Stratigraphy and composition of lava flows in Mare Nubium and Mare Cognitum
Bugiolacchi R., P. D. Spudis, and J. E. Guest.
Meteoritics & Planetary Science 41, Nr2, pp. 285-304(20). 2006.
Three major periods of basaltic activity characterize the infill of
the basins. Each of these periods was itself punctuated by discrete phases
widespread magma eruptions: three during both the Late Imbrian Epoch and
the early Eratosthenian Period and then two in the late Eratosthenian Period.
We found the youngest lavas off the eastern border of the Fra Mauro peninsula
and, mantling a much larger area, over most of the central western Nubium
basin. Our results place the Nubium/Cognitum basalts in the low-Ti category
(1-5 wt% TiO2).The data indicate that the majority (˜90%) of the mare terrain
has iron content between 18 and 22 wt%. In particular, FeO contents tend
to concentrate toward two compositional poles, each of ˜20 wt%, and a much
smaller one of ˜15 wt%. These values are typical of nearside lunar maria.
To complement our compositional data, we present a census of craters larger
than 500 m using Orbiter IV images. The result was a crater count average
with frequency 5.6 × 10-2km-2, translating into an inferred mean age
of 3300 Ma for the exposed lava flows. By combining lava chemistry with age,
we find a possible correlation between the ages of the most prominent flow
units and their estimated titanium content, with younger basalts becoming
progressively Ti-richer with time (from 2-3 to 4-5 wt% TiO2).