CHAPTER-2 PETROGRAPHY, CHEMICAL COMPOSITION AND CLASSIFICATION OF DERGAON METEORITE

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1 CHAPTER-2 PETROGRAPHY, CHEMICAL COMPOSITION AND CLASSIFICATION OF DERGAON METEORITE 2.1. Introduction: Meteorites are rocks of quasi-solar compositions. They show high enrichment of elements like Fe, Ni and Cr compared to crustal elemental abundances. They are mainly classified on the basis of their mineralogy, structure and chemical composition. [Mason 1962, Wasson et al 1988, Anderson 2007]. One of the most important features of meteorite analysis is to distinguish sample from that of a terrestrial one and classify meteorite regarding its origin. The standard method for identifying meteorites is to compare the chemical composition of the sample with the meteoritic rock previously studied. Inter-metallic elemental concentration ratios like Al/Si, Mg/Si, Ni/Si, Ca/Al, Al/Mg and Ca/Mg are frequently used for classification (Wasson et al 1988). Elemental abundances of platinum group elements (PGE) such as Ir, Ru, Rh, Pd, Os and Pt along with Re & Au play an important role in the study of meteorite. On 2 nd March, 2001, in the eastern part of Assam, India, observed a multiple fall of meteorite at 16:40 local time. The fall was visible over a distance of approximately 40 km, stretching from the river island of Majuli to west of town of Dergaon (fig.1) Eye witnesses a fireball accompanied by two land detonations. The largest fragment weighing 10.3 kg was collected from a sugarcane field near Ahajali Bil, around 20 km. south west of Dergaon town of Golaghat District, Assam ( // E / 50 // N). The 10.3 kg fragment created a crater, approximately 40 cm in diameter and 60 cm deep. Additional smaller fragment were recovered in the village of Koilaghat (piece weighing ~ 1.4 kg) and two pieces each weighing < 1 kg were recovered from Majuli. There was possibility that some fragment may fell in the Brahmaputra River channels present in the area of fall. The meteorite had been registered as Dergaon

meteorite by the International Meteoritic Society which was announced online in May 2001.The Dergaon meteorite is the third documented meteorite from the North-Eastern region of India.

2 meteorite by the International Meteoritic Society which was announced online in May 2001.The Dergaon meteorite is the third documented meteorite from the North-Eastern region of India. The other two are Assam (1846) of type L5 and Goalpara (1868) of type ureilite. Fig. 1 : Map showing the locations of three fragments recovered from the multiple fall of the Dergaon meteorite and location map of Balidua fragment. In this chapter, the petrography and mineralogical studies are done and given in section 2.2. Chemical composition of Dergaon meteorite is given in

3 section 2.3. Isotopic composition of Oxygen is discussed in section 2.4. In section 2.5 classification, discussion and conclusion are given. 2.2: Petrography and Mineral Composition Of Dergaon Meteorite. Megascopic Observation: The Dergaon meteorite is hard and massive measuring 17 cm along its longest axis, brownish black in color and Shows black streak. It is coarse grained with observable intergranular poroses and rounded vesicles. The surface is black forming a thin fusion crust measuring 1 mm. It also shows a rusting tint. The largest piece of Dergaon meteorite is shown in fig.2. and also the crater near which it was recovered. Microscopic Study: To determine the mineral composition and texture, five thin sections of the meteorite were studied by Petrological microscope. Texturally Dergaon meteorite is a heterogeneous aggregate of chondrules of varying texture and the matrix made up of chondrule fragments. Apparently 35% mass of the meteorite is composed of chondrules fragments [fig 3A 3G]. However, well formed intact chondrules are absent. Chondrules boundaries are seen to diffuse into matrix. Chondrules are characterized by Ferro magnesium silicate chiefly olivines and pyroxenes and rare metallic phases set in a matrix of extremely time grained recrystallized glass. On the basis of microscopic study, the differing textures of chondrules of Dergaon meteorite can be divided into five basis type. 1. Porphyritic Chondrule (fig.3b and 3C) 2. Barred Chondrule (fig. 3D) 3. Granular Chondrule (fig. 3B)

and metallic phases in order of abundance in table 1. Fig.

4 4. Radiating Chondrule (fig. 3E & 3F) 5. Glassy Chondrule (fig. 3G) The matrix of the meteorite consists of Olivine, Pyroxenes and metallic phases in order of abundance in table 1. Fig.2 : The main fragment of the Dergaon meteorite and its collection site

5 - 31 -

6 The result of electron probe analyses (Duorah et al, 2002) shows olivine are fosterite (Fa 79.9%), Pyroxenes are dominantly bronzite (Fs 17.8%) and metallic phases include kamacite and taenite although very rarely troilite is seen as small lamellae in metallic phases in some part. Table 1: Modal composition of the Dergaon meteorite. Sl. No. Olivine Pyroxene Metallic phase Glassy Matrix Average [Source Duorah et al (2002)] 2.3: Chemical Composition Of Dergaon Meteorite. The compositions of Dergaon meteorite are determined by X-Ray Fluorescence technique (XRF), Scanning electron microscope (SEM) and absorption Spectroscopy (AAS). These experiments are respectively done at Dept. of Instrumentation an SAIF, Gauhati University, Dept. of Instrumentation IIT, Guwahati and Dept. of Instrumentation, NEHU, Shillong. The value of XRF analysis of Dergaon meteorite is also taken from National Geophysical Research Institute (NGRI), Hyderabad. The experimental process for X-ray Fluorescence is as follows. Here both qualitative and quantitative analysis are done. 1. Experimental: XRF studies of Dergaon meteorite was done with the help of Axiol spectrometer. The spectrometer has a dual anode (Au Cr) tube. For identification of the element in the sample, LiF and Ge was used as analyzing crystal

7 1a. Qualitative Analysis. Sample Preparation: The Dergaon meteorite sample was meshed with the help of agate mortar. The sample which was thoroughly meshed was mixed with 0.5g boric acid. The boric acid serves as a binder. An aluminum cup of top diameter 3.9 cm and bottom 3.2 cm was taken. The cup was filled boric acid I.P. Then the cup was mounted on a hydraulic press with ram diameter 6 mm. A pressure of ( ) kn was applied for 2 minutes. Under pressure the sample was solidified as pellet. The pellet was then mounted in the sample holder of XRF spectrometer. Measurement: The X-Ray path was kept at vacuum. The tube voltage and current set at 60 kv and 40 ma. The spectrometer was allowed to scan continuously within scanning range 5 0 to with scan speed 0.08 deg/sec. The graph that was different elements present in the graph was identified with the help of computer software ( X40uu). 1b. Quantitative Analysis: Sample preparation: One gm of sample was mixed with 0.5 gm boric acid powder. They are thoroughly mixed in a agate mortar the sample so produced was placed on a bed of boric acid and pelletized load at ( ) kn. Measurement: For quantitative analysis, the XRF spectrometer is equipped with X40uu software, Rh tube, scintillation and proportional flow counter detector with five analysis crystal. The reference standard samples with known concentration of an analytic element were fed sequentially into X-Ray spectrometer and intensities of fluorescent X-Ray emission line emitted by the analyzed element

8 occurring in different concentrations were measured against the corresponding concentration. Programmed for quantitative Fig. 4 : Quantitative analysis of Dergaon meteorite sample by XRFS

9 estimation of major and minor element oxides in wt. percent (SiO 2, Al 2 O 3, Fe 2 O 3, MnO, MgO, Na 2 O, K 2 O, TiO 2 and P 2 O 5 ) were made separately with the help of X40uu software. The result so obtained is given in table 2. Table 2:Chemical Composition of Dergaon meteorite from quantitative analysis by XRFS. Major & Minor Oxides Value in Wt percent Sample I* 1 Sample II* 1 Sample III* 2 Sample IV* 2 SiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O TiO P 2 O Total *1 Sample I & II analyzed at SAIF, Gauhati University. *2 Sample III & IV analyzed at NGRI, Hyderabad. The XRFS is mainly done at Dept. of Instrumentation and USIC, Gauhati University under supervision of Dr. S. Bordoloi. The quantitative analysis of Dergaon meteorite was also done at National Geophysical Research Institute (NGRI), Hyderabad [Dr. V. Balaram (personal communication)]. The quantitative analysis result is given in table 2. One of the important features for chemical composition of a meteorite is its high content of Fe compared to crustal rock. A comparative treatment can established for major and minor element oxides for Dergaon meteorite and a

10 crustal rock from study of standard crustal rock (avail from the site of Dergaon meteorite fall area) by XRFS is given in table 3 and fig.5. Table 3: Major and Minor oxides in crustal rock. Major & Minor oxides SiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O TiO 2 P 2 O 5 Crustal Rock Concentration (wt%) Fig 5: Comparison of major and minor oxides of Dergaon meteorite and Crustal rock

11 ii) Scanning Electron Microscope (SEM) analysis of Dergaon meteorite was mainly carried at Dept. of Instrumentation IIT, Guwahati. The analysis is mainly a qualitative analysis. The instrument used here is Leo 1430 vp. The obtained graph is given in fig.6. iii) Atomic absorption spectrometer: Atomic absorption spectroscopy (AAS) technique was used to obtain bulk composition of the meteorite. The experimental analysis was done at Dept. of Instrumentation and SAIF, NEHU, Shillong. For analysis, clean fragment of the Dergaon meteorite were crushed and powdered in Agate mortar. The aliquots of measuring 200 mg dissolved in HF-HCL for AAS technique. The result thus obtained is given in table 4. Table 4: Chemical composition of Dergaon meteorite. Element Si Al Ca Mg Fe Ni K Na Co Cr Concentration 17.3( wt%) 1.20 (wt%) 1.08 (wt%) 14.6 (wt%) 27.3 (wt%) 1.80 (wt%) 352 (ppm) 7061 (ppm) 931 (ppm) 1182 (ppm)

12 Fig.6: Quantitative analysis of Dergaon Sample by SEM-EDX

13 2.4: Oxygen isotope in Dergaon meteorite. Overview: Meteorites have long fruitful information about the origin of our planet because most date from pre-planetary times. Chondrites were believed as the most primitive meteorites within any meteorites because chondrite consists of undifferentiated constituents, with the various proportions of refractory inclusions, chondrules and matrices [Grossman et al 1988]. In this section, we mainly study oxygen isotope constituents in Dergaon meteorites. Among all the chemical elements, oxygen has a combination property that makes it uniquely important in cosmochemistry. Oxygen is the most abundant element in most minerals and rocks, i.e., solid component in our solar system. Oxygen is a lightelement, so that its three stable isotopes are subject of large mass dependent fractionation that affect in both equilibrium and kinetically controlled processes. Oxygen isotopes are formed in different nucleosynthesis processes. The abundant isotope 16 O (99.76%) is produced in nuclear synthesis by helium burning. The rare isotope 17 O (0.039%) and 18 O (0.20%) are produced by hot CNO cycles in zone rich in H and He, respectively in both novae and supernovae. Therefore, it would be expected the presolar carries characteristic, that is, which has the anomalous abundances of the isotope of oxygen. However, the isotopic composition of oxygen have not identified as presolar carries since Clayton et al 1973) had established the anomalous oxygen isotopic abundance from meteorites. At present state of research cosmochemist have put out to study the oxygen isotopic compositions meteorites with conventional and new developed techniques applied to the measurements [e.g. Clayton 1973, Itoh et al, 2003] Previous studies show that individual meteoritic components and host meteorites has various O isotopic composition widely distributed both in time and space in solar system

14 The oxygen isotopic anomalies distinguished well with a three isotope ) by Clayton et al (1973). It is conventional to express variation in abundances of the isotopes of oxygen in terms of isotopic ratios, relative to an arbitrary standard, called SMOW (Standard Mean Ocean Water) as follows- 18 O = 17 O = The isotope fractionation resulting from a normal chemical process depends on the mass differences among isotopes. The fractionation takes place along a line with a slope of ½ on the oxygen three isotope diagrams or fractionation of 18 O relative to 16 O occurs twice as much as compared to fractionation of 17 O relative to 16 O. In case of terrestrial sample have a slope of ½. This line is termed terrestrial fraction line (TFL). However, the O isotope ratio of CAI minerals observed by Clayton (1973), on the other hand, distributed along a line called Carbonoceous Chondrite Anhydrous Minerals (CCAM) line having a slope unity. The three isotope diagram for different meteorites indicates that each asteroid body and /or similar one or different regions of early solar system. In other word, each group of meteorite attributes to each distance from the sun. It is believed that at least two distinct oxygen isotope reservoir existed in the solar nebula (Clayton, 1993). One of the reservoir resembles, but it is not identical to the O-isotopic composition of the Earth and another is enriched in 16 O relative to Earth by 4% (Clayton 1993) to 8% (Kobayashi et al, 2002) The O-isotopic composition of the material in the solar system can be derived by mixing of the two reservoirs, the only exception is presolar grain (Nittler 2003, Nagashima et al, 2004). These two reservoirs can t be derived from each other by mass dependent isotope fractionation. However isotope fractional processes were not depending on the mass differences (mass independent isotope

15 fractionation) in nature, that was not fully understood. Therefore the origin of these reservoirs is still controversial, but several processes have been proposed, including nucleosynthesis (Clayton, 1993), isotopic symmetry effect (Thiemens 1999) and self-shielding effects (Thiemens and Heidenreich, 1983, Clayton, 2002). The isotopic composition of oxygen in meteorite, therefore plays an important part regarding classification scheme. So in this section, the study of oxygen isotope is an important aspect to establish Dergaon meteorite under specific group (H5 Chondrite). The measured oxygen isotopic compositions in samples of Dergaon meteorite is given in table 5. In both Balidua as well as Koilaghat fragment, the oxygen isotopic compositions were analyzed by laser fluorination technique. The oxygen isotope value for both the fragment of Dergaon meteorite is taken from Shukla et al (2005). Table 5: Oxygen isotope data for two fragments of Dergaon meteorite. Sample 17 O% 18 O% 17 O%* 1 Balidua Koilaghat *1 Deapartures from the terrestrial fractionation line as represented 17 O, give a unique feature for different group or class of meteorite. The schematic diagram for each group present a specific position from TFL. Therefore this is one of the important factors regarding the classification of meteorite. Table 6 represents value of 17 O for different classes of meteorite

16 Table 6: Oxygen isotopic composition for different group of meteorite Chondrite 17 O (%) CR to Lodranites to Acapulcoites to 1.22 Winonaites -0.4 to HED Branchinites Augrites Moon 0.00 Earth 0.00 Venus 0.00 to 0.3 H (4-6) L (4-6) LL (4-6) Discussion: Classification The purpose of a classification is to group together related objects in order to facilitate comparative investigations.a classification should consists of as few categories as possible without forcing together objects for which the evidence of a relation is inadequate. Here classification is mainly done on the basis of petrography, bulk elemental compositions and oxygen isotopic composition a. From Petrography- From petrographic analysis, the composition of olivine (Fo ~ 80%) from EPMA data (Duorah et al 2002) of Dergaon meteorite as determined from whole rock analysis. These values of Dergaon meteorite matches well with the value of H Chondrite (VanSchmus and Wood, 1967). Further, absence of well formed intact chondrules and their indistinct and diffused boundaries as well as presence of irregular grains of metallic phases indicate some degree of

17 recrystallization (Wood 1963, Dodd 1969) which forms the basis of the petrological classification of the meteorite (VanSchmus and Wood 1967) and accordingly it is classified as type 5. Further a comparison with other known meteorite shows that Dergaon meteorite is an ordinary H5 chondrite with all petrographic features well within the range of those of other meteorite of its class and petrologic type [Frederiksson and Wlotzka, 1985] Further, occasional irregular and fluid flow like features and planar fractures and undulatory extinction of olivine seen in thin section studies are indicative of a low shock level experienced by Dergaon meteorite. 2.5.b. From chemical classification- From table 3 it may be noted that the major elements present in the sample of Dergaon meteorite in decreasing order of concentration are Fe, Si, Mg, Ni, Al, Ca, Na, Cr, Co and K. Chondrite meteorite are inherently inhomogenous because of chondrule and matrix element. Inter comparison of the analytic data with some standard chondritic meteorite is carried out in order to classify Dergaon meteorite according to its elemental content. Table 7 present the values reported for the average H- Chondrite. This is one of the important bases to establish Dergaon meteorite, which is according to petrography as well its Fe content shows that it belongs to H5 group. Table 7: Inter-elemental ratios for Dergaon meteorite and average H- chondritic meteorite.* [Source Wasson and Kellemeyn, 1988] Sample / Element ratio Al/Si Mg/Si Ca/Al Al/Mg Ca/Mg Ni/Si Dergaon H- chondrite As compared the Al/Si, Mg/Si, Ca/Al, Ni/Si, Al/Mg and Ca/Mg ratio as in table 7 with an average H-chondrite. The value shows a good agreement

18 Hence it shows a strong indication that Dergaon meteorite belongs to H- chondrite. The chemical composition of Dergaon meteorite when normalized to mean H chondrite composition (Wasson an Kellemeyn, 1988), observed a graph which is given in fig.7. However, the graph shows a low content of K compared to average K content in H- chondrite. Fig. 7 : Chemical composition of Dergaon meteorite normalized to the mean H composition (Wasson and Kallemeyn, 1988). chondrite The composition of Dergaon meteorite indicate that it is a H-chondrite. The only deviation that is readily observed (fig 7) is the distinctly lower content of K compared to mean value of K in H- chondrite (Wasson & Kellemayn, 1988). The lower value of K, makes Dergaon meteorite unique meteorite amongst the H-chondrite. The possibility that potassium lost during the higher degree of metamorphism experience by Dergaon meteorite ruled out in the absence of any significant depletion of other volatile element relative to H-chondrite [Shukla et al,2005]. Hence Dergaon meteorite can be classified as H group of chondrite having low K content. The unique low content of K gives a possibility that this meteorite belongs to some other group. Such as acapulcoites or ladranites that have same chemical similarities to ordinary chondrites but have suffered higher degree of metamorphism and differentiation

19 Fig. 8 : Schematic locations of various chondrite classes in the isotope graph However, the data of oxygen isotope falls close to H-chondrite [Clayton et al 1991] as clearly seen from table 5 and table 6. But there is an intrasample difference in the 17 O, which is quite significant. This may arise due sample heterogeneity. But this value is quite different from the average 17 O for acapulcoites and lodranites (from table 7). This firmly establish that Dergaon meteorite belongs to H group of chondrite. This can also readily seen from the schematic representation of the location and ranges of whole rocks isotopic compositions of major chondrite classes, from fig.8. Therefore on basis of oxygen isotopic analysis, it can be concluded Dergaon meteorite does not belong to any other class of meteorite, but it belongs to H group of chondrite. Hence Dergaon meteorite can be classified as anomalous H chondrite with a unique K composition

20 Bibliography Anderson D.L.(2007) New Theory of Earth, Cambrige University Press, U.K. Clayton R.N., Grossman, L., Mayeda, T.K (1973).A component of primitive nuclear composition in carbonaceous meteorites. Science 182: Clayton, R.N.,Kieffer, S.W.(1991).Oxygen isotopic thermometer calibrations. In Stable Isotope Geochemistry,ed. H.P. Taylor Jr., J.R. O Neil, I.R. Kaplan pp3-10. Clayton R.N. (1993). Oxygen isotopes in meteorites. Annu. Rev. Earth Planet Sci.21: Clayton R.N.(2002) Nature,415, Dodd R.T.,Jr. (1969). Metamorphism of the ordinary chondrites:a review. Geochimica. Cosmochimica. Acta Duorah K., Mazumdar A.C., Phukon P., Phukan S. and Duorah H.L.(2002). Petrography and classification of Dergaon Meteorite. Proceedings of the 3rd Regional Conference on Physics Research in the North East Frederiksson, K. and Wlotzka, F.(1985). An equilibrated H5 chondrite. Meteoritics, 20, Grossman J.N., Rubin A.E. and MacPherson(1988).ALH85085 a unique volatile poor carbonaceous chondrite with possible implications for nebular fractionation processes. Earth Planet Sci. Lett. 91, Itoh, S., Rubin, A.E., Kojima, H.,Wasson, J.T. and Yurimato, H. (2002). Amoeboid Olivine Aggregates and AOA bearing chondrules from Y CO3.0 chondrite: Distribution of Oxygen and Magnessium Isotopes, Lunar Planet Sci.XXXIII,#1490.The Lunar Planetary Institute, Houston. Kobayashi S., Imai H., Yurimato H.(2003) Geochem. J, 37,

21 Mason B. (1962). Meteorites. New York: John Wiley & sons. Nagashima K., Krot A.N. and Yurimato H.(2004) Nature, 428, Nittler L.R., (2003) Earth Planetary Science Letters, 209, Shukla P.N.,Shukla A.D., Rai V.K., Murty S.V.S., Bhandari N., Goswami J.N.,Mazumdar A.C, Phukon P., Duorah K., Greenwood R.E. and Franchi I.A.(2005). The Dergaon(H5) Chondrite. Meteoritics& Planetary Science 40, 4, Thiemens M.H., (1999). Atmospheric Science- mass independent isotope effects in planetary atmospheres and the early solar system.science 283, Thiemens M. H., Heidenreich J.E.(1983) The mass-independent Fractionation of Oxygen Science,219, VanSchmus, W.R. and Wood, J.A.(1967).A chemical-petrologic classification for chondritic meteorites. Geochim. Cosmochim. Acta 31, Wood J.A.(1963) Origin of chondrules and chondrites. Icarus 2, Wasson J.T. and Kellemeyn G.W.(1988) Composition of Chondrites. Philos Trans R. Soc. London A325,

AMHERST COLLEGE Department of Geology Geology 41: Environmental and Solid Earth Geophysics

AMHERST COLLEGE Department of Geology Geology 41: Environmental and Solid Earth Geophysics Lab 1: Meteorites EQUIPMENT: notebook and pen only In this lab, we will examine thin sections and hand samples