Revisiting chondrule formation ages using plagioclase chemistry as an indicator of thermal metamorphic overprint

Chondrule Al-Mg chronometry is a key tool for reconstructing the timing and duration of chondrule formation. Previous datasets have been interpreted to indicate a prolonged chondrule formation, up to ∼2 million years (Myr), among chondrules within a single chondrite group. Such prolonged chondrule formation, however, can be difficult to reconcile with the specific chemical and isotopic characteristics of chondrules across chondrite groups and with constraints from accretion and subsequent thermal evolution of the parent body. This paper summarizes a mineral-chemical framework for evaluating whether chondrule Al-Mg systems remained effectively closed after primary crystallization. Plagioclase can incorporate a vacancy-coupled excess-silica component, [ ]Si₄O₈ (“excess silica”), first recognized in lunar samples. Experimental studies have shown that [ ]Si₄O₈ can be incorporated into plagioclase at the 5–10 mol% level at 1 atm under anhydrous conditions and high temperatures (1200–1500 °C) along the anorthite-SiO₂ join, indicating that excess silica is a primary high-temperature signature. In chondrules, [ ]Si₄O₈-bearing plagioclase is reported from the least-altered carbonaceous chondrites, whereas its occurrence tends to be less frequent in meteorites that experienced more progressive thermal metamorphism, the latter of which is consistent with partial loss or re-equilibration during secondary processing. We outline EPMA-based quantification of [ ]Si₄O₈ component and discuss how excess silica, together with MgO systematics in plagioclase, can be used to screen chondrules for secondary modification prior to interpreting Al-Mg ages. Chondrules that preserve primary plagioclase signatures define a narrower formation interval than previously thought, suggesting that part of the previously reported age dispersion reflects post-formational modification on the parent body rather than prolonged primary chondrule formation.