Isotopic Constraints on Earth System Processes. Группа авторов

al. (2008).

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	Скачать книгу В начало < 34 35 36 37 38 39 40 41 42 43 > В конец