L impact-melt rock
(H/L6 impact-melt clast in MetBull 90)
(H/L6-melt rock in MetBull Database)

standby for northwest africa 4150 photo
Purchased before 2006
no coordinates recorded

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 initially determined that NWA 4150 is a unique melt lithology belonging to the transitional H/L-chondrite group. This initial classification has been revised based on continued study (Wittmann et al., 2011).

Northwest Africa 4150 has an average olivine Fa value of 24.1 and an average pyroxene Fs value of 19.9, values which are in the range of the L-chondrite group. The metal abundance of 5% matches that of L chondrites, and the concentration of Co in kamacite matches that group as well. A wide variety of chondrules are represented over a much reduced surface area due to the chondritic melt, and they have average dimensions indistinguishable from those of L chondrites. Along with a quenched melt mesostasis, the chondrules and microcrystalline chondrule fragments exhibit an excellent state of preservation, and surviving lithic clasts indicate a petrologic type of L3–L4 (Wittmann et al., 2011). An Fe–S–Si melt covers a significant area, with a cooling rate estimated to be 0.05°C per year, similar to that of L5 Cat Mountain, and corresponding to the metallographic cooling rate at a depth of 1 km. Shock features are prevalent in chondrule fragments, including undulous extinction, planar fractures, mosaicism, and brown olivine. The crystallized impact-melt component contains rounded metal and sulfide particles and aggregates very similar in shape to those in L6 Wickenburg. It is suggested that NWA 4150 formed within a thick breccia lens of a crater measuring 5–7 km in diameter.

It might be a reasonable assumption that the L impact-melt rock NWA 4150 could be paired with the five member pairing group comprising NWA 4152–4156 which were initially classified as H/L6 chondrites, 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. Nevertheless, there are some petrographic features of NWA 4150 that might be 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). Further analysis is required to make a conclusion on their pairing status.

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 (Fa19.920.6). 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 different size scales, and this may shed further light on the question of pairing. They found that the Fa values for the chondrites studied were heterogeneous on km-sized scales and above, but homogenous on meter-sized scales and below. It could be inferred that the Fa values of all stones that are derived from a single multi-fragmented meteoroid should all be identical, and therefore, the difference in Fa values between the L impact-melt rock NWA 4150 and the H/L6-NWA pairing group is inconsistent with their pairing.

The oxygen isotopic composition of NWA 4150 and its possible pairing group of NWA 4152–4156 will help establish their correct groupings. In addition, K–Ar data will be utilized to provide the age of impact. 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.