BEENHAM


L5
standby for beenham photo
Found 1937
36° 13' N., 103° 39' W.

Many masses and fragments of this meteorite were recovered in township 25, range 30E., in Union County, New Mexico; which is 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, after which a period of slow cooling ensued that might be indicative of its re-accretion into a rubble-pile asteroid. Evidence attesting to this disruption has been found in the form of a high abundance of relict, fossil meteorites (101 found to date; Schmitz et al., 2014) in mid-Ordovician marine (i.e., seafloor) 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 and the Lynna River in Russia; some of these chromite grains have been demonstrated to be regolith-derived (Meier et al., 2013).

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 chondritic parent body, and that remnant chondrule abundances, textures, and mean diameters support this finding (Bridges et al., 2007 and references therein). 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 on the parent body by means of varying degrees of metamorphism, and this range 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 discrimination between petrologic types is not absolute, it serves 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 the percentages represented by recent L-chondrite falls, possibly reflecting differences in the nature of the ejected material (Bridges et al., 2007).

In a new method to resolve the chondrite group to which these fossil meteorites derive, Alwmark and Schmitz (2009) studied 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 that are encapsulated in chromite grains, and discovered that for many of these mineral inclusions the primary compositions have been retained, thus enabling these chromite inclusions to be employed as an additional classification method. Despite the fact that extensive sub-solidus re-equilibration of these silicate inclusions 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 the 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. In addition, an anomalously high volume of extraterrestrial chromite grains was found during a search of sediments within this 3.2 m stratigraphic interval. This interval represents 1–2 m.y. of accumulation, and the initial count indicated a 100-fold increase in the meteorite flux during this specific period (Schmitz et al., 2003). A subsequent thorough study was conducted at the Vernadsky Institute in Russia (V. A. Alexeev, 2014) to ascertain the abundances of both extraterrestrial and terrestrial sediment-dispersed chromite grains that are present in these stratographic layers at the various quarries. The results indicate that the actual increase in extraterrestrial chromite grains compared to those of terrestrial origin was only a factor of ~3–4.

In 2001, workers at the Thorsberg quarry discovered a single fossil meteorite in the uppermost bed that was different from all of the other L-chondrite fossils (Schmitz et al., 2014). This fossil meteorite Österplana 065, designated 'the mysterious object', has been determined to be most similar to winonaites, although it has a number of unique characteristics. Its O-isotopic value plots along the trend for winonaites which suggests formation within a common reservoir, and its elemental ratios compare reasonably well with this group as well. Notably, a xenolithic inclusion from the L6 chondrite Villalbeto de la Peña shares similar isotopic and compositional characteristics with this unusual fossil meteorite. Österplana 065 has the same gas retention age as the other L chondrites of 470 m.y., and its CRE age of ~1 m.y. matches that of chromite grains derived from the fossil L chondrites from the same uppermost strata in the quarry. These chromite grains also exhibit abundant planar shock features comparable to those in highly shocked L chondrites (Rout et al., 2017). These facts along with other evidence indicates that Österplana 065 is likely associated with the same catastrophic breakup event as the L6 chondrites, and it was conjectured that this winonaite-like meteorite could represent the impactor that disrupted the L chondrite parent body 470 m.y. ago. Interestingly, Martin and Schmitz (2016) have identified a chrome-spinel grain that has an elemental composition similar to chrome spinel grains from the Österplana 065 fossil meteorite. This grain was among over 500 extraterrestrial grains collected and analyzed from 100 kg of rock obtained from the Komstad Limestone Formation in Killeröd, Scania, Sweden; this location has proven to be exceptionally rich in chromite grains associated with the L-chondrite breakup event. Heck et al. (2017, #1694) conducted a further study of relict grains deposited in sediments at the Lynna River in Russia 467 Myr ago (prior to the L-chondrite breakup event). Employing a coupled Δ17O vs. TiO2 diagram, they identified a single chrome-spinel grain which plots in compositional space similar to Österplana 065.

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, becoming solar gas rich during their rapid transit and delivery to Earth (Heck et al., 2008). Studies of chromite grains from contemporary Mid-Ordovician limestone beds in both China (Alwmark et al., 2012) and northwestern Russia (Lindskog et al., 2012) have demonstrated that the same high concentrations exist and contain similar implanted solar Ne, attesting to a global accretion from the breakup of the L-chondrite parent body 470 m.y. ago.

By considering both solar and galactic cosmic-rays, a very approximate 21Ne-based CRE age was calculated for the chromite grains, with an age of 0.046–9.6 m.y. considered 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 an asteroid would have rapidly injected fragments into a nearby strong orbital resonance such as the Jupiter 5:2 mean motion resonance, which would provide very short transit times from the inner asteroid belt to Earth in a matter of 50 t.y. to 2 m.y. (Bottke et al., 2009). According to their model, recent delivery of this shocked L chondrite material from the main belt to Earth-crossing orbits would occur 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 would likely assist 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. Although the Gefion family was once considered a plausible source of L chondrites due to the timing of breakup and to its proximity to efficient resonances, recent VNIR spectra of 11 Gefion family asteroids show a mixture of chondrite types are present (Roberts et al., 2015). In a subsequent NIR spectral study of five Gefion family asteroids, McGraw et al. (2017) employed laboratory spectral calibrations to ascertain that two of them (1839 Ragazza and 2521 Heidi) have silicate compositions similar to H-chondrites, two of them (2386 Nikonov and 3860 Plovdiv) are similar to an acapulcoite–lodranite-like primitive achondrite, and one (2373 Immo) is similar to an HED-like basaltic achondrite.

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.


For additional information on these fossil meteorites, read these PSRD articles:
"Tiny Traces of a Big Asteroid Breakup", by Linda M. V. Martel, March 2004
"Searching for Ancient Solar System Materials on the Moon, Earth, and Mars", by G. Jeffrey Taylor, November 2016