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

Autor(es)

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

• Tipo de documento: Artigo Científico
• Fonte: JOURNAL OF PETROLOGY; v. 64, n. 11, p. 22-pg., 2023-11-01.

Resumo

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)

• Processo FAPESP: 19/22084-8 - A Província Magmática Paraná: petrogênese, cronologia e impacto ambiental do magmatismo toleítico, alcalino e silícico Cretáceo na Plataforma Brasileira
• Beneficiário: Valdecir de Assis Janasi
• Modalidade de apoio: Auxílio à Pesquisa - Temático
• Processo FAPESP: 18/16755-4 - Estudos experimentais de partição de elementos traço entre minerais máficos e fusões alcalinas
• Beneficiário: Andres Fabian Salazar Naranjo
• Modalidade de apoio: Bolsas no Brasil - Doutorado