|Evolution in the News - December 2011|
|by Do-While Jones|
New ages for old moon rocks
Since Rod brought up the topic in this month’s email, here’s the abstract from a current article about the age of the moon from a respected, peer-reviewed scientific journal.
Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth. The Earth’s Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type. Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems. By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the 207Pb–206Pb, 147Sm–143Nd and 146Sm–142Nd isotopic systems to be 4,360 ± 3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism. 1
The body of the article tells how they “made improvements” to the technique (i.e. fudged the data) and contains these significant sentences:
A few fractions fall above the isochron as well, and are interpreted to have a well known, if poorly understood, lunar Pb contaminant. 2
The advantage of this scenario is that it does not require dismissing the handful of older ages determined on lunar samples and it can potentially account for the variable (if somewhat suspect) ages and widespread distribution of FANs on the lunar surface. The disadvantage is that the lunar magma ocean theory is strongly founded upon the petrological, geochemical and isotopic characteristics of FANs such as 60025. Therefore, if 60025 is not a product of the magma ocean, petrologically similar rocks from the FAN suite cannot be used to characterize its initial solidification. Furthermore, the potential for planetary differentiation by magma ocean solidification on all rocky bodies is weakened if the very rocks that led to the development of the magma ocean theory are themselves not its byproduct. 3
In plain English, the magma ocean theory is based on moon rocks that might not have come from a magma ocean. Furthermore, the reason why some of the data points don’t fall where they should on isochron plots is because of “well known, if poorly understood” lead contamination on the moon, so fudging the data is acceptable.
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Borg, et al., Nature, 1 September 2011, “Chronological evidence that the Moon is either young or did not have a global magma ocean”, pages 70-72, http://www.nature.com/nature/journal/v477/n7362/full/nature10328.html