The High Possil and Strathmore L6 chondrites fell in Scotland in 1804 and 1917 respectively. Unravelling their cosmic-ray exposure (CRE) ages provides crucial information about when they were ejected from the parent body, how they were delivered to Earth and is ultimately important for understanding the dynamics of small bodies in the solar system. Here we use new measurements of the Ne and Ar isotopic composition to determine CRE ages of both meteorites. Duplicated cosmogenic 21Ne and 38Ar concentrations yield CRE ages of 44.6 ± 4.6 Ma for High Possil and 15.4 ± 1.3 Ma for Strathmore. These coincide with well-established peaks in the ejection record for the L6 chondrites. They yield 40Ar gas retention ages in excess of 3.15 Ga, which is consistent with both meteorites originating at depth within the parent body at the time of asteroidal break-up.
Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research
The High Possil and Strathmore meteorites are two of the three meteorite falls recorded in Scotland (Bevan et al. 1985; Meteoritical Bulletin 110 (2021)). The fall of the High Possil (5 April 1804) meteorite was heard by the workers at Possil sandstone quarry (55° 4′ N, 4° 14′ W) and observed throughout Glasgow. The impact left a 38 cm diameter hole that filled with water leaving the meteorite fragments embedded into the soft sandy bedrock. Only two outer parts of the meteorite were recovered. Based on the reported dimensions from Prior and Hey (1923) the total weight of the meteorite was estimated to be 4.5 kg (Bevan et al. 1985 and references therein). The larger of the two recovered fragments was donated to the Hunterian Museum, University of Glasgow.
The Strathmore meteorite fell on 3 December 1917 around Coupar Angus and Blairgowrie. It consists of four stones totalling 13.2 kg. Three fragments were recovered in Perthshire, at Easter Essendy (56° 35′ N, 3° 15′ E) (two fragments: 9.911 and 0.021 kg), Carsie (1.085 kg) and Keithick (1.172 kg). One fragment was found at South Corston (1.066 kg) in Angus (Bevan et al. 1985). Detailed descriptions of the fall are available at https://www.nms.ac.uk/explore-our-collections/stories/natural-sciences/strathmore-meteorite/.
Both meteorites have petrographic, mineralogical and chemical compositions consistent with the L-group chondrites (McLintock and Ennos 1922; Bevan et al. 1985). They have abundant plagioclase feldspar which puts them in the petrologic type 6 of the Van Schmus and Wood (1967) classification. The L6 chondrites are characterized by low iron content and have been metamorphosed at P–T conditions sufficient to homogenize mineral chemical compositions. They likely originate in S-type asteroids. The level of silicate alteration due to shock-loading is variable, but not intense, suggesting that shock pressures were in the range of 10–25 GPa. Strathmore has a greater proportion of deformed grains than High Possil, consistent with shock loading to higher pressures (Bevan et al. 1985).
Cosmic-ray exposure (CRE) ages of meteorites provide fundamental constraints against which models for the origin and delivery of meteorites from the asteroid belt to Earth are tested (e.g. Nesvorny et al. 2009). Cosmogenic nuclides in meteorites are produced by nuclear interactions of high-energy galactic cosmic ray (GCR) protons from outside the solar system, and energetic solar cosmic ray (SCR) protons emitted by the Sun. Cosmogenic nuclide production rate is governed by the meteorite composition, the shape and size of the irradiated body and the position of the analysed sample within the parent body (Eugster 1988). L6 meteorites tend to originate from deep within their parent body so do not acquire a significant cosmogenic nuclide load prior to ejection into space. Consequently, the CRE age records the transit time of a small object from its parent body to Earth after parent body ejection.
Cosmogenic 21Ne and 38Ar are widely used to reconstruct the irradiation history of meteorites in space (Wieler 2002). Here we use new measurements of Ne and Ar isotopes to determine the concentration of the cosmogenic Ne and Ar in the High Possil and Strathmore L6 chondrites in order to estimate the pre-atmospheric size of the meteorites and to reconstruct the exposure time in space. The concentration of radiogenic 40Ar provides an insight into the thermal history of the parent body.
Samples and experimental techniques
In this study we report data on samples of the larger High Possil stone, provided by the Hunterian Museum, and the Keithick fragment of Strathmore, which was from the National Museum of Scotland collection. The specimens looked fresh under the binocular microscope and preserved fusion crust (Fig. 1). Chips (4.6 to 6.7 mg) were prised from the larger sample using tweezers taking care to avoid fusion crust. They were encapsulated in Pt packets, placed in recesses in a Cu pan then evacuated to ultra-high vacuum and baked for 12 h at c. 80°C to remove adsorbed atmospheric gases. The samples were degassed at c. 1300°C for 30 minutes using a 75 W 808 nm diode laser in apparatus identical to that described in Stuart et al. (1999). The gas released was purified by two SAES GP50 getters operated at 250°C and one at room temperature. Argon was trapped for 20 minutes on an activated charcoal-filled stainless-steel finger cooled to −196°C using liquid nitrogen. Neon was then trapped on activated charcoal in a cryostatic cold head at 30 K for 10 minutes; the unabsorbed He was pumped out then the Ne was released at 100 K prior to isotope analysis in static mode in a MAP-215-50 mass spectrometer (Williams et al. 2005). The Ar was subsequently released from the charcoal trap at room temperature and analysed using the same instrument.
System blanks were determined by heating an empty platinum tube to c. 1300°C. They never exceeded more than 0.2% of any isotope in the sample so blank corrections were not made. The sample packets were reheated and in all cases Ne and Ar concentrations did not exceed 1% of the gas released in the main step. Sensitivity and instrument mass discrimination are based on repeated measurements of an air standard. Neon isotopes have been corrected from isobaric interferences from H218O+, HF+ and 40Ar2+ at 20Ne+, CO22+ at 22Ne+ and 66Cu3+ at 21Ne+ following procedures in Codilean et al. (2008). Ne and Ar isotopes were determined in several splits of homogenized powder of the Millbillillie eucrite before and after these samples as a secondary check on mass spectrometer sensitivity and mass discrimination.
Results and discussion
Cosmogenic Ne and Ar
Neon isotopes were measured in three chips of High Possil and two chips of Strathmore (Table 1). The 20Ne/22Ne and 21Ne/22Ne ratios in each sample overlap within uncertainty (Fig. 2) and overlap the composition of cosmogenic Ne in chondrites (Eugster et al. 2007). This suggests that the contribution from atmospheric, solar or primordial Ne is negligible, and the Ne inventory is dominantly cosmogenic in origin.
Argon isotopes were measured in two aliquots of both meteorites (Table 2). 36Ar/38Ar of 0.88 ± 0.07 and 1.63 ± 0.33 for High Possil and Strathmore respectively are higher than the cosmogenic 36Ar/38Ar of L chondrites (c. 0.63; Wieler 2002). This is likely a consequence of the presence of significant contributions of solar wind and/or atmosphere-derived Ar, more so in the case of Strathmore. The low solar wind Ne and Ar content of L6 chondrites (Alexeev 2005) leads us to assume that the non-cosmogenic component is atmospheric in origin (36Ar/38Ar = 5.319) perhaps a consequence of incomplete degassing of adsorbed air prior to analysis. The correction for air-derived 38Ar is c. 5% in the case of High Possil and 14–28% for the two Strathmore splits. Variation in 40Ar/36Ar is also consistent with a minor atmospheric contribution. For the component deconvolution we did not consider the contribution of 36Ar generated by the decay of neutron-capture produced 36Cl. This will tend to increase the 36Ar/38Ar and reduce the cosmogenic 38Ar concentration (e.g. Huber et al. 2008).
The noble gas data alone are not sufficient to determine the pre-atmospheric size of the meteorite and so determine the sample depth. The empirical correlation of Bhandari et al. (1980) facilitates the determination of the pre-atmospheric mass and radius although it is only valid for samples that are from the interior of the meteorites where the variations of Ne ratios are relatively small. The relatively high 21Ne/22Ne of High Possil (0.87) and Strathmore (0.91), in combination with the cosmogenic 20Ne/22Ne ratio, is consistent with only a few centimetres of shielding (Garrison et al. 1995) and legitimizes the use of the empirical correlation between the cosmogenic 22Ne/21Ne and the pre-atmospheric mass of the meteorite developed by Bhandari et al. (1980). Using the average density for L-chondrites of 3.35 g cm−3 (Britt and Consolmagno 2003), we calculate pre-atmospheric radii of 14 and 19 cm for High Possil and Strathmore respectively (Table 3). The recovered masses for High Possil and Strathmore (4.5 and 13 kg, respectively; Bevan et al. 1985) suggest mass loss of around 89 and 87% during atmospheric entry. This is in line with the typical mass ablation for L-chondrites (Bhandari et al. 1980; Alexeev 2004). In this case the pre-atmospheric masses are c. 40 and 100 kg for High Possil and Strathmore, respectively.
As the pre-atmospheric radii of both meteorites was less than 65 cm we can apply the calculation method of Dalcher et al. (2013) to determine the CRE ages of both meteorites. This uses the empirical correlations between the cosmogenic 21Ne (21Necos) and 38Ar (38Arcos) production rates and the (22Ne/21Ne)cos ratio as a shielding indicator. From the measured (22Ne/21Ne)cos (Table 1) we obtain 21Ne production rates of 0.28 and 0.39 × 10−8 cm3STP g-1 Ma-1 for High Possil and Strathmore, respectively. This leads to average CRE ages of 41.1 ± 1.4 and 16.1 ± 0.9 Ma (Table 4). Applying the (22Ne/21Ne)cos to the calculation of the 38Ar production rate we obtain production rates of 0.038 and 0.047 × 10−8 cm3STP g-1 Ma-1 for High Possil and Strathmore, respectively. This produces CRE ages of 49.9 ± 1.9 for High Possil and 24.9 ± 2.5 to 13.8 ± 1.4 Ma for Strathmore (Table 4).
In the case of High Possil the five CRE ages agree within 1 uncertainty and yield an average age of 44.6 ± 4.6 (Fig. 3). This overlaps the large c. 40 Ma peak in CRE ages of L6 chondrites that indicate a major collisional event on the L-chondrite parent body (Wieler 2002; Herzog and Caffee 2014). Three of the four CRE ages of Strathmore overlap a mean age of 15.4 ± 1.3 Ma (Fig. 3). The single old cosmogenic 38Ar exposure age is a statistical outlier and is excluded. The mean Strathmore CRE age overlaps the major 15 Ma peak in the L chondrite CRE age inventory (Wieler 2002; Herzog and Caffee 2014).
The K–Ar gas retention age of chondrites records the time since the meteorite experienced a thermal event causing leakage and provides an indication of the position of the meteorite within the parent body. Radiogenic 40Ar concentrations can be combined with previously published K concentrations to determine the Ar retention age. Bevan et al. (1985) reported K concentrations of 1000 and 1200 ppm for High Possil and Strathmore, respectively. These values are slightly higher than the average for L-chondrites of 858 ppm (Kallemeyn et al. 1989). Using a K concentration of 1000 ± 200 ppm and the measured 40Ar concentration (Table 2) the gas retention ages of the two High Possil splits are 3467 and 3935 Ma, and for Strathmore are 3389 and 3145 Ma (1 uncertainties are 20% predicated on the uncertainty in the assumed K content). More precise K concentration data are required in order to distinguish whether the two meteorites have different gas retention ages. In any case, both meteorites have high gas retention ages that imply they were not close to the surface of the parental body when the asteroidal break-up event occurred at 470 Ma (e.g. Swindle and Kring 2008; Terfelt and Schmitz 2021).
The 21Ne and 38Ar CRE ages of the High Possil and Strathmore meteorites are both consistent with major peaks in the L6 chondrite age record that are indicative of impact events that led to the ejection of meteorites from the parent body (e.g. Wieler 2002; Herzog and Caffee 2014).
The low CRE age of Strathmore coincides with an old gas retention age. This is an exception to the general trend observed in L6 chondrites where the young CRE ages are associated with low gas retention ages that are indicative of relatively recent catastrophic disruption events on the L chondrite parent body. The high 40Ar gas retention ages are an indication of the internal location within the parental body when the asteroidal break-up occurred.
This project was part of AC's PhD. We would like to thank Peter Davidson from the National Museum of Scotland for donating a piece of Strathmore meteorite. We would also like to thank the reviewers and the editor for their thoughtful comments and effort to improve our manuscript.
AC: data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), writing – original draft (lead); FMS: project administration (lead), supervision (lead), writing – review & editing (equal); LDN: formal analysis (supporting), methodology (supporting), writing – review & editing (equal); JWF: resources (supporting), writing – review & editing (supporting)
This project was part of AC's PhD and was supported by the Natural Environment Research Council and Scottish Universities Environmental Research Centre (SUERC)
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
All data generated or analysed during this study are included in this published article.
Scientific editing by Martin Kirkbride