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Petrography and mineral chemistry of Escalón meteorite, an H4 chondrite, Mexico

 

Petrografía y química mineral del meteorito Escalón, una condrita H4, México

 

Adela M. Reyes–Salas^1,*, Gerardo Sánchez–Rubio¹, Patricia Altuzar–Coello², Fernando Ortega–Gutiérrez¹, Daniel Flores–Gutiérrez³, Karina Cervantes de la Cruz¹, Eugenio Reyes⁴, and Carlos Linares⁵

 

¹ Universidad Nacional Autónoma de México, Instituto de Geología, Del. Coyoacán, 04510 México D.F., Mexico. *E–mail: adelars@servidor.unam.mx.

² Universidad Nacional Autónoma de México, Centro de Investigación en Energía, Campus Temixco, Priv. Xochicalco s/n, 62580 Temixco Morelos, Mexico.

³ Universidad Nacional Autónoma de México, Instituto de Astronomía, Del. Coyoacán, 04510 México D.F., Mexico.

⁴ Universidad Nacional Autónoma de México, Facultad de Química, Del. Coyoacán, 04510 México D.F., Mexico.

⁵ Universidad Nacional Autónoma de México, Instituto de Geofísica, Del. Coyoacán, 04510 México D.F., Mexico.

 

Manuscript received: December 2, 2008.
Corrected manuscript received: October 15, 2009.
Manuscript accepted: November 23, 2009.

 

ABSTRACT

The Escalón meteorite, a crusted mass weighing 54.3 g, was recovered near Zona del Silencio in Escalón, state of Chihuahua, México. The stone is an ordinary chondrite belonging to the high iron group H, type 4. Electron microprobe analyses of olivine (Fa_18.1) and pyroxene (Fs_16.5), phosphate, plagioclase, opaque phases, matrix and chondrule glasses are presented. The metal phases present are kamacite (6.08%Ni), taenite (31.66%Ni), high nickel taenite (50.01%Ni) and traces of native Cu. The chondrules average apparent diameter measures 0.62 mm. X–ray diffraction pattern shows olivine, pyroxene and kamacite. Alkaline–type glass is found mainly in chondrules. This meteorite is a stage S3, shock–blackened chondrite with weathering grade W0.

Key words: chondrite, H4 type, electron microprobe analyses, Chihuahua, México.

 

RESUMEN

La meteorita Escalón, una roca con corteza de fusión de 54.3 g de peso, se recuperó cerca de la Zona del Silencio en el poblado de Escalón, Estado de Chihuahua, México. La roca es una condrita ordinaria que por su alto contenido en fierro pertenece al grupo H. Se realizaron análisis con microsonda electrónica, obteniendo olivino (Fa_18.1) y piroxena (Fs_16.5). Otros minerales analizados son fosfatos, plagioclasa, fases opacas y vidrios, tanto en matriz como en condros. Las fases metálicas presentes son kamacita (6.08%Ni), taenita (31.66%Ni), tetrataenita (50.01%Ni) y trazas de cobre nativo. Con difracción de rayos X se identificaron los minerales olivino, piroxena, plagioclasa, troilita, y metales de Fe–Ni. El promedio del diámetro aparente de los condros es de 0.62 mm. El vidrio es de tipo alcalino y se encuentra principalmente en condros. Los patrones de difracción de rayos X muestran la presencia de olivino, piroxena y kamacita. El estado de metamorfismo de choque en Escalón es S3 y el grado de intemperismo es W0.

Palabras clave: condrita, H4, metamorfismo, microsonda electrónica, Chihuahua, México.

 

INTRODUCTION

Meteorites recovered following observed falls are called falls; those which can not definitely be associated with observed falls are called finds. Based on their bulk compositions and textures, meteorites can be divided into two major categories chondrites and achondrites or nonchondritic meteorites; the latter include the primitive achondrites and igneously differentiated meteorites (Krot et al., 2003). Chondrites are broadly ultramafic in composition, consisting largely of iron, magnesium silicon and oxygen. The most abundant constituents of chondrites are chondrules, which are igneous particles that crystallized rapidly in minutes to hours. They are composed largely of olivine and pyroxene, commonly contain metallic Fe Ni, are 0.01–10 mm in size and are rounded (Scott and Krot, 2003).

The primary divisions of chondrite classification include the carbonaceous (C), ordinary (O) and enstatite (E) classes, each of which contains distinct groups that are closely related. The O class is divided into three groups, H, L and LL. H chondrites have high total Fe content, L chondrites have low total Fe content, and LL chondrites have low metallic Fe relative to total Fe, as well as low total Fe content.

A classification scheme that identifies the degree and nature of secondary alteration (petrologic type) experienced by a chondrite was introduced by Van Schmus and Wood (1967). In this scheme, type 3 is the least altered material, types 2 to 1 represent increasing degrees of aqueous alteration and types 4 to 6 represent increasing degrees of thermal metamorphism (Brearley and Jones, 1998).

The degree of shock metamorphism recorded in a chondrite is determined from a variety of mineralogical and textural parameters. Stöffler et al. (1991) defined increasing shock stages of S1 to S6 for ordinary chondrites. An additional classification parameter is used to identify the degree of terrestrial weathering that a chondrite has experienced; Wlotzka (1993) suggested progressive alteration stages, based on features in polished sections, from W0 to W6.

One piece of Escalón meteorite was found in 1979 by Dr. Luis Maeda Villalobos in the surroundings of the Escalón railroad station, which is located in southern Chihuahua, northern México at 26°45' N, 104°20' W (Sánchez–Rubio et al., 2001) (Figure 1). This unique and fresh looking piece shows an intact black fusion crust, which suggests that the stone did not suffer terrestrial weathering. It has a rounded shape and 54.3 grams in weight. The finder, Dr. Maeda, unfortunately, failed to search the area for others specimens. The fresh cut surface is black in color and, under the petrographic microscope, it shows an extremely unsorted pseudosedimentary material where chondrules form about 60%of the whole mass and show well defined shapes, with size ranging up to 2 mm in diameter.

The name Escalón meteorite was submitted to the Nomenclature Committee of the Meteoritical Society, and the find as well as the classification of the stone as an H4 chondrite, were published in the Meteoritical Bulletin (Wlotzka, 1994).

The meteorite was classified as an ordinary chondrite H4, according to its chemical composition and petrography, following the scheme devised by Van Schmus and Wood (1967) (Sánchez–Rubio, in Wlotzka, 1994). The purpose of this paper is to present a detailed petrographic and mineralogic study to confirm the classification of the meteorite as H4. These data include studies of its texture and composition of main minerals. To complete this classification, schock stage (Stöffler et al., 1991) and weathering grade (Wlotzka, 1993) were studied, indicating a slight shock stage of S3, and weathering grade W0.

 

ANALYTICAL PROCEDURES

Sample of Escalón were obtained by request from the Meteorite working group of Instituto de Geología, UNAM. X–ray diffraction analyses were obtained from bulk powders. One polished thin section was prepared and studied under the polarizing microscope. Quantitative analyses of minerals and glasses were performed on the polished thin section, using a JEOL superprobe 733 of the National Institute of Polar Research, Japan. A JEOL JXA–8900R with a quantitative wavelength–dispersive analyses and also a JEOL jsm 35c with a Tracor–Noran energy dispersive X–ray analysis system attached to the SEM, using natural and synthetic minerals as standards in the premises of the institutes of Geophysics and Geology, at the main campus of the Universidad Nacional Autónoma de México, in México City. Analyses were conducted at 20 keV accelerating potential with a probe current of 20 nA, a beam size of 1 μm and 10–40 s counting times. ZAF correction was made to all analytical data obtained. The polished thin section of the Escalón meteorite was also examined with scanning–electron microscopy (SEM) using back–scattered electron (BSE) image techniques. XRD analysis were performed on pulverized bulk rock samples. X–ray diffraction (XRD) were collected using a Siemens D5000 diffractometer (40 kV, 30 mA) with Cu Kα radiation (λ_Kα1= 1.5406 Å, λ_Kα2 = 1.54439 Å), 2θ range between 20° and 70°, step size of 0.02 °2θ, 1.2 s/step, divergence slit = 0.5 mm, and receiving slit = 0.3 mm.

 

RESULTS

Petrography and mineral chemistry

Optical microscopy

The chondrite consists of chondrules (spherical or subspherical, commonly sub–milimetric petrographic objects that show evidence of a molten or partially molten origin), crystal fragments, and matrix (fine grained aggregates of material filling the interstices between chondrules and crystal fragments) (Figure 2). The identified mineral phases in Escalón are olivine, pyroxene, plagioclase, merrilite and chlorapatite as well as troilite, taenite, kamacite, chromite and pentlandite. Olivine, pyroxene, troilite, kamacite and taenite are the main constituents besides chondrules.

Olivine and pyroxene are the predominant silicates; minor plagioclase shows characteristic albite twinning. Olivine is resorbed and silicates show euhedral to anhedral shape, with common wavy extinction. There is no visible oxidation of metal or sulfide, and only a limonitic staining may be noticeable in transmitted light.

Electron microscopy

Olivine and pyroxene grains were analysed by electron microprobe to determine their compositions and degree of equilibration. Representative analyses are given in Tables 1 and 2. The composition is quite uniform; grains inside chondrules have the same composition as those outside chondrules. Olivine (34 analyses) with an average fayalite (Fa) content of 18.1± 0.4 mol%; low–Ca pyroxene (32 analyses) show an average ferrosilite (Fs) content of 16.5 ± 0.35 mol%, whereas wollastonite content (Wo) in orthopyroxene is 1.5 ± 0.2 mol%; the average composition of four high–Ca pyroxene grains is Wo 41.7 ± 2.7 mol%. This composition is similar to most analyses in the H group range (Scott et al., 1986; Brearley and Jones, 1998).

Pyroxenes are mainly orthopyroxene, but less than 5% of the grains are clinopyroxene with Wo_41.7 (%mol), which is similar to diopside composition. Growth steps are visible on the faces of olivine and pyroxene crystals. Other minerals identified in the polished thin section are pentlandite, merrilite, chlorapatite, native copper as inclusion in taenite, and tetrataenite.

Plagioclase occurs in the matrix, with composition Ab_85.2±1.1, An_12.8±1.3 , Or_5±0.5 (Table 3), which is near the mean equilibrated H chondrite plagioclase composition (Van Schmus and Ribbe, 1968).

Chloroapatite and merrilite are common accessory minerals in equilibrated ordinary chondrites and they usually occur interstitially among ferromagnesian silicates in the matrix and associated with metal and sulphide phases (Yabuki and Goresy, 1986).

The phosphates in Escalón are mostly merrillite and up to 200 µm in size; merrillite was found in the matrix associated with kamacite, troilite and chromite; it was also found as relatively large crystals in the groundmass of barred olivine chondrule (Figure 3a) and as crystals attached to the surface of chondrules. Merrillite is also mixed with chlorapatite (Figure 3b).

Opaque minerals make up less than 20% of the meteorite, with troilite more abundant than Fe–Ni grains and minor chromite, merrillite, pentlandite and copper metal.

Chromite crystals are frequent. The composition of chromite (Table 4) shows a Fe/(Fe+Mg) of 0.847 ± 0.03, which is within the range of those reported for H chondrites (Bunch et al., 1967; Nehru et al., 1997).

The chromite forms individual anhedral to subhedral grains, 20–100 μm in size, isolated from other opaque phases or close to metals and troilite. Also, there are local aggregates up to 300 μm, associated with plagioclase glass (Figures 3c, 3d) and similar to CPA (chromite–plagioclase assemblages) of Rubin (2003). Chromite in Escalón meteorite also occurs as small inclusions (blebs) <3 μm in chondrules and in olivine crystals (Figure 3f).

A few mafic silicate grains in Escalón contain Fe–Ni blebs (Figure 3e) and small chromite blebs (Figure 3f). Similar chromite blebs have been reported in shocked ordinary chondrite (Rubin, 1992).

Modal analysis, based on 300 points counts on an area of a thin section by using SEM–EDS thechnique, were obtained for metals and sulfides, showing the following values: troilite 52%; kamacite 35%; taenite 10%; tetrataenite 3% and scarce native copper and pentlandite.

Kamacite can be observed on large grains up to 1 mm in diameter, as tiny granules (1–5 μm) inside some chondrules and also in the matrix. The larger grains have irregular margins and some of them present growth steps typical of group H chondrite (Michel–Levy, 1981), and perhaps it also contains olivine inclusions (Figure 4a), or other opaque phases such as native copper (Figures 4b, 4c).

The average cobalt concentration in kamacite is 0.318–0.432 wt.%. The results for Ni and Co are similar to other H group meteorites.

Taenite is present in less proportion that kamacite and tends to form slightly smaller grains, in places associated with troilite, kamacite and tetrataenite (Figures 4d, 4e, 4f). Ni–rich zones known as clear taenite (tetrataenite) that surround taenite display a characteristic M–shaped (Wood, 1967; Clarke and Scott, 1980) Ni concentration profile, with the highest Ni content concentrated at the edge (± 50 wt%), and the lowest in the center (± 30 wt.%). Tetrataenite occurs as grains of 5–50 μm, some also as rims 1–20 μm wide on taenite grains (Figures 4e, 4f) and in places with troilite grain inclusions. In Escalón meteorite, kamacite and taenite also occur intergrown, forming the phase plessite (Figures 4d, 4f) (Brearley and Jones, 1998).

Troilite is more abundant than metal grains and can be seen with sizes of 10–200 μm. It is associated with metal and chromite grains and also occurs inside metal grains. Representative analyses of Fe–Ni and troilite are given in Tables 5 and 6.

Scanning electron images were obtained from grains of opaque phases in the Escalón meteorite. Although some grains appear homogeneous, other show complex intergrowths of Ni–rich areas (Figure 4f).

Chondrules tend to show well defined boundaries. The measurement of the apparent diameters of about 50 chondrules showed an average of 0.62 mm and a range of 0.23–2.25 mm apparent diameters. The mean diameter of 0.62 mm was larger than the normal for H chondrites (300 μm for H chondrites ; Grossman et al., 1988).

The chondrules are spherical or oval in shape, sometimes polysomatic (Figure 5a), often display rims of fine–grained olivine, pyroxene and opaque phases in a glass matrix (Figure 5b). Chondrules in the Escalón meteorite are of two main types: BO (barred olivine) (Figures 3a, 5a, 5b, 6b, 6c, 7c, 7e, 7f) and RP (radial pyroxene) (Figure 6f). The BO chondrules may also have orthopyroxene combined with minor amounts of Fe–Ni metal grains distributed throughout the chondrule (Figure 7f). Some interstitial glass was also observed (Figure 7f). A readily distinguished porphyritic olivine and pyroxene (POP) chondrule, whith some olivine grains showing ondulatory extinction, is also present (Figure 5d).

Others chondrules were GO (granular olivine) with well–defined borders (Figure 6a); some are porphyritic (with phenocrysts), a common texture termed POP (porphyritic chondrules composed of olivine and pyroxene) (Figure 6e), and finally some are C (cryptocrystalline) (Figure 6d). Apart from these typical petrological textures, other chondrules in Escalón show unusual textures such as probably barred pyroxene with glass (Figure 7a), a transitional texture BO with pyroxene radial texture (Figure 7f), Radial olivine and pyroxene (ROP) chondrules with a complex texture of radial olivine and pyroxene where the fine grained fibers grow in different orientations (Figure 5c). It includes an unusual matrix–rich chondrule with chromite and high–Ca pyroxene (Figure 7b), some glass–rich chondrules (Figure 7c, 7e), and one POP chondrule has a large central euhedral olivine crystal with a discontinuous chromite veinlet (possible restitic grain?), which is rimmed by orthopyroxene grains (Figure 7d).

Glass is found in chondrules (10–29%) and in the matrix (~5%). All analysed glasses (Table 7) are similar in composition and they are highly recrystallized in some instances. This glass is rich in Na, Al, and Ca, and poor in Fe and Mg. The glass analyses reveal an alkaline composition (Figures 8a, 8b). Normative mineralogy includes albite, anorthite, nepheline, orthoclase and corundum (Table 7).

Troilite, kamacite and chromite were identified also within the glass phase.

X–ray diffraction

X–ray diffraction (XRD) pattern recorded for Escalón samples are shown in Figure 9. The XRD pattern from the powdered sample shows three phases: pyroxenes (enstatite) (PDF 19–0768); olivine (forsterite) (PDF 34–0189); and iron (kamacite) (PDF 37–0474).

The most abundant mineral in Escalón is olivine (~50%), followed by pyroxene (enstatite) (~32%), and iron (kamacite) (~18%). The semiquantitative analysis were estimated from the main peak intensity of each mineral phase: Olivine (forsterite) (d = 2.4567 Å), pyroxene (enstatite) (d = 2.8700 Å) and , iron (kamacite) (d = 2.0280 Å).

 

DISCUSSION

Based on olivine and low–Ca pyroxene composition, Escalón meteorite can be classified as an H group chondrite. The olivine and orthopyroxene compositions are within the range of the H chondrites (fayalite content of 18.1 mol% for their olivines and ferrosilite with 16.5 mol% for their orthopyroxenes). The silicate mineral composition homogeneity and the well defined chondrule boundaries suggest that Escalón belongs to petrologic type 4 (classification of Van Schmus and Wood, 1967).

Kamacite Co concentrations vary systematically in the chondrite chemical groups; kamacite in H, L, LL group chondrites contain 0.4–0.5, 0.7–1.0, and 1.5–3 wt.% Co, respectively (Rubin, 1990; Reisener and Goldstein, 2003). The kamacite Co content (0.318–0.432) in Escalón meteorite is almost similar to H group chondrites.

There are no visible oxidation of metal or sulfide, and only a limonitic staining may be noticeable in transmitted light, which characterize a weathering grade W0, according to the classification scheme proposed by Wlotzka (1993) for ordinary chondrites.

The average diameter in Escalón chondrules of 0.62 mm is larger than normal for H chondrites, which show the smallest sizes among ordinary chondrite (Rubin, 2005). Possibly, chondrule precursors were larger or more densely distributed in the environment where this meteorite formed as part of a primitive asteroid.

Glass in Escalón is alkaline and similar to igneous rocks of the alkaline series (Figures 8a, 8b). Normative albite content is dominant and resemble that of other ordinary chondrites , in particular to Dimmitt H chondrite (Krot and Rubin, 1994). Glass in Escalón chondrules and matrix has the same composition, maybe because it is a quench product solidified at very high temperatures. According to Varela et al. (2006) all meteorite glasses are the result of an unique universal process, representing the remnants of the liquid that facilitated growth of crystals from primary liquid condensation; they reasoned that, in contrast to mayor elements, trace element abundances of all glasses display the same pattern.

The opaque phases study is important because they registred the thermal histories of meteorites (Wood, 1967; Reisener and Goldstein, 2003). The spherical shape of metallic phases inside some silicate grains in Escalón (Figure 3e) indicate an origin as liquid droplets that may be interpreted as relicts of a primary solidification structure by the rapid, non–equilibrium cooling of the Fe–Ni–S melts to temperatures of about 700 °C, similar to some textures in Tieschitz meteorite (Bevan and Axon, 1980). The plessitic texture also observed in this chondrite, possibly was formed at low temperatures from the decomposition of high temperature taenite, which was retained during slow cooling (Brearley and Jones, 1998).

Zoned taenite has received much attention because the Ni profiles are useful for calculating metallographic cooling rates (Reisener and Goldstein, 2003). Tetrataenite, which forms by ordering of Fe and Ni atoms in taenite, makes rims around taenite grains in chondrites below 350 °C and slow cooling (Clarke and Scott, 1980). Taenite in the Escalón meteorite shows the highest Ni content at the edge (±50 wt.%), and the lowest at the center (±30 wt.%) (Figure 4e).

The metallic Ni–Fe phases of this meteorite are typical of slowly–cooled ordinary chondrites. The kamacite is monocrystalline and has Neumann bands. Most of the zoned taenites are isolated from kamacite; larger zoned taenite grains have martensitic interiors. Tetrataenite, an ordered FeNi phase (Clarke and Scott, 1980), occurs at some kamacite borders, and also surrounds some of the zoned taenite grains that are associated with troilite.

In ordinary chondrites, metallic copper is rare compared to metallic Fe–Ni alloy, and it occurs in several petrographic assemblages (Rubin, 1994); In Escalón, metallic Cu, 10–16 μm in size, occurs enclosed in metallic Fe–Ni (taenite), and it is adjacent to small irregular troilite grains inside taenite. This type of assemblage is reported as the most common occurrence in ordinary chondrites, and it may have formed by localized shock melting or remelting of a metal–troilite assemblage (Rubin, 1994).

Shock metamorphic effects on Escalón correspond to shock stage S3, based on the presence of planar fractures in olivine (not shown by all grains) and undulatory extinction in all olivine grains, irregular fractures, and the presence of thin opaque veins. The black color of the hand specimen can be the result of numerous fine–grained metallic Fe–Ni–troilite particles, which may be associated with the shock effects (Stöffler et al., 1991). The other petrographic shock signatures in Escalón occur as chromite–plagioclase assemblages, and polycrystalline troilite, metallic Cu, Cr and Fe–Ni blebs inclusions in olivine are relict shock features, which are consistent with a history of shock and annealing (Rubin, 2003, 2004; Bennett and McSween, 1996).

Other H4 chondrite with similar characteristics to Escalón are Menow and Ste. Marguerite (Michel–Levy, 1981).

 

CONCLUSIONS

Escalón is a high iron chondrite (H) as indicated by the average olivine composition (Fa₁₉) and average composition of pyroxene (Fs_13.6–16.3).

Classification as petrologic type 4 is indicated by the readily discernible chondrules, modest recrystallization and the Wo content of low–Ca pyroxene, which is within the range of typical type 4 chondrites.

The occurrence of plessitic texture and tetrataenite points to a slow cooling at low temperature.

Growth steps were observed in olivine, pyroxene and troilite crystals, whereas shock effects determined in Escalón were undulatory extinction of most olivine grains, and the occurrence of planar fractures, as well as some opaque shock veins indicating stage 3 of Stöffler et al. (1991). Other petrographic shock indicators observed in Escalón and stated by Rubin (2004) in equilibrated ordinary chondrite include: chromite–plagioclase assemblages, polycrystalline troilite, metallic Cu and Fe–Ni blebs inclusions in olivine, confirming shock stage S3 for Escalón.

 

ACKNOWLEDGEMENTS

This project received funds from Conacyt Project 43227, which are gratefully acknowledged. The authors acknowledge to M. Naito of the National Institute of Polar Research, Japan, for the use of the electron–probe microanalyser; Dr. Hugo Delgado for allowing the use of electronprobe at Laboratorio Universitario de Petrología, UNAM, México; Juan T. Vázquez–Ramírez prepared one polished thin section and L. Baños provided technical support in the XRD laboratory of IIM–UNAM. The authors thank Dr. Mikhail Ostrooumov and an anonymous reviewer for valuable comments and suggestions for improving the manuscript.

 

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