Many masses and fragments of this meteorite were recovered in township 25, range 30E., in Union County, New Mexico—a very arid region. The total recovered weight was 44.4 kg.
The L chondrite parent body experienced a catastrophic breakup event 470 (±6) m.y. ago. This refined age estimate was determined by utilizing multiple 40Ar–39Ar isochrons to successfully resolve the various trapped components and then correct for any excess Ar (Korochantseva et al., 2007). This violent planetesimal breakup resulted in the loss of a primordial gas component, and a period of slow cooling ensued, possibly as a rubble pile. Evidence attesting to this disruption has been found in the form of a high abundance of relict, fossil meteorites (at least 89 found to date; Heck et al., 2009) in mid-Ordovician Swedish marine limestone quarries, including the Thorsberg quarry in southern Sweden (e.g., Österplana), the Hällekis quarry, the Gullhögen quarry in southern Sweden, and the Gäde quarry in central Sweden (e.g., Brunflo). Sediment-dispersed extraterrestrial chromite grains have been found in these quarries, as well as in coincident sediment beds from the Puxi River in south-central China.
It was determined that the elemental and O-isotopic compositions of the surviving chromite grains from the fossil meteorites were most consistent with an L-group parent body, and that remnant chondrule abundances, textures, and mean diameters support this finding as well (Bridges et al., 2007, and references therein). Furthermore, the O-isotopic composition of the sediment-dispersed extraterrestrial chromite grains was determined by Heck et al. (2009) through a SIMS study. Measurements of several fossil meteorites along with numerous chromite grains obtained from various quarries have oxygen three-isotope values most consistent with modern L chondrites, but overlap with LL chondrites. The size range of the chromite grains was established by means of varying degrees of parent body metamorphism, and it was shown to have a direct correspondence to the petrologic type of the fossil meteorites as follows:
Chromite Diameter vs.
Petrologic Type
Diameter (µm)
L3
34–50
L4
87–150
L5
76–158
L6
253–638
Although this method of discriminating between petrologic types is not absolute, it does serve to illustrate the approximate range of petrologic types represented by the fossil meteorites from the mid-Ordovician Swedish marine limestone quarries. It was determined that a greater percentage of low petrologic types are present in the quarries compared to percentages from recent L-chondrite falls, possibly reflecting differences in the nature of the ejected material (Bridges et al., 2007).
In a new method to establish the chondrite group to which these fossil meteorites derive, Alwmark and Schmitz (2009) utilized relict chromite grains from fossil meteorites recovered in the Thorsberg quarry (Österplana) and Gärde quarry (Brunflo), as well as from sediment samples from similar quarries. They studied <1–15 µm-sized olivine, pyroxene, and other inclusion types which are encapsulated in chromite grains. It was discovered that for many of these mineral inclusions their primary compositions have been retained, thus enabling these chromite inclusions to be employed as an additional classification method in such instances. Despite the fact that extensive sub-solidus re-equilibration of the silicate inclusions in the relict chromite grains has occurred over hundreds of thousands of years, resulting in their having a higher content of Cr and a lower Fe/Mg ratio, a comparison was still able to be made between these relict inclusions and analogous inclusions in chromite grains from recent chondrites. It was demonstrated that the Fa and Fs values associated with the fossil chromite host meteorite are all consistent with an L chondrite heritage. Moreover, Heck et al. (2009) found that the elemental compositions of chromite grains they analyzed were consistent with those of modern L chondrites.
The age of the meteorites found in these quarries has been precisely established in accord with the Geologic Time Scale 2004 to be 467.3 (±1.6) m.y., based on the appearance of certain species of early marine conodonts. An anomalously high volume of extraterrestrial chromite grains was also found during a search of sediments within this 3.2 m stratigraphic interval. This interval represents 1–2 m.y. of accumulation and indicates a 100-fold increase in the meteorite flux during this specific period (Schmitz et al., 2003).
Cosmic ray exposure age measurements of the fossil meteorites, based on cosmogenic 21Ne, indicate that their ages correspond to their recovered depth within the sediment, with longer CRE ages found in the younger strata (Heck et al., 2004). It was determined that meteorite samples arrived on Earth within one to several hundred thousand years after parent body breakup.
A significant number of chromite grains recovered from the Sextummen bed at the Thorsberg quarry were found to contain solar-wind-implanted Ne and He (Meier et al., 2008). This finding provides positive evidence that the chromite grains did not permeate the meteorite before its delivery to Earth only to be released into the limestone sediments by the action of terrestrial weathering processes. Rather, the chromite grains were established initially as particles comprising one to several ~100-µm-sized grains at the time of parent body breakup, and became solar gas rich during their rapid transit and delivery to Earth (Heck et al., 2008).
By considering both solar and galactic cosmic-rays, a very approximate 21Ne-based CRE age was calculated for the chromite grains; an age of 0.046–9.6 m.y. was determined to be the best estimate. Extensive analyses of cosmogenic noble gases in these chromite grains, compared to those in fossil meteorites recovered from similar quarries, have demonstrated that the CRE ages are comparable, and significantly lower than those of recently fallen L chondrites.
A catastrophic break-up of the 100–150 km Gefion family parent body, thought to be the source of the shocked L chondrites (~20% of all meteorite falls), occurred 469.6 (±5.4) m.y. ago. The break-up rapidly injected fragments into a nearby strong orbital resonance such as the Jupiter 5:2 mean motion resonance, which provided very short transit times from the inner asteroid belt to Earth from 50 t.y. to 2 m.y. (Bottke et al., 2009). According to their model, more recent delivery of this shocked L-chondrite material from the main belt to Earth-crossing orbits occurs through the efficient Jupiter 3:1 mean motion resonance in 5–100 m.y. (most in 30–40 m.y.). In addition, orbital decay via the Poynting–Robertson drag mechanism likely assisted the rapid transit of chromite dust grains to Earth (Heck et al., 2008). An increase in the cratering rate on Earth at this time may be apparent, but crater ages need to be better resolved before a definite association can be established.
At least one further impact event on the L-chondrite parent body is recorded ~20 m.y. ago, which produced abundant fragments and caused severe shock effects and significant radiogenic gas loss. A portion of the fragments eventually found their way into Earth-crossing resonances, and they are currently one of the most well represented meteorite groups seen to fall. The specimen of Beenham shown above is a 29.0 g partial slice with a large troilite inclusion at its center.
