1 Introduction

The Santa Lucia (2008) L6 chondrite fell in San Juan province, Argentina, on 23 Jan. 2008 at 17.20 h. Around 6 kg was recovered. The fresh surface shows a grey homogenous colour with grains of fresh metal and sulphides. The stone investigated is part of one piece of fusion-crusted black stone weighting 1900 g.

2 Materials and Methods

Five thin polished sections were studied using optical microscope, scanning-electron microscopy (SEM) and electron microprobe. Major element chemical compositions were obtained with a JEOL 6400 analytical scanning electron microscope (ASEM) (NHM; Vienna) and a ARL-SEMQ (WDS) (ICATE, Argentina) electron microprobe. Electron microprobe analyses (EMPA) were performed using a 15 kV acceleration potential and 15 nA sample current. Estimated precision for major and minor elements is better than 3 %, for Na about 10 %. Natural and synthetic standards were used for calibration and a ZAF correction was applied to the data.

Bulk trace element analyses were performed using Instrumental Neutron Activation Analysis (INAA) following the procedure described by Joron et al. (1997). The irradiation was done under Cd-cover and performed using the CEA/Saclay Osiris reactor. After 1 week cooling the first counting for 3000 s allow determination of 140La, 153Sm, 24Na, 76As, and 198Au. After 1 month cooling, samples are counted again for 20 and 40 s for: 46Sc, 54Mn, 58Co, 60Co, 65Zn, 85Sr, 86Rb, 95Zr, 110Ag, 131Ba, 134Cs, 141Ce, 147Nd, 152Eu, 160Tb, 169Yb, 181Hf, 182Ta and 192Ir.

The noble gas and nitrogen analyses were performed on PRL-Noblesse multi collector mass spectrometer. Noble gases and nitrogen were measured in 78.52 mg specimen using step wise heating method. The sample was wrapped in aluminium foil is preheated for about 48 h at 150 °C to desorb loosely bound atmospheric noble gases. Gases were extracted in three heating steps, 600 (45 min), 1200 (45 min) and 1700 °C (25 min) in resistance furnace. Extracted gas was divided into two fractions, one for nitrogen analysis and other for noble gas analysis. Nitrogen fraction was cleaned by oxygen at 2 torr pressure and collecting the volatiles in cold finger with liquid nitrogen trap. Noble gas fraction was cleaned by exposing the gas to getter at 750 °C. Heavy noble gases (Ar–Kr–Xe) were collected on charcoal by liquid nitrogen and the He–Ne in gas was measured. Liquid nitrogen trap was maintained during He–Ne measurements to remove background gases and interfering species. Blanks were run under the same conditions as that of sample run. Typical blanks at 1700 °C (in cm3 STP for noble gases and ng for nitrogen) are 4He = 26 × 10−10, 22Ne = 1.9 × 10−12, 36Ar = 2.4 × 10−11, 84Kr = 5.1 × 10−14, 132Xe = 1.1 × 10−14, N2 = 5.3. The measured noble gas concentrations, as listed in Table 1, are accurate to ~10 %. Data reported are corrected for blanks, interferences and mass discrimination. Errors reported include uncertainties derived from the all the correction including volume calibration. For cosmogenic corrections (38Ar/36Ar)tr = 0.188 and (38Ar/36Ar)c = 1.5, (20Ne/22Ne)c = 0.8 and (21Ne/22Ne)c = 0.9 (Wieler 2002) are used. 40Ar is assumed to be entirely radiogenic.

Table 1 Representative EMP analysis of olivine in Santa Lucia (2008)
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3 Results and Discussion

3.1 Petrology and Chemical Composition

In hand specimen almost all pieces are fusion crusted. Broken surfaces expose a light grayish interior with dispersed opaque minerals and scarce delineated oval and spherical chondrules.

Under optical inspection Santa Lucia (2008) shows a chondritic texture similar to other ordinary chondrites with poorly defined chondrules often intergrown with the matrix. It is a monomict breccia with strongly re-crystallized texture. It consists mainly of chondrule fragments enclosed in a crystalline matrix. The matrix consists of tiny subhedral and anhedral crystals and opaque minerals intergrown with broken chondrules. The chondritic texture is poorly defined. Only a few chondrules (mainly radiating type, Fig. 1a) are visible. The essential minerals are olivine (Fa24.4) and low-Ca pyroxene (En77.8 Fs20.7 Wo1.6) (Tables 1, 2). Accessory minerals are plagioclase, Fe–Ni metal phases and polycrystalline troilite grains (Fig. 1b).

Fig. 1
figure 1

a Photomicrographs in transmitted plane polarized light showing a radial chondrule in Santa Lucia (2008). b Photomicrographs in reflected light under XP showing the polycrystalline grains of troilite

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Table 2 Representative EMP analysis of low-Ca pyroxene in Santa Lucia (2008)
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Chemical data indicate that Santa Lucia (2008) is a member of the low iron L chondrite group (e.g., Brearley and Jones 1998). The observed texture and mineral phases led us to classify it as an equilibrated petrologic type 6 (Van Schmus and Wood 1967; Rubin 1990). The shock features of the minerals (e.g., undulatory extinction, planar structure) as well as the presence of twinned plagioclase and troilite occuring as polycrystalline grains suggest that the meteorite has been weakly shocked.

The bulk lithophile trace element abundance of Santa Luica (2008) is around chondritic (1 − 2 × CI) (Fig. 2a), showing also chondritic abundances for the siderophile Ir, Mo, Ni, Co, Fe, As, Au and Ag (Table 3).

Fig. 2
figure 2

CI-normalized (Lodders and Fegley 1998) trace element abundances of Santa Lucia (2008), a Litophile elements, b Siderophile elements

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Table 3 Bulk trace element analysis (INAA)
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3.2 Noble Gases and Nitrogen

The concentrations and isotopic ratios of the noble gases and nitrogen are given in Tables 4 and 5. The measured 20Ne/22Ne ratios (0.77–0.82) indicate that the Ne composition in Santa Lucia (2008) is purely cosmogenic. There is no solar-wind gases. The measured 36Ar/38Ar and 40Ar/36Ar range between 1.8 to 0.97 and 4066 to 239 respectively, indicates the presence of cosmogenic, radiogenic and trapped components. The concentrations of cosmogenic, radiogenic and trapped components for noble gases and CRE ages are given in Table 6. Trapped He and Ne is negligible, small amounts of trapped 36Ar, 84Kr and 132Xe are present, indicating carrier of trapped noble gases only contain Ar, Kr and Xe. As there is absence of solar-wind implanted noble gases, the radiogenic 4Her was calculated after subtracting the cosmogenic component. The measured 40Ar concentration is taken entirely as radiogenic produce. Radiogenic 4He and 40Ar concentrations are 2.99 × 10−6 and 2.54 × 10−5 cm3STP/g respectively. Using U and Th concentrations, 0.014 and 0.02 (Table 3), we obtain a U–Th–He age of 1.2 Ga. Using K = 850 ppm, concentration of chondrites (Wasson and Kallemeyn 1988), we obtain a K–Ar age of 2.96 Ga. These gas retention ages are lower than the crystallization ages of ordinary chondrites of about 4.5 Ga, and are due to gas loss during impact event on L chondrite parent body. Furthermore, the ratio T4/T40 = 0.4 is less than unity, lower T4 from T40 may be due the partial loss of radiogenic helium due to solar heating (Eugster et al. 1993). The measured 129Xe/132Xe = 1.169 ± 0.050 is similar to chondritic value, indicating presence of radiogenic 129Xe* (from decay of 129I). The radiogenic 129Xe* is 9.9 × 10−12 cm3STP/g is small, indicating Xe retention temperature of the parent body attained late. The cosmogenic (82Kr/83Kr)c ratio estimated for Santa Lucia (2008) is 1.27, assuming Kr-Q as trapped composition (Busemann et al. 2000), is much higher that the pure spallation value expected for chondritic composition, 0.765 (Marti et al. 1966) suggest the presence of excess 82Kr by neutron reaction, 81Br(n, γβ) 82Kr. In Fig. 3 where the ratio 82Kr/84Kr is plotted against 83Kr/84Kr, the data points fall above the spallation-Q mixing line implies the excess in 82Kr due to neutron reaction. The low 22Ne/21Ne cosmogenic ratio, 1.05 in Santa Lucia (2008) is indicative of deep shielding in large meteoroid. Both Ne and Kr suggest that the meteoroid was large enough to generate epithermal neutrons to produce 82Krn (Göbel et al. 1982). However, the recovered mass is only ~6 kg, indicates major loss of mass due to ablation during fall. Xenon in Santa Lucia (2008) is mixture of Q and spallation. As shown in 124Xe/132Xe versus 136Xe/132Xe plot (Fig. 4) the data falls on Q and spallation mixing line. The elemental ratios 36Ar/132Xe = 67 and 84Kr/132Xe = 0.73 also indicates the presence of Q type trapped component (Busemann et al. 2000) typical for ordinary chondrites. The total nitrogen content in Santa Lucia (2008) is 0.9 ppm is within the range of ordinary chondrites (Sugiura et al. 1998) with δ15N = 17.22 ± 0.11 ‰. The trapped nitrogen signature is δ15N = 8.1 after correcting the cosmogenic, using the cosmogenic production ratio of (15N/21Ne)c = 4.5 for chondrites (Mathew and Murty 1993), is similar to nitrogen observed in ordinary chondrites (Suguira et al. 1998).

Table 4 Measured Noble gas and nitrogen in Santa Lucia (2008)
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Table 5 Xenon in Santa Lucia (2008), ratios × 100
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Table 6 Cosmogenic, radiogenic and trapped components in Santa Lucia (2008)
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Fig. 3
figure 3

Plot of 82Kr/84Kr against 83Kr/84Kr. Trends expected for component mixture of Q (trapped) and spallation, and neutron produce are shown

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Fig. 4
figure 4

Xenon 124Xe/132Xe versus 136Xe/132Xe three isotope plot of Santa Lucia (2008). Trapped component (Q) and trends of two component mixture, trapped and spallation and HL type are shown

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3.3 Cosmic-Ray Exposure History

We calculate the CRE ages from the cosmogenic 3He, 21Ne and 38Ar concentrations and production rate methods of Marti and Graf (1992), which are independent of elemental composition. The 4π CRE ages obtained from 3Hec, 21Nec and 38Arc are T3 = 18.4 ± 0.2, T21 = 24.4 ± 0.6 and T38 = 17.2 ± 0.6 Ma (Table 6). The average CRE age obtained from 3Hec, 21Nec and 38Arc is 20.0 ± 0.9 Ma for Santa Lucia (2008). This CRE age, 20.0 Ma is consistent to the central peak of CRE age distribution of L chondrites (Alexeev 2005).

4 Conclusions

Santa Lucia (2008) consists of chemically homogeneous (in terms of Fe and Mg) major phases with chondritic bulk trace element abundances. Taking into account the average composition of olivine (Fa24.4) and low-Ca pyroxene (En77.8 Fs20.7 Wo1.6) this recent fall is thus classified as an equilibrated ordinary chondrite (L6) with a weak shock stage and a degree of weathering W0. The meteorite name was approved by the NomCom of the Meteoritical Society (Meteoritic Bulletin, no. 97).

Based on cosmogenic 3He, 21Ne, and 38Ar, Santa Lucia (2008) shows a cosmic ray exposure (CRE) age around 20 Ma. The radiogenic K–Ar age is of 2.96 Ga, while the young U, Th–He are of 1.0 Ga suggesting that Santa Lucia (2008) lost radiogenic 4He more recently. Our results show the absence of solar wind noble gases. Heavy noble gases (Ar/Kr/Xe) indicate presence of Q-type gases. Neon and krypton suggest large pre-atmospheric size of the meteoroid.