PASAMONTE


Eucrite-like basaltic achondrite, ungrouped
(previously Eucrite-polymict)
standby for pasamonte photo
Fell March 24, 1933
36° 13' N., 103° 24' W.

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). By 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 might 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 that 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 relatively 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 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 paired brecciated, vesiculated basalts PCA 82502 and PCA 91007 have O-isotopic compositions which are virtually identical to Pasamonte (see diagram below), and they have similar anomalously high abundances of certain siderophile elements (Ni, Ir, Os) as well; it could be inferred that they formed in a common nebular region (Scott et al., 2009).

standby for pasamonte-pca o diagram
Diagram credit: Mittlefehldt et al., 47th LPSC, #1240 (2016)

As presented by Sanborn and Yin (2014) [#2018], a Δ17O vs. ε54Cr diagram is one of the best available diagnostic tools for determining genetic (parent body) relationships among meteorites, constrained by the degree to which isotopic homogenization occurred on their respective parent bodies. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. Currently, a number of anomalous eucritic meteorites are known, including Ibitira, Pasamonte, NWA 1240, PCA 82502/91007, Bunburra Rockhole, A-881394, EET 92023, and Emmaville, each of which are resolved from typical eucrites and the HED parent body both isotopically and compositionally; notably, the latter four anomalous eucritic meteorites share close similarities in their O-isotopes and might be genetically related (Barrett et al., 2017; see O-isotopic diagram).

Another useful tool to help resolve potential genetic relationships among meteorites is the Fe/Mn ratio. While Fe and Mn do experience nebular fractionations they are not readily fractionated during parent body igneous processing, and therefore different Fe/Mn values are inherent in different parent objects. Mittlefehldt et al. (2017) utilized a number of eucrites and anomalous eucrite meteorites, including A-881394, EET 92023, Ibitira, and Emmaville, to compare the Fe/Mn and Fe/Mg ratios in low-Ca pyroxenes. Contrary to the O-isotopic results, these four meteorites plot in separate locations on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies (see top diagram below). Moreover, despite the fact that Pasamonte and PCA 82502/91007 are similar in both Δ17O and ε54Cr values (see Sanborn et al., 2016, #2256), these two meteorites are resolved on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies as well (see bottom diagram below).

standby for fe/mn vs. fe/mg diagram
Diagram adapted from Mittlefehldt et al., 47th LPSC, #1240 (2016)

standby for fe/mn vs. fe/mg diagram
Diagram credit: Greenwood et al., 48th LPSC, #1194 (2017)

It is known that ureilites, generally considered to originate from a common parent body, have a relatively wide degree of variability in Δ17O, but a relatively narrow degree of variability in ε54Cr. By comparison, Sanborn et al. (2014) inferred that the similar degrees of variability that exist among these anomalous eucritic meteorites could likewise reflect a common origin from a single Vesta-like parent body distinct from typical eucrites (see diagram below). Several exceptions to this hypothesis have since been identified including the following: NWA 1240 plots away from the common HED field; PCA 82502/91007 is resolved from the other anomalous eucrites by both O-isotopes and pyroxene Fe/Mn ratio; A-881394 has significantly different oxygen isotopes, Ti/Al and Fe/Mn values, and bulk composition compared to HEDs (Mittlefehldt et al. (2015); and EET 92023 exhibits significant differences in O-isotopes, Cr-isotopes (Sanborn et al., 2016, #2256), and pyroxene composition compared to HEDs and other anomalous achondrites. EET 92023 shares similar O- and Cr-isotopes to A-881394 and Bunburra Rockhole indicating that they each formed within a common isotopic reservoir. Under the hypothesis that Δ17O values serve equally well as a discriminator compared to ε54Cr values, all of these anomalous meteorites could derive from numerous unique parent bodies distinct from Vesta (see diagram below). Furthermore, although Bunburra Rockhole and A-881394 have the same oxygen and chromium isotope compositions, new in-depth analyses of Bunburra Rockhole conducted by Benedix et al. (2017, and references therein) have revealed that these two meteorites have very different textures and mineral chemistries; e.g., Bunburra Rockhole has plagioclase with An8790, while A-881394 has plagioclase with An98. Based on their results, they consider it likely that these two meteorites also derive from separate parent bodies.

standby for pasamonte o-cr diagram
Diagram adapted from Sanborn and Yin, 45th LPSC, #2018 (2014)

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 presumed HED parent body Vesta. The photo above is a crusted fragment of an 18.93 g specimen of Pasamonte acquired from the Robert Haag Collection.

standby for sonic boom photo standby for pasamonte photo
The left photo above captures the shock-generated condensation cloud at the moment when a jet breaks the sound barrier. Compare this to the Pasamonte fireball photo on the right. 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.
standby for pasamonte photo