopic Studies on Extraterrestrial Materials

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Radiogenic isotopes and trace elements in meteorites preserve evidence on the types of pre-solar and solar materials present in our solar system, nebular processes, early parent body processes, the earliest differentiation history of their parent bodies, and how their parent bodies have evolved to the present. Our present and near-future work will concentrate on two tasks: 1) High precision Os isotope measurements on extraterrestrial materials, 2) Lu-Hf, Rb-Sr, and Sm-Nd isotope chronology of achondrites.

The degree of isotopic mixing in the solar nebula and the nature of solar and pre-solar components that have contributed to our solar system remain subjects of vigorous debate. Isotopic anomalies have been identified in inclusions in chondrites. This indicates that the pre-solar components were not completely homogenized or processed away at high temperatures. I have initiated a program to investigate these issues using Os isotopes. Osmium is one of the first elements to condense from the solar nebula and therefore provides a distinct perspective on the debates over nebular heterogeneity compared to other elements studied to present. High precision measurements of Os isotopes show that some bulk carbonaceous chondrites have Os isotopic compositions consistent with a ‘missing’ s-process component when compared solar isotopic abundances as exemplified by ordinary chondrites and mantle-derived materials from Earth (shaded envelopes). These first results were published in Science (Brandon et al. 2005). We interpreted this heterogeneity as resulting from incomplete access of a pre-solar s-process carrier phase during digestion of the meteorites. Interestingly, the s-process carrier necessary to balance the Os isotopes to the solar abundances is deficient in 186Os and provides clues to the conditions within the star where this s-process component was produced.

This investigation has been expanded and most recent work has been on enstatite chondrites and iron meteorites (van Acken et al., 2011; Wittig et al. 2012). In the near future high precision Os isotope measurements will be obtained on additional sample suites to examine the affects of cosmic ray exposure on Os isotopic compositions, as well as to further address the issues of isotopic heterogeneity in the solar nebula, with ramifications on nebular mixing models and the types of pre-solar components that contributed to our solar system with implications for nucleosynthesis.

We are examining the petrological and geochemical characteristics of martian meteorites, lunar meteorites, and Apollo Moon rocks. These data provide the crystallization age, its relation to other lunar and martian basalts, and aids in constraining lunar sources and mantle evolution models. These investigations set the framework for future chronological work on other achondrites from differentiated terrestrial bodies, taking advantage of the new Triton for Rb-Sr and Sm-Nd isotope measurements. For example, the Rb-Sr isochron age we obtained for LAP 02205 (Rankenburg et al., Geochimica et Cosmochimica Acta, 2007), is approximately an order of magnitude more precise (2946±14 Ma) than those previously obtained for Apollo basalt samples. These studies will be directed towards newly discovered meteorites, carefully selected meteorites and lunar rocks from the present collection, and also the goal is to measure materials from sample return missions from the Moon, Mars, and potentially, smaller bodies. This more precise chronological information will provide an assessment of the timing and duration of magmatism in relation to early-formed terrestrial body mantle sources, and the thermal evolution of the interior of these bodies.

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In addition, Dr. Brandon developed techniques for precise measurement of 142Nd/144Nd for constraining Sm/Nd fractionation during early (i.e. within ~300 million years of accretion, or 5 half-lives of parent 146Sm @ 67 Ma) differentiation of the Moon, Mars, and the eucrite parent body, employing the new Triton TIMS. With the revelation that terrestrial basalts are not chondritic in 142Nd/144Nd, important new insights into early planetary differentiation and dynamics are to be gleaned from the precise measurement of this ratio. Our results so far for early Earth rocks, martian meteorites , and lunar basalts reveal key constraints on early differentiation of Mars and the Earth-Moon system (Rankenburg et al., 2006; Debaille et al., 2007; Bennett et al., 2007, Brandon et al., 2009, Murphy et al., 2011).

Apollo mare basalts collected during the Apollo 12, 15 and 17 missions are being studied in the UH TIMS lab in order to evaluate the early differentiation history of the Moon. This work builds on the previous work of Dr. Brandon. Results published to date (see diagram above, modified from McLeod et al., 2014, EPSL) allow for further assessment of the Low-Ti, high-Ti and KREEP mantle source reservoirs within the context of a single closure age for all versus a multi-stage history. New data collected at UH demonstrates that by evaluating a wider array of lunar basaltic compositions the linear correlation between calculated source ε143Nd–μ142Nd as observed in previous studies, does not change. A model age of c. 4.34 Ga can be reconciled with a scenario in which the Moon formed early c. 4.55-4.47 Ga and where the model age of c. 4.34 represents a later, potentially Moon-wide remelting event associated with an impact event. This scenario is consistent with models of Lunar Magma Ocean crystallization timescales, U-Pb constraints from Earths Jack Hills zircons and recently published geodynamic models. These precise 142Nd measurements have opened up new lines of inquiry regarding early Earth-Moon evolution and a variety of sample suites continue to be studied in our lab to extend these results.

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