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doi: 10.7185/geochemlet.2014 The first nuclear detonation, at the Trinity test site, is an analogue for high temperature volatile loss during planet formation processes. We report Cu isotope and abundance data, comparing them with Zn and K isotopes in trinitite glasses sampled with distance from the detonation centre. Copper concentrations drop, and isotopic compositions become ~0.3 ‰ higher with proximity to the detonation. Relative sensitivity to high temperature evaporative isotopic fractionation processes occur in the order: Zn > Cu >> K. Lower volatility of K relative to Cu and Zn results from its low activity coefficient in silicate melts. Lunar mare basalt and some tektite Cu, Zn and K isotope compositions can be modelled using empirical fractionation factors (α) derived from trinitite. Combined with isotopic variations measured in mare basalts, the results support volatile loss during a magma ocean phase for the Moon. Received 12 October 2019 | Accepted 23 March 2020 | Published 16 April 2020 1. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA 2. Institut de Physique du Globe de Paris, Université de Paris, 1 rue Jussieu, 75005, Paris, France 3. Institute of Geochemistry and Petrology, ETH Zürich, Sonneggstrasse 5, CH-8092, Zürich, Switzerland 4. McDonnell Center for the Space Sciences, Washington University, Saint Louis, MO 63130, USA 5. Lyndon B. Johnson Space Center, Houston, Texas, 77058, USA * Corresponding author (email: jmdday@ucsd.edu) Introduction The moderately volatile elements (MVE), including Cu, Zn and K, trace volatile behaviour in planetary bodies. The MVE have 50 % nebular condensation temperatures (TC50) at solar nebula conditions (10-4 bar) of between 1134 K and 665 K (Lodders, 2003), and have low element-element bond energies (Albarède et al., 2015). Stable isotope MVE compositions have been utilised to examine fractionation effects induced by evaporation or condensation, and to assess stable isotope differences between Earth and the Moon (e.g., Paniello et al., 2012; Day and Moynier, 2014; Boyce et al., 2015; Wang and Jacobsen, 2016; Dhaliwal et al., 2018). An advantage of examining the MVE relative to more volatile compounds such as H2O is that they generally occur in sufficient quantity within rock-forming minerals and glasses that their isotopic composition can be precisely measured. Copper (TC50 = 1037 K) is a useful tracer of planetary volatile histories, being both moderately volatile and chalcophile during planetary processes (e.g., Moynier et al., 2017). Copper isotopic fractionations (expressed as δ65Cu = ([65Cu/63Cu) sample/[Cu/Cu]SRM976 1) × 1000) of planetary materials do not match predicted kinetic isotopic fractionation factors, but are instead consistent with subdued fractionation factors approaching unity (Moynier et al., 2017). A problem in interpreting volatile element isotope fractionation has been that laboratory experiments are unable to reproduce large scale volatile processes, making empirical assessment of volatile element loss during planet formation challenging. This is particularly troubling since debate surrounds the extent of volatile loss experienced by planets and when this occurred (Paniello et al., 2012; O’Brien et al., 2014; Wang and Jacobsen, 2016). To examine the behaviour of Cu during high temperature processes, and to compare it with two other MVE, we investigated melted glasses (trinitite) formed during nuclear detonation at the Trinity Test site, New Mexico, USA. During this event, local conditions exceeded 8430 K, leading to vapourisation of the arkosic desert sand substrate (Supplementary Information). Temperature estimates for the explosion are above those anticipated for typical basaltic eruption temperatures. The conditions of formation were likely to be reducing since the absolute logfO2 (bar) defined by the FMQ buffer is -6.3 at 1673 K, but is -1.8 at 2273 K. In contrast, air has logfO2 = -0.68 at both temperatures, resulting in reducing conditions relative to solid-solid buffers.

Moderately volatile element behaviour at high temperature determined from nuclear detonation