Elsevier

Geochemistry

Volume 78, Issue 2, May 2018, Pages 248-253
Geochemistry

Petrology, phase equilibria modelling, noble gas chronology and thermal constraints of the El Pozo L5 meteorite

https://doi.org/10.1016/j.chemer.2017.12.003Get rights and content

Abstract

We present the results of physical properties, petrography, bulk chemistry, mineral compositions, phase relations modelling and Noble gases study of the meteorite El Pozo. The petrography and mineral compositions indicate that the meteorite is an L5 chondrite with a low shock stage of S2-S3. Heterogenous weathering was preferentially along shock structures. Thermobarometric calculations indicate thermal equilibrium conditions between 768 °C and 925 °C at ∼4 to 6 kb, which are substantially consistent with the petrological metamorphism type 5. A pseudosection phase diagram is relatively consistent with the mineral assemblage observed and PT conditions calculated. Temperature vs. fO2 diagram shows that plagioclase compositional stability is very sensitive to Tschermack substitution in orthopyroxene, clinopyroxene and XAn plagioclase during the high temperature metamorphic process. Based on noble gases He, Ne, Ar and K contents a cosmogenic exposure age CRE of 1.9 Myr was calculated. The 21Ne would be totally cosmogenic, with no primordial Ne. The 21Ne/22Ne value (0.97) is higher than solar value. According to the cosmogenic Ne content, we argue that El Pozo chondrite originally had a pre-atmospheric mass of 9–10 kg, which would have been produced by a later collision after the recognized collision of the L-chondrite parent body ∼470 Ma ago.

Introduction

Metamorphic or equilibrated chondrites (e.g. petrologic type 4–6), provide understanding about size and heating events acquired during accretional, collisional and fragmental processes (Grossman and Brearley, 2005; Huss et al., 2006). Although the degree of metamorphism of chondrites groups is commonly referred to reliable classification of petrologic types (Van Schmus and Wood, 1967), many textural and mineral reaction details are not well understood (Dunn et al., 2010). Ordinary chondrites show a vast variety of bulk chemistry that can lead to significant differences during the multiple metamorphic reaction series. Consequently, detailed textural and metamorphic phase equilibria modelling studies of natural ordinary chondrites could provide new insights about reaction of solids metamorphic chondrites (Johnson et al., 2016).

Two pieces totalling 460 g of the El Pozo meteorite were found in the Chihuahua State, Mexico (Grossman, 2000). Based on the preliminary mineralogy: olivine Fa23.6 and pyroxene Fs22.2 (Sánchez–Rubio and Reyes–Salas in Grossman 2000; Sánchez-Rubio et al., 2001), El Pozo meteorite is considered to be an ordinary chondrite belonging to the L group and petrologic type 5. Hernández-Bernal and Solé (2010) reported a whole rock K–Ar age of 3103 ± 16 Ma. A comparative Raman (RMP), infrared (IR) and X-ray diffraction (XRD) study identified a common mineral assemblage of ordinary chondrite (Ostrooumov and Hernández-Bernal, 2011). However, petrography, modal abundances of minerals and bulk chemistry in the El Pozo chondrite are not yet described.

In this work, we provide the first physical, chemical, petrological and isotopic study of the El Pozo meteorite (See Appendix A for supplementary material, and Analytical Methods in Sm.1). Based on bulk and mineral chemistry, conventional thermobarometry and phase diagram technique we discuss the relationships between chondrule textures and P-T phase conditions of metamorphic reactions during thermal processes of the El Pozo chondrite. Finally, considering the 21Ne and 3He isotopic contents we discuss the implications of early irradiation cosmic rays on the El Pozo-asteroid before entering the Earth's orbit.

Section snippets

Physical description and petrography

The El Pozo meteorite is black to dark brown with reddish haloes of oxidation distributed preferentially along shock structures. Specific bulk density value is G = 3.441 g/cm3. The apparent porosity is 10.674% and the magnetic susceptibility obtained was χ = 58158.1 × 10–8 m3/kg. (See detail in Sm.2).

The study of the polished thin section under the polarising microscope reveals a chondritic texture with heterogeneously sized components showing a chaotic arrangement (Fig. 1). However, the

Concluding remarks

Based on the whole texture, physical properties, bulk and mineral chemistry, and calculated thermal conditions between 768 °C and 924 °C and pressures at ∼4–6 kbar, we support that the El Pozo meteorite can be classified as an ordinary L chondrite with petrological type 5 of metamorphism. Since weakly impact deformation structures are S2–S3 degree (Stöffler et al., 1991), the main thermal features were not erased. However, more than one-third of the area shows from moderate to complete

Acknowledgements

The authors acknowledge Instituto de Geologia of Universidad Nacional Autónoma de Mexico for supplying the meteorite used in the present study. Paolo Gentile is thanked for SEM and EDS microprobe access. At the early stage of this research, L. Folco has contributed with specific ideas. Mexican Grant CeMIE Geo Project P15 and Spanish GrantCGL2013-40785-P and Andalusian GrantP12.RNM.2163. Prof. Alex Deutch, Prof. Klaus Keil and two anonymous reviewers are thanked for reviews which improved this

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