Portales Valley

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H IMB (metallic melt breccia)
(H6 in MetBull 83)
standby for portales valley photo
Fell June 13, 1998
34° 10.5′ N., 103° 17.7′ W.

At 7:32 on a Saturday morning, accompanied by numerous sonic booms and smoke trails, a bright fireball fragmented and produced a meteorite shower over eastern New Mexico. The meteorite was observed by a Piper Cherokee pilot and his passengers as the meteorite fell toward Portales, New Mexico. At the same time, while having their morning coffee, Nelda Wallace and Fred Stafford heard loud explosions. They ran out onto the porch in time to witness a fragment land with a thud 100 yards away, kicking up a large cloud of dust. Within a crater ten inches deep, they recovered a 16.5 kg, metal-veined fragment. In another case, a 530 g fragment penetrated the roof of Gayle Newberry’s barn and embedded itself in the north wall, verifying the meteorite’s southwest to northeast trajectory (on a course of ~62°).

Remarkably, famed meteorite hunter Skip Wilson heard the sonic booms and witnessed the corkscrew-shaped contrail produced by the meteorite as it approached. He later recovered several specimens from the area. Approximately 51 fragments were recovered over several weeks ranging in weight from 12 g to 34 kg (see photos below), having a combined weight of over 100 kg. The fragments delimit a strewnfield 12 km-long by 2.5 km-wide, and contrary to normal fall patterns, the largest fragments fell at the beginning of the strewnfield rather than at the end — a phenomenon similar to that of the Johnstown diogenite fall. This unusual pattern can be explained by a multiple breakup in which the largest fragment is disrupted late downrange.

Portales Valley is an unusual recrystallized, H chondrite, metallic-melt breccia—a unique classification recently proposed by A. Ruzicka et al. (2005). This meteorite contains a low abundance of relict chondrules (2.6 vol%) and large, mm- to cm-sized, metal-rich veins exhibiting a fine-to-medium Thomson (Widmanstätten) structure, the first chondrite found to contain this feature (see photo below). The large metal-rich veins have a composition similar to H-chondrite metal with calculated Re–Os and U–Pb ages of formation of 4.56 b.y. An absolute I–Xe age, calculated relative to Shallowater (4,562.3 [±0.4] m.y.), was calculated to be 4,559.9 (±0.5) m.y. (Bogard and Garrison, 2009). The Ar–Ar age, representing the last significant heating event, is calculated to be 4,477 (±11) m.y. The chondritic component of Portales Valley is depleted in metal (4.4%) compared to normal H-chondrite material (15–19%), suggesting that the metal-rich veins might be derived from the host silicate component through impact mobilization of an already hot to partially molten source.

Studies of sub-mm-sized feeder veins and portions of silicate-rich material reveal evidence for a significant heating and geochemical fractionation event, probably by an endogenous heat source. This may account for the major depletion of Sm–Nd and the significant LREE enrichment. A model which best expains the trace element data favors equilibrium partial melting with incomplete separation of melt from solid, and reflects variations in the proportion of melt and solid that existed within both silicate-rich and large vein areas. The degree of metallic partial melting was <~40%, while the silicate melt fraction was ~13%, corresponding to temperatures of ~940–1150°C. Portales Valley is the first documented metallic-melt breccia, presumed to have been created through partial melting and mobilization of the metal phase. As such, it was proposed by Ruzicka et al. that it could represent a new type of meteorite—a portalesite. Other meteorites were also formed as a result of impact processing on the H-chondrite parent body, including the FeNi- metal meteorites Sacramento Wash 005 and Meteorite Hills 00428. These are the first recognized Fe- and S-rich meteorites to have been formed by impact on the H-chondrite parent body; notably, both are distinct from the IIE-iron meteorite group (Schrader et al., 2010).

One formation scenario presupposes that the REE and actinides mechanically segregated into phosphate and metal phases. This segregation occurred simultaneously with large, metal-rich vein formation, and probably with brecciation of the rock during an impact event ~1.16–1.85 b.y. ago (Sm–Nd). However, this young Sm–Nd age may reflect a sampling bias, or perhaps a mixture of components having discordant ratios. Other shock indicators present in Portales Valley include silicate darkening, chromite–plagioclase assemblages, chromite veinlets, metallic Cu, and irregular troilite grains in FeNi-metal, which are all indicative of an earlier shock stage of S6 that was followed by postshock annealing to a shock stage slightly higher than S1 (Rubin, 2004). An alternative scenario was asserted by Ruzicka et al.. It invokes only a weak shock event (S1–S2, 5–10 GPa) accompanied by shear forces to mobilize already internally heated metal into vein formation, as well as to cause deformation and brecciation, which was followed by slow cooling and annealing.

In their microstructure study of nonmagmatic iron meteorites, Tomkins et al. (2013) expanded upon the latter scenario, and at the same time proposed a mechanism for a more rapid core segregation process that occurred under mostly disequilibrium conditions. They presume that multiple impacts into an already hot to partially molten body led to ponding of FeNi-metal melt and mixing of metal and silicate. As a network of increasingly-smaller fractures is created by the impacts, pressure gradients are established through which melt rapidly migrates away from high-pressure, compressed regions and into low-pressure, hydraulically dilated macro and micro fractures. During this fracturing process, lower-viscosity metal–sulfide melt develops along the margins of silicate melt flows creating a lubricating effect, thus driving a more rapid melt migration. As the concentration of sulfide within the melt steadily increases through fractionation, it is drawn into the finest fractures and between grain boundaries of the silicate component through capillary action. In Portales Valley, slow cooling at depth resulted in a chalcophile enrichment of silicates and a siderophile enrichment of metal. Eventually, as the planetesimal grew large enough to become insulating to radiogenic heat, and widespread partial silicate melting took hold, these sulfide-depleted FeNi-metal melt accumulations were rapidly gravitationally extracted into a core.

One incidental fact that Tomkins et al. (2013) have elucidated from their disequilibrium scenario above is that previous Hf–W-based age studies would no longer be accurate for meteorite irons derived from parent body cores. If the large FeNi-metal accumulations that were rapidly formed through deformational impacts did not have time to allow for migration of W from silicates into the metallic melt, then the core material is not constrained in time to a very early formation, generally accepted to have occurred within 1.5 m.y. of CAIs; i.e., the W-isotopic ratios would necessarily reflect only the initial nebular ratios identical to those of CAIs, and therefore would not be a useful chronometer for core iron meteorites.

An olivine dislocation density analysis was conducted on Portales Valley by Hutson et al. (2007), with the results suggesting that shock effects were nonuniform at the grain-scale level, and that the overall shock intensity was on the low end of the scale at S2–S3 (5–20 GPa); the meteorite subsequently experienced annealing to produce features consistent with stage S1 (Ruzicka and Hugo, 2011). The large grain size with minimal spacing results in a porosity of 1.12 (±0.58) vol% (M. Strait, 2010).

The N-isotopic signature of fine-grained metal extracted from a silicate portion of Portales Valley was determined. This silicate metal has a N signature that is isotopically unlike that of the metal from the large veins. Instead, the N in the silicate metal is very similar to that found in IAB iron meteorites, which coincidentally, also share a similar cooling rate. It could be inferred that the silicate portion was mixed with IAB-type metal associated with an impact-melt event, while metal in the larger veins was mobilized from within the parent body, consistent with typical IIE-type FeNi-metal. It was shown through Re–Os systematics that the metal of the postulated impact melt is more recently disturbed than the metal of the large veins.

Portales Valley differs from other H chondrites in having enrichments of FeNi-metal (probably non-representative samples), troilite, and phosphate, while showing depletions of olivine, orthopyroxene, and plagioclase. This composition is consistent with the addition of P-rich metal into H-chondrite precursor material, which then underwent redox reactions with clinopyroxene (oxidative) and olivine (reductive) at temperatures of ~975°C down to ~725°C to form excess phosphate (merrillite) and orthopyroxene. Other phosphate phases could have crystallized directly from P-rich metallic melts.

It was proposed by Rubin et al. (2001) that the enrichment of troilite in Portales Valley occurred as a result of the condensation of a S-rich vapor, which was produced during the impact-induced formation of a metallic melt at temperatures of at least 1477°C. In a like manner, it was found by Ruzicka et al. that the excess S present in silicate-rich areas is compensated for by the S-free coarse veins. It was argued that S-enriched metal migrated out of the metallic melt contained within the coarse veins as these veins underwent crystallization; at the same time, the S-enriched melt infiltrated the silicate-rich component. Consistent with this interpretation is the presence of thin troilite veinlets (conduits) that are connected to the coarse metal veins.

Two cm-size graphite nodules, previously undiscovered in any ordinary chondrite, have been identified in a specimen of Portales Valley. They are entirely enclosed in coarse vein FeNi-metal, and their margins are intermingled with the metal, suggesting that they were emplaced at the same time. The graphite nodules are thought to have crystallized at high temperatures from liquid metal that was enriched in disolved carbon, incorporated during its flow through large volumes of rock.

Isotopic systematics demonstrate that the metal–silicate breccia that constitutes Portales Valley was formed in an impact event early in Solar System history, 4,559.9 (±0.5) m.y. ago. It was initially rapidly quenched, followed by very slow cooling from high temperatures as a metal–silicate mixture. Very slow cooling and annealing has erased most optical evidence of shock in olivines greater than S1, but some plagioclase indicates shock stages of S2–S3, and some silicate clasts and relict chondrules contain curvilinear trails of kamacite and chromite blebs and veinlets which are indicative of shock stages S3–S6 (Rubin et al., 2001). In addition, the high abundance of metallic Cu grains in silicate clasts attests to significant shock pressures.

The inferred cooling history constrains the location of the precursor material of Portales Valley to a depth of at least 7 km under a large crater measuring ~20 km in diameter. This material was covered by an insulating blanket of brecciated fallback material. This crater would have been about 10% of the diameter of the H-chondrite parent body. Another plausible scenario that avoids the need for such an extensive accumulation of fallback material on a small planetesimal calls for the mobilization of metal during a low-velocity impact during accretion. Subsequent accretion would result in the burial of the material at the required depth to establish an appropriately slow cooling rate (a few °C/m.y.) and associated high metamorphism. The most efficient conversion of impact kinetic energy into heat would occur on a highly porous asteroid, which is a plausible scenario for the structure of the H-chondrite parent body (note the highly porous H chondrite Sahara 98034 on this website).

Texturally, Portales Valley may be transitional between the ordinary chondrites and the silicated IIE-iron group, having many similarities to the high Fe/Si, H chondrite Rose City, and the IIE-An meteorite Netschaëvo. However, Netschaëvo is more reduced than H chondrites and has a different O-isotopic plot, indicating that although it experienced a similar petrogenesis to Portales Valley, it may have been derived from a separate parent body. Other group IIE members, as well as some IAB Complex and EL-group members, share many similar features with Portales Valley and may have experienced similar formation processes.

Based on a cosmic-ray exposure age for Portales Valley of ~40 m.y., nuclear track densities are consistent with a meteoroid diameter of at least 60 cm. The Portales Valley specimen shown above is an 11.1 g partial slice with fusion crust, and exhibiting large angular chondritic clasts.

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Close-up of etched metal
Photo courtesy of Michael Farmer—Mike’s Meteorites & Tektites

The composite photo below shows an impressive 2,113 g etched complete slice, sectioned from the 34 kg main mass, which is shown both in situ and after cleaning. The slice is in the collection of Dr. J. Piatek, acquired from the R. A. Langheinrich Collection, previously obtained from the finder of the mass, Robert Woolard.
standby for portales valley photo

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