Enstatite achondrite, ungrouped
(impact melt rock, anomalous aubrite)
Found July, 1936
33° 42' N., 101° 56' W.
A single stone weighing 4.65 kg was found in Lubbock County, Texas. Despite its anomalous characteristics, Shallowater was classified as an igneous aubrite. Shallowater is a rare unbrecciated aubrite, and is the only aubrite composed primarily of coarse-grained orthoenstatite (ordered orthopyroxene) crystals (~80 vol%) rather than disordered enstatite common to the typical brecciated aubrites. It contains a second component of xenolithic material (~20 vol%) present in interstices and as inclusions in the orthoenstatite, which represents the melt-entrapped remnants of a solid impactor. This xenolithic material comprises EH-like minerals including weathered opaques (8 vol%), FeNi-metal (3.3 vol%), troilite (2.9 vol%), forsterite (2.9 vol%), plagioclase (2.5 vol%), low-Ca clinoenstatite (1 vol%), and schreibersite (0.4 vol%), along with traces of niningerite (instead of the typical alabandite) and oldhamite (Keil, et al., 1989). The plagioclase in Shallowater comprises two types: the majority is oligoclase (An17.6, Ab79.8, Or2.6), and the minor constituent is Ab-rich anorthoclase (An0.4, Ab88.2, Or11.4).
The metallic component in Shallowater is much larger (up to 9 vol%; Keil et al., 1989) than that of any other aubrite with the exception of Mt. Egerton and the probable aubrite-related Horse Creek, and consists primarily of homogeneous troilite (van Acken et al., 2010). It follows that Shallowater (and Mt. Egerton) contains the highest concentrations of HSE among aubrites (van Acken et al., 2012). Compared to typical aubrites which contain ~13 vol% diopside, Shallowater contains none, possibly reflecting low O and high S fugacities during crystallization from a different source magma (Fogel, 1997). The moderately volatile element Zn in Shallowater is isotopically heavier than it is in all other aubrites (Moynier et al., 2010), possibly reflecting evaporation of the lighter isotopes during a severe impact event. Contrariwise, aubrites are both highly depleted in Zn and enriched in the light isotopes of Zn, a circumstance inconsistent with an evaporation scenario. The mineralogy of Shallowater is inconsistent with an origin from either of the enstatite-chondrite parent bodies (EH and EL), or from the aubrite parent body, and it is considered to represent a fourth enstatite parent body. Studies of Shallowater have revealed a unique and complex cooling history, described in three stages by Keil, 1989:
The first stage reflects rapid cooling (supercooling) from a melt temperature of ~1580°C to ~712°C. This is consistent with the collisional breakup of a differentiated body, consisting primarily of an enstatite melt phase, as a result of an impact with a solid enstatite-like object. The disruption and incorporation of cold impactor material into enstatite melt resulted in very rapid quenching. This was a low-velocity collision that permitted the gravitational reassembly of the disrupted body and the incorporation of xenolithic material from the solid body.
Cooling rate data indicate that a long period of very slow cooling was then initiated which lasted for several million years, until temperatures reached ~680°C. This reflects the deep burial (e.g. 40 km deep on a 100 km-diameter body) of the Shallowater rock within the rubble-pile.
A final stage of fast cooling to ~300°C commenced following another impact event. This incident excavated the Shallowater rock to within ~5 m of the asteroid surface, possibly through a second breakup and reassembly event, or to within ~5 m of the surface of an ejected fragment.
The sizes of the differentiated Shallowater and aubrite parent bodies are constrained by those processes which led to their melting. Arguments suggesting that the heat source was the decay of short-lived radionuclides like 26Al have not been reconciled with the apparent low Al and plagioclase contents in Shallowater and the aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.00000040.0000005 calculated for the angrites Sah 99555 and D'Orbigny based on their 26Al26Mg isochrons is too low to have generated any significant melting without an additional heat source. Some have suggested that relatively small planetesimals might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.
In studies of the IXe system of the Shallowater and EL parent bodies, evidence was obtained that both bodies experienced a similar period of heavy bombardment that was concordant in time, suggesting that they formed in a similar region of the solar system 4.566 (±0.002) b.y. ago (Brazzle et al., 1999); this precise IXe closure isochron of Shallowater enstatite was subsequently adopted as a reference standard for calibation of the IXe technique against the PbPb chronometer. Since then, a new absolute age for the closure of Shallowater enstatite was calculated by Gilmour et al. (2006) based on the IXe and PbPb systems to be 4.5633 (±0.0004) b.y., and this was further refined by Gilmour et al. (2009) to be 4.5623 (±0.0004) b.y., which is 1 m.y. earlier than previously determined. A succeeding high precision isotopic study conducted by Pravdivtseva et al. (2016) led them to suggest a further refinement of the absolute IXe age to 4.5624 (±0.0002) b.y. Additional data points based on IXe studies of Ibitira and NWA 7325 have now been merged into the Shallowater calibration, resulting in a new absolute closure age of 4.5627 (±0.0003) b.y., which is 0.3 m.y. older than the previous calculation (Gilmour and Crowther, 2016 and references therein).
Radiometric dating techniques utilizing 39Ar40Ar data have determined an average degassing age of 4.53 b.y, likely representing an intense impact event. This is similar to ArAr ages determined for some EH and EL meteorites such as Happy Canyon, but still older than for others, which suggests that the complex cooling history of Shallowater occurred very early after its formation. The CRE age of Shallowater is 28 (±4) m.y., which forms a cluster with the anomalous aubrites Mt. Egerton and Happy Canyon, as well as the normal aubrite ALHA78113 (and pairings).
A study of HW systematics in various aubrites was conducted by Petitat et al. (2008). They found that in contrast to all other aubrites studied, metal in Shallowater has an unradiogenic W isotopic composition more similar to that in ordinary chondrites. They argue that Shallowater therefore could not have experienced the late heating event(s) which caused radiogenic W diffusion into metal.
On an oxygen 3-isotope diagram, the O-isotopic composition of Shallowater is indistinguishable from that of the aubrites and the EL chondrites. Interestingly, the EH chondrites define a slightly steeper slope, an anomaly that may reflect parent body metamorphism (Newton, 2000). Since the REE abundances measured in plagioclase were too low to account for the bulk REE content of Shallowater, a better candidate for the REE carrier was sought. The discovery of trace amounts of oldhamite provided the answer; it was proposed that the weathering products of oldhamite were now the carriers of much of the bulk REE (Heavilon, 1989).
Petrologic, mineralogic, and geochemical investigations relating to Shallowater have ruled out an igneous or impact-melt origin on the aubrite parent body, as well as an impact-melt origin on the EH or EL parent bodies, and it is presumed that it represents a fourth enstatite parent body. The Kr and Xe noble gas isotopic compositions are Q-like, and considered to be primordial due to the unbrecciated nature of the meteorite (Miura et al., 2006). The photo shown above is a 0.574 g specimen of Shallowater, which was sectioned from the 50.54 g partial slice shown below.
Photo courtesy of the Macovich Collection
The two photos shown below provide high resolution close-up views of both a prepared and an unprepared face of a Shallowater section (note the reflective free-metal inclusions conspicuous in the prepared face).