  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 that
condensed in the remnants of core-collapse Type II supernovae (CCSNe) -
isolated from the Murchison meteorite. Energy dispersive X-ray (EDX) data were
used to determine the Mg and Al contents of the X SiC grains for comparison
with the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS).
Previous SIMS studies have used O-rich standards in the absence of
alternatives. In this study, the correlated isotopic and elemental data of the
X SiC grains enabled accurate determination of the initial 26Al/27Al ratios for
the grains. Our new grain data suggest that (i) the literature data for X
grains 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 sigma
uncertainties) lower than estimated based on the SIMS analyses that used O-rich
standards. The lowered Al/Mg ratios result in proportionally higher inferred
initial 26Al/27Al ratios for presolar SiC grains. In addition, the suppression
of asteroidal/terrestrial contamination in this study leads to the observation
of negative trends for 12C/13C-30Si/28Si and 26Al/27Al-30Si/28Si among our CCSN
grains. We discuss these isotope trends in the light of explosive CCSN
nucleosynthesis models, based on which we provide new insights into several
non-traditional CCSN nucleosynthesis processes, including explosive H burning,
the existence of a C/Si zone in the outer regions of CCSNe, and
neutrino-nucleus reactions in deep CCSN regions.
