0.05
Note: All reported where difference between BSE to SRM915a is 0.95 (Antonelli and Simon, 2020), cf. Simon and DePaolo (2010) saw an intrinsic effect in SRM915a, which leads to ~0.1 per mil increase in measured values reported relative to SRM915a (see text).
More than four different reference materials are currently used to define δ44Ca (see inter‐conversions in Antonelli & Simon, 2020). These are igneous samples that represent BSE, e.g., unmetamorphosed peridotites, komatiites, and basaltic rocks from Earth and other terrestrial planets, carbonate standard SRM915a, synthetic carbonate standard SRM915b, and modern seawater. In this study, reported values assume BSE = 0.0‰ (reported as deviations from 44Ca/40Ca = 0.0212094 ± 3 and 43Ca/40Ca = 726.840 ± 45; see Simon & DePaolo, 2010). This follows the approach of DePaolo (2004). When multiple measurements were made, the values listed in Table 3.1 are weighted means with uncertainties of two standard errors in the mean and are corrected for age and/or intrinsic 44Ca/40Ca ratio based on measurements reported by Simon et al. (2009). The reported uncertainties are typically less than the 2SD long‐term reproducibility of the SRM915a standard, which is ± 0.14 and ± 0.20‰ for δ44Ca and δ43Ca, respectively. Based on my experience, this is the current resolution of the technique and, therefore, I focus on δ44Ca differences that are greater than ~0.15‰. When only an individual measurement is reported, I assign the 2SD reproducibility of the SRM915a standard.
Measured values of SRM915a were δ44Ca = −0.97‰ and δ43Ca = −0.77‰. Correcting for their intrinsic 44Ca/40Ca ratio that is slightly higher than most planetary materials (see Mills et al., 2018; Simon et al., 2009), shifts the values to δ44Ca = −0.88‰ and δ43Ca= −0.68‰. All data in Table 3.1 and the electronic supplement are reported corrected to the value of SRM915a (δ44Ca = –0.95‰) as recommended by Antonelli and Simon (2020). For reference, the measured difference between the average of repeat analyses of the well‐known SRM915a standard and estimates of the BSE for this work (Δ44CaBSE‐RM915a = 0.97 ± 0.07‰, 2σ, n = 11) is similar to the difference found elsewhere, e.g., Table 3.2.
3.2.2. Igneous Samples Characterized for Calcium Isotope Composition
Four Central American arc basalts were selected for this study. The recent volcanism along the Central American volcanic arc results from subduction of the Cocos Plate beneath the Caribbean Plate (Fig. 3.1) and consists of two regions of magmatism, the volcanic front and back‐arc volcanism (Carr et al., 1990). Three of the studied arc samples come from the volcanic front and one is from a back‐arc volcano. From north to south the samples include: Atitlan volcano, Guatemala (AT‐50), back‐arc Yohoa volcano, Honduras (YO1), San Cristobal volcano, Nicaragua (SC2), and Telica volcano, Nicaragua (TE1). Trace elements ratios (e.g., Ba/La, Ba/Th, Sr/Nb, U/Th) measured in bulk samples and in olivine‐bearing melt inclusions for these samples exhibit a significant amount of the geochemical variation seen both locally and regionally along the volcanic arc that is thought to reflect the influence of subducted sediments (Fig. 3.2; Patino et al., 2000; Sadofsky et al. 2008). Characterization of sedimentary core sampled off the coast at Deep Sea Drilling Program Site 495 indicates early carbonate deposition followed by later hemipelagic deposition involving large differences in incompatible elements and element ratios (e.g., Ba/Th). These differences can be used to identify sediment contribution and even possibly to distinguish between carbonate deposition and hemipelagic deposition (see Fig. 3.2). In particular, the carbonate sediments are believed to provide a distinctive geochemical signature that has been used to track the flux of sedimentary addition to the arc magmas (Patino et al., 2000). Bulk powders of the samples were obtained from M. Carr. Their major, trace element, and isotopic compositions have been reported by Bolge et al. (2009), Carr et al. (1990), Feigenson (1986), Leeman et al. (1994), and Patino et al. (1997; 2000), see electronic supplement.
Table 3.2 Difference between igneous rocks used to estimate δ44Ca of BSE and SRM915a standard for various research laboratories.
| Lab Facilities | Method | 44Ca/40CaBSE‐SRM915a | 2σ | Source |
|---|---|---|---|---|
| Center for Isotope Geochemistry, University of California at Berkeley | DS‐TIMS | 0.97 | 0.07 | Simon & DePaolo (2010); this work |
| Center for Isotope Cosmochemistry & Geochronology, NASA Johnson Space Center | DS‐TIMS | 0.96 | 0.04 | Simon et al. (2017) |
| Isotope Research Geo‐ and Cosmochemistry Group, Harvard University | DS‐TIMS | 0.97 | 0.04 | Huang et al. (2011) |
| State Key Laboratory of Isotope Geochemistry, Guangzhouz Institute of Geochemistry, Chinese Academy of Sciences | DS‐TIMS | 0.94 | 0.04 | Kang et al. (2017) |
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