NORTHWEST AFRICA 4150

Two pieces of a single stone weighing together 436.4 g were found in Northwest Africa and sold in Erfoud, Morocco to meteorite collector C. Anger of Austromet. A sample was submitted for analysis and classification to the Museum für Naturkunde (A. Greshake and M. Kurz), and it was determined that NWA 4150 is a unique melt lithology belonging to the transitional H/L-chondrite group.

The H/L-chondrite group comprises only a couple of dozen members, of which some or all may represent a distinct ordinary chondrite parent body. Northwest Africa 4150 has an average olivine Fa value of 24.1 and an average pyroxene Fs value of 19.9, values which are consistent with those of the L-chondrite group. Identification by the investigators of other chemical and petrographic characteristics, e.g., the Co content in kamacite, which are consistent with a transitional H/L grouping have not yet been published, but such information would provide insight into the classification guidelines utilized for NWA 4150.

ordinary chondrite Fa and Fs ranges
	Fa	Fs
H	16–20.4	14.5–18.1
H/L	19.5–21.8	17.2–21.2
L	22–26	18.7–22
L/LL	25.5–26.5	—
LL	26–33	22–26

H/L-chondrite ranges derived from published values for equilibrated H/L members.

The petrographic features of this meteorite were determined to be consistent with metamorphic type 6, a rare petrologic type for this transitional group shared by only three other meteorites—DaG 591, SaU 324, and the pairing group of NWA 4152–4156.

It may be a reasonable assumption by many that the very rare H/L6-melt rock NWA 4150 may be paired with the five H/L6 group members constituting the NWA 4152–4156 pairing group, especially considering that all were found at virtually the same location in the same timeframe, purchased by the same collector, submitted to the same classification lab, and assigned virtually consecutive NWA-series numbers. Moreover, similar scenarios have occurred before, in which a very small percentage of shock melted stones were discovered associated with a majority of non-melted stones of a common fall, e.g., Gao-Guenie. That said, there are several petrographic and chemical features of NWA 4150 that are inconsistent with such a pairing; its weathering grade of W0/1 is significantly lower than that of the NWA pairing group (W2–4), and its impact melt features reflect a much greater shock event than that commensurate with the NWA pairing group (S3–4).

Another important factor relevant to the pairing question is the olivine Fa content that characterizes NWA 4150 (Fa24.1) compared to that of the NWA pairing group (Fa19.9–20.6). A recent study by Rubin et al. (2008), addresses the question of how the oxidation state of ordinary chondrites (represented by the mean olivine Fa value) varies over size scales, and this may shed further light on the question of pairing. They found that the Fa values for the chondrites they studied were heterogeneous on km-sized scales and above, but were homogenous on the meter-size scale and below. It can be inferred that the Fa values of all stones which are derived from a single multi-fragmented meteoroid should all be identical, and therefore, the difference in Fa values between the H/L6-melt rock NWA 4150 and the H/L6-NWA pairing group is inconsistent with their pairing. The determination of the CRE ages of these meteorites could help bring a definitive answer to this puzzle. The photo of NWA 4150 shown above is a 3.21 g end section acquired from Christian Anger which exhibits a ring-shaped, metallic-melt swirl pattern in the upper left corner. Thank you dearly departed friend.