Sen Li, Andrew Schauer, Alexis Licht, Jie Liang, Kate Huntington, Kangning Peng. Clumped Isotope Analysis of Calcite and Dolomite Mixtures Using Selective Acid Extraction. Journal of Earth Science, 2023, 34(3): 726-734. doi: 10.1007/s12583-022-1630-4
Citation:
Sen Li, Andrew Schauer, Alexis Licht, Jie Liang, Kate Huntington, Kangning Peng. Clumped Isotope Analysis of Calcite and Dolomite Mixtures Using Selective Acid Extraction. Journal of Earth Science, 2023, 34(3): 726-734. doi: 10.1007/s12583-022-1630-4
Sen Li, Andrew Schauer, Alexis Licht, Jie Liang, Kate Huntington, Kangning Peng. Clumped Isotope Analysis of Calcite and Dolomite Mixtures Using Selective Acid Extraction. Journal of Earth Science, 2023, 34(3): 726-734. doi: 10.1007/s12583-022-1630-4
Citation:
Sen Li, Andrew Schauer, Alexis Licht, Jie Liang, Kate Huntington, Kangning Peng. Clumped Isotope Analysis of Calcite and Dolomite Mixtures Using Selective Acid Extraction. Journal of Earth Science, 2023, 34(3): 726-734. doi: 10.1007/s12583-022-1630-4
Qingdao Institute of Marine Geology, Qingdao 266237, China
2.
Department of Earth and Space Sciences, University of Washington, Seattle 98195, USA
3.
Centre Européen de Recherche et d' Enseignement des Géosciences de l' Environnement (Cerege), UMR 7330, Aix-Marseille Université, CNRS, IRD, INRAE, Aix-en-Provence 13545, France
4.
Shandong Provincial Territorial Spatial Ecological Restoration Center, Jinan 250014, China
Acid extraction methods have been used in the last half century to selectively extract the CO2 produced from different carbonate minerals in mixed samples. However, these methods are often time-consuming and labor intensive. Their application to clumped isotope (Δ47) analysis has not been demonstrated. We propose here an acid extraction method with phosphoric acid for bulk stable and clumped isotope analysis that treats mixtures of calcite and dolomite the same regardless of the proportional composition. CO2 evolved from calcite is extracted by allowing a reaction with phosphoric acid to proceed for 10 min at 50 ℃. We then extract CO2 evolved from dolomite by rapid ramping the acid temperature from 50 to 90 ℃ and allowing the reaction to complete. The experimental results show that our method yields accurate calcite and dolomite Δ47 values from mixed samples under different proportional compositions. Our method also displays equal or higher accuracy for calcite δ13C and dolomite δ13C and δ18O values from mixtures when compared to previous studies. Our approach exhibits higher sample throughput than previous methods, is adequate for clumped isotopic analysis and simplifies the reaction progression from over 24 h to less than 2 h, while maintaining relatively high isotopic obtaining accuracy. It yet poorly resolves calcite δ18O values, as found with previous methods.
Coexistence of carbonate minerals such as calcite, dolomite, ankerite, siderite and magnesite are commonly observed in sedimentary rocks, hydrothermal deposits and carbonatites (Baudrand et al., 2012; Ray and Ramesh, 1998). The carbon (δ13C) and oxygen (δ18O) isotopic composition in these carbonates provides clues on the salinity and temperature of aqueous fluids involved during carbonate growth and later diagenesis (Dean et al., 2015; Luzon et al., 2009; Leng and Marshall, 2004) and helps determining the genesis of ore deposits (Yang et al., 2009; Maglambayan et al., 2001). However, measuring the isotopic composition of distinct carbonate phases occurring in natural mixtures is not straightforward because the different phases cannot be physically separated in most cases (Baudrand et al., 2012). The differential reactivity of CaCO3 (calcite and aragonite) and CaMg(CO3)2 (dolomite) with phosphoric acid is commonly used to selectively extract the CO2 released by the two phases and track the evolution of its isotopic composition; it has thus been widely adopted in the last half century for carbonate phase extraction methods (Bustillo et al., 2017; Chen et al., 2016; Aloisi et al., 2013; Clayton and Jones, 1968; Epstein et al., 1964).
The selective acid extraction method with phosphoric acid has been refined through time as researchers discovered that reaction rate and isotopic composition are significantly affected by grain size, reaction temperature, and dolomite stoichiometry (Liu et al., 2018; Baudrand et al., 2012; Kyser et al., 2002; Ray and Ramesh, 1998; Al-aasm et al., 1990; Wada and Suzuki, 1983; Walters et al., 1972). A three-step extraction procedure was gradually developed, with varying experimental parameters (Table 1). First, the mixture reacts with phosphoric acid for 20 min to 2 h at 25 ℃, and the CO2 evolved from this reaction is considered to carry the isotopic signature of calcite. Then, the acid digestion is continued for up to 24 h, discarding the produced CO2 because it is considered a mixture of calcite and dolomite. Finally, the remaining carbonate is further reacted with acid for a long period (up to 24 h) and sometimes at higher temperature (50 ℃), and the produced CO2 in this step is considered to carry the isotopic signature of dolomite.
Table
1.
A summary of previously published selective acid extraction procedures
However, this 3-step procedure is labor intensive and time-consuming (Liu et al., 2018) and is sometimes considered as only suitable for samples containing 1 : 1 calcite-dolomite pairs (Ray and Ramesh, 1998). The suggested grain-size (i.e., 5–44 and 0.5–5 μm) or additional preparation steps (i.e., pre-concentration of calcite to > 50%) in various versions of the procedure add complexity to the process (Table 1). X-ray diffractometry (XRD) analysis of samples is required for optimizing experimental parameters such as reaction temperature and time (Liu et al., 2018; Ray and Ramesh, 1998). Changes of δ13C and δ18O values for calcite and dolomite with varying duration of the different steps are recurrently documented in previous works (Liu et al., 2018; Baudrand et al., 2012; Yui and Gong, 2003; Al-Aasm et al., 1990), indicating kinetic isotope effects induced by an only partial reaction of calcite during steps 1 and 2.
In addition to these experimental limitations, the effect of selective phosphoric acid method on "clumped isotope" (i.e., Δ47) analysis remains to be addressed. Carbonate clumped isotope provide an independent proxy for carbonate growth temperature, which completes the conventional isotopic toolkit (Eiler, 2014, 2011). Combined with carbonate δ18O values and thermometry equations, carbonate clumped isotope thermometry can be used for paleoclimate reconstruction (Frantz et al., 2014; van de Velde et al., 2013; Affek, 2012), paleoaltimetry (Huntington and Lechler, 2015; Lechler et al., 2013), and the study of burial diagenetic processes and fluids (Dale et al., 2014; Bristow et al., 2011). In recent years, the impact of solid-state diffusion on Δ47 values (Lloyd et al., 2017), temperature-time paths (Cong et al., 2021; Mangenot et al., 2018; Staudigel et al., 2018), kinetic and other disequilibrium effects (Guo, 2020) have been explored. Understanding the effect and uncertainty of selective phosphoric acid methods on clumped isotope values is necessary to acquire different clumped isotope data from samples with more than one phase of carbonate.
These limitations call for further optimization of the acid extraction procedure. In this study, we propose a new selective acid extraction method that is valid for both clumped and bulk isotope analysis of carbonates. We evaluate the uncertainty and accuracy of clumped and bulk isotopic analysis for calcite and dolomite phases by optimizing the procedure with a series of artificial carbonate mixtures with different calcite : dolomite ratios.
1.
MATERIALS AND METHODS
1.1
Sample Preparation
Pure calcite and dolomite crystals were used to prepare calcite-dolomite mixtures (Table 2). The calcite end-member is from a 100% pure calcium carbonate (i.e., C64-Certified ACS, Calcium Carbonate, Lot 775662, Fisher Scientific, Schauer et al., 2016). The dolomite end-member is from a 100% pure dolostone (as assessed through detailed microscopic observation) obtained from the minerals collection of the Department of Earth and Space Sciences, University of Washington. Both samples were crushed and ground to powder, and were sieved to save the fraction between 75 and 84 μm. Such uniform fine-grained powder was used to enhance the reaction rate and reduce the grain size effect on the reaction (Liu et al., 2018). The δ13C, δ18O and Δ47 data of pure calcite and dolomite at different reaction temperatures are listed in Table 2 as "expected values" throughout the paper. We chose two weighted proportions of the two carbonate powders to represent the possible range of calcite-dolomite ratios: 70% calcite : 30% dolomite (Mix-7v3), 30% calcite : 70% dolomite (Mix-3v7). The powders were weighed respectively according to the above proportions, put into 5 mm × 9 mm silver capsules, and stirred with a utensil.
Table
2.
Calcite and dolomite endmembers used in the present study. The isotopic values at different reaction temperatures are referred to as "expected values" throughout the text. The dolomite 50–90 treatment indicates ramping the reaction temperature from 50 to 90 ℃ rapidly, which is described below
The CO2 produced from individual minerals of mixtures was collected and purified using a carbonate digestion/CO2 purification vacuum line in IsoLab described previously (Burgener et al., 2016). To separately extract CO2 from calcite and dolomite, we modified the routine sample preparation procedure by optimizing the reaction parameters rather than discarding the mixed products. Our procedure is divided into two-steps.
Step 1: Extraction of calcite CO2. We set the acid temperature at 50 ℃ and wait for 10 min reaction time for the calcite-only reaction (duration of the reaction time tested in Test 2 below), and then extract the CO2.
Step 2: Extraction of dolomite CO2. After the extraction of the CO2 in step 1, we isolate the acid bath and ramp its temperature to 90 ℃. We then let the sample reacting at 90 ℃ for ~55 min. This approximate time roughly corresponds to the time needed for (1) the purification of the evolved CO2 extracted in step 1 in our purification vacuum line (method described in Burgener et al., 2016; pure CO2 is cryogenically separated from water on an automated stainless steel vacuum line using an ethanol-dry ice slush trap, isolated in a liquid N2 trap, passed through a Porapaq Q trap, and finally sealed in a Pyrex break seal); and (2) the automated cleaning of the vacuum line. We then open the outlet valve of the acid bath and wait for an additional 10 min. The total dolomite reaction time in step 2 is ~65 min. The CO2 extracted from step 2 is then purified as done during step 1.
We made a series of tests to optimize the procedure that are synthesized in Table 3.
Test 1: This test is on pure calcite and pure dolomite samples only. For pure calcite, we tested the impact of reaction temperature (50 and 90 ℃) for a given reaction time (10 min). For pure dolomite, we investigated the adequate reaction time (10, 20, and 30 min) at 90 ℃ to completely digest the sample, and tested the effect of different acid temperature ramp times (13.5 and 6 min).
Test 2: The purpose of test 2 is to study the impact of varying reaction times (6 and 10 min) for the calcite-only reaction (step 1). This test was only done with dolomite-rich mixtures (Mix-3v7).
Test 3: The purpose of test 3 is to study the influence of the acid bath ramp temperature pace in step 2. We tested two paces for the ramp: 13.5 and 6 min. Once 90 ℃ is reached, the reaction time at 90 ℃ for slow ramp (13.5 min) is ~51.5 min and rapid ramp (6 min) is ~59 min, to keep a total reaction time of ca. 65 min for step 2.
1.3
Isotopic Analyses
Carbon, oxygen and clumped isotopic composition of the collected CO2 were measured in IsoLab, University of Washington as described in Schauer et al. (2016). The Δ47 values were calculated according to Petersen et al. (2019). A series of carbonate reference materials were used to place all δ13C and δ18O on the VPDB and VSMOW scales, respectively, and a suite of reference frame gases were used to place Δ47 values on the CDES scale (Schauer et al., 2016).
Individual measurements are summarized in Table 4. In order to compare whether there are significant differences in isotopic composition between CO2 produced from different mixtures, we calculated 2-sample t-tests at 95% confidence using IBM SPSS® software version 19.0 for calculation.
Table
4.
Bulk and clumped isotopic compositions of pure carbonate samples and artificial mixtures under different analytical conditions. N indicates number of replicates, SE. Standard Error, SR. slow ramp, RR. rapid ramp
2.1
Test 1: Impact of Reaction Time and Heating Strategy on the Isotopic Composition of Pure Calcite and Pure Dolomite
According to Liu et al. (2018), pure dolomite at 75–80 μm grain-size yields only ~8% CO2 after 45 min at 50 ℃, while pure calcite reaches 100% yield in 5 min. Pure dolomite at 90 ℃ reacts much faster, and reaction times of 10, 20 and 30 min yield similar results in our experiments. δ13C, δ18O and Δ47 values of pure dolomite reacted for these different reaction times are statistically indistinguishable (Fig. 1). An independent sample t-test on the isotopic values obtained through 10- and 20-min reaction times yields p-values of 0.569 for δ13C, of 0.696 for δ18O, and of 0.665 for Δ47. Besides, the CO2 % yield of individual replicates obtained with a 10-min reaction (99.2%, 98.8%, 91.2%, 71.3%, 68.8%), 20-min reaction (100%, 99.4%, 84.9%) and 30-min reaction (100%, 99.7%, 91.0%) are all close to 100%. Our > 50 min reaction time at 90 ℃ for step 2 ensures the complete digestion of dolomite in mixed samples. These tests show that shorter reaction times (down to 10 min) are also adequate.
Figure
1.
Pure calcite or pure dolomite isotopic values under different experimental conditions. Error bars stand for standard errors. SR. Slow ramp; RR. rapid ramp.
Our test also shows that short temperature ramp times for step 2 increase the accuracy of the isotopic results for the dolomite phase. The δ13C, δ18O and Δ47 values of CO2 produced from pure dolomite is closer to their expected values at 90 ℃ for the short ramp time (6 min) than for the long ramp time (13.5 min; Fig. 1).
The δ13C value obtained from pure calcite at 50 ℃ and pure dolomite at 90 ℃ with short temperature ramp time are statistically indistinguishable from their expected value, obtained at 90 ℃, with a 0.093 p-value for calcite and 0.085 for dolomite. In contrast, the δ18O value from pure calcite at 50 ℃ and pure dolomite are statistically different from their expected values (Fig. 1b). These results emphasize that changing reaction temperatures has little impact on carbon isotopic values but significantly impact oxygen isotopic values. The calcite oxygen isotope difference between phosphoric acid reaction temperature at 50 ℃ and that at 90 ℃ is 0.94‰ in our data (Fig. 1b). The difference of dolomite δ18O values between short ramping and the expected value at 90 ℃ is 1.13‰, and 0.33‰ for rapid ramping (Fig. 1b).
Similar to oxygen isotopes, changing reaction temperature introduces a fractionation in Δ47 that must be accounted for prior to the calculation of carbonate growth temperature (Defliese et al., 2015). Previous work has already quantified the phosphoric acid correction factors relative to reaction at 25 ℃ for calcite, aragonite, and dolomite across a temperature range from 25 to 90 ℃ (Bernasconi et al., 2021; Peterson et al., 2019; Defliese et al., 2015; Henkes et al., 2013; Wacker et al., 2013). In the most recent quantification effort, Petersen et al. (2019) calculated a phosphoric acid correction compared to digestion at 25 ℃ of 0.066‰ at 70 ℃, 0.088‰ at 90 ℃ reactions, respectively. Our data support these results, with an average difference between reactions at 50 and 90 ℃ of 0.046‰ for calcite (Fig. 1c). The difference of Δ47 value with and without temperature ramping is 0.020‰ (long ramp), and 0.016‰ (short ramp; Fig. 1c), but these values cannot be compared with acid temperature correction factors, as most of the step 2 reaction subsequent to the temperature ramp is achieved at 90 ℃.
2.2
Bulk and Clumped Isotopic Composition of Calcite and Dolomite from Mixtures
2.2.1
Results of test 2
This test was performed on the dolomite-dominated mixture (Mix-3v7). The calcite δ13C and Δ47 values with 6- and 10-min reaction time for step 1 are statistically indistinguishable from their expected values (p-value = 0.168 and 0.906, respectively; Fig. 2), while the calcite δ18O value is different and offset from its expected value regardless of reaction time. The dolomite δ13C is also statistically indistinguishable from its expected value (p-value = 0.157) regardless of reaction time; while the Δ47 value is statistically indistinguishable from its expected value for the long (10 min) reaction time only (p-value = 0.091), while the dolomite δ18O value is different and offset regardless of reaction time. These results highlight that the long reaction time (10 min) is best to acquire accurate δ13C and Δ47 values but does not allow a great accuracy for δ18O values. The accuracy in δ18O values decreases with increasing dolomite content (Fig. 2b), as observed in previous studies (see next subsection).
Figure
2.
Bulk and clumped isotopic composition of calcite and dolomite from mixtures with different reaction tests. Results from pure calcite at 50 ℃ (in green shade, 1 S.E.) and pure dolomite at 90 ℃ (with short 50 to 90 ℃ temperature ramp time; in red shade, 1 S.E.) are also displayed. Samples in black have varying reaction time for step 1 (6/10 min) and slow ramp time for step 2 (13.5 min); samples in orange have 10 min reaction time for step 1 and slow ramp time for step 2 (13.5 min); samples in blue have 10 min reaction time for step 1 and rapid ramp time for step 2 (6 min). Error bars are 1 S.E.; SR. slow ramp; RR. rapid ramp.
Unsurprisingly, changing ramp time has no impact on step 1 results but significant impact on step 2. Dolomite δ13C and Δ47 values are gradually offset from their expected values with decreasing dolomite content in the mixture (Fig. 2c), but this offset is minimized while using the rapid (6 min) ramp time. For dolomite-dominated mixtures with the rapid ramp time, δ13C (p-value = 0.360), δ18O (p-value = 0.100), and Δ47 values (p-value = 0.067) are all statistically indistinguishable from expected values. For calcite-dominated mixtures with the rapid ramp time, δ13C (p-value = 0.073) and δ18O (p-value = 0.887) values are statistically indistinguishable from their expected values, but the accuracy decreases for the Δ47 value (p-value = 0.012). The long (13.5 min) ramp time yield greater deviations from the expected value, especially for δ18O and Δ47 values (Fig. 2).
2.2.3
Summary
We conclude from the above tests that accurate calcite and dolomite δ13C and Δ47 values in different mixtures can be acquired with the proposed parameters for step 1 (10 min of reaction time at 50 ℃) and fast ramp time for step 2 (6 min). Using the fast ramp time helps acquire more accurate dolomite Δ47 and δ18O values; the accuracy of δ18O values for the calcite phase significantly decreases with increasing dolomite content, and accurate δ18O values are difficult to obtain for step 1.
2.3
Comparison with Previous Studies
We compare the accuracy of our δ13C and δ18O values for the calcite and dolomite components in Table 5 with the accuracy of previous methods (Liu et al., 2018; Baudrand et al., 2012; Ray and Ramesh, 1998). The δ18O values in VSMOW scale are converted to the VPDB reference frame through the following equation: δ18OVPDB = 0.970 01 × δ18OVSMOW – 29.99‰ (Brand et al., 2014).
Table
5.
Carbon and oxygen isotopic values from different mixtures obtained by previous studies and this study. Δ13CECD and Δ18OECD are the absolute difference between the expected δ13C (or δ18O) of pure calcite and the expected δ13C (or δ18O) of pure dolomite. The results are in VPDB scale (‰)
To estimate gains in accuracy, previous studies (Liu et al., 2018; Baudrand et al., 2012) only compared the difference between the measured and expected δ13C (Δ13C = δ13Cexpected – δ13Cmeasured) and δ18O values (Δ18O = δ18Oexpected – δ18Omeasured) for calcite and dolomite from different mixtures. However, the amplitude of Δ13C and Δ18O should significantly change depending on how wide is the difference between the isotopic composition of pure calcite and pure dolomite in these studies. Instead, we compare the Δ13C/Δ13CECD, where Δ13CECD is the difference between the expected δ13C of calcite and the expected δ13C of dolomite in different studies (same for δ18O): Δ13CECD = δ13Cexpected for calcite – δ13Cexpected for dolomite (Fig. 3).
Figure
3.
Accuracy of isotopic analysis for calcite and dolomite from various mixtures in this and previous studies. Δ13Ccalcite and Δ13Cdolomite are the absolute difference between the measured calcite (or dolomite); δ13C from different mixtures and expected pure calcite (or dolomite); δ13C values (same for Δ18Ocalcite and Δ18Odolomite). Δ13CECD and Δ18OECD are the absolute difference between the expected δ13C (or δ18O) of pure calcite and the expected δ13C (or δ18O) of pure dolomite.
We observed an overall increase in accuracy for calcite δ13C and δ18O values with increasing calcite content (Figs. 3a, 3c). Calcite δ13C and δ18O measurement accuracy for the method of Liu et al. (2018) is the highest regardless of the calcite: dolomite ratio; the accuracy of this method for calcite δ18O results is yet among the lowest. The accuracy for calcite δ13C measurements reached by our approach falls within the range of previous studies.
Overall, the accuracy of dolomite δ13C and δ18O values gradually decrease from their expected values with increasing calcite content. The accuracy for dolomite isotopic measurements reached by our approach is better than most previous studies, particularly for δ18O values (Fig. 3d). Our method performs thus as well, if not better than previous acid extraction methods.
3.
CONCLUSION
We developed a new method for the selective extraction of CO2 from of calcite and dolomite mixtures that we subsequently tested with artificial calcite: dolomite mixtures. Our method reduces the previous three-step procedure to only two steps and thus greatly decreases the experimental time from over 24 h to less than 2 h. Our tests show that our method provides a similar accuracy for bulk stable isotope measurement than previous methods; we also show that our method is accurate for clumped isotopic analysis. The new method, like previous methods, fails providing accurate δ18O values when one carbonate phase overwhelms the other; the method is thus best designed for high throughput analyses that do not require high accuracy results for stable oxygen isotopic data.
ACKNOWLEDGMENTS:
This project was funded by the fellowship of the China Postdoctoral Science Foundation (No. 2020M682134), the National Natural Science Foundation of China (Nos. 41872149, 42076220), and the Shandong Postdoctoral Innovation Research Project. The final publication is available at Springer via https://doi.org/10.1007/s12583-022-1630-4.
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Table
2.
Calcite and dolomite endmembers used in the present study. The isotopic values at different reaction temperatures are referred to as "expected values" throughout the text. The dolomite 50–90 treatment indicates ramping the reaction temperature from 50 to 90 ℃ rapidly, which is described below
Table
4.
Bulk and clumped isotopic compositions of pure carbonate samples and artificial mixtures under different analytical conditions. N indicates number of replicates, SE. Standard Error, SR. slow ramp, RR. rapid ramp
Table
5.
Carbon and oxygen isotopic values from different mixtures obtained by previous studies and this study. Δ13CECD and Δ18OECD are the absolute difference between the expected δ13C (or δ18O) of pure calcite and the expected δ13C (or δ18O) of pure dolomite. The results are in VPDB scale (‰)
Figure 1. Pure calcite or pure dolomite isotopic values under different experimental conditions. Error bars stand for standard errors. SR. Slow ramp; RR. rapid ramp.
Figure 2. Bulk and clumped isotopic composition of calcite and dolomite from mixtures with different reaction tests. Results from pure calcite at 50 ℃ (in green shade, 1 S.E.) and pure dolomite at 90 ℃ (with short 50 to 90 ℃ temperature ramp time; in red shade, 1 S.E.) are also displayed. Samples in black have varying reaction time for step 1 (6/10 min) and slow ramp time for step 2 (13.5 min); samples in orange have 10 min reaction time for step 1 and slow ramp time for step 2 (13.5 min); samples in blue have 10 min reaction time for step 1 and rapid ramp time for step 2 (6 min). Error bars are 1 S.E.; SR. slow ramp; RR. rapid ramp.
Figure 3. Accuracy of isotopic analysis for calcite and dolomite from various mixtures in this and previous studies. Δ13Ccalcite and Δ13Cdolomite are the absolute difference between the measured calcite (or dolomite); δ13C from different mixtures and expected pure calcite (or dolomite); δ13C values (same for Δ18Ocalcite and Δ18Odolomite). Δ13CECD and Δ18OECD are the absolute difference between the expected δ13C (or δ18O) of pure calcite and the expected δ13C (or δ18O) of pure dolomite.
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