Stable Isotope Geochemistry: Course Overview

1. GE 140A, Winter 2019 Lecture 1: INTRODUCTION AND OVERVIEW

2. GE 140a: Stable Isotope Geochemistry 2019 Syllabus and course information Instructor: John M. Eiler; 105 N. Mudd; x6942; eiler@gps.caltech.edu T.A.: Sang Chen; scchen@caltech.edu Course components: • Three 1-hour lectures per week, M-W-F, 10-11 am, held in 251 Arms • Approximately once-weekly help sessions/recitations run by Sang; these will be organized and scheduled by him some time during the first week of class. • Eight homework sets (6 % of grade each, plus 2 % bonus if you turn them all in); each due 1week after being assigned. These will be distributed by e-mail and posted on the course web site. Each set will include one or more assigned papers from the literature and a short essay response to a question regarding that paper. • Assigned and recommended readings to be distributed periodically by e-mail and posted on the course web site. • One midterm and one final exam (take-home; closed note, closed book; 20 and 30 % of grade, respectively) • Near the end of term, we will hold a faux research proposal competition (primarily to give you all experience writing, presenting and evaluating research proposals). This will involve 2 pages of writing and a joint panel evaluation to distribute up to 1 million dollars worth of funding among the proposals.

3. GE 140a: Stable Isotope Geochemistry 2019 Syllabus and course information Reading materials: Weekly papers: Either easily available through Web of Science, or distributed by e-mail as pdfs and posted on the class web site. Text: Isotopes – Principles and applications. Faure and Mensing. This is used as a supplement and background material, not as the main source of assigned reading. A copy will be available in the library Assigned reading: You will be periodically assigned readings copied from the following sources; all will be distributed by e-mail as pdf’s and posted on the class web site. Reviews in Mineralogy volume 16: Stable Isotopes in High Temperature Geological Processes. Reviews in Mineralogy, volume 43: Stable Isotope Geochemistry Reviews in Mineralogy, volume 55: Geochemistry of Non-Traditional Stable Isotopes Criss, Principles of Stable isotope distribution Honor code policy: You may discuss assigned home works with the instructor, TA or fellow students currently enrolled in the class. You are encouraged to discuss the assigned short papers with fellow students as part of your homework collaboration. However, all submitted work must be your own (i.e., it should not be transcribed from or otherwise based on others’ completed work). One may not read or otherwise examine other student’s work without their knowledge and permission, regardless of circumstances. No collaboration is permitted on exams.

4. GE 140a: Stable Isotope Geochemistry 2019 Syllabus and course information Content and structure: The purpose of this course is to provide a foundation in the chemical physics, analytical technologies and natural distributions of the stable isotopes, at a level that will permit you to engage in cutting-edge research applying these concepts to any discipline of the experimental, theoretical, natural or applied sciences. Major components of the lecture portion of class include: • Historical development and nomenclature of the field • Isotopic fractionations arising from classical physical phenomena • Fractionations arising from chemical isotope effects and other quantum-mechanical phenomena • Isotopic fractionations in complex networks of chemical and physical processes, including biology • Illustrative applications to the Earth, planetary and environmental sciences • Technologies of isotopic measurement and engineered enrichment The following slides provide an overview of the order of material to be covered (some topics will be spread over 2 or more days):

5. History • Stable isotope chemistry emerged from the revolution in nuclear and chemical physics in the first decades of the 20th century; most of the concepts and phenomena that are key to our field were discovered in this period. Thompson’s ‘positive ray device’ • These advances came to a crescendo during the Manhattan project, which served as a training ground for most of the founders of the field, and involved large-scale experiments that explored all of the major classes of naturally significant isotopic fractionations. AO Nier looking weird after helping discover 235U

6. Nomenclature • Stable isotope geochemistry is plagued by abstruse, redundant, recondite nomenclature. You must learn all of it in order to talk about our subject without constantly making mistakes. [isotope i] = Ri [reference isotope] Risample -1} x 1000 distandard= { Ristandard Rij Rik aj-k = ∆ij-k = dij - dik [HDO][H2] [H2O][HD] K = d2sample = d1sample + d21 + (d1sample)(d21) 1000 ∆17O = d17OVSMOW — 0.52xd18OVSMOW Rimeasured Ristochastic [ -1]x 1000 ∆i =

7. Electromagnetism • Charged atoms and molecules (ions) can be manipulated using electric and magnetic fields, following rules of classical electromagnetic theory and Newton’s laws of motion (m/q) = (B2r2)/(2V) r = (mv2/qE) • This is the first instance where theoretical understanding, laboratory experiment practical application converged for some form if isotopic separation or discrimination • Natural electromagnetic fractionations are rarely encountered in applied isotope geochemistry; an Important exception is ’inefficient coulomb drag’ in the solar wind

8. Diffusion • Isotopic fractionation by gaseous diffusion is perhaps the easiest mechanism of isotopic separation to understand, and inspired some of the first experiments following the discovery of isotopes • The core concept is that gaseous molecules possess the same average kinetic energy at a given temperature, and therefore have average velocities that vary inversely as m1/2, as described by classical kinematic theory; thus, any process that separates atoms by their speed of motion has the potential to fractionate isotopes • Complexities arise in complex mixtures (where one gas diffuses through another), condensed materials, or at pressures where diffusing molecules interact with each other or their surroundings through chemistry, electromagnetic attraction and repulsion, etc.

9. Gravitation • When a column of gas is allowed to come to thermodynamic equilibrium in a gravitational field, with no forced stirring or convective overturn, it will sort itself so that heavy molecules concentrate at the bottom and light ones concentrate at the top Enriched in 235U • These effects are very subtle over lab distance scales, but can shift isotope ratios by parts per thousand over geological domains of 10’s -100’s of meters, up to 10’s of per cent over atmospheric length scales of 100’s of kilometers UF6 in Depleted in 235U • Gravitational effects are important in sand dunes, glacial firnand low-density parts of planetary atmospheres • These principles are also the basis of the most efficient and disruptive technology for nuclear isotope enrichment: the gas centrifuge

10. Common chemical isotope effects: The effect of isotopic substitution on electronic and atomic motions To first order, chemistry is about atomic electronic structure, which mostly depends on the charge of the nucleus but not its mass. That’s why we consider the isotopes to be members of the same elemental ‘family. So we it would be reasonable not to expect a ‘chemistry’ of isotopes. But the ‘reduced mass’ of the motion of the electron around the nucleus does differ subtly between isotopes, and this leads to very small changes in chemical behavior Much more importantly, isotopes differ in the reduced masses of their interatomic motions across chemical bonds (molecular vibrations), and this leads to relatively large differences in chemical behavior

11. The manifestations of vibrational chemical isotope effects Heterogeneous equilibria Site-specific equilibria CO2 Methyl site Carboxyl site 12C Energy 13C CH4 12C 12C 13C 12C 13C 13C Clumped-isotope equilibria Kinetic isotope effects Transition state H2 12C 13C Reactant H2 HD Product 12C D2 13C 12C 13C

12. Complex fractionation networks Many natural processes that cause isotopic variations combine several different elementary fractionations, often with counter-intuitive net effects. Describing these processes correctly requires both a solid understanding of each elementary step, and careful treatment of the transport and mass balance that link multiple fractionations to each other.

13. Biosynthetic fractionations Biosynthesis presents the most complex systems of isotopic fractionation, often involving dozens of individual compounds that interact with each other through a diverse mix of kinetic and equilibrium chemical isotope effects, diffusion-limited transport, and mixing among external and internal reservoirs.

14. Exotic chemical fractionations Photochemistry Quantum tunneling Nuclear volume effects Nuclear spin effects 6s

15. Themes of application Earth and planetary history Environmental science Life and Medicine

16. Themes of technology Isotopic analysis Engineering isotope enrichment