The silicon isotope composition of granites

Abstract

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...

Fe and Si isotope variations at Cedar Butte volcano; insight into magmatic differentiation

“…In assessing δ 30 Si variations in granites, Savage et al (2012Savage et al ( , 2014 found A-and I-type granites generally conform to the igneous magmatic differentiation trend but that S-type granites deviated to significantly lower δ 30 Si. The light isotopic signatures of the latter are attributed to the partial melting of isotopically light metasedimentary components (Savage et al, 2012(Savage et al, , 2014 during the S-type granite genesis.…”

“…Thanks to recent advances in mass spectrometry, several stable isotopic systems including Fe (e.g., Poitrasson and Freydier, 2005;Heimann et al, 2008;Schuessler et al, 2009) and Si (e.g., Douthitt, 1982;Ding et al, 1996;Savage et al, 2011Savage et al, , 2012Zambardi and Poitrasson, 2011) now show analytically significant and systematic variations during magmatic differentiation. While these data provide new insight into magmatic differentiation, explanations of how the fractionations occur remain debated, in part due to different isotopic systems being used in separate studies.…”

The silicon isotope composition of the upper continental crust

“…The two Archaean-age Pilbara shales from Australia are 259 both isotopically much lighter than igneous rocks (sample Pg-7 displays the lightest δ 30 Si value 260 analysed in this study, viz. The data described above show that, with respect to Si isotopes, the UCC is heterogeneous 277 relative to the mantle and igneous rocks (Savage et al, 2010(Savage et al, , 2011(Savage et al, , 2012. The following discussion 278 concerns how these data can be used to constrain the Si isotopic composition of the modern day UCC, 279 as well as interpreting the Si isotope composition of clastic sedimentary material in terms of what is 280 already known about possible sources and Si isotope fractionation as a result of low temperature, 281 critical zone, processes.…”

The silicon isotopic composition of fine-grained river sediments and its relation to climate and lithology

“…While pristine (unweathered) silicate rocks usually display uniform Si isotopic compositions on continents (d 30 Si $ À0.3 ± 0.2‰; e.g. Savage et al, 2010Savage et al, , 2012), their alteration leads to weathering products having very distinctive Si isotopic ratios: a dissolved pool characterized by positive d 30 Si values (up to $+4.7‰; e.g. De la https://doi.org/10.1016/j.gca.2018.03.015 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved.…”