ALH84001

This is a summary of petrographic observations of carbonates in probe mount thin sections of ALH 84001. I examined five thin sections, 64, 82, 86, 88 and 90. Section 82 is a thick section, nominally about 100 µm thick, and petrography of this section is more difficult. All thin sections contain carbonates, although the abundance varies. I estimate that on average carbonates make up on the order of a percent by area of the thin sections. Note that I do not consider my abilities at areal estimates to be very good. This should not be considered a detailed petrographic study of carbonates, as I didn't have the time I would like to do this work.

The carbonates can be divided into four basic textural types:

1. Rounded, zoned blobs (rzb). These are the "classic" ALH 84001 carbonates that get the most discussion in the literature. They are typically 150-200 100 µm in diameter, with orangish brown centers, a thin black inner rim, a white, ~10mm thick middle rim and a thin black outer rim. The black rims are on the order of a couple of 100 µm in thickness. In thin section the rzb are seldom complete circles, unlike what I've observed on hand sample. They can occur singly or in clusters of touching rzb. They are frequently in interstitial areas between large orthopyroxene grains, associated with maskelynite. However, from the few I saw in thin section this may not be truly general.
   
   
2. Polkilitic, zoned blobs (pzb). These are approximately equidimensional regions in the crushed zones containing orangish brown carbonates that appear to mimic the zoning trend seen in rzb. The pzb include small rounded grains of orthopyroxene. Black material is present as a discontinuous rim around the pzb, and this may be the equivalent of the black rims on the rzb. In some, l believe I saw some white, magnesian carbonates in the outer regions of the pzb. The size of individual carbonate areas inside the pzb is of the order of 10-20 mm. The pzb are probably equivalent to the carbonates shown by Treiman (1995) in Fig. 7, a back-scattered electron image.
   
   
3. Unzoned, fine-grained patches (ufp). These are orangish-brown carbonates in crushed zones and are composed of ~10-20 um grains interspersed with the rounded ~10-20 100 µm orthopyroxene grains. Unlike the pzb, the ufp tend to be more irregular in shape, and do not have the discontinuous rims of black material. The patches can be several hundred 100 µm across.
   
   
4. Fracture filling patches (ffp). These are common in thin section. They consist of true veins of orangish brown carbonates in wider fractures and trains of orangish brown carbonate grains in narrower fractures in large orthopyroxene grains. The texture gives the appearance that the carbonates are replacing the orthopyroxene. However, it is possible that they simply grew in fractures.