Robust methods, tricky materials: Challenges in dating high common-Pb perovskite from Cretaceous Brazilian kimberlites

Author(s)

Azzone, R. G. ; Pearson, D. G. ; Sarkar, C. ; Cetina, L. M. T. ; Chmyz, L. ; Luo, Y. ; Ruberti, E. ; Guarino, V. ; Cabral-Neto, I.

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 670, p. 19-pg., 2024-09-13.

Abstract

Kimberlites and related rocks are the focus of research and exploration in the Alto Paranaiba Igneous Province (APIP; Brazil) due to their potential correlation with adjacent diamond fields. New U-Pb perovskite ages were obtained using recommended protocols for error propagation and data interpretation. Despite the utility of perovskites for dating, their incorporation of significant common-Pb (common-Pb of new APIP data: 52-84 % for in situ analyses, 75-95 % for TIMS concentrate analyses) limits the age resolution of this dating approach. Consequently, the APIP U-Pb ages neither confirm nor discredit previously proposed geodynamic models that invoke age progression or distinct magmatic episodes. Comparing the new ID-TIMS and LA-ICP-MS ages of the same intrusions, the LA-ICP-MS data have lower uncertainties, largely due to accessing a greater spread in common-Pb, improving regressions, and tend to yield older ages. For perovskite grains collected from weathering profiles, we found evidence of Pb-loss. This process leads to higher uncertainties in individual analyses, increased data scatter, and consequently higher uncertainties in age estimates. For fresh samples with different degrees of crustal contamination from a single intrusion, we found an indication of some variability in( 207)Pb/Pb-206 and U-238/Pb-206 ratios of the perovskite data, with a tendency towards older ages for more contaminated sample. For almost fresh intrusions with geochemically and isotopically similar mantle sources, related to the same event and in a close geographic area, we prefer to use a unique local Pb-207/Pb-206 ratio as anchoring for each TeraWasserburg plot. This approach is particularly preferred when perovskite data of an intrusion are too clustered and the Pb-207/Pb-206 ratio from the unanchored T-W differs from that predicted from theoretical Pb-isotope evolution model. Despite the high-common Pb in APIP perovskite crystals, our new data using these strategies clearly indicate that APIP magmatism is restricted to the Cretaceous, with most cases crystallized in Upper Cretaceous, but some from Early Cretaceous, such as the diamondiferous occurrence of Catalao-1b (120.6 +/- 6.7 Ma). Different triggers of APIP magmatism are related to both episodes, mainly attributed to supercontinent break-up stages. (AU)

• FAPESP's process: 17/21397-7 - Xenocrysts and cognate crystals in kimberlites from the Alto Paranaíba Alkaline Province (MG-GO, Brazil): petrologic contribution from elemental and isotopic in situ micro-analyses
• Grantee: Rogério Guitarrari Azzone
• Support Opportunities: Scholarships abroad - Research
• FAPESP's process: 23/12323-0 - Revisiting the ages of the Poços de Caldas Alkaline Massif: 40Ar/39Ar dating of key samples to solve a conundrum.
• Grantee: Luanna Chmyz
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• 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: 17/03768-8 - Xenocryst and Antecryst compositional influence in basic to ultrabasic alkaline magmas
• Grantee: Rogério Guitarrari Azzone
• Support Opportunities: Regular Research Grants
• FAPESP's process: 23/11675-0 - Basic-ultrabasic magmatic bodies of SiO2-undersaturated affinity from the Brazilian platform: study of sources and open-system evolution processes in complex plumbing systems using microanalytical approach
• Grantee: Rogério Guitarrari Azzone
• Support Opportunities: Regular Research Grants