P-T-fO2-X constraints on the partitioning behavior of trace elements between clinopyroxene and alkali melts: An experimental study

Author(s)

Salazar-Naranjo, Andres Fabian ; Vlach, Silvio Roberto Farias

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 677, p. 22-pg., 2025-01-29.

Abstract

Titanian Ca-clinopyroxene (Ti-Cpx) is one of the most characteristic phases and main carriers of trace elements in alkali ultrabasic to intermediate magmas, making it a key mineral for geochemical studies on the origin and evolution of this type of system. Starting from natural basanite and tephrite compositions, we conducted a series of experiments under one atmosphere pressure (1-atm) with oxygen fugacity (fO2) controlled from QFM-2 to QFM+2 and moderate pressure (0.5-2.0 GPa) with fO2 around the CCO buffer. The results allowed a better examination of the P, T, fO2, and compositional controls on DCpx/melt element i , and indicated that: (1) the Lattice Strain Model (LSM) successfully fits the divalent and trivalent cations into the M2 site and trivalent cations into the M1 site; (2) divalent cations are strongly influence by clinopyroxene composition, with their compatibility decreasing at the M1 site and increasing at the M2 site as the DiHd component rises; (3) redox-sensitive elements, such as Cr, V, and Eu exhibit systematic variation with & fnof;O2 and T; (4) high-field strength elements (HFSEs) replace Mg via the Tschermak reaction, resulting in a variation in DCpx/melt HFSE along the clinopyroxene compositional trend from Mg-rich augite to ferroan diopside; (5) the crystal and melt compositions determine the compatibility of the rare earth elements (REEs), where the latter particularly substitute Na instead of Ca; (6) the thermodynamic model based on the activity of the Na0.5(REE)0.5MgSi2O6 component reproduces closely the observed partitioning of the REEs under 1-atm. Esseneite (CaFe3+AlSiO6) component increases with the oxygen fugacity and facilitates the incorporation of REEs, meaning that Ti-Cpx crystallized under oxidizing conditions will be REE-rich (except Eu). (DTa/DNb)Cpx/melt and (DZr/DHf)Cpx/melt remains almost constant, while (DU/DTh)Cpx/melt correlates positively with the EnFs component, generating fractionation degrees variable of U/Th and constant of Ta/Nb and Zr/Hf during the evolution of crystal-melt systems. Trace element ratios such as V/Sc, V/Yb, and Sr/Y can be used as proxies of oxygen fugacity and pressure in Ocean Island Basalts (OIBs). This new experimental dataset, analyzed under controlled and extensive settings, particularly fO2, offers a valuable tool for studying magmatic processes in alkali ultrabasic to intermediate systems through Ti-Cpx under crustal and upper mantle conditions. (AU)

• FAPESP's process: 19/22084-8 - The Paraná Magmatic Province: petrogenesis, chronology and environmental impact of Cretaceous tholeiitic, alkaline and silicic magmatism in the Brazilian Platform
• Grantee: Valdecir de Assis Janasi
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 18/16755-4 - Experimental studies on trace element partitioning between mafic minerals and alkaline melts
• Grantee: Andres Fabian Salazar Naranjo
• Support Opportunities: Scholarships in Brazil - Doctorate