MILES

A mass of of about 265 kg was found by Mr. Frank Timms on open shrub farmland near Miles, in Queensland, Australia. About 100 kg was exported to the USA by M. Killgore and submitted for analysis. Miles has been classified as a group IIE iron belonging to the "fractionated IIE" grouping, as distinct from the "normal IIE", "IIE-An", and "ungrouped iron" group types distinguished by Wasson and Wang (1986).

Miles contains 10–20 vol% coarse-grained, globular silicate inclusions which generally form an interconnected network. They have been described as feldspathic orthopyroxenitic or pyroxene-enriched basalt/gabbro. The major silicate phases include the clinopyroxene augite, orthopyroxene, and plagioclase feldspar in a ratio of 1:1:2. In addition, albitic glass, whitlockite, chlorapatite, merrillite, schreibersite, and chromite occur, along with accessory tridymite, K-feldspar, antiperthite, and rare FeS. The low abundance of sulfide has been attributed to evaporative loss of S in an FeS melt near the surface (Ikeda and Prinz, 1996).

The inclusions were likely derived from both slowly cooled equilibrium processes that occurred between cumulate and melt phases which lasted ~20 t.y., and from a more rapidly cooled, late-stage (~98% crystallization) fractional crystallization phase of much shorter duration (Ruzicka and Hutson, 2009). This rapid cooling phase is consistent with the occurrence at this time of a catastrophic collision which disrupted the planetesimal, resulting in the re-accretion of a smaller body composed predominantly of metallic melt with some silicate mush; it is this secondary melt body from which the fractionated IIE meteorites such as Miles likely originated.

Another type of inclusion is present in lower abundance as compared to the gabbroic inclusions. It has a cryptocrystalline texture thought to have formed by late-stage impact-shock of albite-rich inclusions which were cooled quickly. In addition, orthopyroxenite inclusions and a rhyolitic assemblage have also been identified (Ruzicka and Hutson, 2009). While there is large variation in bulk compositions among the inclusions, they have similar major- and trace-element compositions consistent with derivation from a common precursor. In Miles and other differentiated IIE irons, a wide variety of shock features are present, corresponding to shock stage S4 and postshock heating to ~300°C.

The small, globular silicates present in Miles (as well as Colomera, Weekeroo Station, Kodaikanal, and Elga) likely formed in a hot, highly differentiating environment, such as within a multi-km-deep regolith. At peak temperatures of 1250°C the proportion of liquid metal exceeded that of solid metal by as much as a 2:1 ratio. Other IIE irons contain melted but undifferentiated (primitive) silicates (Watson, Techado), or even unmelted, chondrule-bearing silicates (Netschaëvo, Mont Dieu), the latter providing evidence for mechanical disruption and mixing of very diverse lithologies on an H-like chondritic parent body. Both the wide range of cooling rates and the varying degrees of differentiation exhibited by the silicates suggest a near-surface cooling environment having a large temperature gradient, or alternatively, multiple impact events of varying scales.

Miles contains silicates that formed at significant depth and which experienced a high degree of fractional crystallization at higher temperatures and slower cooling conditions. The presence of undevitrified albitic glass inclusions in some IIE members implies a more rapid cooling (within a few days) from a melt at lower temperatures. A rapid cooling history is also necessary to explain the lack of segregation of low-density silicates from the high-density FeNi-metal host, as well as the small size of the taenite crystals (Wasson, 1972). In a thorough study by Ruzicka et al. (2006) examining the petrogenesis of Sombrerete, a silicated iron grouped within the IAB complex, they argued that the evolved members of the IIE irons, and probably some silicated irons from other groups, experienced a two-stage formation history similar to that of Sombrerete.

An alternative mechanism for the formation of the inclusions and metal host in IIE irons has been proposed by Kurat et al. (2007). They believe that the many contradictory features observed in these irons, such as the discordance in age between inclusions and metal, the chemical and isotopic disequilibria among various components, the Eu and Yb abundance anomalies, the presence of glasses, and the ability for components with such highly contrasting densities to form a pore-free, uniform assemblage, to be the result of nebular fractionation and metasomatic processes rather than through an impact-shock scenario. These investigators argued that pyroxenes and apatite which are embedded in the glass inclusions obviously crystallized from the same precursor liquid phase, but that their compositions are now far out of equilibrium with the present glassy mesostasis, which is especially evident as a high depletion of REE in the glass. They propose that the silicate inclusions were initially formed from a high-temperature, refractory-element-rich liquid that condensed from a non-fractionated nebula gas under reducing conditions, and were thereafter quenched to glass. Metasomatic processes, occurring in a nebular region similar to that in which ordinary chondrites formed, resulted in the Si-rich, alkali-rich, REE-depleted composition we observe. The metal phase is believed to have condensed around the inclusions under low-temperature conditions, possibly from carbonyl breakdown.

The radiometric Ar–Ar age of Miles, reflecting its time of crystallization from a melt, was calculated to be ~4.41 b.y. At least four of the other IIE members share this approximate age (e.g., Colomera, Weekeroo Station, Tarahumara, and Techado), while Watson, Kodaikanal, and Netschaëvo share younger ages of ~3.68 b.y., ages which were likely reset by a common localized impact event. Cosmic-ray exposure ages for the IIE members also plot into distinct groups, with one group again constituting Watson, Kodaikanal, and Netschaëvo with CRE ages of ~3–15 m.y., and another group constituting the remaining IIE members having much older CRE ages, up to ~400 m.y. Based on these age distinctions, it is reasonable that Watson, Kodaikanal, and Netschaëvo may have been ejected from a unique location during a common impact event. Any differences in the CRE ages for these three can be interpreted as being derived from shielding or late breakup events, or in the case of Netschaëvo, from forging to >1000°C after its recovery.

There have been a number of formation models presented to explain the mixing of metal and silicates observed among the spectrum of IIE meteorites:

Scenario 1

Both metal and silicate fractions underwent low degrees of partial melting (~30%) and incomplete segregation as a result of endogenous heating to peak temperatures of ~1250°C. Silicate melts underwent fractional crystallization prior to being trapped within small metallic melt sheets or pods (similar in size to IAB iron subgroups; Wasson and Scott, 2011) within the silicate crust or upper mantle, forming the IIE meteorites.

Scenario 2

Through the impact of an FeNi-metal object from the core of a differentiated, H-chondrite-like body, both metal and silicate fractions formed in near-surface impact-melt sheets or pools on a porous chondritic object similar to the H chondrite parent body (Gaffey and Gilbert, 1998). This was followed by mixing in subsequent impacts and by rapid lithification to produce IIE meteorites.

Scenario 3

A hybrid of the above scenarios has been suggested in which endogenous heating of an H chondrite-type parent body produced low degrees (~7%) of partial melting resulting in the migration of basalt, FeNi-metal, and FeS melts, leaving olivine–pyroxene residues. Varying degrees of differentiation resulted in a variety of lithologies that were then collisionally mixed within metallic melt sheets or pods. Impact heating and heat from molten FeNi-metal remelted plagioclase and pyroxene to produce the albitic glass. The entire composite was then rapidly cooled to produce IIE meteorites. Three members of the IIE group have young radiometric ages; Watson and Netschaëvo likely experienced age resetting by large-scale impacts long after their initial formation, while evidence suggests that Kodaikanal experienced late-stage impact melting and mixing to produce its differentiated silicates.

Scenario 4

As outlined in a paragraph above, Kurat et al. (2007) propose a model by which the components of the IIE group were formed within a condensing nebula region rather than through impact-shock processes on accreted planetesimals. They submit that nebular metasomatic processes were the mechanism for alteration of the composition to that which we observe today.

Many models have been developed that place the origin of the IIE iron group on the H-chondrite parent body, considered likely to be the S-IV type asteroid 6 Hebe. A surface consisting of 40% FeNi-metal and 60% H5 chondrite would match the S-type spectrum of 6 Hebe. Asteroid 6 Hebe is located next to the ν6 and 3:1 resonances serving as a major source of meteorites to Earth. O-isotopic compositions (McDermott et al., 2010, 2011), petrographic evidence, and some CRE ages link these two groups and suggest a model for their joint formation:

A plausible scenario for the formation of Miles was proposed by Ikeda et al. (1997) and Ebihara et al. (1997). Large impacts onto a porous, metal-rich, H-type chondritic asteroid produced partial melting of ~25%, forming localized melt sheets and pods at the bottoms of craters. The melt consisted of both a silicate phase and an Fe–Ni–S–P phase, along with a residual phase of olivine and orthopyroxene. Incipient crystallization of the silicate melt phase produced a crystal mush consisting primarily of phenocrysts of pigeonite, orthopyroxene, and plagioclase, which was then mixed with the Fe–Ni–S–P melt phase to produce the gabbroic (low proportion of residual melt) and cryptocrystalline (high proportion of residual melt) inclusions within the host metal. Half of the phosphorus from the Fe–Ni–S–P melt phase was utilized in the reduction of the silicate inclusions to form metal and phosphates, while the phosphate formation in turn utilized much of the CaO from anorthite in the plagioclase. During the reduction process, Cu behaved as a chalcophile element and was sequestered into sulfide, while Ga was transferred into host metal in higher than normal abundances. Rapid cooling at low temperatures near the surface and near the edge of the melt pool resulted in glass formation in the inclusions, and the eventual exsolution of the remaining phosphorus to form schreibersite around silicate inclusions. Those areas that cooled more slowly underwent extensive fractional crystallization, accounting for the large variations in incompatible element abundances (Lindsay et al., 2003).

In contrast to the nonmagmatic IAB complex irons, the IIE precursor material contained a lower abundance of volatiles such as S and C, and consequently, the melting temperatures were higher, resulting in silicates with nonchondritic compositions (Wasson and Wang, 1986). The metal–sulfide-rich H6 meteorite, Y-791093, contains both chondritic and metal–sulfide components which are texturally, mineralogically, and compositionally similar to members of the H chondrite group. It may be transitional between the H chondrites (e.g., Rose City) and primitive IIE irons with silicate inclusions (e.g., Netshaëvo) (Ikeda et al., 1997). Like Miles, it lacks a Thomson (Widmanstätten) structure and probably formed at a shallow depth rather than in a core.

In their studies of the differentiation of the IIE iron group, Teplyakova et al. (2012) determined that, based on comparisons of siderophile elements between IIE metal and H-chondrites, all of the IIE irons are consistent with formation as solid metal that had precipitated from a metallic liquid of H-chondrite composition. They showed that even metal from Miles, which has the most fractionated siderophile pattern of the group, precipitated from the parental liquid after ~70% crystallization of solid metal. These findings are contrary to a scenario of quenching within cooler silicates following an impact, but rather, suggest crystallization from a melt within a reduced, endogenous environment.

New techniques utilized for measuring O-isotopic data found nearly identical mean Δ17O values for IIE irons and H chondrites (McDermott et al., 2011). However, despite this finding and the data presented above, several factors suggest a different conclusion for the origin of the IIE group. Certain undifferentiated silicates in some IIE members contain FeO abundances that are below the range for H chondrites. Moreover, IIE O-isotopic compositions may only appear the same as those in H chondrites due to a lack of precision in the estimates and the wide range of values. The O-isotope values collected by Clayton et al. (1991) and Clayton and Mayeda (1996) were 0.14‰ lower for IIE irons than for H chondrites. In addition, Wasson and Scott (2011) documented the significant difference that exists between the metal in Portales Valley H chondrite and in IIE irons on Co–Au and Ga–Au plots. Moreover, olivine in the IIE irons with chondritic silicates have distinct Fa and Fs ranges. Furthermore, the CRE ages of the majority of the IIEs are much older than any H chondrite. Therefore, it may be reasonable to conclude that the two groups formed on similar but separate parent bodies. The IIE meteorites may originate from an H chondrite-like parent body that experienced more extensive differentiation, was more reduced, had higher abundances of mafic silicates and metal, higher abundances of siderophile elements, and had slightly different O-isotopic compositions—an "HH" asteroid (Teplyakova et al., 2012).

Prior interest in asteroid 6 Hebe as the source of H chondrites, and possibly the genetically related IIE irons, has lost some favor after hydrocode model data showed inconsistencies exist between expected and observed CRE ages based on the scenario of direct injection into resonances. Current studies by Rubin and Bottke (2009) on the subject have led to the conclusion that family-forming events resulting in large meteoroid reservoirs having homogenous compositions, which are located near dynamical resonances such as the Jupiter 3:1 mean motion resonance, are the most likely source of the most prevalent falls such as H chondrites and HED achondrites (especially howardites). See further details on the Abbott page.

Further studies will help resolve some of the ambiguity concerning the origins of the IIE irons as well as their relationship to the H-chondrite parent body. The specimen of Miles shown above is a 95 g partial slice showing abundant globular silicate inclusions.

---