Combining clumped isotope and trace element analysis to constrain potential kinetic effects in calcite
Introduction
Since the pioneering experiments of McCrea (1950) that suggested that precipitation rate influences carbon and oxygen isotope fractionation in the CaCO3-HCO3– system, the consensus has been that the δ18O temperature proxy can be affected by non-equilibrium fractionation effects such as composition of the fluid (e.g. carbonate concentration and pH), biological effects, and/or precipitation rate (McCrea, 1950, Tarutani et al., 1969, De Villiers et al., 1995, Adkins et al., 2003, Kim et al., 2006, Dietzel et al., 2009). However, the magnitude and direction of this non-equilibrium fractionation is still not fully constrained. In laboratory inorganic carbonates precipitates, δ18O values are variously reported to be independent of precipitation rates (Tarutani et al., 1969, Kim and O’Neil, 1997, Jiménez-López et al., 2001) or to decrease with increasing precipitation rate (Kim et al., 2006). On the other hand, in natural biogenic carbonate, the effect of growth rate on carbonate δ18O is noticeable and a rapid growth rate results in a lower δ18O values (McConnaughey, 1989, De Villiers et al., 1995, Adkins et al., 2003).
Non-equilibrium effects are even less well understood in carbonate clumped isotopes (Ghosh et al., 2006, Schauble et al., 2006). Developed in the last decade, clumped isotopes (Δ47) are based on measuring the abundance of 13C-18O bonds in the carbonate lattice of a mineral relative to the stochastic distribution of isotopologue with this isotopic composition (Ghosh et al., 2006, Eiler, 2007, Dennis et al., 2011). The clumped isotope composition of a carbonate only depends on the temperature of the fluid, not its isotopic composition, making this at least in theory a more direct proxy for precipitation temperatures. However, since the clumped isotope thermometer is also based on equilibrium thermodynamics, it potentially suffers from the same shortcomings as oxygen isotope thermometry (Watkins and Hunt, 2015), especially for natural carbonates which often grow in non-equilibrium conditions (Dietzel et al., 2009). For example, in speleothems fast carbonate precipitation often does not allow sufficient time for isotopic equilibration and leads to disequilibrium effects (Affek et al., 2008, Daëron et al., 2011, Wainer et al., 2011, Kluge and Affek, 2012).
Here, we try to evaluate potential kinetic effects on clumped isotopes associated with precipitation rates in inorganic calcite in the surficial burial regime, within near-surface fracture network. For this, we selected three large crystal samples growing along fracture planes in the carbonate carapace of a salt dome (Jebel Madar, Oman). Samples at Jebel Madar are ideal for our study, in part because of the availability of published data (Immenhauser et al., 2007, Vandeginste et al., 2017) that can complement the current research, but also because the size of the crystals (>20 cm) allows for detailed transects across a single crystal. We measured clumped isotope values and trace element compositions along high-resolution transects in three crystals, starting from the host rocks and going to more recent growth phases on the outer layers of the mineral. The abundance and distribution of trace elements is often used to constrain diagenetic history in carbonates (Veizer et al., 1983, Bruhn et al., 1995), but the motivation behind our approach is that the incorporation of Mn2+ and Fe2+ into calcite is known to be influenced by growth kinetics (Lorens, 1981, Mucci, 1988, Dromgoole and Walter, 1990a), temperature (Veizer, 1974) and solution composition (Mucci, 1988). The current consensus is that the slower the precipitation rate, the closer trace element partitioning is to chemical equilibrium (e.g Dromgoole and Walter, 1990b). Hence, by investigating trace elements and clumped isotopes in the same samples we hope to find a qualitative or semi-quantitative way to identify disequilibrium precipitation, and by extension be able to interpret the clumped isotope temperature estimates from unknown samples with greater confidence. Our results strongly suggest that the coupling of clumped isotopes and trace elements analysis can indeed reveal possible kinetic fractionations, at least for calcitic fracture infills.
Section snippets
Site and materials
Jebel Madar is a salt-cored dome structure in the Adam Mountains, the foothills of the Oman Mountains (Fig. 1). The stratigraphy of the carapace at Jebel Madar consist of Late Triassic to Cretaceous carbonates, and it is exposed approximately 500 m above the surrounding Quaternary plain deposits (Claringbould et al., 2013). Jebel Madar formed by diapirism of the Precambrian-Cambrian Ara salt of the Ghaba Salt Basin (Mount et al., 1998) during the Miocene (Claringbould et al., 2013). The main
Clumped isotopes thermometry
A total of twenty-one samples were analyzed for clumped isotope thermometry in the Stable Isotope Laboratory at Imperial College London. Powdered calcite samples were used to perform three replicate measurements per sample using fully automated system (the prototype IBEX system developed in-house). The IBEX uses helium as a carrier gas and a trap configuration similar to our manual line (Dale et al., 2014). The advantages of using the IBEX is efficiency, as the system only requires 3–4.5 mg of
Results
The range in bulk oxygen isotope composition of the calcite (δ18OCalcite [VPDB]) is as follows: −10.86 ± 019 to −14.61 ± 0.04‰ for sample JMF 1, −14.5 ± 0.02 to −15.58 ± 0.05‰ for sample JMF 12, and −2.68 ± 0.07 to −12.13 ± 0.02‰ for sample JMG 1 (Table 1). The carbon isotopic ratio (δ13CCalcite [VPDB]) range from −6.2 ± 0.05 to −2.2 ± 0.16‰ for JMF 1, −7.61 ± 0.04 to 4.79 ± 0.01‰ for JMF 12, and 0.46 ± 0.12 to 2.93 ± 0.03‰ for JMG 1 (Fig. 2). Hence, macro-columnar calcite samples yield more
Relationship between calculated δ18OFluid and clumped isotope temperatures: Recrystallization or potential kinetic effects?
From our result we also note that macro-columnar calcite results show a significant (R2 = 0.79) correlation between calculated δ18O Fluid [VSMOW] and clumped isotope temperature (Fig. 5), but temperature does not seem to decrease in correlation with the calcite phases (i.e. further away from the host rock, from ‘A’ to ‘H’ on Fig. 5). Sample JMF 1 A is an outlier plotting outside the observed trend, and we interpret this as mixing between the host-rock and calcite phase during micro-drill
Conclusions
The two types of samples previously recognized (macro-columnar calcite and thin-vein calcite) differ in their clumped isotope temperatures and calculated δ18OFluid SMOW: macro-columnar calcite show apparent lower clumped isotope temperatures and lighter δ18Oliquid SMOW, whereas thin-vein calcites show apparent higher apparent clumped isotope temperatures and heavier δ18Oliquid SMOW. Pairing clumped isotopes with trace element analysis allow us to conclude that our lower temperature
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Department of Earth Science and Engineering of Imperial College for provide the funding. All members of Carbonate Group Imperial College London for help in the lab and constructive discussion to improve this paper resulting its current form.
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