Mass-dependent triple oxygen isotope variations in terrestrial materials

doi: 10.7185/geochemlet.1815 High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δ́17O δ 1́8O space that can be explained by well understood fractionation processes. The Δ́17O δ 1́8O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a λ value of ~0.528, although a better fit to waters with δ18O values >-20 ‰ is δ’17O = 0.52654 (±0.00036) δ’18O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δ́17O δ 1́8O space with high δ 1́8O and low Δ́17O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ’18O values at low fluid/rock ratios, and finally higher Δ́17O when F/R ratios are greater than 1. In order to make the triple isotope data tractable to the entire geological community, consensus on a reporting scheme for Δ’17O is desirable. Adoption of λRL= 0.528 (λRL = slope of δ 1́7O δ 1́8O reference line, the ‘Terrestrial Fractionation Line’ or TFL) would bring the ‘rock’ community in line with well established hydrological reporting conventions. Received 10 February 2018 | Accepted 5 May 2018 | Published 1 June 2018 1. University of New Mexico, Albuquerque, NM 87131, USA 2. Georg-August-Universität, Göttingen, D-37073, Germany * Corresponding author (email: zsharp@unm.edu) Introduction The early practitioners of stable isotope geochemistry recognised that the three isotopes of oxygen followed mass-dependent rules, such that fractionation in 17O/16O was approximately half that of 18O/16O for both equilibrium and kinetic processes (Craig, 1957). Measuring the 17O/16O ratios therefore provided no additional information to the 18O/16O values alone. Mass-independent fractionations have since been found in primitive meteorites (Clayton et al., 1973) and many atmospheric components such as O3, CO2, H2O2 which are explained by photochemical processes (Thiemens, 2006). With improved precision, it is now recognised that there are small, but non-zero deviations from a simple best fit line in δ17O δ18O space that are related to mass-dependent processes (Luz and Barkan, 2010; Pack and Herwartz, 2014). As more data are collected, general trends are being observed. Here we compile the bulk of published, and our own unpublished, high precision triple oxygen isotope data and categorise distinct trends in δ 1́8O – Δ́17O space for different processes. The Terrestrial Fractionation Line and Relevant Notation A fit through δ18O δ17O values of terrestrial materials defines the ‘Terrestrial Fractionation Line’ (TFL) with a slope of ~1⁄2. The general expression for the TFL is δ 1́7O = λTFL δ 1́8O + γTFL Eq. 1 where λTFL is the best fit slope and γTFL is the y-intercept of the best fit line. Equation 1 and all data in this study are presented in the linearised notation given by δʹ (see Miller, 2002 and Supplementary Information ). There is no ‘correct’ or unique TFL, as different equilibrium or kinetic processes result in slightly different triple isotope fractionations. For example, meteoric waters fall on a trend that does not exactly overlap with the trend defined by most rocks and minerals (Luz and Barkan, 2010; Pack and Herwartz, 2014; Pack et al., 2016). A δ 1́7O δ 1́8O plot of published data from natural samples is shown in Figure S-1. Virtually all data plot exactly on the same line at the resolution of the figure. To better visualise subtle deviations from the TFL reference line, the Δ́17O value has been introduced. The Δ́17O term is defined as Δ́17O = δ 1́7O λRL δ 1́8O + γRL Eq. 2 λRL is the reference line slope and γRL is the y intercept. In this study, δ 1́7O and δ 1́8O values from different publications are normalised to VSMOW (rocks and minerals) and VSMOW/ SLAP2 scale (waters; SLAP2 from Schoenemann et al., 2013) and derived Δ́17O to a reference line with slope 0.528 (λRL) and zero intercept (γRL = 0). Rock samples are normalised to the VSMOW scale with the Δ́17O value of the broadly adopted San Carlos Olivine standard (SCO) having δ 1́8O and Δ́17O values of 5.4 ‰ and -0.05 ‰ relative to a λRL = 0.528 and γRL = 0 (Sharp et al., 2016). See Pack et al. (2016) for details of normalisation. Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2018) 7, 27-31 | doi: 10.7185/geochemlet.1815 28 Figure 1 Characteristic δ 1́8O – Δ́17O fields for different materials. Meteoric waters have uniquely high Δ́17O values and generally low δ 1́8O values. Marine carbonates and silica plot in a narrow band of high δ 1́8O and low Δ́17O values. Manganese oxides have unusually low Δ́17O values due to incorporation of dissolved O2. Hydrothermal alteration drives igneous and metamorphic rocks to lower δ 1́8O values. All data are standardised to VSMOW, where solid samples are normalised to Δ́17O of San Carlos olivine = -0.05 ‰. See Figure S-1 for data sources. Small variations in Δ́17O become apparent in a Δ́17O vs. δ 1́8O plot (Fig. 1). Waters have positive Δ́17O values, while rock samples have negative Δ́17O values that tend to decrease with increasing δ 1́8O. Hydrothermally altered samples have the lowest δ 1́8O values of any rock samples. Characteristic Δ́17O δ 1́8O Fields and Trends A number of generalisations can be made from the data shown in Figure 1. A clear separation in the Δ́17O values between waters and rocks is immediately apparent. The mantle has a very tight range of Δ́17O values of -0.03 to -0.07 ‰, with most samples plotting at -0.05 ± 0.01 ‰. The 0.04 spread in Δ́17O in mantle samples is due to mantle metasomatism or alteration and in some cases analytical error. True variability within pristine mantle is likely much smaller and is centered on Δ́17O = -0.05 ‰. The δ’18O values of more evolved high silica igneous rocks are higher than the typical mantle value of ~5.4 ‰, but the Δ́17O values remain relatively constant. This is because 1) the shift in δ’18O is small and 2) the associated high-T θ value (where θ = ln(α17O) / ln(α18O); see Supplementary Information) for minerals and melts is close to the reference slope value of 0.528, thereby moving more siliceous samples to higher δ 1́8O, but constant Δ́17O values. Meteoric waters. Meteoric water data vary greatly in Δ́17O δ 1́8O space (Fig. 2). Samples with δ18O values between -55 and -15 ‰ have an average Δ́17O value of 0.041 ± 0.006 and fall on a line with slope λ = 0.528. The offset in Δ́17O relative to the value of 0 ‰ for the ocean is related to the kinetic fractionation associated by transport of vapour from the saturated layer into the free, undersaturated atmosphere (Luz and Barkan, 2010). Samples from the Vostok ice core (pale blue points in Fig. 2) are fit with a higher λ value of 0.5309 ± 0.0001, which is attributed loosely to variable humidity and wind speed at the ocean source and supersaturation conditions during condensation in extremely cold environments (Landais et al., 2008) or direct condensation as ‘diamond dust’ (Miller, 2018). For δ’18O values greater than -20 ‰, the Δ́17O values of meteoric waters show the reverse trend, decreasing with increasing δ 1́8O. Rain re-evaporation and mixing have been explained as the principle driving force towards the low Δ́17O values (Landais et al., 2010; Risi et al., 2013; Li et al., 2015). The best fit line to meteoric water samples with a δ 1́8O value greater than -20 ‰ is δ’17O = 0.52654 (±0.00036) δ’18O + 0.014 (±0.003). Eq. 3 Most waters outside of extreme polar regions have δ18O values that are greater than -20 ‰, so that Equation 3 is a rough fit to the Global Meteoric Water Line for waters from non-polar sample regions. Low temperature sediments. A compilation of data from low temperature sediments shows decreasing Δʹ17O values with increasing δ 1́8O (Fig. 3). This trend is explained by the temperature effect on the equilibrium fractionation between minerals and seawater. The triple isotope equilibrium θ value decreases with decreasing temperature, (Cao and Liu, 2011; Sharp et al., 2016). The curved line in Figure 3 is the equation governing the Δ́17O δ 1́8O values of silica in equilibrium with ocean water as a function of temperature (Sharp et al., 2016). Most marine silica samples fall on or near this curved line. Samples that have equilibrated with meteoric waters consequently plot towards lower δ 1́8O and, in general, higher Δ́17O values, consistent with a lighter meteoric water source. Our unpublished manganese oxide data from a deep sea nodule off Hawaii have very low Δ́17O values in relation to