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Abstract:
Grenville Turner’s pioneering Ar-Ar work on lunar highland
rocks has shaped debates on the ages of lunar basins and the
early lunar bombardment history. Although interpretations of
some of the data were challenging, the predominance of ages
near 3.9 Ga and more scarce older dates were major results that
were confirmed by many other studies. Over the last two
decades, in situ U-Pb dating methods have been applied more
often. An advantage of in situ methods is that they can utilize
textural, mineralogical and microchemical information to
understand the formation and modification of the chronometer
minerals and their local environment.
A few examples illustrate the response of different
chronometers, in impactites with different thermal histories. For
instance, in the lunar granulite 78155, zircon and Ca-phosphates
yield similar Pb-Pb dates of 4.2-4.3 Ga (this work), which
partially overlap earlier Ar-Ar dates and conventional Pb-Pb
isochron dates (between 4.1 and 4.2 Ga). The consistently old
ages reflect effective thermal annealing of preexisting breccia by
hot ejecta, presumably from a basin-forming impact, and little
influence by later heating events. In contrast, breccias like 15455
and 67955 show textural evidence for a multi-stage heating
history and display bi- or multimodal distributions of dates.
Zirconium minerals in these breccias yield old ages and
relatively homogeneous distributions at 4.2 (in 15455 and 67955)
and 4.33 Ga (in 15455). In contrast, Ca-phosphate dates tend to
cluster near 3.92 Ga (15455), or spread on the concordia between
3.9 Ga and older dates, due to variable discordance (67955).
Published Ar-Ar data on 15455 show disturbed systematics by
heating events that occurred at or after 3.9 Ga. Thus, these and
data on other samples show clear biases of chronometer systems
in complex breccias, with Ca-phosphates and Ar-Ar data
preferably recording younger heating events, whereas zircon and
baddeleyite record old ages because they were not disturbed by
later heating events. The abundant occurrence of 3.92, 4.2 and
4.3 Ga ages at multiple landing sites and auxiliary information
from samples, geological data and modeling of ejecta
distributions, suggest that these age clusters reflect basin forming
impacts.
