MILES


Iron, IIE, silicated octahedrite
(HH chondrite related)
standby for miles photo
Found 1992
27° 50' S., 150° 20' E.

A mass 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" categories distinguished by Wasson and Wang (1986). Researchers divided the eight known (at the time) silicate-bearing IIE irons into five groups based on silicate inclusion types: 1) chondritic clasts (Netschaëvo), 2) partially melted but undifferentiated clasts (Techado), 3) completely melted clasts with a loss of metal and sulfide (Watson 001), 4) plagioclase–orthopyroxene–clinopyroxene basaltic partial melts (Miles and Weekeroo Station), and 5) plagioclase–clinopyroxene partial melts (Colomera, Kodaikanal, and Elga) et al., 1998). Other researchers (e.g., Ruzicka, 2014) have followed a more simplified classification system for the ten silicate-bearing IIE irons known at the time, recognizing only two subgroups—unfractionated (Netschaëvo, Techado, and Watson 001) and fractionated (Miles, Weekeroo Station, Colomera, Kodaikanal, Elga, Tarahumara, and NWA 5608). As a result of their classification of the silicated IIE Mont Dieu, Van Roosbroek et al. (2015) suggest that a five-stage division from most primitive to most differentiated is most useful as follows: 1) Mont Dieu and Netschaëvo; 2) Techado; 3) Watson; 4) Miles and Weekeroo Station; 5) Kodaikanal, Colomera, and Elga. On the other hand, an investigation of silicated IIE irons by McDermott et al. (2015) led them to propose a classification scheme that comprises only four categories, from most primitive to most differentiated as follows: primitive chondritic (Netschaëvo, Mont Dieu, Garhi Yasin, Techado) → evolved chondritic (Watson 001) → differentiated with high opx (>10 vol%; Weekeroo Station, Miles, Tarahumara) → differentiated with low opx (<10 vol%; Kodaikanal, Colomera, and Elga).

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 plagioclase feldspar, clinopyroxene (augite), and orthopyroxene in a ratio of 2:1:1. 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 of the parent body (Ikeda and Prinz, 1996).

Studies based on W-isotopic systematics by Schulza et al. (2012) indicate that IIE irons experienced multiple metal segregation events at ~3, ~13, and ~28 m.y. after solar system formation. The two later events occurred after radiogenic heating had diminished and are best attributed to impact heating. They also argue that Sm isotope data is most consistent with the precursor material of the Miles meteorite not being exposed on the parent body surface.

The inclusions were likely derived from both slowly cooled equilibrium processes that occurred between cumulate and melt phases lasting ~20 t.y., and from a more rapidly cooled, late-stage (following ~98% crystallization) fractional crystallization phase of much shorter duration (Ruzicka and Hutson, 2009). This latter rapid cooling phase is consistent with a catastrophic collision that disrupted the moderately melted and partially differentiated (structurally zoned) planetesimal, which retained a chondritic crust. This was followed by the re-accretion of one or more smaller second-generation asteroids composed predominantly of metallic melt with some silicate mush; it is these heterogeneous second-generation asteroids from which the silicated IIE meteorites such as Miles likely originated (Ruzicka, 2014).

Another type of inclusion has a cryptocrystalline texture and is present in lower abundance compared to the gabbroic inclusions. It is 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 IIE irons with differentiated silicates, a wide variety of shock features are present corresponding to shock stage S4 and postshock heating up 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), and these groups provide 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. The discovery of quenched impact-melt clasts in Netschaëvo led Roosbroek et al. (2016) to conclude that this IIE silicated iron is actually a breccia. They recognized two different metal/silicate lithologies which experienced distinct petrogenetic histories: i) a metamorphosed (type 6–7), chondrule-bearing lithology formed through early indigenous radiogenic heating, and ii) an impact-melt rock (IMR) lithology most likely derived from the former during the late-stage impact event dated at ~3.7 b.y. ago.

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). Based on a thorough investigation into the petrogenesis of Sombrerete, a silicated iron tentatively grouped within the IAB iron complex, Ruzicka et al. (2006) 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, reflect nebular fractionation and metasomatic processes rather than formation through an impact-shock scenario. These investigators argued that the pyroxenes and apatite that 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 which thereafter was quenched to glass. The occurrence of metasomatic processes in this nebular region, similar to that in which ordinary chondrites were formed, resulted in the Si-rich, alkali-rich, REE-depleted compositions we observe. The metal phase is posited 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 five of the other IIE silicated irons share this approximate age (e.g., Colomera, Weekeroo Station, Tarahumara, Techado, and Mont Dieu), while Watson, Kodaikanal, and Netschaëvo share younger ages of ~3.68 b.y., all of which likely experienced resetting by a common localized impact event. Cosmic-ray exposure ages for the IIE members also plot into distinct groups, with one group again comprising Watson, Kodaikanal, and Netschaëvo sharing CRE ages of ~3–15 m.y., and another group comprising 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 could have been ejected from a unique location during a common impact event. Any CRE age differences that do exist for these three meteorites can be interpreted as being the result of 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 becoming 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 derived 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 planetesimal similar to the H chondrite parent body (Gaffey and Gilbert, 1998). This scenario is consistent with W-isotopic data which indicate late metal segregation events occurring ~13 and ~28 m.y. after solar system formation. 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-like parent body produced low degrees (~7%) of partial melting followed by 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 adjacent molten FeNi-metal) remelted plagioclase and pyroxene to produce the albitic glass. The entire composite was then rapidly cooled to produce silicated IIE meteorites. The three members of the IIE group which have young radiometric ages, Watson and Netschaëvo, likely experienced age resetting by large-scale impacts long after their initial formation. Evidence suggests Kodaikanal experienced late-stage impact melting and mixing to produce its differentiated silicates.

Scenario 4

As outlined 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 melting processes on accreted planetesimals. They submit that nebular metasomatic processes were the mechanism for the compositional alteration observed today.

Many models have been developed that place the origin of the IIE iron group on the H-chondrite parent body, considered to be an S-IV type asteroid such as 6 Hebe or a similar H chondrite-like parent body. 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 which serve as a major source of meteorites delivered to Earth. Mo-isotopic anomalies are correlated with Ru-isotopic anomalies related to s-process material synthesized in low-mass AGB stars (Dauphas et al., 2004). The ε100Ru values for IIE irons overlap with those of ordinary chondrites, consistent with a genetic relationship with H chondrites (Fischer-Gödde, 2015). 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 chondrite-type 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 this 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 pools resulted in glass formation within the inclusions, and the eventual exsolution of the remaining phosphorus to form schreibersite around silicate inclusions. Those areas which cooled more slowly underwent extensive fractional crystallization, accounting for the large variations in incompatible element abundances (Lindsay et al., 2003).


Schematic diagram of an impact melt pool origin for IIE irons
Diagram credit: McDermott et al., 45th LPSC #1910 (2014)

Utilizing precise Hf–W chronometry in a study of nine IIE irons, Fisher-Gödde et al. (2016) ascertained ε182W values which attest to the occurrence of three separate metal–silicate segregation events on the parent body:

  1. 3.7–5.3 m.y. after CAIs (Colomera, Barranca Blanca, Arlington, Mont Dieu)
  2. 10–15 m.y. after CAIs (Weekeroo Station, Watson, Miles, Kodaikanal)
  3. ~27 m.y. after CAIs (Tarahumara)

In addition, on a CRE-corrected coupled ε100Ru vs. ε92Mo diagram, these nine IIE irons plot with ordinary chondrites indicating a probable genetic relationship; i.e., IIE irons formed through impact-generated melting on an ordinary chondrite parent body during several impact events over an extended period of time. In addition, the IVA irons plot with the IIE irons and ordinary chondrites, and all of these groups likely have a genetic connection (see diagram below).

standby for IIE-OC coupled diagram
Diagram credit: Fisher-Gödde et al., 47th LPSC, #2704 (2016)

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 might be transitional between the H chondrites like Rose City and the primitive IIE irons with silicate inclusions like Netshaëvo (Ikeda et al., 1997). Similar to Miles, Y-791093 lacks a Thomson (Widmanstätten) structure and probably formed at a shallow depth rather than in a core.

Teplyakova et al. (2012) conducted studies on the differentiation of the IIE iron group. Based on comparisons of siderophile elements between IIE metal and H-chondrites, they determined that that all of the IIE irons are consistent with formation as solid metal which had precipitated from a metallic liquid of H-chondrite composition. It was determined 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 involving quenching within cooler silicates following an impact, but instead suggest crystallization from a melt within a reducing, endogenously-heated environment.

Utilizing new analytical techniques for measuring O-isotopic data, McDermott et al. (2011) found nearly identical mean Δ17O values for IIE irons and H chondrites. 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 to the wide range of values. The O-isotopic 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 significant differences between the metal in the Portales Valley H chondrite and in IIE irons on Co–Au and Ga–Au plots. Moreover, olivine and chromite grains in the IIE irons which contain relict chondrules (e.g., Netschaëvo, Mont Dieu) have Fa and Fs ranges as well as O-isotopic compositions distinct from H chondrites (Schrader et al., 2017). Furthermore, the CRE ages of the majority of the IIEs are much older than any H chondrite. Therefore, it is 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, and which had higher abundances of mafic silicates and metal, higher abundances of siderophile elements, and slightly different O-isotopic compositions—an "HH" chondritic asteroid (Teplyakova et al., 2012; J. T. Wason, 2017).

In an updated study of H chondrites and IIE silicated irons based on high precision O-isotopic data, McDermott et al. (2015) suggest that H chondrites (as well as HH chondrites) might derive from multiple source objects, with some possibly sharing a common parent body with IIE silicated irons. They argue that all of the IIE irons likely formed on a common large H/HH chondritic body which was heated internally by radiogenic elements but had not experienced significant differentiation. Following a severe impact disruption event, rapid cooling ensued preventing metal/silicate unmixing in some regions, succeeded by burial from re-accreting material which initiated a period of slow cooling. Late, less severe impact events occurred which established the ~3.7 b.y. old chronometric age of Netschaëvo and others, and fragments were ejected on a trajectory to Earth.

Prior interest in asteroid 6 Hebe as the source of the H chondrites has lost some favor after hydrocode model data revealed inconsistencies between expected and observed CRE ages based on the scenario of direct injection into resonances. Studies by Rubin and Bottke (2009) on this subject have led them to conclude that family-forming events resulting in large meteoroid reservoirs having homogenous compositions, and 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. See further details on the Abbott page. The specimen of Miles shown above is a 95 g partial slice showing abundant globular silicate inclusions.