Shallowater

Enstatite achondriteUsed in past as synonym for Aubrites. Present definition from the Meteoritical Bulletin states that this rare class is an "enstatite-rich  achondrite  that has not yet been classified into a group". Click on Term to Read More, ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More
(impact melt rockRock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as "impact melt spherules" that are splashed out of the impact crater, and larger pools and sheets of melt that collect in Click on Term to Read More, anomalous aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More)

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 orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More) crystals (~80 vol%) rather than disordered enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More common to the typical brecciated aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More. 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%), troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More (2.9 vol%), forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More (2.9 vol%), plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More (2.5 vol%), low-Ca clinoenstatite (1 vol%), and schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More (0.4 vol%), along with traces of niningerite (instead of the typical alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More) and oldhamiteMn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such as many enstatite chondrites, and some aubrites and enstatite achondrites. Oldhamite in enstatite chondrites likely formed by solar nebular gas condensation. CaS Oldhamite was also found in the most fresh Click on Term to Read More (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 ~1–3 vol% diopside, Shallowater contains none, possibly reflecting low O and high S fugacities during crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More from a different source magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. Click on Term to Read More (Fogel, 1997). The moderately volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More Zn in Shallowater is isotopically heavier than it is in all other aubrites (Moynier et al., 2010), possibly reflecting evaporationProcess in which atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. Click on Term to Read More 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 bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More, 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:

1. 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.
2. 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.
3. 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 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source. Some have suggested that relatively small planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More might have been just the required size to allow heating by induction in the plasmaFourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow. Click on Term to Read More environment of the T Tauri SunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the.

In studies of the I–Xe systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with 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 SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. 4.566 (±0.002) b.y. ago (Brazzle et al., 1999); this precise I–Xe closure isochron of Shallowater enstatite was subsequently adopted as a reference standard for calibation of the I–Xe technique against the Pb–Pb chronometer. Since then, a new absolute age for the closure of Shallowater enstatite was calculated by Gilmour et al. (2006) based on the I–Xe and Pb–Pb 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 I–Xe age to 4.5624 (±0.0002) b.y. Additional data points based on I–Xe 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 39Ar–40Ar data have determined an average degassing age of 4.53 b.y, likely representing an intense impact event. This is similar to Ar–Ar 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 H–W systematics in various aubrites was conducted by Petitat et al. (2008). They found that in contrast to all other aubrites studied, metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More in Shallowater has an unradiogenic W isotopic composition more similar to that in ordinary chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More. They argue that Shallowater therefore could not have experienced the late heating event(s) which caused radiogenic W diffusionMovement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Because diffusion is thermally activated, coefficients for diffusion Click on Term to Read More 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 REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. Click on Term to Read More 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 gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More isotopic compositions are Q-like (see the Yilmia page for further details about Q-gases), and are considered to be primordial due to the unbrecciated nature of the meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More (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).

click on image for a magnified view

Photography by R. Elliott—Fernlea Meteorites UK