Mesosiderites

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

As Sm, Nb can be used to place constraints on the site of p-process nucleosynthesis. After a first hint (Harper 1996 Sardoup et al. 2000), its presence was established in an ordinary chondrite and a mesosiderite (Fig. 9f Schonbachler et al. 2002). The solar system initial Nb/ Nb ratio was between 10 and 3x10 a value in the same order of magnitude as most of the other extinct radioactivities. 

Ganguly J., Yang H., and Ghose S. (1994) Thermal history of mesosiderites quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxenes. Geochim. Cosmochim. Acta 58, 2711-2723. 

Stony irons are nonchondritic meteorites that contain roughly equal proportions of silicate minerals and metal. Two types of stony irons - pallasites and mesosiderites - are distinguished. Pallasites consist of approximately equal amounts of metal and olivine (one small group contains pyroxene as well). Mesosiderates also have approximately equal proportions of metal and silicate, but the silicate fraction is basalt. 

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

The stony-iron meteorites are intermediate between chondrites and irons. These very rare meteorites are equal mixtures of iron/nickel alloys and silicate minerals. Pallasites are striking examples of this type of meteorites, consisting of green olivine crystals in a matrix of metallic iron. Another type of stony-iron meteorite, called mesosiderites, contain pyroxene and plagioclase feldspars, minerals that are common on Earth. 

As illustrated in Fig. 10, europium anomalies are found not only in lunar samples but also, for example, in the silicate phase of mesosiderites. This is strong evidence that these meteorites have undergone extensive magmatic fractionation. Of course, a low partial pressure of oxygen is required in order to keep europium in the 2+ state. 

Fig. 14 includes terrestrial and meteoritic samples which show that the good correlation of La and K is not restricted to the Moon. It is interesting to note that the correlation line for the Moon falls close to that of the basaltic achondrites and that furthermore the silicate phases of mesosiderites and the hypersthene chondrite (diogenite) Johnstown fall on the line of the basaltic achondrites. 

HED meteorites howardites, eucrites, diogenites Mesosiderites Pallasites... 

Angrites Mesosiderites pallasites Aubrites Brachinites Ureilites HEP... 

Pallasite and mesosiderite Metal, troilite, schreibersite, graphite Coarse-grained, generally exsolved Fractional crystallization of core... 

The mesosiderites are divided into three petrologic classes (Hewins, 1984, 1988) based on... 

Chaunskij is an unusual, highly metamorphosed, heavily shocked, metal-rich, cordierite-bearing mesosiderite (Petaev et al., 2000). 

Figure 23 (a) The type-1 A mesosiderite Mount Padbury having numerous centimeter-sized silicate and metal clasts... 

Figure 24 Orthopyroxene (wt.%) versus plagioclase (wt.%) for mesosiderites showing the generally greater basaltic component, indicated by plagioclase, in the compositional class-A mesosiderites compared to the more ultramafic-rich class-B mesosiderites (source Mittlefehldt et aL, 1998) (reproduced by permission of the Mineralogical Society of America from Reviews in Mineralogy 1998, 36, 1-495).

Table 7 Summary of criteria for textural types of mesosiderites.

Floran R. J. (1978) Sihcate petrography, classification, and origin of the mesosiderites review and new observations. Proc. Lunar Planet. Sci. Conf. 9, 1053-1081. 

Hewins R. H. (1983) Impact versus internal origins for mesosiderites. Proc. 14th Lunar Planet. Sci. Conf Part I. J. Geophys. Res. 88, B257-B266. 

Hewins R. H. (1988) Petrology and pairing of mesosiderites from Victoria Land, Antarctica. Meteoritics 23, 123—129. 

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