Gb. Dalrymple et G. Ryder, ARGON-40 ARGON-39 AGE SPECTRA OF APOLLO-17 HIGHLANDS BRECCIA SAMPLES BY LASER STEP HEATING AND THE AGE OF THE SERENITATIS BASIN/, J GEO R-PLA, 101(E11), 1996, pp. 26069-26084
We have obtained high-resolution (21-63 steps) Ar-40/Ar-39 age spectra
using a continuous laser system on 19 submilligram samples of melt ro
cks and clasts from Apollo 17 samples collected from the pre-Imbrian h
ighlands in the easternmost part of the Serenitatis basin. The samples
include poikilitic melt rocks inferred to have been formed in the Ser
enitatis basin-forming impact, aphanitic melt rock whose compositions
vary and whose provenance is uncertain, and granulite, gabbro, and mel
t clasts. Three of the poikilitic melts have similar age spectrum plat
eau ages (72395,96, 3893 +/- 16 Ma {2 sigma}; 72535,7, 3887 +/- 16 Ma;
76315, 150, 3900 +/- 16 Ma) with a weighted mean age of 3893 +/- 9 Ma
, which we interpret as the best age for the Serenitatis basin-forming
impact. Published Ar-40/Ar-39 age spectrum ages of Apollo 17 poikilit
ic melts are consistent with our new age but are much less precise. Tw
o poikilitic melts did not give plateaus and the maxima in their age s
pectra indicate ages of greater than or equal to 3869 Ma (72558,7) and
greater than or equal to 3743 Ma (77135,178). Plateau ages of two poi
kilitic melts and two gabbro clasts from 73155 range from 3854 +/- 16
Ma to 3937 +/- 16 Ma and have probably been affected by the ubiquitous
(older?) clasts and by post-formation heating (impact) events. Platea
u ages from two of the aphanitic melt ''blobs'' and two granulites in
sample 72255 fall in the narrow range of 3850 +/- 16 Ma to 3869 +/- 16
Ma with a weighted mean of 3862 +/- 8 Ma. Two of the aphanitic melt b
lobs from 72255 have ages of 3883 +/- 16 Ma and greater than or equal
to 3894 Ma, whereas a poikilitic melt clast (of different composition
from the ''Serenitatis'' melts) has an age of 3835 +/- 16 Ma, which is
the upper limit for the accretion of 72255. These data suggest that e
ither the aphanitic melts vary in age, as is also suggested by their v
arying chemical compositions, or they formed in the 72255 accretionary
event about 3.84-3.85 Ga and older relict material is responsible for
the dispersion of ages. In any case the aphanitic melts do not appear
to be Serenitatis products. Our age for the Serenitatis impact shows,
on the basis of the isotopic age evidence alone, that Serenitatis is
>20-25 Ma and probably >55-60 Ma older than Imbrium (less than or equa
l to 3870 Ma and probably less than or equal to 3836 Ma [Dalrymple and
Ryder, 1993]). Noritic granulite sample 78527 has a plateau age of 41
46 +/- 17 Ma, representing a minimum age for cooling of this sample in
the early lunar crust. So far there is no convincing evidence in the
lunar melt rock record for basin-forming impacts significantly older t
han 3.9 Ga.