3.2. ANALYTICAL METHODS AND SAMPLES
3.2.1. Double‐spike Thermal Ionization Mass Spectrometry Calcium Isotope Measurements
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 |
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