MESOSIDERITE
Mesosiderites make up one of the two groups of stony-iron meteorites. Unlike
the pallasites, the mesosiderites appear to consist of impact-generated
silicate-metal breccias - they're mixtures of asteroidal core & mantle
& crustal materials. The silicate-rich portion consists of olivine,
pyroxene, and calcic plagioclase feldspar, with a texture ranging from
unmetamorphosed angular clasts to recrystallized silicates to melt
matrix. The metal portion is iron-nickel alloy, having the minerals
kamacite & taenite. The sample shown below also has a fair amount of
troilite (FeS - iron monosulfide) in close association with the iron metal.
Genetically, meteoriticists have noticed that some of
the silicates in mesosiderites have close chemical-mineralogical similarities
to HED meteorites (rocks from the asteroid Vesta). The iron metal portion
is chemically similar to group IIIAB iron meteorites.
So, what's the origin of mesosiderites? These
rocks appear to be the result of impact mixing.
Impactor
- an asteroidal iron core stripped of its crust and mantle
Impactee
- Vesta or Vesta-like asteroid having basaltic-gabbroic crust
The rock shown below is from the Estherville
Meteorite, a type A3/4 mesosiderite having a highly recrystallized silicate
matrix. This meteorite fell to Earth at 5 PM on 10 May 1879 near the town
of Estherville in northwestern Iowa, USA.
Published isotopic dates of Estherville silicates
indicate crystallization ages from 4.422 to 4.556 billion years. The
impact event has been dated to 4.4 billion years, and a significant reheating
event occurred at 3.6 billion years.
Mesosiderite (above & below; 2.9 cm across) - cut & polished slice of the
Estherville Meteorite. The blackish material is Fe-Ni metal. The
rest of the rock is a mixture of pyroxene, plagioclase feldspar, olivine, and
some troilite.
Mesosiderite - cut & polished slice (2.9 cm across), tilted to show silver
reflectivity of Fe-Ni metal component & golden-brown color of the troilite
(see labels below).
(More info. on the
Estherville Meteorite)
Mostly
synthesized from Norton (1998) and Hutchinson (2004).