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We report C, N, Si, and Al-Mg isotope data for 39 presolar X silicon carbide(SiC) and four silicon nitride grains - a group of presolar grains thatcondensed in the remnants of core-collapse Type II supernovae (CCSNe) -isolated from the Murchison meteorite. Energy dispersive X-ray (EDX) data wereused to determine the Mg and Al contents of the X SiC grains for comparisonwith the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS).Previous SIMS studies have used O-rich standards in the absence ofalternatives. In this study, the correlated isotopic and elemental data of theX SiC grains enabled accurate determination of the initial 26Al/27Al ratios forthe grains. Our new grain data suggest that (i) the literature data for Xgrains are affected to varying degrees by asteroidal/terrestrial contamination,and (ii) the Al/Mg ratios in SiC are a factor of two (with +/-6% 1 sigmauncertainties) lower than estimated based on the SIMS analyses that used O-richstandards. The lowered Al/Mg ratios result in proportionally higher inferredinitial 26Al/27Al ratios for presolar SiC grains. In addition, the suppressionof asteroidal/terrestrial contamination in this study leads to the observationof negative trends for 12C/13C-30Si/28Si and 26Al/27Al-30Si/28Si among our CCSNgrains. We discuss these isotope trends in the light of explosive CCSNnucleosynthesis models, based on which we provide new insights into severalnon-traditional CCSN nucleosynthesis processes, including explosive H burning,the existence of a C/Si zone in the outer regions of CCSNe, andneutrino-nucleus reactions in deep CCSN regions.

Explosive Nucleosynthesis in Core-Collapse Type II Supernovae: Insights from new C, N, Si, and Al-Mg isotopic compositions of presolar grains