Magnesium isotope fractionation during carbonatite magmatism at Oldoinyo Lengai, Tanzania

Li, Wang-Ye; Teng, Fang‐Zhen; Halama, Ralf; Keller, Jörg; Klaudius, Jurgis

doi:10.1016/j.epsl.2016.03.034

Cited by 35 publications

(24 citation statements)

References 58 publications

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“…This was identified in natrocarbonatites at Oldoinyo Lengai which have δ 26 Mg compositions of 0.13–0.37‰40. Although the heavy Mg isotope has been demonstrated to be preferentially incorporated into silicate relative to carbonate melts during liquid immiscibility, such fractionation is relatively small (−0.3 to 0.09‰)40. Due to the high δ 26 Mg composition of the lower, middle (−0.26 to −0.21‰)42 and upper continental crust (−0.52 to 0.92)43, crustal contamination will increase the Mg isotope ratio in mantle-derived magmas.…”

Section: Resultsmentioning

confidence: 97%

“…suggested that the light Mg isotope preferentially enters into crystallizing carbonates, leading to differentiated carbonatite enriched in the heavy Mg isotope. This was identified in natrocarbonatites at Oldoinyo Lengai which have δ 26 Mg compositions of 0.13–0.37‰40. Although the heavy Mg isotope has been demonstrated to be preferentially incorporated into silicate relative to carbonate melts during liquid immiscibility, such fractionation is relatively small (−0.3 to 0.09‰)40.…”

Section: Resultsmentioning

confidence: 99%

“…The carbonatites in the southernmost Qinling host a large LREE deposit39. Both carbonatites have significantly lower Mg isotope ratios than typical mantle derived rocks, including Mid Ocean Ridge Basalt (MORB), Ocean Island Basalt (OIB) and peridotite xenoliths (−0.25 ± 0.07‰)37, as well as intra-continental rift-hosted carbonatites from Oldoinyo Lengai (0.13–0.37‰)40, with δ 26 Mg ranging from −1.28 to −1.07‰ and −1.89 to −1.37‰ at NQ and Miaoya, respectively (Fig. 5).…”

Section: Resultsmentioning

confidence: 99%

“…There is insignificant Mg isotopic fractionation during partial melting of the mantle, or from olivine and pyroxene crystallization374041. Li et al 40. suggested that the light Mg isotope preferentially enters into crystallizing carbonates, leading to differentiated carbonatite enriched in the heavy Mg isotope.…”

Section: Resultsmentioning

confidence: 99%

“…Coloured bars show the reference compositions of mantle peridotites37, MORB37, OIB37, upper43 and lower crusts42, and Oldoinyo Lengai rifting-carbonatites40, featuring with distinct Mg isotopic compositions, which are higher than those of the orogenic carbonatites. The carbonatites have similar δ 26 Mg values to marine dolomites44.…”

Section: Figurementioning

confidence: 99%

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Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China

Song

Smith

et al. 2016

Sci Rep

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Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ26Mg values (−1.89 to −1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.

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Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Figurementioning

confidence: 99%

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Origin of unusual HREE-Mo-rich carbonatites in the Qinling orogen, China

Song

Smith

et al. 2016

Sci Rep

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High‐precision magnesium isotope analysis of geological and environmental reference materials by multiple‐collector inductively coupled plasma mass spectrometry

Gao

et al. 2019

Rapid Comm Mass Spectrometry

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Rationale High‐precision magnesium (Mg) isotopic analysis for geological and environmental reference materials is a prerequisite to ensure data quality before using Mg isotopes to trace geochemical and environmental processes. However, the Mg‐isotopic ratios of many commonly used reference materials, especially sediments, have rarely been reported. Furthermore, published values for some commonly used reference materials exhibit a significant inconsistency across laboratories and thus need more data comparison. Methods We developed different Mg purification schemes for silicate rocks, high‐Ca carbonates and carbonatites, and high‐Mn samples because of their significantly different matrices. We then used synthetic solutions to evaluate potential effects on measurement using multiple‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The accuracy and precision of our procedures were assessed by measurement on both synthetic solutions and well‐studied geostandards. Results The three different schemes for routine, high‐Ca, and high‐Mn samples can remove matrices efficiently with nearly 100% Mg yield. However, the presence of acid molarity and concentration mismatch, matrix elements, and fluctuations in room temperature can significantly affect the precision and accuracy of Mg isotope analysis, and must be avoided. The Mg isotopic ratios of reference materials obtained in this study are identical to the previously published values within ±0.06‰, verifying that our procedures are robust. Conclusions This study presented a thorough set of tests for high precision and accuracy of Mg isotope measurements using MC‐ICP‐MS, which demonstrate reproducibility and accuracy better than 0.05‰ for δ25Mg values and 0.06‰ for δ26Mg values. We reported high‐quality Mg isotopic data for 16 geological and environmental reference materials to aid the inter‐laboratory calibration of Mg isotope measurements in the future.

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