Sanislav, Ioan, and Sami, Mabrouk (2027) A two-population lattice strain model for H+-coupled REE incorporation in zircon: calibration, detection threshold, and implications for LREE anomalies. Chemical Geology, 727. 123802.

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Abstract

The classical single-site lattice strain model fails systematically for light and middle rare earth elements (LREE and MREE) in zircons from hydrous, phosphorus-undersaturated magmatic systems, predicting concentrations one to three orders of magnitude below observed values. We develop a two-population model in which the bulk partition coefficient is the mole-fraction-weighted sum of contributions from the ideal ZrO8 site and a second, enlarged site generated by REE3+–H+ coupled substitution. Both sites are parameterised by independent Brice parabolas; the fractional occupancy of the defect site, fd, follows a Langmuir saturation function of water content with half-saturation constant H0 = 422 ppm. Initial calibration of defect-site parameters against the 32 published SIMS spot analyses of Vema Fracture Zone oceanic gabbro zircons reported by De Hoog et al. (2014) gives an unconstrained Nd/Lu optimum of r 0 REE − H = 0.9916 Å, EREE–H = 592.1 GPa, D 0 REE − H = 344.5 (RMSE = 0.154 log10 units), whereas physically constrained weighted global optimisation, within-grain REE-pattern inversion, and DFT-consistent modest site expansion together indicate a preferred effective defect radius near 0.984 Å with an admissible envelope of ~0.975–0.993 Å. Forward modelling defines a detection threshold in temperature–[H2O] space below which the defect-site signal is unresolvable from geological noise, requiring [H2O]zrc > ~30–80 ppm depending on temperature. The model predicts that in hydrous, P-undersaturated systems the REE3+–H+ mechanism alone can produce Lazrc exceeding 0.1 ppm through genuine lattice incorporation, demonstrating that a fixed La threshold is not a universal inclusion-free criterion but depends on temperature, water content, and melt La concentration. The charge-balance ratio f d CB = (REY–P)/REY, recoverable from routine LA-ICP-MS data without water measurement, provides an independent estimate of defect-site occupancy and correlates with water-based estimates where the REE3+–H+ mechanism is active, thereby providing a quantitative test of mechanism activation across natural systems. The model is applicable specifically to P-undersaturated, hydrous, low-temperature magmatic systems where the REE3+–H+ mechanism has been independently confirmed by FTIR spectroscopy and charge-balance analysis.

Item ID:	94578
Item Type:	Article (Research - C1)
ISSN:	1872-6836
Copyright Information:	© 2026 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Date Deposited:	09 Nov 2026 07:31
FoR Codes:	37 EARTH SCIENCES > 3703 Geochemistry > 370302 Inorganic geochemistry @ 20% 37 EARTH SCIENCES > 3705 Geology > 370505 Mineralogy and crystallography @ 45% 37 EARTH SCIENCES > 3705 Geology > 370503 Igneous and metamorphic petrology @ 35%
SEO Codes:	28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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