AMHERST COLLEGE Department of Geology Geology 41: Environmental and Solid Earth Geophysics

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1 AMHERST COLLEGE Department of Geology Geology 41: Environmental and Solid Earth Geophysics Lab 1: Meteorites EQUIPMENT: notebook and pen only In this lab, we will examine thin sections and hand samples of meteorites and compare them with terrestrial rocks. The meteorite samples used in this lab are irreplaceable, so please handle them with care. Thin-sections are set-up on microscopes. Please move from scope to scope to view them rather than moving the thin-sections. The purpose of the lab is TO SEE AND EXAMINE meteorites OURSELVES. Meteorite samples like these are our only (non-lunar) extra terrestrial samples and our best analogs for the starting materials for Earth. Look carefully at each sample to try to see what it might tell us about the solar system and the early history of Earth. Iron meteorites: These meteorites are composed of an octahedral (Widmanstätten) intergrowth of kamacite (Nipoor α-iron) and taenite (Ni-rich γ-iron). The Widmanstätten structure forms by the exsolution of Ni-rich taenite from a intermediate-ni taenite when the parent body of the meteorite cooled form a temperature above 800 C. The width of the exsolution lamellae is related to the rate of cooling of the parent body through a temperature of approximately 500 C. Slower cooling rates allowed the exsolution lamellae to grow thicker. The coarse lamellae that are visible in the meteorite hand samples is kamacite. In most of the samples the taenite lamellae are too time to be seen with the naked eye. The Fe sulfide troilite (FeS) is a common minor constituent of iron meteorites. Classification of iron meteorites is based largely on the nature of the kamacite exsolution lamellae Iron meteorite structural classification kamacite width abbreviation Hexactahedrite (only kamacite) >50 mm H Octahedrite (both kamacite and taenite in Wittmanstatten pattern) coarsest mm Ogg coarse mm Og medium mm Om fine mm Of finest <0.2 mm Off plessitic <0.2 mm Opl Ataxite (no structure) D

2 Look carefully at each of these samples of Fe meteorites determine the approximate width of the kamacite bands and rank the samples in terms of relative cooling rate of the parent bodies. What can you infer about the parent bodies from this information? 1) Toluca meteorite: hand sample 2) Henbury meteorite: hand sample 3) El Sampal: hand sample 4) Canyon Diablo: hand sample Stony-Iron meteorites Stony-iron meteorites contain abundant metal and silicates. Stony-iron meteorites are relatively rare (only 1.2 % of observed falls) and appear to be mechanical mixtures of metal and silicates. Pallasites are stony-iron meteorites with olivine crystals and clasts in a continuous metal matrix which frequently has a Widmanstätten texture. Mesosiderites contain a wide range of silicate clasts (olivine, pyroxene, feldspar) in a mesh-textured matrix of metal. Examine the 2 samples of stony-iron meteorites. Are these pallasites or mesosiderites? Do you think that these might be more closely related to Fe-meteorites or chondrites? Why? 1) Springwater: hand sample 2) Albin: hand sample Chondritic meteorites

3 Chondritic meteorites are distinguished from all other types of rocks by the presence of chondrules, small spherical silicate masses. Chondrules may be composed of a single mineral such as olivine, or pyroxene, or they may be composed of aggregates of silicates, metal and silicate glass (sometimes devitrified). Most chondrites contain a variety of different types of chondrules. The chondrules sit in a matrix of silicates with minor amounts of metal. Chondrites are classified into enstatite (E-chondrites), ordinary (H, L, & LL-chondrites) and carbonaceous (C-chondrites) on the basis of bulk meteorite and mineral chemistry. Bulk-rock ratios of Mg/Si, Al/Si, Ca/Si increases from enstatite to ordinary to carbonaceous chondrites. Bulk chemical variations between chondritic meteorite classes (and sub-classes) suggest that each class or sub-class formed from a separate parent body.

4 Chondrites can be further classified based on their textures and variations in mineral composition 1) The following 3 samples (2 terrestrial, 1 meteorite) contain more or less the same mineralogy. Wellman (meteorite) C-8 Stillwater complex SEM-2 S ophiolite The Stillwater complex is a mafic layered igneous complex. The minerals in sample C-8 settled to the base of a crystallizing magma chamber. SEM-2 is a sample from the oceanic upper mantle. It has flowed in the solid state. How do the textures in the meteorite compare with the textures in the two terrestrial samples? Describe textures that are found in both the terrestrial samples and the meteorite. Describe textures found only in the meteorite.

5 2) Examine the chondrules in the following meteorite sample. Are they easy to distinguish from the matrix? Are they all similar, or are there distinct types of chondrules? Describe 3 different types of chondrules. Do any chondrules have textures similar to those seen in the terrestrial samples (C-8 or SEM-2)? Parnalee 3) Meteorites often contain shock features that formed when they were ejected from their parent bodies. Look at the chondrules and matrix of the following meteorite samples. Is it easy to distinguish the matrix from the chondrules? What evidence is there for shock metamorphism of these meteorites? Plainview Kelly New Concord hand sample

6 4) Carbonaceous chondrites are considered to be the most primitive type of meteorites. They have been influenced less by processes that occurred after accretion of their parent bodies than other types of meteorites (i.e., they came from planetary bodies with very simple geological histories). Chondrules and calcium-aluminum inclusions (CAI) are present in a fine-grained dark carbonaceous matrix. The CAI s are composed of relatively to very fine grained spinel, melilite(ca 2 Al 2 SiO 7 ), anorthite and pyroxene. These minerals are the phases that would condense first from a gas of solar composition and may represent primary condensates from the solar nebula. Describe the texture of a CAI. Allende