PASAMONTE

At 5:00 on a March morning, ~100 stones totaling 3–4 kg were heard and seen to fall in New Mexico, after putting on a display for observers in New Mexico, Colorado, Kansas, Oklahoma, and as far away as Texas and Wyoming. The fireball left a thick, twisting dust trail, perhaps a mile wide and hundreds of miles long, comprising perhaps thousands of tons of material. Grabbing his Kodak Brownie camera, a rare photo of the actual fireball in flight was taken by the quick-acting Charles M. Brown as it spiralled towards Earth (see below), and other images of the remnant twisted dust cloud were captured. Data was written in a note by Harvey Nininger describing the scene as photographed by Charles Brown:

Great meteor of Mar. 24 1933. Photo by Chas. M. Brown and copyrighted by him. Nininger survey demonstrated that meteor was visible for 15 to 22 seconds. Cloud remained visible 3 hrs. or more. Diameter of luminous sphere was about 6 miles. Diameter of spiral train was about 1 mile. Meteorites from this fall were strewn along a path of 28 miles having a width of about 2 to 3 miles wide. The fall was in an E.N.E. to W.S.W. direction beginning about 25 mi. W.S.W. from Clayton New Mexico. Meteorites were preserved in America Meteorite Museum, U.S. 66, west of Winslow Arizona.

The small stones were collected by ranchers along a distance of 28 miles near the Pasamonte Ranch, and these were subsequently identified as meteorites by Harvey Nininger, who had independently located the strewnfield after spending many months conducting eyewitness interviews.

Pasamonte was determined to be an unequilibrated (Type 2 in the metamorphic sequence of Takeda and Graham, 1991) basaltic meteorite that has retained some primary Fe–Mg zoning in pigeonite grains. While it was previously classified as an unequilibrated monomict eucrite representing the type specimen for "Pasamonte-type" lithologies in polymict eucrites, detailed examination has resulted in its reclassification as a polymict breccia. Pasamonte contains a variety of basaltic lithologies, granulites, granulitic breccias, and impact melt breccias. In addition, it contains pyroxene of both equilibrated and unequilibrated types with differing zoning types. Pasamonte exhibits evidence of mild thermal annealing by the variation it exhibits in pyroxene lamellar wavelengths, a factor related to cooling rate. This feature, along with the Fe-enriched zones adjacent to pyroxenes fractures and reversed zoning in pyroxenes, provides evidence supporting a low degree post-magmatic metasomatic equilibration process associated with an Fe-rich dry vapor lasting ~60 years (Schwartz and McCallum, 2003; McCallum et al., 2004; Barrat et al., 2011). This duration can be contrasted to the 25,000 years of annealing experienced by the highly metamorphosed eucrite Haraiya (Type 7). In contrast, the eucrite NWA 049 represents a sample that experienced a high degree of post-magmatic metasomatic equilibration. Pasamonte has a very old crystallization age of 4.58 b.y., and a young cosmic ray exposure age of only 7.7 m.y.

In 1981, the Basaltic Volcanism Study Project (BVSP) assigned Pasamonte, along with Nuevo Laredo and Lakangaon, to the Nuevo Laredo Trend eucrites, which were formed from fractional crystallization of Main Group melts. However, this assignment of Pasamonte may have been based on incomplete data. The three subgroups of the noncumulate group of eucrites have been separated based on the molar Mg/(Mg+Fe) (here abbreviated Mg#) versus an incompatible element such as Ti, as follows:

1. Main Group (primary basalt): Mg# ~ 0.38–0.41; Ti ~ 3–4 mg/g
   
2. Stannern trend (primary partial melt): Mg# similar to main series; Ti up to 5.7 mg/g
3. Nuevo Laredo trend (fractional crystallization): Mg# extends from Main Group to 0.32; Ti = 5.7 mg/g

A plot of the three subgroups shows a convergence at the center of the Main Group, implying a genetic relationship (i.e., same parent body) exists among them, and a possible derivation of the two trends from the primary melts of the Main Group. Currently, the Main Group is combined with the Nuevo Laredo Trend to form a single series, while the Stannern Trend represents Main Group magma that has been contaminated by a crustal partial melt.

It has been demonstrated that the HED parent body was homogenous in its O-isotopic composition. In a study of a number of eucrites having anomalous O-isotopic ratios and/or anomalous chemical compositions, textures, or ages, evidence was presented indicating that Pasamonte must have originated on a parent body distinct from that of the other HED meteorites (Scott et al., 2008, 2009). For example, its significant displacement from the Eucrite Fractionation Line (EFL)—plotting ~4.7 standard deviations from the eucrite/diogenite mean Δ17O value—cannot be reasonably explained by the admixture of foreign impactor contaminates, by terrestrial weathering processes, or by an isotopically heterogeneous parent body. Pasamonte has a pyroxene Fe/Mn ratio of 29, which is at the lower range (28–40) of typical eucrites. Moreover, its chromites have compositions which are much more Al rich and Ti poor than those in other eucrites. It is reasonable to assume that Pasamonte was derived from one of many Vesta-sized asteroids that likely existed early in Solar System history prior to the Late Heavy Bombardment period 3.5–4.1 b.y. ago. Notably, the eucrite PCA 91007 contains O-isotopic compositions that are virtually identical to Pasamonte, and they both have anomalously high abundances of certain siderophile elements (Ni, Ir, Os) as well; it could be inferred that they share a common origin (Scott et al., 2009).

Because there are now a number of eucrite-like meteorites that are not grouped with normal eucrites for various reasons, it was proposed that the term ‘eucrite’ be used as a description of a rock type rather than to imply an origin on the HED parent body, Vesta. Pictured above is a crusted fragment of the recently distinguished ungrouped achondrite, Pasamonte, weighing 18.93 g (previously in the Robert Haag Collection).

This actual photo captures the exact moment when a jet breaks the sound barrier. Compare this to the Pasamonte fireball photo above. The corkscrew appearance of the dust train attests to the spinning motion of the incoming object over an extended period of time. The photo below shows the persistent dust cloud of the Pasamonte meteorite showing the effects of adiabatic processes.