According to this publication, the standard marker for comparing moon rocks with those of earth or meteorites is the oxygen isotope, O-17, with the isotope, O-16. The ratio: O-17/ O-16 has a characteristic value common to bodies of earth-like planets within the Solar System. Oxygen is the most abundant element in the earth's crust, having three isotopes with values presented in the following table, which shows isotope percentages from three samples each for the earth and moon.

Table Footnotes:
a. Oceanic plus terrestrial crust; b. "Inorganic Geochemisrty", P. Henderson, 1986, Pergamon Press; c. "Chemistry of the Elements", N. N. Greenwood and A. Earnshaw, 1980, Pergamon Press; d. Significant figures based on instrumental accuracy only, not on accuracy of O content in the samples; e. Values not reported but inferred from other reported values; f. From references within Table 3.4, page 56, of reference b.
The average O-17/O-16 ratios are 0.00038 +/- 0.0001 and 0.00038(3) +/- 0.0001 for earth and moon respectively. According to the publication, the ratios obtained from freshly prepared moon-rocks were shifted to 0.00039(2), from the 0.00038 average found in earth samples. This is only a 2.6 % difference, which is well within the variation in values presented in the above table. Nonetheless, the publication maintains that this difference provides support for the Theia impact hypothesis of the moon's formation. My question is WHY? The reported variation in the O-17/ O-16 ratio is masked by the inconsistencies in the raw data themselves !
The real answer to the moon's formation comes from the Bible, Genesis 1: 16-18, which clearly states that the Moon, along with the Sun, was created on the 4th Day, just some 6,000 years ago, not by some impact on the earth billions of years ago.

Blessings in his Holy Name, Ed.
Dr. Ed Boudreaux, RMCF President