HOLBROOK


L/LL6
standby for holbrook photo
Fell July 19, 1912
34° 54' N., 110° 11' W.

Accompanied by detonations heard for miles around, local residents observed a smoke trail over Navajo County, Arizona at 7:15 in the evening. A large terminal explosion, which was heard at least 40 miles away, was followed by a rapid series of smaller explosions and a long-lasting thunder-like rumbling, leaving a smoky trail in its wake (Holbrook Argus, July 26, 1912). Multiple fragmentations occurred, with an estimated 16,000+ stones falling near the Aztec railroad yard, pelting the metal roofs and raising clouds of dust in the surrounding desert. Then seventeen-year-old Pauline McCleve later recalled, "that was the loudest sound I ever heard in my life". A combined weight of over 536 pounds was recovered, ranging in size from 14.5 pounds to pea-sized and smaller. The famous Adamana, or "Venus Stone", has been speculated by some to be the nosecone of the Holbrook meteoroid.

The disruption occurred at a high enough altitude to enable fusion crust to form on even the smallest fragments. To this day specimens of this meteorite continue to be found within the strewnfield, located about 6 miles east of Holbrook at the southern end of Sun Valley Rd. off of I–40. It encompasses an area about 1/2 mile to the north and south of the Sante Fe RR tracks covering an area about 1 mile west and 2 miles east along the tracks from Sun Valley Rd; i.e., an ellipse with dimensions of ~ 1.5 × 3.5 miles (Dave Andrews—Holbrook hunter). An oxygen isotopic study comparing the earliest recovered specimens to those recovered most recently has demonstrated that the heavier isotopes have increased measurably over the past 100 years (Pillinger et al., 2013).

In a study of Holbrook conducted by Rubin (1990), it was determined that the mean olivine Fa content (Fa 25.6) is at the high range for equilibrated L chondrites (Fa 23.0–25.8). In addition, the Co abundance in kamacite grains (10.4 mg/g) was found to lie between the values for both equilibrated L and LL chondrites (7.0–9.5 mg/g and 14.2–370 mg/g, respectively). Moreover, Clayton et al. (1991) determined that the O-isotopic composition of Holbrook (Δ17O = 1.25–1.32‰) lies at and beyond the upper range of equilibrated L chondrites (Δ17O = 0.80–1.25‰), but within the range of equilibrated LL chondrites (Δ17O = 1.02–1.44‰). Therefore, Holbrook has been included within the transitional L/LL group.

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio, grain density, and magnetic susceptibility, and these Amean values all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.

standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3)

From rare gas studies, it is thought that the parent body of Holbrook underwent a catastrophic collision 340 (±50) m.y. ago. The largest Holbrook individual that was recovered, weighing about 14.5 pounds, has resided at the American Museum of Natural History in New York since Warren Foote obtained it soon after the fall. It was only recently returned to Arizona, and can be viewed in the Bateman Physical Sciences Center at Arizona State University in Tempe.

The photo shown above is a 1.0 g, fusion-crusted, pea-sized specimen labeled as AMNH collection #3241, along with an interior cut fragment weighing 1.2 g. Pictured below is an artist's impression of a "deadly hail of stones" at the Holbrook fall in Arizona, as illustrated by Walter Molino in an October 6, 1946 edition of the Italian newspaper La Domenica Del Corriere.