Pallasites- These meteorites are mixtures of olivine and FeNi-metal that probably formed through impact events in the upper mantle region of small, differentiated asteroids. Later collisions exposed this mixed zone and delivered samples to Earth. To date, three compositional clusters and three ungrouped pallasites have been identified thus far, each likely representing one or more distinct parent bodies:
1. Main-group
olivine with Fa contents between 10.5 and 13%; metal with 8–12% Ni
2. Eagle Station subgroup
olivine with Fa contents between 19 and 20%; metal richer in Ni, Ir, and Ge and poorer in Au, As, and Ga than main-group
3. Pyroxene subgroup
pyroxene coexists with olivine, metal ungrouped and different from MG and ES
4. Ungrouped: Milton, Choteau, NWA 10019
Utilizing the pressure constraints on the tridymite inclusions present in the Fukang pallasite, the maximum size limit for the main-group pallasite parent body would be ~600 km in diameter; a minimum size still large enough to enable differentiation would be ~40 km (Della-Giustina et al., 2011).

Mesosiderites- The mesosiderites are complex polymict assemblages of FeNi metal and brecciated silicates including orthopyroxene, plagioclase, and olivine. Lithic clasts of cumulate and basaltic eucrites, diogenites, and dunites are present. Mesosiderites possess specific thermal histories ranging from little recrystallized to melted (subgroups 1–4), reflecting different cooling rates based on their burial depth following a rapid heating event, probably an impact. Group 1 cooled the fastest at ~1°C/m.y. with group 3 having the slowest cooling rate of ~0.1°C/m.y. Group 4 reached the highest temperature of ~1350°C before quickly cooling. Recognition of clear differences in bulk compositions among these groups led to further group divisions (see the Bondoc page for details regarding these subdivisions).

There is not yet a consensus for the origin of the mesosiderites, and different theories currently exist to explain their formation. A recent model based on smoothed-particle hydrodynamics calls for the disruption and re-accretion of a 200–400 km differentiated asteroid with a molten core. The impactor is calculated to have been a 50–150 km body with an impact speed of 5 km/s. This event initially caused rapid cooling (~0.1°C/y.) from thermal equilibration, followed by very slow cooling (~0.5°C/m.y.) as the brecciated material was deeply covered by a massive debris blanket. The Ar–Ar ages of mesosiderites of 3.7–4.1 b.y. reflect this very slow cooling. Weakly shocked olivine was sequestered into the core at the time of the catastrophic impact. Subsequently, molten metal was mixed with crustal fragments during re-accretion. The O-isotopic values for the mesosiderites are virtually identical to those of the HED suite meteorites, which implies that a genetic link (i.e., same parent body) exists between these meteorite classes (Greenwood et al., 2006). Stony-irons represent only 2.8% of the total known meteorites.

[PART I] Chondrites
[PART II] Achondrites
[PART III] Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification

© 1997–2017 by David Weir