Introduction
The Carancas meteorite fell on 15 September 2007 approximately 10 km south of Desanguadoro, near Lake Titicaca, Peru. The impact excavated a crater having an overall diameter of 13.5 m, a 7.5 m central hole, 1 m high ejecta rim and a shallow (0.6 m deep) groundwater pond (del Prado et al. Reference del Prado2008). The estimated timing of the fall was recorded at approximately 11:45 AM (local time) (16:45 UT) (Macedo & Macharé Reference Macedo and Macharé2007), but later claimed to be 11:40 AM (16:40:14.1 UT) (Tancredi et al. Reference Tancredi2009). The meteorite was thought to travel from east to west, and the angle at which the meteorite entered the atmosphere was estimated to be between 45 and 60 degrees from the horizontal (Tancredi et al. Reference Tancredi2009). According to the initial report (Macedo & Macharé Reference Macedo and Macharé2007), the locals claimed that a luminous object was seen about 1 km from the Earth's surface before a strong explosion lasting 15 minutes was heard. The fireball consisted of a head with strong bright light and a white smoky tail. The locals also reported that immediately after the explosion, a massive cloud of dust was seen followed by a ‘sulphurous’ smell from the site and groundwater evaporation. Neither explosion nor ‘rained down’ debris was reported to take place in the sky, suggesting that the meteorite remained intact on entering the atmosphere. According to the media reports, many local witnesses became ill and several of them had to be hospitalized after they came into contact with the glowing rocks (Tancredi et al. Reference Tancredi2009). The symptoms of the illness have been reported to include headache, dermal injuries, dizziness, nausea and vomiting. Arsenic vapour from the contaminated water in the crater was thought to be responsible for the outbreak of the sickness.
The mineralogical analysis of the fragments indicate that the meteorite is a chondrite composed of fine-grained materials with light grey coloration, rich in iron particles, pyroxene, olivine and kamacite (Macedo & Macharé Reference Macedo and Macharé2007; Tancredi et al. Reference Tancredi2009). Other minerals, such as forsterite, enstatite and troilite, have also been detected in the fragments (del Prado et al. Reference del Prado2008). The Carancas meteorite has been proposed to be an ordinary H4/5 chondrite (del Prado et al. Reference del Prado2008; Tancredi et al. Reference Tancredi2008, Reference Tancredi2009). The meteorite was found to survive the atmosphere entry without major fragmentation (Le Pichon et al. Reference Le Pichon, Antier, Cansi, Hernandez, Minaya, Burgoa, Drob, Evers and Vaubaillon2008). The trajectory of its fall has recently been reviewed by Tancredi et al. (Reference Tancredi2009) ; they have suggested that the crater was formed as a result of a hypervelocity impact. Earlier reports have presented several transmitted light optical images of the fragments (Macedo & Macharé Reference Macedo and Macharé2007; Harris et al. Reference Harris, Schultz, Tancredi and Ishitsuka2008; Tancredi et al. Reference Tancredi2009). However, so far there has been little information on the microstructural characteristics and elemental composition of the particles that formed the Carancas fragments.
Materials and methods
Collection and preparation of Carancas samples for analysis
The samples were donated by Dr. Barry E. DiGregorio to Prof. Wickramasinghe at the Cardiff Centre for Astrobiology. They were collected from the surrounding creater soil and transferred into sterile bags, which were then immediately sealed before being transported to UK laboratories for analysis. Handling and subsequent processing of the samples were conducted in a double air-filtered chamber. The Carancas fragment was cut open using a sterile scalpel and its peripheral layer carefully removed with sterile pointed forceps. The remaining part of the sample was taken for analyses.
Scanning electron microscopy
The Carancas samples were mounted onto a metal stub using a piece of carbon double-sided sticky tape, and coated with gold in an Emscope sputter coater. The samples were examined using a Philips scanning electron microscope (SEM) XL 20 operated at 20 kV accelerating voltage.
Transmission electron microscopy
Particles taken from the internal part of the Carancas fragment were embedded in LR White resin (London Resin Co. Ltd, UK) at 55°C for 48 hours, and the resulting polymerized resin blocks were cut into thin sections (100 nm thick) using a diamond knife on a Reichert E ultracut microtome (Reichert-Jung, Austria). The thin sections were collected on pioloform-coated copper grids and examined using a Philips EM 208.
Energy dispersive analysis of X-rays analysis
A Philips CM12 transmission electron microscope (TEM), fitted with an energy dispersive analysis of X-rays (EDAX) system (EM-400 Detecting Unit, EDAX, UK), was used to analyse the thin sections. The following operating conditions were maintained constantly throughout the analyses: electron beam size (20 nm), condenser aperture (30 μm), magnification (13 000×) and analysis live time (200 cps). During the analyses, the electron beam was focused onto the regions of interest, and a total of 25 similar measurements were randomly taken for each region. Both qualitative and quantitative analyses were performed using a ‘Genesis’ software.
Fourier Transform Infrared spectroscopy
The samples (0.15 mg) were prepared in KBr powder (20 mg). The mixture was transferred into a small blending mill (SPECAC mill™) for grinding, and into the Evacuable Pellet Dies (SPECAC Ltd) under a pressure of 10 tons cm−2 to make a KBr disc. The KBr embedded sample was analysed using a Fourier Transform Infrared (FTIR) spectrometer (JASCO Model FT/IR-660 Plus series).
Results
Scanning electron microscopy
The Carancas particles are mostly minerals having variable sizes and shapes (Figs 1(a) and (b)). The most common features include conchoidal, cuboidal and irregularly shaped bodies (Fig. 1(b)). The surface of some individual particles displays a mud crack-like texture. At high magnifications, this texture contains ovoid and rod-shaped objects, which are similar to microfossils of terrestrial origin (Figs 2(a) and (b)). A large number of particles have crusts on their surface (Fig. 1(c)), whereas many others show fractures (Fig. 2(d)) and cavities (Figs 2(c) and (d)). Some particles exhibit three zones, distinguishable by their differences in both electron opacity and texture: a light zone showing a layered or sheet-like structure, a darker zone at the centre displaying a pronounced radiating pattern and an intermediate zone with a smooth texture (Fig. 1(d)).
Fig. 1. Scanning electron micrographs of the Carancas meteorite. (a) Carancas particles. (b) A general view of the central region of the particle showing minerals in various forms and sizes. (c) A partial view of the peripheral region of the particle showing a mud crack-like texture; streaks of fusion crusts on the surface (arrow head); the inset (C1) shows an enlarged picture of the mud crack-like structure. (d) Aparticle showing three clearly delineated regions: a light zone (Z1) shows a layered structure; a darker zone (Z2) at the centre displays a pronounced radiating texture; an intermediate zone (Z3) is distinguishable by its smooth surface structure where cracks can be seen as shown by the arrows.
Fig. 2. SEM micrographs of the Carancas particles. (a) A high-magnification electron micrograph of the peripheral region of a particle: ovoid and elongated features are apparent. (b) An electron micrograph of the same region as (a); the inset (B1) illustrates the close-up of microfossil-like features. (c) An electron micrograph of the layered texture showing cracks (black arrows) and cavities (white arrows). (d) An electron micrograph of the central region of a particle showing fractures (arrows) and numerous cavities.
Transmission electron microscopy
Most of the sections cut from the resin-embedded particles were brittle and damaged by the electron beam bombardement. The ultrastructual observations of these sections reveal the presence of numerous holes, indicating that the samples were poorly embedded. As shown in Fig. 3, some well-embedded particles show three zones, distinguishable by their differences in electron density and structural characteristics: (i) a light zone displays a layered or thin sheet-like structure; (ii) a darker region shows aligned ‘chatter pits’; and (iii) an intermediate zone has a surface structure that appears to be rather smooth.
Fig. 3. A transmission electron micrograph of particles in the well-embedded resin section. Three large particles appear to have a similar structure. Each particle shows three clearly defined zones: a light zone (Z1) (dark arrows), a darker zone (Z2) (white arrows) and an intermediate zone (Z3).
X-Ray nanoprobe analysis
The results obtained from the X-ray nanoprobe analysis of the thin sections show that the resin-embedded particles are composed of three different regions, distinguishable by the differences not only in structural characteristics but also in chemical composition (Table 1). For convenience, they are labelled in the present study as Z1, Z2 and Z3. During the analyses, attempts were also made to determine whether arsenic element (As) was present in the sample. As Fig. 4 shows, the spectra do not reveal the presence of the characteristic X-ray emission peak of arsenic (As) element, which should normally be located at 10.5 KeV energy. The chemical composition of Z1 is similar to that of Z3, but the former contains less magnesium and silicon than the latter. The amounts of major elements in Z1, such as sulphur, iron and nickel, are much higher than those in Z2 and Z3. On the other hand, the chemical composition of Z2 is unique, since it contains no nickel, but is rich in magnesium and silicon as compared to Z1 and Z3. Traces of sodium and calcium are detected in Z2, but they are not present in Z1 and Z3.
Fig. 4. X-ray spectra obtained from the analyses of a thin section. (a) The X-ray intensity of the elements detected is variable depending on the zones. Carbon, iron, sulphur and magnesium are detected in Z1, Z2 and Z3. Nickel is found only in Z1 and Z3. The presence of sodium and calcium is confined to Z2. The copper peak represents the element from the copper grid used for mounting thin sections. (b) A background area in the same section containing no sample. The X-ray intensity of carbon and chlorine is negligible.
Table 1. Relative concentrations (wt%) of major elements that make up the three regions of the particle
Values±standard deviations (SDs) represent the average of 25 measurements. The data are computed by the ‘Genesis’ thin section software, which calculates the weight percentages of the elements based on the known concentrations of the standard samples.
Fourier Transform Infrared spectroscopy
The analysis by FTIR spectroscopy of the Carancas grains embedded in KBr shows a spectrum containing a series of absorption features located at 9.5, 9.8, 10.2, 10.5, 10.8, 11.3, 11.9, 13.7, 14.5, 15.6, 16.8, 18.4, 20.0 and 24.4 μm (Fig. 5). Several small peaks are located within the range 2–3 μm and 6–7 μm.
Fig. 5. FTIR spectrum of the Carancas meteorite.
Discussion
The scanning electron microscopy of the Carancas particles indicates that they have a stony appearance and are composed of a large population of mineral particles in variable shapes and forms (Figs 1(a) and (b)). Some particles show numerous cavities (Fig. 2(d)), which may suggest the occurrence of a steam phase. Many particles also show fractures (Figs 2(c) and (d)), probably as a result of the impact brecciation. Furthermore, the surface of many particles is covered in places with crusts, most of which show fusion melt along the fissures (Fig. 1(c)). These findings support an earlier observation made by Miura (Reference Miura2008) that the Carancas meteorite was subjected to two major step processes: melting in the atmosphere and steaming after impacting Earth. The melting process resulted in the formation of fusion crusts rich in Fe and Si and melt flakes of Fe-Ni composition. The steam phase occured as a result of the reaction between the hot meteorite metal and cold groundwater at the excavation site. Fractured or broken fragments were discovered around the crater, suggesting that the Carancas meteorite fragmented on impact with Earth. The impact velocity on the ground was estimated at between 3 and 6 km s−1 (Tancredi et al. Reference Tancredi2009).
Some particles show a mud crack-like texture intermixed with ovoid and elongated features, which appear similar to microfossils of terrestrial origin (Figs 2(a) and (b)). These features have also been found in Martian meteorite ALH84001 (McKay et al. Reference McKay, Gibson, Thomas-Keptra, Vali, Romanek, Clemett, Chillier, Maechling and Zare1996) and in other carbonaceous chondrites (Hoover et al. Reference Hoover, Jerman, Rozanov and Davies2003; Hoover Reference Hoover2009). The origin of these objects is, however, uncertain and remains to be properly determined. One possible explanation is that the mud crack-like structure may originate from the terrestrial contamination. It is possible that some of the fragments were exposed to the terrestrial muddy water or wet ground at the excavation site. This explanation is further supported by the observation that the Carancas fall was immediately followed by a massive cloud of dust and groundwater evaporation from the impact crater (Macedo & Macharé Reference Macedo and Macharé2007; del Prado et al. Reference del Prado2008; Miura Reference Miura2008; Schultz et al. Reference Schultz, Harris, Tancredi and Ishitsuka2008).
Concerning the ovoid and elongated features, an explanation could be offered that these may be the broken products of carbonate crystallization, since they appear similar to the bundles of carbonate crystal precipitation induced by bacteria, as suggested by Buczynski & Chafetz (Reference Buczynski and Chafetz1991) and McKay et al. (Reference McKay, Gibson, Thomas-Keptra, Vali, Romanek, Clemett, Chillier, Maechling and Zare1996).
The FTIR analysis of grains in the Carancas meteorite reveals a waveband stretching from 2.5 to 25 μm showing infrared (IR) absorption peaks centred at 9.5, 9.9 and 10.2 μm, whose characteristics are of amorphous (9.5 μm) and crystalline (9.9 and 10.2 μm) silicates (Knacke & Kraetschmer Reference Knacke and Kraetschmer1980). The presence of many bands around 10 μm may be indicative of the band shift from amorphous to crystalline silicates, and this shift may result from the annealing of amorphous silicates at high temperatures. This process is believed to take place in the hot inner region of the solar nebular (Bockelée-Morvan et al. Reference Bockelée-Morvan, Gautier, Hersant, Huré and Robert2002). Amorphous silicates are found in abundance in interstellar medium (Zolensky Reference Zolensky2006) and interplanetary dust particles (IDPs) (Molster & Kemper Reference Molster and Kemper2005), whereas crystalline silicates are predominant in primitive meteorites (Nuth et al. Reference Nuth, Brearley and Scott2005; Busemann et al. Reference Busemann, Young, Alexander, Hoppe, Mukhopadhyay and Nittler2006). On the other hand, the C1 and C2 chondrites are rich (50–60%) in hydrated silicates, which show a sharp absorption at 2.71 μm in the Orgueil and Murchison meteorites (Knacke & Kraetschmer Reference Knacke and Kraetschmer1980). Furthermore, the carbonaceous chondrites have been shown to display strong absorptions at 6.1 and 6.8 μm (Knacke & Kraetschmer Reference Knacke and Kraetschmer1980). Recently, Rauf et al. (Reference Rauf, Hann and Wickramasinghe2010) have detected these absorption features in the Tagish Lake meteorite. The 6.1 μm feature is known to be related to water absorption in many clay minerals (Farmer Reference Farmer1974), whereas the 6.9 μm is caused by carbonate minerals (Knacke & Kraetschmer Reference Knacke and Kraetschmer1980). Unlike these carbonaceous chondrites, the Carancas meteorite shows considerably weaker absorptions in the IR range 2.5–4.0 μm and 5.0–7.0 μm (Fig. 5). The occurrence of such small humps indicates that hydrated silicates (probably phyllosilicate minerals), hydroxyl (OH) groups and carbonates are less common. There is no evidence of aliphatic functioning groups, as confirmed by the absence of a 3.4 μm peak. However, aromatic hydrocarbons may be present in the Carancas meteorite, as shown by the presence of a band centred at 11.3 μm, which is known to be the absorption feature of aromatic hydrocarbons in many astronomical objects (Cataldo & Keheyan Reference Cataldo and Keheyan2003; Rauf & Wickramasinghe Reference Rauf, Hann and Wickramasinghe2010; Rauf et al. Reference Rauf, Hann and Wickramasinghe2010), including comets and 81P/Wild 2 in particular (Sandford Reference Sandford2008). The IR bands observed in the 10–20 μm region may be due to the presence of the olivine group (Saikia & Parthasarathy Reference Saikia and Parthasarathy2009).
Attempts to section the resin-impregnated particles were met with little success. Most of the sections showed perforations and became brittle, suggesting that the particles were poorly embedded. However, there are some small particles or grains that were reasonably well preserved. As shown in Figs 1(d) and 3 and Table 1, these grains are composed of three phases distinguishable not only by the differences in their electron density and texture, but also their chemical composition. The light zone (Z1), showing a mica sheet-like structure, is composed of sulphide minerals rich in Fe and Ni. In contrast, the darker region (Z2) displays a spectacular pattern of ‘chattering pits’, rich in Mg and Si (the major elements of high-temperature minerals, such as forsterite Mg2SiO4 and enstatite MgSiO3). The richness of chemical elements, such as Mg, Si, and Fe, in Z2 also suggests the existence of olivine and pyroxene. The third zone (Z3), showing a rather smooth surface structure, is composed of crystalline silicates rich in Mg, Fe and Si (the major elements of olivine and pyroxene). The X-ray microanalysis also confirms that there is no evidence for the presence of the arsenic element (Fig. 4 and Table 1), thus ruling out the claim that the illness experienced by the locals in Carancas was arsenic related.
The results obtained from the elemental analysis, electron microscopy and FTIR spectroscopy suggest that the chemical composition of some particles in the Carancas meteorite is similar to that of particles of the 81P/Wild 2 comet, where both amorphous and crystalline silicates are present in abundance (Keller Reference Keller2006; Zolensky Reference Zolensky2006). The 8 μm terminal particle (known as Sitara) of this comet is composed of bright regions of sulphides rich in iron and nickle, a central grey region of enstatite marked by aligned ‘chatter pits’ and a smooth grey region rich in crystalline Mg-silicates (Brownlee et al. Reference Brownlee2006).
Using FTIR spectroscopy, EDAX analysis and electron microscopy to analyse the Carancas particles, the current investigation presents the following findings: (1) the meteorite is composed of a large population of minerals that are abundant in space; (2) it is rich in chemical elements that are composed of olivine, pyroxene and high-temperature minerals, such as fosterite and enstatite; (3) the richness of these minerals and iron metal support the early proposal that the meteorite is an ordinary H4/5 chondrite (del Prado et al. Reference del Prado2008; Tancredi et al. Reference Tancredi2008, Reference Tancredi2009); (4) some particles show structural characteristics and chemical composition of minerals similar to the 81P/Wild 2 comet; and (5) the ovoid and elongated features look like microfossils found in other meteorites, many of which resemble bacterial microfossils of terrestrial origin.
