New Experimental Constraints for the Evolution and Thermobarometry of Alkali Ultrabasic to Intermediate Igneous Rocks

Author(s)

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

• Document type: Journal article
• Source: JOURNAL OF PETROLOGY; v. 64, n. 11, p. 22-pg., 2023-11-01.

Abstract

We report new experimental data from ultrabasic basanite and ultrabasic tephrite as starting material compositions in the 1350 degrees C to 1000 degrees C temperature range. Crystallization experiments under low-to high-pressure (0.5-2.0 GPa) were carried out under reduced conditions (approximate to CCO buffer), while one-atmosphere, anhydrous, experiments were performed ranging from reduced to oxidized conditions (-2 < AQFM < +2). The results highlight the fO(2) role on the silica saturation of the alkali liquids differentiated from these primary ultrabasic magmas, on the mineral assemblage, and its composition. The liquid lines of descent (LLDs) from basanite are sodic and strongly SiO2 undersaturated, whereas from tephrite, the LLDs are sodic-potassic/potassic for both weakly SiO2-undersaturated and SiO(2-)saturated compositions, being more silica saturated under oxidized conditions. At the lowest temperature experiments, the percentage of liquid remaining is significantly higher in the basanite-derived products (ca. 35 wt.%) than in tephrite, indicating that the equivalent magmas are more prone to produce larger quantities of evolved melts. The best obtained Fe-Mg olivine/melt and clinopyroxene/melt exchange coefficients for these alkali compositions considering the new and available data are KDOl-Alkali melt Fe2+-Mg = 0.285 +/- 0.014 and KDCpx-Alkali melt Fe2+-Mg = 0.245 +/- 0.008, slightly lower than those observed in tholeiitic melts. Clinopyroxene compositions are Ti-Al-rich and Si-poor as compared with common clinopyroxenes in subalkali systems. We suggest that Ti should be allocated in the tetrahedral sites substituting for Si and that its contents are inversely correlated with pressure. Our results allow a simple new barometer based on clinopyroxene-only compositions, as follows: P (+/- 0.16 GPa, 1 sigma) = 16.028 (+/- 1.042) Na - 6.715 (+/- 0.727) Ti Ti + Al(t) + 0.494 (+/- 0.441) Si + 0.144 (+/- 0.079) where Na, Ti, Al(t), and Si are molar proportions relative to 6O. This formulation accounts for the jadeite (NaAlSi2O6) component, herein computed from the Na contents, corrected for the Ti-diopside (CaMgTi2O6) component in clinopyroxene and also considers the evolutionary trend from Mg-augite to ferroan diopside. It applies to alkali ultrabasic to intermediate compositions in the examined P-T-fO2 range, resulting in more accurate estimates than the available calibrations. The MgO-in-melt thermometer was optimized for the studied compositions at one-atmosphere pressure and anhydrous conditions, as follows: T (+/- 5 degrees C, 1 sigma) = 27.35 (+/- 0.65) (MgOliq) + 984 (+/- 4) which provides much more reliable liquidus temperatures for these alkali systems. Given data restriction, this formulation may be expanded to include the pressure effects for relatively low-H2O (< 3 wt.%) systems as: T (+/- 20 degrees C, 1 sigma) = 27.35 (+/- 0.65) (MgOliq) + 80.20 (+/- 5.50) P(GPa) + 981 (+/- 4) (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