"Rock of the Month # 116, posted for February 2011" ---

The olivine-metal mixtures known as pallasites are, by many accounts, the most handsome of all meteorites. Only about 1 meteorite in 500 is a pallasite (84 of 40,000-plus individually-identified meteorites), and one-third (28) of these were recovered from Antarctic ice fields, including 19 recovered from the Cumulus Hills during the ANSMET 2003 season. In Canada, just three have been recovered, each one a find, not a witnessed fall. These are: Springwater (1931, 67.6 kg, Saskatchewan; Nininger, 1932); Giroux (1954, 4.28 kg, Manitoba) and Southampton (2001, 3.58 kg, Ontario). Recently additional finds have been made in the Springwater strewnfield. In 2009 a team of English researchers found 100 kg of new pieces, including a newly-found 53-kg main mass, now at the Royal Ontario Museum. The largest surviving piece of the 1931 find, ~20 kg, is at the Natural History Museum in London (Ruf, 2010).

Springwater is a well-documented meteorite, possibly the most-described and researched in Canada after Abee, Tagish Lake and Bruderheim. Detailed studies have been made of constituent minerals, such as olivine (Zhou and Steele, 1993), metal, chromite and phosphates, and of chemical components, including phosphorus, chromium, rare earth elements and rare gases.

Some pallasites are seen as having formed at the core-mantle boundaries of differentiated asteroids, though others may be impact melts. Rounded olivine grains in examples such as Brenham, Springwater and Krasnojarsk may be interpreted as evidence of metal-silicate liquid immiscibility generating fabrics analogous to net textured sulphides (ore textures seen on Earth: Scott and Taylor, 1990). Not all pallasites formed the same way, but these with coarse, rounded olivines may indeed display cumulus olivines settled at the core-mantle boundary atop a metallic asteroidal core.

According to Buseck (1977), an "average" pallasite contains 65 volume percent olivine and 50.5 weight percent total iron. The iron resides largely in metal phases such as kamacite, and in olivine. An ion microprobe study of seven pallasites, including Springwater, showed that there was a much stronger partitioning of nickel into the metal phase, with just 22-41 ppm Ni in olivine (Reed et al., 1979), just 1% of the Ni content of olivine in many terrestrial mafic intrusions. Davis and Olsen (1989) noted that about two-thirds of pallasites carry one or more of the primary phosphate minerals farringtonite, stanfieldite and whitlockite. Low concentrations of incompatible elements in major phosphates indicate that the phosphates were formed by redox reactions between olivine and molten metal, the phosphorus originating within the metal.

References

Buseck,PR (1977) Pallasite meteorites - mineralogy, petrology, and geochemistry. Geochim.Cosmochim.Acta 41, 711-740.

Davis,AM and Olsen,EJ (1989) The origin of phosphate minerals in the Eagle Station and Springwater pallasites. Lunar and Planetary Science 20, 220-221.

Nininger,HH (1932) The Springwater meteorite. Amer.Mineral. 17, 396-400.

Reed,SJB, Scott,ERD and Long,JVP (1979) Ion microprobe analysis of olivine in pallasite meteorites for nickel. Earth Planet.Sci.Letts. 43, 5-12.

Ruf,C (2010) Rare meteorite lands at ROM. National Post, p.A12, 22 July.

Scott,ERD and Taylor,GJ (1990) Origins of pallasites at the core-mantle boundaries of asteroids. Lunar and Planetary Science 21, 1119-1120.

Zhou,Y and Steele,IM (1993) Chemical zoning and diffusion of Ca, Al, Mn, and Cr in olivine of Springwater pallasite. Lunar and Planetary Science 24, 1573-1574.