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The approach and analytical methods employed in this work are identical to those described in Simon and DePaolo (2010) and analyses were made during the same period as this earlier study. Briefly, bulk rock powders (~25 mg) were dissolved in a mixture of mineral acids and combined with a ⁴²Ca‐⁴⁸Ca spike prior to chemical separation. Calcium was purified on cation exchange columns (AG‐50 W‐X8). A ⁴³Ca single spike was used to determined column blanks and yields, which were ~10–15 ng and ~99.5%, respectively. About 3 μg of purified calcium was loaded in dilute HNO₃ onto rhenium filaments with dilute H₃PO₄ for each mass spectrometric analysis. Calcium isotope ratios were measured with a Thermo‐Finnigan Triton thermal ionization multi‐collector mass spectrometer (TIMS) at the University of California in the Center for Isotope Geochemistry (Table 3.1). The ³⁹K, ⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca, ⁴⁸Ca, and ⁴⁹Ti ion beams were measured in a multi‐step static cup configuration. The magnitude of mass interference from ⁴⁰K and ⁴⁸Ti was monitored and found to be insignificant; no corrections for ⁴⁰K and ⁴⁸Ti were applied. The ⁴²Ca‐⁴⁸Ca double spike method, e.g., Russell et al. (1978), was employed to correct for instrumental mass‐fractionation. The tracer ⁴²Ca/⁴⁸Ca ratio of 0.8364±29 used in this study was determined by isotopic measurements of tracer–standard mixtures, and by assuming that the ⁴²Ca/⁴⁴Ca ratio of the calcium standard is 0.31221, the value obtained by Marshall and DePaolo (1982) and Russell et al. (1978). Due to the higher abundances of ⁴⁰Ca (96.94%) and ⁴⁴Ca (2.09%) stable calcium isotope variations are commonly reported as δ⁴⁴Ca = (⁴⁴Ca/⁴⁰Ca)_sample/(⁴⁴Ca/⁴⁰Ca)_standard – 1)·1000, where it is important to note that the most abundant isotope, ⁴⁰Ca, can also be produced by the radioactive decay of ⁴⁰K (half‐life of ~1.25 Ga). This is typically not a concern for young mafic rocks, but in old rocks, stable calcium isotope variations (δ⁴⁴Ca) must be corrected for potential radiogenic ingrowth of ⁴⁰Ca.

Table 3.1 Mass‐dependent calcium isotope compositions of igneous rocks and standards.

SampleBSE	Age (Ma)	44Ca/40Ca	2σ	43Ca/40Ca	2σ	n	Source
Peridotite (avg)Peridotite (avg) Komatiite (avg)		0.010.00–0.02	0.040.050.16	–0.05––	0.07––	2147	Simon and DePaolo (2010)Kang et al. (2017) Amsellem et al. (2019)
Basalts
BCR‐1	15	–0.08	0.04	–0.05	0.07	2	Simon and DePaolo (2010)
BCR‐2	15	–0.09	0.06			1	Simon et al. (2017)
OIB (avg)	1	0.00	0.04	–	–	7	Huang et al. (2010)
Koolau OIB (avg)	1	–0.15	0.06	–	–	6	Huang et al. (2011)
Mahukona OIB (avg)	1	–0.02	0.19	–	–	2	Huang et al. (2011)
Manua Kea OIB (avg)	1	–0.02	0.02	–	–	2	Huang et al. (2011)
BHVO‐1 OIB	1	0.01	0.05	–	–	1	Huang et al. (2011)
BHVO‐2 OIB	1	–0.05	0.05	–	–	1	Bermingham et al. (2018)
average		–0.05	0.02
Arc Lavas
AT‐50		–0.08	0.14	–0.01	0.10	1
YO1		–0.16	0.14	–0.18	0.11	1
TE1		–0.12	0.14	–0.15	0.10	1
SC2		0.06	0.05	0.07	0.05	4	Simon and DePaolo (2010)
average		–0.07	0.05	–0.07	0.06
Carbonatites
Laacher See 129OL lava 2007OL lava 4‐7‐08	0.0129000.0000130.000011	–0.39–0.130.05	0.140.130.14	–0.27–0.060.06	0.250.100.10	121
Standards
SRM915aSeawater (IAPSO) Seawater (GeoB 9506‐4)		–0.950.810.94	0.040.160.04	–0.77––	0.06––	111	this study Simon et al. (2017)Bermingham et al. (2018)

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 (Δ⁴⁴Ca_BSE‐RM915a = 0.97 ± 0.07‰, 2σ, n = 11) is similar to the difference found elsewhere, e.g., Table 3.2.