Is MIL 05029 an impact-melted L chondrite, or a differentiated rock from an as yet unsampled parent body with L chondrite affinities? Most chondritic impact melts have a small grain size due to rapid cooling and contain relict clasts with shock features. MIL 05029 does not have these distinguishing features, despite having L chondrite affinities and an oxygen isotopic composition well within the range observed for L chondrites. Moreover, its ancient Ar-Ar age and unusual petrologic properties suggest that it constitutes a unique record of the early evolution of its parent asteroid, regardless of which formation scenario is correct.
Oxygen isotope measurements indicate L chondritic affinity for MIL 05029. The compositions of the major mineral constituents (olivine and low-Ca pyroxene) are mostly similar to those in L chondrites. Higher CaO contents in low-Ca pyroxenes of MIL 05029 probably indicate high-temperature equilibration conditions (Lindsley and Andersen 1983). Fodor and Keil (1975, 1976) interpreted increased CaO contents in poikilitic low-Ca pyroxenes of the L chondrite melt rock Shaw and in similar melt rock fragments in the LL chondrite Ngawi and the H chondrite Plainview as due to equilibration of superheated impact melts. Furthermore, the relative abundances of olivine and low-Ca pyroxene phases also confirm the relationship with L chondrites.
Texturally, it is difficult to determine definitively the formation environment of MIL 05029. The medium-grained poikilitic texture of MIL 05029 implies complete crystallization of a melt in a relatively stable environment, which is also indicated by pyroxene crystals that frequently exhibit 120° junctions. Although this texture is consistent with cooling at a relatively slow rate in an insulated environment, it does not distinguish between plutonic magmas and an impact melt setting. If MIL 05029 was created in an impact, the lack of shock features is not surprising because it is a clast-free melt.
The meteorite has an extremely old age, indicating that the melt formed within tens of Ma of solar system formation. This means MIL 05029 could potentially be sampling an asteroid with L chondrite affinities that was heated enough to cause differentiation. The HED (howardites, eucrites, and diogenites) meteorites are well-studied examples of differentiated meteorites. The oldest cumulate and unbrecciated eucrites give a tight cluster of Ar-Ar ages around 4.48 Ga, which have been interpreted as recording closure of the Ar-Ar system by excavation from depth during an impact (Bogard and Garrison 2003). As MIL 05029 reached its closure temperature at least 30 Ma before the oldest eucrites, there are two scenarios that would enable it to be part of an unsampled parent body. The first is that it was large enough to differentiate, but small enough to cool below the closure temperature faster than the HED parent body, or MIL 05029 was also excavated from depth. Both of these scenarios are unsubstantiated, but possible.
Although an old age opens the possibility of magmatic formation, oxygen isotope data and mineral composition are consistent with an L chondrite. Recent thermochronologic modeling suggests that an L chondrite parent body was never sufficiently heated to experience differentiation and magmatism because accretion of the OC parent asteroids was delayed for approximately 2 Ma after formation of calcium-aluminum-rich inclusions (CAIs) (Hevey and Sanders 2006; Kleine et al. 2008). This deprived the 100–200 km diameter parent asteroids of sufficient radiogenic heating from short-lived isotopes to initiate endogenous magmatism. Because no evidence for multiple parent bodies with L chondrite affinities as potential sources for MIL 05029 is available, the petrological data suggest an impact on the L chondrite parent body as the most plausible formational scenario for MIL 05029.
The meteorite appears depleted in metal and sulfide, both in thin section and in hand sample, although it must be kept in mind that preliminary work on the Shaw meteorite also reported metal and sulfide depletion. While it is true that cm-sized regions were depleted in Shaw, other areas were enriched such that the overall abundances were within the normal limits (Taylor et al. 1979). If we assume that the metal and sulfide depletion in MIL 05029 occurs throughout the entire meteorite, the impact event probably either stripped metal and sulfide from the shock-metamorphosed target rocks (e.g., by shear), or created a melt volume that was sufficiently large and remained sufficiently hot long enough to allow for density-driven segregation of metal-sulfide from silicate melt. Below, we consider these two possibilities.
Shearing during an impact (Stöffler et al. 1991) has been seen in Portales Valley (Kring et al. 1999), and is a very reasonable explanation for the metal/sulfide depletion in MIL 05029. However, shearing does not explain the feldspar enrichment in the silicate fraction seen in Table 2. If the metal/sulfide depletion was instead caused by density separation, then if fractional crystallization also occurred it might allow the mafic silicates to separate by density, enriching the remaining melt in the feldspathic component. The silicate fraction in Table 2 does not show depletion of olivine or low-Ca pyroxene, but does appear to show depletion in high-Ca pyroxene. Nonetheless, we calculate separation speeds for all relevant mineral species.
A simple calculation using the Stokes velocity equation (Turcotte and Schubert 2002) can be applied to the metal/sulfide, olivine, and pyroxene grains. The velocity is then
where Δρ is the density difference between the mineral of interest and the silicate melt, r is the grain radius of the mineral of interest, g is the gravity on the asteroid, and η is the viscosity of the silicate melt. As we are interested in the maximum speed (giving a minimum separation time), we used the largest estimated parent body size of 190 km diameter with zero porosity (Bennett and McSween 1996) which gives a g of 0.09 m s−2. By using equations which determine viscosity as a function of composition and temperature, we calculate a viscosity of 0.05 Pa·s for an L chondrite silicate melt at 1600 °C (Bottinga and Weill 1972; Shaw 1972). An Fe-Ni metal grain of radius 0.55 mm would sink at a rate of approximately 3.3 m h−1, whereas an olivine grain of radius 100 μm would sink at 1.5 × 10−2 m h−1. Pure troilite of radius 0.55 mm would descend at approximately 1.1 m h−1. Although the pyroxene grains in MIL 05029 are much larger than the olivine grains, much of the growth could have been solid-state. Hence, we assume pyroxene grains of a similar size to the olivine grains, and because of the similar density of low-Ca pyroxene, high-Ca pyroxene, and olivine, the pyroxene grains would sink at a rate close to that of the olivine.
Even if the proposed mafic silicate depletion was not a meteorite-wide event, the thin section we examined shows that migration of at least a centimeter (the size of the thin section) could have occurred. If the mafic silicates migrated 1 cm via density separation at this speed, it would take at least 0.7 h to occur. If the depletion is throughout the entire meteorite (or if the migration took place at a temperature <1600 °C), it implies the meteorite remained liquid for an even longer period of time. If density separation did deplete the mafic silicates, it raises questions about the metal/sulfide abundance. In the amount of time it takes the mafic silicates to travel 1 cm, the metal would have traveled 2.2 m, which should have removed nearly all the metal. This issue becomes even more severe upon examining the whole-rock specimen, which has metal grains up to 2.5 mm in radius (Fig. 1) that would descend at 68 m h−1.
Both shear and density separation leave unanswered questions. Shear does not explain the feldspar enrichment, while density separation requires a fine balance between the grain size of metal/sulfide and pyroxene such that the pyroxene can be depleted without completely removing the metal/sulfide. At this time that balance, and how it would leave the olivine abundance unaltered, cannot be confirmed.
Using the thermal history to distinguish between a melt dike and a melt pool will be difficult because the old age means the parent body might have been up to 800 °C at the time of the impact (Bennett and McSween 1996), depending on when the impact occurred and the emplacement depth. In the melt pool scenario, the initial temperature of the parent body is somewhat mitigated because the breccia surrounding the melt pool would have been heated by the impact. The solidification time would then be controlled by conduction to the surrounding breccia, and the subsolidus cooling rate would be controlled by conduction to the surface and surrounding wall rock, and this energy would in turn be radiated to space. In the melt dike scenario, the solidification time would be less than the melt pool scenario as the thermal diffusion distance is less, but if the impact occurred into a region that was 800 °C, the cooling time scale may still be sufficiently slow to develop a medium-grained igneous texture. The subsolidus cooling would then be controlled by the parent body radiating leftover radiogenic energy to space, and would be identical to all material located at that depth.
The lack of relict clasts helps distinguish between these two scenarios. PAT 91501, while texturally similar to MIL 05029, contains rare relic clasts. For this reason it was interpreted to be a sample of a melt dike, albeit one from the center due to the small number of clasts (Benedix et al. 2008). MIL 05029, in contrast to many chondritic impact melt breccias, e.g., Shaw (Taylor et al. 1979), Cat Mountain (Kring et al. 1996), Abee (Rubin and Scott 1997), and PAT 91501 (Mittlefehldt and Lindstrom 2001), is not associated with clastic material. In this regard, MIL 05029 is much like the total impact melt Ilafegh 009, although this impact melt formed on an enstatite parent body (McCoy et al. 1995). Last but not least, slow cooling indicates deep burial, further indicating that MIL 05029 was involved in a large-scale impact. Comparison with large-scale terrestrial impact craters indicates that the amount of impact melt that occurs in dikes is dwarfed by the volume of melt that is retained as coherent pods, melt sheets, or as components of suevites (Wittmann et al. 2010). Thus, the lack of clasts in MIL 05029 implies a thick volume of melt, suggesting a melt pool rather than a melt dike, because MIL 05029 must have remained liquid long enough to assimilate clasts by melting.
Regardless of the formation scenario, because the metallographic cooling rate of approximately 14 °C Ma−1 is about what is inferred for the transitional conditions between typical L5 and L6 chondrites (Taylor et al. 1987), it appears to be a reasonable assumption that MIL 05029 cooled in the thermal regime of the L5–L6 boundary. Thermal modeling of the L chondrite parent asteroid (Bennett and McSween 1996) suggests that the minimum depth for this region is 5–12 km below the surface of a 100–200 km diameter L chondrite asteroid. Scaling laws for simple craters on asteroids and the Moon indicate depth-to-diameter ratios of 1:5 (Sullivan et al. 1996). Excavation to depths of 5–12 km in a low-gravity environment would therefore require minimum crater diameters of 25–60 km. On a parent body 100–200 km in diameter, this size impact would at least have caused rheological weakening (Housen 2009), making later shattering (where the pieces remain gravitationally bound) easier. However, it is also likely that such a magnitude impact event itself shattered the nascent L chondrite parent body if scaling relationships of Holsapple et al. (2002) are taken into account. It should be kept in mind that shattering does not necessarily indicate blocks that are put into orbit and later re-accreted. Shattering could also mean an “expansion” of the radius of the parent body, followed by collapse. If shattering occurred, whether by the impact that created MIL 05029 or a later event, it could have rotated blocks in place. This rotation would expose hot type 6 material to space that would cool rapidly, and possibly emplace larger pods of impact melt at depth (Grimm 1985). This shattering scenario would explain why L chondrites do not exhibit a correlation of metamorphic grade with cooling rates as is found in H chondrites (Trieloff et al. 2003), and instead have strongly metamorphosed L6 chondrites that have faster metallographic cooling rates than less metamorphosed L5 or L4 chondrites (Taylor et al. 1987). In summary, MIL 05029 could provide evidence for why the onion shell model does not work for the L chondrite parent body, because shattering would have disrupted the otherwise steady cooling. However, an impact into a thermally metamorphosing L chondrite body is not the only scenario that can produce a slowly cooled impact melt. Recent hydrocode models using conservative parameters show that impacts between cold porous bodies can produce melting followed by subsequent cooling of 10 °C Ma−1 (Ciesla et al. 2009; Davison et al. 2010). Additionally, porous objects will absorb more of the shock energy, requiring larger impacts to disrupt them.
Most L chondrite impact melts have ages close to 500 Ma (e.g., Chico). Previously, only two L impact melts were known with Ar-Ar ages >4.4Ga: Shaw (∼4.23–∼4.46 Ga; Bogard and Hirsch 1980; Turner et al. 1978) and PAT 91501 (4.461 ± 0.008 Ga; Benedix et al. 2008). MIL 05029 cooled much slower, has an even older Ar-Ar age of 4.517 ± 0.011 Ga, and is >5σ away from the oldest previous L impact melt. The age of MIL 05029 is about the same as the Ar-Ar ages of unshocked H5 chondrites (Turner et al. 1978; Trieloff et al. 2003), and as the H and L chondrite parent bodies initially had a similar thermal history, this indicates the impact event that created MIL 05029 occurred while the parent body was still cooling. This means the Ar-Ar age could be dating the end of metamorphism instead of the age of the impact. By incorporating the time required to form the cooling rate seen in the metal, we can somewhat constrain when the impact occurred. The metal tells us that over the temperature range of approximately 700–400 °C, MIL 05029 cooled at approximately 14 °C Ma−1 (Table 4). This means we need to add at least approximately 20 Ma to the Ar-Ar age, putting the impact at approximately 4.537 Ga. If we assume cooling down to the closure temperature of the Ar-Ar system, the impact must have occurred even earlier than this. Newtonian cooling, where the cooling rate is proportional to the temperature difference, will give us a decent estimate of the cooling rate below that measured by the metal. The equation for temperature as a function of time then becomes
where Ta is the ambient temperature, T0 is the initial temperature (700 °C in this case), k is a constant, and t is time. We can determine k from the metallographic cooling time, as the temperature after approximately 20 Ma was 400 °C. The closure temperature of feldspar (a common carrier of K) is typically 240 ± 120 °C (Turner et al. 1978), which for Ta between about −170 and 30 °C would indicate that the impact occurred approximately 40 Ma before Ar closure, setting the impact age at approximately 4.557 Ga. Using the diffusion parameters and cooling rate for the first release of MIL 05029, we get an even lower closure temperature of ∼40 ± ∼20 °C. As a side note, while an apparent closure temperature of approximately 40 °C is low, it is not unique for L chondrites. The L chondrite Mount Brown also has a very low closure temperature of 40–60 °C (Turner et al. 1978). This lower closure temperature, coupled with the Newtonian cooling, would indicate an impact at approximately 4.59 Ga for ambient temperatures of −170 °C. This would indicate an age older than the formation of the solar system, which clearly did not happen. Higher ambient temperatures (even temperatures that decrease with time) require an even older event, making the problem worse.
More likely, the cooling rate was not steady below 400 °C due to shattering, the Ar closure temperature was not 40 °C, or (because the metallographic cooling rates are only accurate to one order of magnitude) the true cooling rate was closer to 100 °C Ma−1. If the first scenario, the cooling rate will not help us calculate an impact age, but will instead only give a lower limit. If the last scenario, the cooling rate indicates that the impact occurred between 4.521 and 4.531 Ga, depending upon the ambient temperature and closure temperature. If it is shown the metallographic cooling rate of approximately 14 °C Ma−1 is correct, it almost requires excavation after the formation of MIL 05029 or a higher Ar closure temperature to explain the apparent Ar-Ar age. Without further work, we can only say the impact occurred sometime between the apparent Ar-Ar age of approximately 4.52 Ga and the accretion of the L chondrite parent asteroid, but our best estimate places it at approximately 4.54 Ga, due to the additional approximately 20 Ma indicated by the metallographic cooling time. To determine accurately the age of impact, isotopic systems with a higher closure temperature, such as Pb-Pb, Sm-Nd, or I-Xe, will be needed.
Miller Range 05029 is not the only meteorite that records a very early impact in solar system history. Happy Canyon, an impact melt breccia on either the EL or EH parent body, has an Ar-Ar age of 4.53 Ga, and an I-Xe age that is indistinguishable from solar system formation (McCoy et al. 1995). The accretion of most asteroids was rapid, typically <10 Ma after the formation of CAIs (Nichols 2006). But we also know that post-primary accretion could have continued for significantly longer, as evidenced by the final formation of the Earth–Moon System approximately 60 Ma after the formation of CAIs (Touboul et al. 2007). MIL 05029, sandwiched between these two important events, is one of only a few meteorites that record an impact during or shortly after post-primary accretion (McCoy et al. 1995).
K Carrier Phase
Mineral separates are typically not extracted from chondrites because of their limited quantity and fine-grained nature. Whole-rock analysis usually leads to difficulty in identification of which phase is being dated. What is known is that meteorites commonly show two releases of Ar, one below the melting temperature of sodium-rich plagioclase (∼1100 °C; Deer et al. 1962), the other above. The lower temperature release is almost certainly plagioclase feldspar, but there is no consensus on what produces the higher temperature release. It has been attributed to K contained in pyroxene, an effect of shock that converts the low-temperature phase into the high-temperature phase, a change of diffusion distance from grain size to melt vein width, and feldspar enclosed in enstatite grains (Bogard and Hirsch 1980; Bogard et al. 1995; McCoy et al. 1995; Kunz et al. 1997). MIL 05029 is special because, in addition to just two releases of Ar, we have also identified two K sources via the microprobe. The next logical step is to see if these two sets can be matched, so that is where we turn our attention next.
From the Arrhenius plot (Fig. 8) and release curve (Fig. 9), it is obvious that there are two major sources of K in MIL 05029, and both phases release their Ar below the melting temperature of feldspar. Examination of the thin section reveals two sources of K, oligoclase and feldspathic melt inclusions. The first release has an E of approximately 25 kcal mol−1, much less than the approximately 44 kcal mol−1 of feldspar (McDougall and Harrison 1999). The E for maskelynite in Allan Hills 84001 was found by Weiss et al. (2002) to be approximately 18 kcal mol−1, and we have found, in work to be published subsequently, shocked Na-rich feldspar to have an E of approximately 24 kcal mol−1 (Weirich et al. 2010). Additionally, if we assume the D0 for shocked feldspar from Weirich et al. (2010) to be the same as the D0 for the first release in Table 6, we get a grain diameter for the first release of 7–20 μm, in agreement with the grain size of the feldspathic melt inclusions. However, the melt inclusions in MIL 05029 are crystallized, not disordered like maskelynite or shocked feldspar. The feldspar in MIL 05029 is much larger and easy to analyze, exhibits twinning and birefringence, and certainly is not glass or maskelynite. We were not able to determine accurate diffusion parameters for the second release, although it is consistent with the activation energy of feldspar. Based upon the diffusion parameters, feldspar is most likely the second release and melt inclusions are the first release, although it is unclear why (or if) the melt inclusions have a disordered structure.
The relative amounts of K in each domain make comparison of the two sets even harder. Calculating the amount of K from the release of Ar is straightforward: 74% of the total 570 ppm K is “released” from 300 to 660 °C, another 20% from 675 to 1000 °C.
To calculate the amount of K in each domain based on petrographic data, we multiplied the average K wt% (Table 3) by the volume abundance (Table 2) and the ratio of domain density to whole-rock density (assumed to be 0.77). In this calculation, the melt inclusions contain 29% of the total 837 ppm K, with the remaining 71% coming from the feldspar.
By comparing the Ar-Ar data to the microprobe data, two things become obvious. First, the Ar-Ar data and the microprobe data (both with uncertainties on the order of ∼10%) disagree on the K abundance by approximately 40%. As long as we are comparing percentages, this will not affect the outcome. Second, the microprobe data clearly indicate that most of the K is located in the feldspar, whereas the Ar data indicate that most of the K is not located in the feldspar. Perhaps, there is an inhomogeneity in the sample, and the splits we used for Ar-Ar analysis had more melt inclusions. This is certainly possible because the samples used for Ar-Ar analysis were approximately 2 mm in diameter, smaller than the typical grain size of low-Ca pyroxene.
Unfortunately, we were not able to find a consistent correlation between the two domains using Ar-Ar and microprobe data. Despite not having an exact match between the two data sets, we can confidently say that two K domains were identified using two independent techniques. Based on the petrographic observations, those domains are oligoclase and melt inclusions.