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Journal of Petrology Volume 42 Number 6 Pages 1219-1220 2001
© Oxford University Press 2001
Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited: A Critical Comment
DEPARTMENT OF GEOLOGY, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T 0200, AUSTRALIA
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INTRODUCTION |
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 TOP INTRODUCTION REFERENCES |
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The lengthy paper on this subject by M. J. O’Hara (2000)
contains an interesting reassessment of lunar evolution, although it differs little in its conclusions from his original views published over a quarter of a century ago. To answer all the arguments in detail would demand a paper of equal length. However, it is possible to discern some basic underlying assumptions among the extensive discussion and I have chosen to concentrate on these to display some of the fundamental fallacies that underlie O’Hara’s case. Crucial to his argument is his statement that ‘there is no positive europium anomaly in the average lunar highland crust’ (p. 1545, see also p. 1551) and this leads him to deny the existence of a magma ocean from which the crust formed by plagioclase flotation. Consequently, ‘there is no negative europium anomaly in the average mantle to be inherited by later mare basalts’ (p. 1545, see also p. 1555).
As these two factors, an enrichment of Eu in the highland crust and a corresponding depletion in the mantle, inherited by the later mare basalts, are cornerstones of the standard model for lunar evolution based on a magma ocean, the remainder of O’Hara’s conclusions stand or fall on their validity.
His claim that there is no overall enrichment of Eu in the average highland crust is based on the data from the Apollo 16 site as interpreted by Korotev & Haskin (1988). The fundamental difficulty is that the Apollo 16 site is more basic than the average highlands. Subsequent extensive coverage of the farside highlands by the Clementine and Lunar Prospector missions has revealed that they are dominated by Ca-rich anorthosite, as discussed by Lucey et al. (1995), a paper that does not appear in the extensive reference list of O’Hara’s paper. Other evidence of the highly feldspathic nature of the highlands comes from studies of the lunar farside (e.g. Pieters & Tompkins, 1999) and studies of crustal thickness (e.g. Neumann et al., 1996). These papers are also not cited. This new evidence has led to a revision of the Al2O3 content of the bulk lunar crust from 24·6% [the value used by Taylor (1975) that O’Hara discusses] to 30% (Lucey et al., 1995).
To account for this amount of feldspar (plagioclase has 36% Al2O3), close to 50% of the Al (and Eu) in the bulk Moon has to be concentrated in the crust, shortly after the formation of the Moon. Flotation of feldspar in a dry magma ocean is the only viable hypothesis, accounting, incidentally for the parallel REE patterns (excluding Eu) observed in most highland samples. Thus much new evidence ignored by O’Hara has only served to substantiate the traditional interpretation.
O’Hara wishes in fact to form the thick feldspathic crust by partial melting of wet peridotite and claims that ‘partial melting in the presence of water, followed by near-surface fractionation and volatile losses, can explain the feldspathic character, high incompatible element concentrations and lack of Eu anomaly in the lunar highlands’ (p. 1545, see also p. 1553). This view, heavily based on terrestrial analogues, is unfortunately negated by the completely anhydrous nature of all lunar minerals and total absence of any evidence for water on the Moon, even at ppb levels (the famous rusty rock 66095 was hydrated in the terrestrial atmosphere; Epstein & Taylor, 1974).
O’Hara also seems unwilling to accept the data that show that there is a great depletion of other volatile elements in the Moon, preferring a scenario, as in much of the rest of the paper, based heavily on terrestrial experience. However, he also appeals to the example of Io as a model for getting rid of volatiles such as Na and sulphur. That unfortunate body, in the close embrace of Jupiter (six Jupiter radii distant from a body of 318 Earth masses), is subject to tidal heating on a scale that could not be invoked for the Moon between 4 and 3 billion years ago when most of the mare basalts were erupted. By that time, the highland crust was cold enough and strong enough to support both mascons and the high mountain ranges (e.g. Apennines) resulting from the giant basin-forming collisions (Neumann et al., 1996). But even Io, with its surface coated with sulphur, illustrates the difficulties of losing volatiles from moon-sized bodies.
The depletion of volatile elements in the Moon, shown most clearly by the lead and strontium isotopic data, long pre-dates the later eruption of the mare basalts. O’Hara’s evocative picture of boiling these elements off from lava lakes is a mechanism too late by a few hundred million years to account for the evidence that rubidium and common lead have always been depleted in the Moon. Indeed, Gibson & Hubbard (1972, PLC 3, 2003) pointed out 28 years ago that on the cold lunar surface where pressures are below 10-9 torr, the lithospheric pressure of the lava exceeds that of the vapour pressure of the elements at depths greater than 10-3 cm so that volatile loss would only be possible from a thin skin.
All basalts derived from the lunar interior display a depletion in Eu, traditionally ascribed to an overall depletion of their source regions by the prior crystallization of the plagioclase now in the highland crust. [A sole exception is some USSR Luna 24 VLT basalt samples where only milligam-size amounts were available, raising serious questions about how representative were the samples that were analysed (Neal & Taylor, 1992).] An exhaustive discussion of other processes to account for the Eu depletion was given by Taylor (1975, pp. 154–159), but none except the prior extraction of Eu into the lunar crust have survived. O’Hara has resuscitated the ancient controversy that plagioclase crystallization in the basaltic magma is responsible for the Eu depletion in the mare lavas. Plagioclase, however, is a late, rather than an early, phase to crystallize and is not a liquidus phase except in some high-Al basalts, according to students of mare basalt petrography (e.g. Longhi, 1992).
O’Hara spends much space on comparisons of the Moon with Mars, Venus, Mercury and the Galilean satellites, but seeking analogues in a Solar System dominated by stochastic events is a hazardous task (Taylor, 1998, 1999). Curiously, Vesta, from which the eucrites come that so closely resemble mare basalts, is scarcely mentioned and there is no reference to the significant paper that discusses this by Ruzicka et al. (1998), although the 492 references that he cites include 27 papers published later than 1998.
In summary, O’Hara has reiterated his previous objections to the standard model without proffering fresh evidence. In the long historical perspective of attempting to understand the Moon since Galileo turned his telescope towards our satellite, progress has always been made through the acquiring of new data, now needed as before, rather than in the revival of ancient arguments. Nullius in verba.
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FOOTNOTES |
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*Email: Ross.Taylor{at}anu.edu.an
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REFERENCES |
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TOPINTRODUCTIONREFERENCES |
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Epstein, S. & Taylor, H. P., Jr (1974). D/H and 18O/16O ratios of H2O in the ‘rusty’ breccia 66095 and the origin of ‘lunar water’. Proceedings of the Lunar Science Conference 5, 1839–1854.
Gibson, E. K., Jr & Hubbard, N. J. (1972). Thermal volatilization studies on lunar samples. Proceedings of the Lunar Science Conference 3, 2003–2014.
Korotev, R. L. & Haskin, L. A. (1988). Europium mass balance in polymict samples and implications for plutonic rocks of the lunar crust. Geochimica et Cosmochimica Acta 52, 1795–1813.
Longhi, J. (1992). Experimental petrology and petrogenesis of mare volcanics. Geochimica et Cosmochimica Acta 56, 2235–2251.
Lucey, P. G., Taylor, G. J. & Malaret, E. (1995). Abundance and distribution of iron on the Moon. Science 268, 1150–1153.
Neal, C. R. & Taylor, L. A. (1992). Petrogenesis of mare basalts: a record of lunar volcanism. Geochimica et Cosmochimica Acta 56, 2177–2211.
Neumann, G. A., Zuber, M. T., Smith, D. E. & Lemoine, F. G. (1996). The lunar crust: global structure and signature of major basins. Journal of Geophysical Research 101, 16481–16863.
O’Hara, M. J. (2000). Flood basalts, basalt floods or topless Bushvelds? Lunar petrogenesis revisited. Journal of Petrology 41, 1545–1651.
Pieters, C. M. & Tompkins, S. (1999). Tsiolkovsky crater: a window into crustal processes on the lunar farside. Journal of Geophysical Research 104, 21935–21949.
Ruzicka, A., Snyder, G. A. & Taylor, L. A. (1998). Giant impact and fission hypotheses for the origin of the moon: a critical review of some geochemical evidence. International Geology Review 40, 851–864.
Taylor, S. R. (1975). Lunar Science: A Post-Apollo View. Oxford: Pergamon Press, 372 pp.
Taylor, S. R. (1998). Destiny or Chance: Our Solar System and its Place in the Cosmos. Cambridge: Cambridge University Press, 229 pp.
Taylor, S. R. (1999). On the difficulties of forming Earth-like planets. Meteoritics and Planetary Science 34, 317–329.
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