The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Shen, Bing; Jacobsen, Ben; Lee, Cin-Ty A.; Yin, Qing‐Zhu; Morton, Douglas M.

doi:10.1073/pnas.0910663106

Cited by 114 publications

(55 citation statements)

References 42 publications

(63 reference statements)

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“…As with Li, Mg isotopes do not appear to distinguish between I-and S-type granites of the LFB (Li et al, 2010), with both groups showing little variation away from mantle values. However, Mg isotopes in granites from China (Li et al, 2010) and California (Shen et al, 2009) appear to reflect, in both cases, mixing between juvenile, mantle-derived material and an evolved (weathered) crustal source.…”

Section: Introductionmentioning

confidence: 99%

The silicon isotope composition of granites

Savage

Georg

Williams

et al. 2012

Geochimica et Cosmochimica Acta

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Because weathering results in preferential incorporation of the lighter Si isotopes into clay minerals, the Si in magmas derived from (meta)sedimentary lithologies enriched in such phases should also be isotopically light, relative to melts derived from igneous sources. With the advent of new high precision techniques it should therefore be possible to resolve such differences. Hence, Si isotopes have to potential to distinguish between, and characterise, different magmatic source regions.To explore this, here we report the Si isotopic compositions of a suite of Phanerozoic I-, S-and A-type granites and mineral separates, as well as samples of associated country rock. The whole-rock data fall between δ30Si = −0.40 and −0.11 ± 0.04‰ (95% s.e.). This range is more limited than that defined by previous low precision measurements of granites, but is significantly broader than that displayed by more recent, high precision, measurements of high-Si extrusive igneous rocks. As predicted, S-type granites are, on average, isotopically lighter than I-and A-type samples, and negative correlations with alumina saturation index (ASI) values and initial 87Sr/86Sr provide evidence that Si isotope variations in granites can be explained by variable incorporation of an enriched sedimentary component. However, the relatively small isotopic perturbations and apparent degree of overlap between granite affinities suggest that Si isotopes are not as sensitive to sedimentary input, relative to more established proxies such as, for example, O isotopes. The average Si isotope value of all granites analysed in this study is δ30Si = −0.23 ± 0.15‰ (2 s.d.), which is entirely consistent with a recently published estimate for the continental crust, based on an andesitic bulk composition and the well defined relationship between Si isotopes and magmatic fractionation. AbstractBecause weathering results in preferential incorporation of the lighter Si isotopes into clay minerals, the Si in magmas derived from (meta)sedimentary lithologies enriched in such phases should also be isotopically light, relative to melts derived from igneous sources. With the advent of new high precision techniques it should therefore be possible to resolve such differences. Hence, Si isotopes have to potential to distinguish between, and characterise, different magmatic source regions.To explore this, here we report the Si isotopic compositions of a suite of Phanerozoic I-, S-and A-type granites and mineral separates, as well as samples of associated country rock. The whole-rock data fall between d 30 Si = À0.40 and À0.11 ± 0.04& (95% s.e.). This range is more limited than that defined by previous low precision measurements of granites, but is significantly broader than that displayed by more recent, high precision, measurements of high-Si extrusive igneous rocks. As predicted, Stype granites are, on average, isotopically lighter than I-and A-type samples, and negative correlations with alumina saturation index (ASI) values and initial 87 Sr/ 86 Sr provide e...

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Section: Introductionmentioning

confidence: 99%

The silicon isotope composition of granites

Savage

Georg

Williams

et al. 2012

Geochimica et Cosmochimica Acta

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“…igneous differentiation, metamorphic dehydration, and continental weathering. [7,9,11,14,15,17,19] Nevertheless, debate continues regarding: (1) whether the earth has a chondritic Mg isotopic composition or not, [7][8][9]13,18,20] and (2) the accuracy of Mg isotopic compositions reported by different labs for certain geostandards, such as BCR-1, BCR-2 and San Carlos olivine. [5,9,10,12,13,16,18,[20][21][22][23][24][25] Both debates may reflect the presence of MC-ICP-MS analytical artifacts in high-precision Mg isotopic analyses conducted at different laboratories.…”

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confidence: 99%

“…[5,6] For example, a precision of AE0.07% ( 26 Mg/ 24 Mg, 2SD) can be routinely achieved for MC-ICP-MS Mg isotopic analysis, [7] which is sufficient to discriminate mass-dependent Mg isotopic variations in natural samples. [6] Recent high-precision Mg isotopic studies have significantly increased our knowledge of Mg isotope geochemistry by documenting the Mg isotopic variations in the mantle and crustal rocks, [5,[7][8][9][10][11][12][13][14][15][16][17][18] and illuminating the processes that may produce these variations, e.g. igneous differentiation, metamorphic dehydration, and continental weathering.…”

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Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis

Ling

Sedaghatpour

Teng

et al. 2011

Rapid Comm Mass Spectrometry

143

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The magnesium (Mg) isotopic compositions of 40 seawater samples from the Gulf of Mexico and of one seawater sample from the southwest Hawaii area were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to investigate the homogeneity of Mg isotopes in seawater. The results indicate that the Mg isotopic composition of seawater from the Gulf of Mexico is homogeneous, both vertically and horizontally, with average values for d 25 Mg = À0.43 AE 0.06 (2SD, n = 90). The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precipitation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Considering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excellent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis.

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“…Дан-ное исследование показывает, что включение избыточного Si на M-позиции в структуре энстатита ком-пенсируется по зарядам отсутствием атомного Mg 2+ . Наличие включений энстатита, обогащенного Si, позволяет предположить, что вмещающие их сапфиры могли кристаллизоваться из контаминированных расплавов в зоне контакта между коровым материалом (обогащенным Si и Al) и базальтовой магмой -производной мантии (источник Mg) [Arndt, Goldstein, 1989;Hofmann, 1997;Hawkesworth, Kemp, 2006;Shen et al, 2009].…”

Section: рис 3 микрофотографии включенийunclassified

MINERAL INCLUSIONS IN SAPPHIRE FROM THE BASALT-RELATED DEPOSIT IN BO PHLOI, (Kanchanaburi, Western Thailand): INDICATION OF THEIR GENESIS

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The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Cited by 114 publications

References 42 publications

The silicon isotope composition of granites

The silicon isotope composition of granites

Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis

MINERAL INCLUSIONS IN SAPPHIRE FROM THE BASALT-RELATED DEPOSIT IN BO PHLOI, (Kanchanaburi, Western Thailand): INDICATION OF THEIR GENESIS

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