Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite

摘要: Abstract TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen magnesium isotopic compositions were measured by secondary ion mass spectrometry spots known chemical composition all major minerals TS34. Using sequence formation dynamic crystallization experiments melilite fassaite, oxygen evolution CAI melt was established. constituent plot along carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly 16O-rich (Δ17O = −22.7 ± 1.7‰, 2SD), while 16O-poor (Δ17O = −2.8 ± 1.8‰) irrespective their akermanite content thus relative time crystallization. fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow Ti-rich to Ti-poor compositions. also show continuous variations Δ17O inferred directions crystal growth, (Δ17O ∼ −3‰) (Δ17O ∼ −23‰), covering full range observed early-crystallized isotope equilibrium formed simultaneously. correlation with Ti imply that evolved during crystallization, presumably due exchange surrounding nebular gas. Formation liquidus phase melts this composition, predates other phases, so its relic earlier stage. Anorthite exhibits between Δ17O ∼ −2‰ −9‰, within those indicating co-crystallization these two earliest intermediate stage growth. yield 26Al–26Mg isochron initial value (26Al/27Al)0 = (5.003 ± 0.075) × 10−5, corresponding age 0.05 ± 0.02 Myr canonical Al–Mg CAIs. These data demonstrate both reservoirs existed solar nebula at least ∼0.05 Myr after birth Solar System.