Haber, Thomas
Description
One of the most important questions in lunar science is whether
there was a lunar cataclysm – a sudden increase in the impactor
flux around 3.9 Ga. The absolute ages of basin-scale impacts are
the key to answering this question. Crystalline impact melt rocks
from the Apollo landing sites can hold vital information about
the lunar cataclysm, as they were potentially produced by such
basin-scale impacts. This study aims to constrain (1) the number
and nature of...[Show more] impact events represented in the Apollo collection
of melt rocks, (2) the chemical and isotopic signatures of the
source regions and impactors, and (3) the processes involved in
the formation of a set of Apollo impact melt rocks.
To achieve this, I combine analyses of major and trace element
chemistry, highly siderophile element abundances, and the
87Rb-87Sr and 147Sm-143Nd isotopic systems, with 40Ar/39Ar dating
of 35 crystalline impact melt rocks from the Apollo 14, 16 and 17
landing sites. To produce a coherent data set and minimise
intrasample compositional variations, the different analyses were
all obtained on the same split of each sample.
The chemical data and petrographic information show that the melt
rocks likely represent multiple impact events, some of which
produced craters (<300 km diameter), whereas others were
basin-forming events (>300 km diameter). The 40Ar/39Ar plateau
ages obtained here reflect the crystallisation ages of the melt
rocks, and thus offer a viable tool for dating the impacts that
produced these deposits. Four different events (3950 ± 24 Ma,
3885 ± 8 Ma, 3815 ± 19 and Ma 3644 ± 42 Ma) are resolved using
40Ar/39Ar plateau ages from the Apollo 16 landing site, and one
plateau age (3829 ± 15 Ma) is obtained for Apollo 14 sample
14310. The combination of chemical, isotopic and age data provide
a strong case for a dominance of melt rocks related to a single
basin-forming event, most likely the Imbrium, at 3885 ± 8 Ma.
The chemical and isotopic data from the Apollo 17 impact melt
rocks are consistent with an origin of these samples in the same
event. The melt rocks associated with the other four resolved
events are likely derived from smaller craters. However,
uncertainty about the nature of these events is greatest for the
youngest and oldest events. These findings weaken the case for a
lunar cataclysm, which requires multiple impact basins to form
around 3.9 Ga.
The age and isotopic data confirm the notion that compositional
variations within related groups of melt rocks are mainly the
result of mixing of different lunar lithologies, reflecting
heterogeneity in their crustal sources. However, in some cases
the chemical and isotopic relationships might be better explained
by igneous differentiation processes in an impact melt sheet.
This study presents the first model that relates a set of
impact-derived lunar whole rock compositions by an igneous
process in an impact melt sheet.
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