Friday, March 07, 2008

Chondrite normalization

Chris and Ron are mostly right. The ‘red’ line is actually 4 overlapping lines, representing 4 different published values for the REE composition of CI chondrites. The blue line is the REE plot for an ordinary chondrite.

Chondrites are the original condensates from the solar nebula- the hot cloud of gas from which the sun and planets formed. As this cloud of gas cooled, the refractory (hard-to-evaporate) elements condensed to form minerals, and the minerals stuck together. There are a number of different types of chondrites, based on texture, temperature, etc.

Geochemists like the REEs (lanthanides). The reason for this is that in natural systems, rare earth elements all have broadly similar chemistry. As a result, changes in relative REE concentrations are easier to interpret than changes in 14 randomly selected elements.

Because scientists are interested in the relative changes, they like to normalize to an initial condition. And that’s where chondrites come in. Chondrites are the solar system’s initial condition- they represent the leftover raw material from which the terrestrial planets were assembled. They trouble is, they aren’t all the same.

Ordinary chondrites are the most common type of meteorite. They consist mostly of silicates and metal, and for most refactory (low vapor pressure) elements they have a composition that is similar to the sun. The generally have various metamorphic textures that indicate variable amounts of post-formational reheating. For volatile elements, ordinary chondrites generally show various degrees of depletion- those elements evaporated as the meteorite headed up.

CI chondrites are a rare type of chondrite. They contain lots of organic matter and structural water, and have no history of reheating. As a result, they have solar composition for almost all non-gaseous elements.

The trouble is, CI chondrites are rare. Of the 36,000 meteorites that have been found and catalogued, we have 5 CI’s.

Because the REE (lanthanides) are all refactory, many earlier papers and studies normalize to ordinary chondrites- they work fine. But as better measurements of CI chondrites became available, and as their importance was realized, most folks started normalizing to CI compositions instead. And for the unwary, this can cause complications.

This is because CI chondrites contain a large amount of water, sulphur, and organic material. As a result, the absolute REE concentrations are somewhat diluted, compared to ordinary chondrites. That makes perfect sense, and is no big deal, AS LONG AS YOU SPECIFY WHICH CHONDRITE YOU USED for normalization.

Let’s see how the professionals did.

This link is a REE search of the figure, report a total of 4 different reported values for normalization. They are:
Anders & Grevese 1989 (2)
McDonough & Sun 1995
Wakita et al. 1971
Anders & Grevese 1989 x 1.36
Anders and Ebihara 1982

The 1.36 multiplication factor is used to calculate a volatile-free CI equivalent. In otherwords, if the CI had been a normal chondrite, that is what the concentration would be. The rationale behind this is explained in this lab’s website, and a decent compilation of early chondrite results is given.

Rimas et al. do not specify which type of chondrite they use for normalization. Floss et al. normalize to CI chondrite values, but their figure captions don’t say which Ci values they use. Both methods sections are blocked, so I can’t check to see if they say there.

Finally, here’s a table of all of the above, normalized to Sun & McDonough 1989:


References:
Anders E. and Ebihara M. (1982) "Solar-system abundances of the elements" Geochim. Cosmochim. Acta 46, 2363-2380.

Anders E. and Grevesse N. (1989) "Abundances of the elements: Meteoritic and solar" Geochim. Cosmochim. Acta 53, 197-214.

McDonough W. F. & Sun S-s. (1995) “The Composition of the Earth” Chemical Geology 120 223-253.

Sun S-s. & McDonough W. F. (1989) “Chemical and isotopic systematics of oceanic basalts: implications for mantle compositions and processes.” In: A. D. Saunders and M. J. Norry (editors). Magmatism in the ocean basins. Geological Society. London. 313-345.

Wakita H., Rey P., and Schmitt R. A. (1971) Elemental abundances of major, minor, and trace elements in Apollo 11 lunar rocks, soil and core samples. Proc. Apollo 11 Lunar Sci. Conf., 1685-1717.

See also:
Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3

1 comment:

Eric said...

I enjoyed this post and the entire blog. Added to my RSS feeds. Thanks!