In geochemical diagrams, granitoids define 'trends' that reflect increasing differentiation or melting degree. The position of an individual sample in such a trend, whilst linked to the temperature of equilibration, is difficult to interpret. On the other hand, the positions of the trends within the geochemical space (and not the position of a sample within a trend) carry important genetic information, as they reflect the nature of the source (degree of enrichment) and the depth of melting. This paper discusses the interpretation of geochemical trends, to extract information relating to the sources of granitoid magmas and the depth of melting....

Sample (material)

10.1130/2010.2472(03)

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Citations (41)

Performing process-oriented investigations involving mass transfer using Rcrust: a new phase equilibrium modelling tool

Figshare (2019)

Matthew Jason Mayne Gary Stevens Jean‐François Moyen Tim Johnson

Files for reproducing Figures 3–7. To open these, download Rcrust from https://www.sun.ac.za/english/faculty/science/earthsciences/rcrust and replace the 'Projects' and 'data' folders with those in the zipped folder. Open each project by double clicking the appropriate x.RData file.

10.6084/m9.figshare.7667912

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The Impact of Measurement Scale on the Univariate Statistics of K, Th, and U in the Earth Crust

Earth and Space Science (2021)

D. Baratoux M. Fall P. Y. Meslin Mark Jessell O. Vanderhaeghe

Abstract The univariate statistics of Potassium (K), thorium (Th), and uranium (U) concentrations, in the Earth’s oceanic and continental crust are examined by different techniques. The frequency distributions of the concentrations of these elements in the oceanic crust are derived from a global catalog of mid‐ocean ridge basalts. Their frequency distributions of concentrations in the continental crust are illustrated by the North Pilbara Craton, and the West Africa Craton. For these two cratons, the distributions of K, Th, and U derived from geochemical analyses of several thousand whole rock samples differ significantly from those derived from airborne radiometric surveys. The distributions from airborne surveys tends to be more symmetric with smaller standard deviations than the right‐skewed distributions inferred from whole rock geochemical analyses. Hypothetic causes of these differences include (a) bias in rock sampling or in airborne surveys, (b) the differences between the chemistry of superficial material and rocks, and (c) the differences in scales of measurements. The scale factor, viewed as consequence of the central limit theorem applied to K, Th, and U concentrations, appears to account for most of the observed differences in the distributions of K, Th, and U. It suggests that the three scales of auto‐correlation of K, Th, and U concentrations are of the same order of magnitude as the resolution of the airborne radiometric surveys (50–200 m). Concentrations of K, Th, and U are therefore generally heterogenous at smaller scales.

Radiometric dating

10.1029/2021ea001786

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Granite petrogenesis and the δ44Ca of continental crust

Earth and Planetary Science Letters (2023)

Michael A. Antonelli Chris Yakymchuk Edwin A. Schauble John Foden Vojtěch Janoušek

Stable Ca isotopes are an increasingly useful tool for understanding the sources and processes leading to the formation of magmatic rocks, yet Ca isotope fractionation during genesis of silicic continental crust is still poorly understood. Here, we present Ca, Sr, and Nd isotope, as well as major- and trace-element whole-rock geochemical data for A-, I-, and S-type granites (n = 30) from Australia/Tasmania, Canada, and France (δ44CaBSE of -0.6‰ to +0.2‰) and compare them to phase-equilibrium models for partial-melting (pelite, greywacke, MORB, enriched Archean tholeiite) and crystallization (hydrous arc basalt, A-type granite) that incorporate novel ab-initio predictions for Ca isotope fractionation in epidote and K-feldspar. The ab-initio calculations predict that epidote has similar δ44Ca to anorthite and that K-feldspar is the isotopically lightest known silicate mineral at equilibrium (Δ44Cakspar-melt of -0.4‰ at 1000 K). Our phase-equilibrium model results suggest that δ44Ca variations in all three granite types can be fully explained through magmatic processes, without necessarily requiring addition of isotopically exotic components (e.g., carbonate sediments). Heavy Ca isotope enrichments in A-type granites from the Lachlan Fold Belt, however, require isotopic disequilibrium between plagioclase and melt, which we use to constrain average plagioclase growth rates in these systems. This also serves to illustrate that whole-rock Ca isotope measurements can be used to estimate crystal growth rates, even in the absence of analyzable phenocrysts. In general, low Ca diffusivities and strong isotopic diffusivity ratios (D44/D40) in low-H2O granitic magmas should lead to resolvable isotopic disequilibrium effects in plagioclase, even at relatively slow growth rates (e.g., > 0.03 cm/yr). Combining our data with those from previous studies, we demonstrate that average granitoids and upper continental crust (with newly estimated δ44CaBSE of -0.25 ± 0.02‰, 2SE) have resolvable low δ44Ca compared to basalts and oceanic crust. Given that pressure has a major influence on Ca isotope fractionation across all of our models, this implies that melts feeding upper crustal granitoids dominantly evolve in the lower crust (10-14 kbar, through either partial-melting or fractional crystallization). This observation also suggests that heavier Ca isotopes are preferentially recycled back into the mantle through subduction and/or lower-crustal delamination events, but this is unlikely to have had a significant influence on the δ44Ca evolution of the upper mantle through geologic time.

Fractional crystallization (geology)

Petrogenesis

Silicic

10.1016/j.epsl.2023.118080

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Citations (16)

Porosity-limited: A phase equilibrium investigation of selected source controls on the composition of melt batches generated by sequential melting of an average metapelite

Figshare (2019)

Matthew Jason Mayne Gary Stevens Jean‐François Moyen

Dataset for porosity-limited scenario.

10.6084/m9.figshare.9197780

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