An introduction to pallasites, their mineralogy, texture and origin

Transcription

1 An introduction to pallasites, their mineralogy, texture and origin Lena Boeck Institut für Mineralogie, TU Bergakademie Freiberg, Germany supervised by Professor Gerhard Heide Piece of the Springwater pallasite (photo by Lena Boeck)

2 2 Boeck, Lena Table of Contents 1. Introduction The sources of meteorites The classification of meteorites Pallasites Definition The Mineralogy of Pallasites Olivine Kamacite and Taenite Minor phases Texture The classification of pallasites Main group pallasites Eagle station trio (Eagle station pallasites) Pyroxene-pallasite grouplet Formation of pallasites The pallasite problem Summary References Appendix A Glossary B List of meteorite groups and their main properties C Petrologic type D Shock stage E Grade of weathering F Figures G References... 27

3 Pallasites Their origin and composition 3 Abstract. Some of the most beautiful meteorites found on Earth are pallasites. These meteorites contain only two major phases, olivine and FeNi-metal. The either rounded or angular, pure, yellow to yellow-green olivine crystals are swimming in a matrix of silver FeNi-metal like raisins in a cake. Based on differences in the chemistry, three pallasite groups were distinguished by meteoritists: the main group pallasites, the Eagle Station pallasites, and the pyroxene pallasites. It is suggested that the unlike association of olivine and metal was generated by metal melt intruding into fragmented olivines. 1. Introduction When scientists want to obtain information about the very beginning of our solar system including Earth, they do not study Earth s interior but look into the sky. In other words, they do research on extraterrestrial material meteorites. In one year nearly 40,000 tons 1 of cosmic material enter Earth s atmosphere and fall as dust particles, which usually can not be collected, or meteorites. Unfortunately, a lot of meteorites are not discovered because they fall in oceans or uninhabited areas. But the theoretical number of samples is very huge. The formation of chondrules (all words written in italics are explained in the glossary), which are found in meteorites, can be dated to a time ~4,564.7 ± 0.6 Ma ago. 2 Contrary to meteorites, the oldest terrestrial material is only 4.2 billion years old, measured by dating the ages of grains of the mineral zircon 3. So meteorites are the oldest witnesses of the origin of our solar system. After Wood and Chang (1985), the materials which formed meteorites are residues of the planetesimals. Most of the planetesimals incorporated afterwards into the planets 4. The most primitive meteorites made of this primordial material are the carbonaceous chondrites. Especially CV3-, CO3-meteorites and UOCs represent the earliest solar nebular material. Some constituents of chondritic meteorites such as chondrules, CAIs and dark inclusions can be dated to measure the time of the accretion. After that a period of mild to moderate heating followed. Holdovers from this time are undifferentiated meteorites (= chondrites). Some asteroids were partly or totally melted (e.g. Vesta) and differentiated into an inner and an outer part which show similarities to the core and the mantle of the Earth. Pieces of this core- and mantle-like material fall as meteorites to Earth. 1 Web 1 2 Davis, 2004, p Kleinschrot, 2003, p Sears, 2004, p. 13

4 4 Boeck, Lena 2. The sources of meteorites Meteoriticists distinguish between different sources of meteorites. There are Martian meteorites, meteorites from the Earth s moon and asteroidal meteorites. Four kinds of Martian meteorites are known: shergottites, nakhlites, chassigny, and ALH From the compositions and properties of these meteorites, it is possible to develop and understand the geological and planetary evolution of Mars. 5 The majority of the lunar meteorites are pieces of the lunar highlands. The lunar meteorites are divided into the types LUN A (lunar anorthositic (feldspathic) breccia), LUN B (lunar mare basalt or gabbro, basaltic or gabbroic breccia), LUN M (mingled breccia (basaltic, and anorthositic clasts)), and LUN K (lunar KREEP basalt, or KREEP-rich mafic breccia) 6. Asteroidal meteorites have their source in the asteroid belt which is located mostly between the orbits of Mars and Jupiter (2-4 AU from the Sun). By reflectance measurements, 26 different asteroid groups were defined. Through spectral reflectance most meteorites can be linked to different, although not necessarily specific, asteroids. Also it is possible to track incoming meteors and meteoroides and calculate the orbits of these objects. These orbites lead to the asteroid belt. 7 Sometimes a fourth source of meteorites is assumed, namely leftovers from comets, but this is unproven The classification of meteorites Scientists are used to classify all objects to make it possible to compare them with other objects. Classifications system of meteorites were created very early in the history of science. Besides various classification schemes there are two long used systems based on different properties. One is the Rose-Tschermak-Brezina classification. It divides the meteorites by their composition and texture. 9 The second system is the Prior classification. It is based on variations of the iron-nickel metal and the iron incorporated in olivines and pyroxenes. These early systems were replaced after the first microanalysis of meteorites was preformed by Klaus Keil and Kurt Fredriksson. This new tool of analysis enabled a classification unattached to the structure and nickel content alone. 10 Today s classification includes different properties which are both primary and secondary. The first step is to characterize the chemical type of the meteorite. The chemical classification divides meteorites into undifferentiated and differentiated 5 Lauretta and McSween, 2006, p. 6 6 Web 2 7 Lauretta and McSween, 2006, p. 5 8 Bevan and de Laeter, 2002, p Norton, 2002, p. 73f. 10 Norton, 2002, p. 75f.

5 Pallasites Their origin and composition 5 meteorites. The undifferentiated meteorites are also called chondrites and include different classes, groups and clans. The classification is given in Figure 1. Undifferentiated Meteorites (chondrites) Ordinary Chondrites (O) Carbonaceous Chondrites (C) Enstatite Chondrites (E) class CI-Chondrites CM-CO-Chondrites CV-CK-Chondrites CR-Chondrites EH-EL-Chondrites clan H-Chondrites (high iron) L-Chondrites (low iron) LL-Chondrites (low metal, low total iron) Figure 1: Classification of undifferentiated chondrites 11 CM- Chondrites CO- Chondrites CI- Chondrites CV- Chondrites CK- Chondritess CR- Chondrites EH- Chondrites EL- Chondrites group To the given groups 2 further classes must be added: the R-chondrites and the K-chondrites whereas the latter group is only a grouplet, because it only consists of 2 members. Chondrites are the most primitive and most common meteorites. Of all known meteorites ca. 85 % are chondrites. 12 The primary classification parameters mentioned above are the bulk chemical composition, the oxidation state, the bulk oxygen isotopic composition, the bulk nitrogen and carbon abundance, as well as their isotopic composition. The secondary classification parameters are the petrologic type, the shock stage and the grade of weathering. The petrologic type was developed by Van Schmus and Wood 1967 and describes the degree of the aqueous and thermal alteration the meteorite experienced. 13 The shock stage, classified by Stöffler, Keil and Scott, 1991, results from hypervelocity collisions on their parent body. 14 Finally the affect of terrestrial weathering can be characterized with the grade of weathering after Wlozka. This grade correlates with the terrestrial age of the meteorite. 15 For further informations see appendix C to E or various literature (e.g. Norton; 2002; p. 73ff., Davis; 2004; 89ff.). The remaining 15 % of all meteorites are differentiated meteorites and belong to achondrites, irons or stony-irons. The parent bodies of differentiated meteorites did undergo total or partial melting and started to differentiate as described in the introduction. In Figure 2 the structure of a basaltic parent body and Earth are compared. The classification of differentiated meteorites is given in Figure Norton and Chitwood, 2008, p. 80; Lauretta and McSween, 2006, p Norton and Chitwood, 2008, p Davis, 2004, p. 87ff. 14 Stöffler and Keil, 1991, p Wlotzka, 1993, p. 460

6 6 Boeck, Lena Figure 2: Structure of the terrestrial planet compared to a basaltic parent body 16 A list of all groups of meteorites can be found in the appendix B. Differentiated Meteorites Irons Achondrites Stony-irons Martian Primitive achondrites Asteroidal achondrites Lunar Shergottites Nakhiltes Chassigny ALH Acapulcoites Lodranites Winonaites Impact breccias Mare basalts Basaltic Angrites Aubrites Ureilites Brachinites Eucrites Diognites Howardites Figure 3: Classification of differentiated meteorites 17 The meteorites, this paper is dealing with, belong to the stony-irons. The stonyiron class includes mesosiderites and pallasites. Mesosiderites are breccias which have approximately equal proportions of silicates and FeNi-metal. The silicate component is made of minerals and lithic clasts of fine-grained fragmental or igneous matrix. 18 The pallasites will be described in more detail below. 16 Norton and Chitwood, 2008, p Norton and Chitwood, 2008, p. 115ff. 18 Davis, 2004, p. 112

7 Pallasites Their origin and composition 7 4. Pallasites 4.1. Definition Pallasites are stony-iron meteorites composed of 2 major phases: Olivine and FeNi-metal. The 2 phases are physically and geochemically dissimilar and for this reason unlikely to be related to each other. 19 The olivine to metal ratio of an average pallasite is 2.40 by volume or 1.02 by weight. Pallasites also contain some minor phases, but their abundance is limited. Minor phases are troilite, schreibersite, chromite, phosphoran olivine, and pyroxenes. The composition of an average pallasite would be 64.9 vol. % olivine, 31.0 vol. % metal, 2.3 vol. % troilite, 1.2 vol. % schreibersite, 0.4 vol. % chromite, and 0.2 vol. % phosphate. 20 Pallasites were the first material, which was recognized and accepted as extraterrestrial material. 19 A picture of a pallasite can be found in Figure 8 (appendix F) The Mineralogy of Pallasites Olivine One of the main phases of pallasites is olivine. The composition of the olivine varies from Fa 11 Fa 20 21, but in a single pallasite the olivine composition does not vary 22. The crystals are very homogenous and show no zoning. 21 Besides in dimensions of some 100 µm 23 around the edges they show a significant chemical gradient in minor elements. 24 The shape of the olivine can either be rounded or angular. The olivine appears as transparent yellow to yellow-green crystals. The green olivine known from terrestrial rocks is surprisingly rare in pallasites. 25 Until today no correlation between the shape and chemical features could be found. Only a correlation to some evidence of deformation is suggested, because the non-rounded olivine is thought to be a product of an event or a series of events of deformation Buseck, 1977, p Buseck, 1977, p Buseck, 1977, p Scott, 1977b, p Huss and Tomiyama, 2005, Introduction 24 Huss and Tomiyama, 2006, Introduction 25 Norton, 2002, p. 203

8 8 Boeck, Lena Kamacite and Taenite The second main phase of pallasites is the iron-nickel metal. The FeNi-metal can appear as 3 different minerals. If the nickel content is low (between 4 and 7.5 wt.%) the mineral is called kamacite. Kamacite is mostly found in irons and stonyirons and only as a minor mineral in some achondrites. 26 The Ni-content of the second mineral varies from 27 to 65 %. It is known as tenite and occurs usually as thin lamellae around kamacite in iron meteorites. If the kamacite and the taenite grow into each other they are forming plessite, the third FeNi-mineral. Plessite can be found in octahedrite and some chondrites. 27 In pallasite all three minerals can be found. The composition of the FeNi-metal is similar to that of iron meteorites. Etching of pallasitic metal shows that it has a Widmannstätten structures. 28 Lovering et al. (1957) suggested a genetical linkage between pallasites and group III irons. This was confirmed by different studies of e.g. the cooling rate which is similar to the IIIB irons 29 or the content of Ga, Ge, Au, As, Ir, Ni and W which overlaps the IIIAB irons field in plots Minor phases Pyroxene is a widespread mineral in meteorites. In pallasites they were unknown for a long time. However, modern analytical techniques lead to their discovery as a minor phase with grains only a few µm in diameter. Pyroxenes commonly occur in 2 kinds of symplectic intergrowth. The first one is as peripheries along olivine crystals. In this case they are in contact with kamacite or schreibersite. Also it can be found in sharply defined contact areas between two olivine grains. The pyroxenes in pallasites have a low Ca-content. Also a common mineral found in meteorites is troilite. It can be found in pallsites next to olivine crystals or as polycrystalline grains in eutectic (or eutecticlike) intergrowth with kamacite. Another mineral found in pallasites is schreibersite. Schreibersite is an ironnickel phosphide which is common in iron and stony-iron meteorites. 27 In pallasites it is always surrounded by kamacite, because of that there are suggestions that the schreibersite incorporated the nickel of the metal. In the metal the size of the grains is around 1 mm and smaller. But schreibersite can also occur as mm- or cm-sized grains adjacent to olivine. 31 One of the most interesting minerals in pallasites is phosphoran olivine. Because phosphor is generally not a constituent of olivine it is unusual that olivines in pallasites can contain sharply defined areas, up to 20 µm wide and several mm 26 Norton, 2002, p Norton, 2002, p Buseck, 1977, p Scott, 1977a, p Scott, 1977a, p Buseck, 1977, p. 727

9 Pallasites Their origin and composition 9 long, with P 2 O 5 contents between 3.8 and 4.9 wt.%. Based on studies of the charge, size and stoichiometry of the phosphorus it can be concluded that it substitutes for silicon. The question of the compensation of the different charge is still open. Known is that the source of the phosphor is external to the olivine as suggested by the appearance of the P-rich areas near the edges. 32 Besides these already described minor phases pallasites can contain accessory phases such as chromite, rutile, magnetite, pentlandite, native copper and copper sulfides, sphalerite, graphite and many more. Usually these phases are known in a limited number of pallasites and not common in all pallasites Texture Pallasites have a very unique texture that is dominated by olivine and FeNi-metal. The mixture of olivine and metal can vary from nearly pure olivine regions to nearly pure metal, in numbers the FeNi-content differ from 28 to 88 wt.%. 25 With this the density of pallasites has also a wide range. E.g. the pallasite Mount Vernon has variations from 4.0 to 6.0 g/cm 3 even in samples of only 1 kg. 34 The olivine crystals in pallasites are either rounded or angular. Angular means in this case fragmental, not euhedral. 22 It must be added that the angular olivines appear also rounded in microscopic scales (see Figure 9, appendix F). It is thought that the rounded shape was produced by an event called grain boundary migration which reduces the boundary energy of the olivine. The grain boundary migration is caused by the unusual Figure 4: Widmannstätten structures in different cutting positions (a - octahedral face, b - cube face, c - rhombbododecahedron face, d - optional 35 high surface energy of olivines in metal that is much higher than the surface energy of olivine in silicate melts. It is expected that this also produces the crystal faces found in pallasitic olivines. 36 Terrestrial olivines have only three dominating crystal faces (faces after {010}, {110}, and {021}). After Kolomensky et al. olivines in pallasites can have up to 21 different faces. 37 The FeNi-metal of the pallasites is a complete connected network. Primary metal is kamacite showing also thin lamellae of taenite and plessite. The Widman- 32 Buseck, 1977, p. 724f. 33 Buseck, 1977, p.727f. 34 Scott, 1977b, p Kleinschrot, 2003, p Scott, 1977b, p Scott, 1977b, p. 703

10 10 Boeck, Lena stätten structure of the metal appears as a medium octahedrite structure (see Figure 4). 25 Pallasites are strongly influenced by weathering which oxidize the metal and olivine. The largest intact pallasite is the Huckitta pallasite. To his main mass of 1411 kg a 900 kg iron shale was discovered nearly completely converted to hematite and a opaque black mass of former olivine The classification of pallasites Meteoritists defined three separate pallasite types based on different silicate mineralogy and composition as well as the metal composition and the oxygen isotopic composition. By these scientists concluded that pallasites formed at least on three different parent bodies. 38 Further the three types will be presented Main group pallasites The main group pallasites contain besides FeNi-metal and olivine minor amounts of low-ca pyroxenes, chromite, phosphates (e.g. farringtonite, whitlockite), troilite, and schreibersite. 38 The composition of the olivine varies from Fa 10.5 to Fa 13 and the Ni content of the metal is between 8 and 12 wt.%. 22 No. Ni (wt.%) Ga Ge Au Fa (µg/g) (µg/g) (µg/g) (mole%) Main group Eagle station trio Table 1: main properties of the main group and Eagle Station pallasites 30 In Table 1 the concentration of different elements in the metal are given based on studies of 34 pallasites. In a Ga-Ge plot, the IIIAB iron field overlaps with the field where the main group pallasites are plotted 30 (plots are given in Figure 10, appendix F). Another sharply defined field can be found in the bulk oxygen isotopic composition. In Figure 5 the plots of the main group pallasites are highlighted in blue. Even if the oxygen isotopic composition of the main group pallasites is alike to this of HEDs it is unlikely that both come from the same parent body. 38 Davis, 2004, p. 113

11 Pallasites Their origin and composition 11 Figure 5: Bulk oxygen isotopic composition of primitive achondrites and differentiated meteorites Eagle station trio (Eagle station pallasites) The red highlighted plot in Figure 5 characterizes the Eagle Station pallasites which have similar mineralogy as the main group pallasites but different chemical compositions. For example, the metal is richer in Ni, Ir, and Ge 22 (see Table 1) and therefore close to the composition of IIF iron meteorites. Meteoriticists suggest that they formed in the same region of the solar nebular. 40 The fayalite content of the Eagle Station pallasites is higher than that of pallasites belonging to the main group pallasites, and the Au, As, and Ga contents are lower. 22 Also with trace element plots (like Ga-Ge) the Eagle Station pallasites can be easily distinguished from the main group pallasites Pyroxene-pallasite grouplet This group was defined after sufficient volumes of pyroxene were discovered in two pallasite. These are referred to as members of a grouplet because at this time, only two members, Y-8451 and Vermillion, are known. These meteorites have between 0.7 to 3 vol.% mm-sized pyroxene grains. The contents of olivine in this grouplet is between 14 to 63 vol.% and the metal content is reduced to 30 to 43 vol.%. The classification can also be made with both the oxygen isotopic composition (highlighted in orange in Figure 5 and Figure 6) and the metal composition which indicates a formation on a third pallasite parent body Davis, 2004, p Davis, 2004, p Scott, 1977a, p. 352ff.

12 12 Boeck, Lena Figure 6: Bulk oxygen composition (enlarged from Figure 4) Formation of pallasites In pallasites meteoriticists found evidences for three different events which influence their formation. The first event was the fracturing of an earlier formed olivine mass with crystals that were at least 30 cm in size. It is thought that this was closely related to the second event where the olivine fragments were mixed with the FeNi-metal. In this step the olivines were separated and reoriented. Finally the metal started to solidify while the olivine-metal boundaries migrated and formed macroscopically or microscopically rounded shapes The pallasite problem Many scientists suggested for a long time that pallasites represent the core-mantleboundary of asteroids. But in today s literature there is extensive discussion about the formation of the pallasites including the mechanism as to how olivine can occur next to metal. So actually four different regions inside a parent body are described where pallasites formed. First, near the surface, second close to the center of the asteroid, third inside of isolated metal pools which had a contact zone to an olivine layer and finally at the core-mantle boundary. 43 Buseck (1977) suggests that at least one or more melts formed the pallasites. First a silicate melt which formed the olivines and second a metal melt forming the surrounding network of FeNi-metal. But isolated metal grains proof that the metal melt already existed when the olivine crystallized. The higher melting point of pallasitic olivine speaks for a coexistence of solid olivine and a FeNi-melt. Also a residual melt is proposed because of the presence of troilite and phosphate Scott, 1977b, p. 705ff. 43 Web 4 44 Buseck, 1977, p. 733

13 Pallasites Their origin and composition 13 The pallasite problem itself includes not only the formation of the pallasite minerals but also the mechanism of mixture. Because of the immiscibility of silicate and metal melts olivine and FeNi-metal have a strong tendency to separate quickly from each other. The Encyclopedia of meteorites describes two possible mixture processes. Either a shock wave induced driving of olivine into the metal core or a convective instability which forced the Figure 7: Formation of Pallasites 45 metal into olivine cumulates above (see also Figure 7). Both are only short-lived and temporary processes after which the separation would start again. So they conclude that the pallasites [ ] formed after differentiation but before complete solidification of the core.. 45 Scott 1977b gives 3 explanations for the pallasite problem. The first is after Wood 1963 who suggested that the separation of floating olivine crystals and a metal melt was prohibited by an overlying olivine layer or a solid crust of metal. The second explanation suggests that the metal was already solid and acted plastic while it intruded into the fractured olivine as developed by Merrill The third and by Scott most favored idea is that the metal was close to the freezing point or maybe even supercooled while the olivine was marginally cooler, so that differentiation and segregation was prevented. 46 Today s research is focusing on the minor and trace element zoning of pallasitic olivine as well as different isotopic analyses. The Widmannstätten structures found in pallasitic FeNi-metal are known to develop between C and C with a cooling rate of a few degrees per million years 47 (0.5 2 C/10 6 a 48 ). Cooling rates based on the olivine zoning were measured as C/a from 1100 to 600 C. The wide range of these cooling rates are caused by different best fitting calculated cooling rates for different analyzed elements. 49 From these cooling rates a thermal history of a two step cooling was suggested. Miyamoto (1997) discusses two different interpretations. First a high-temperature formation of chemical zoning, where the first step is characterized by fast cooling at high temperatures and the formation of the chemical zoning. And the second step is a slow cooling at low temperatures event formed the Widmannstätten structure. The other interpretation is a low-temperature formation process to form the chemical zoning. This theory explains the chemical zoning by a migration of ele- 45 Norton, 2002, p Scott, 1977b, p Huss and Tomiyama, 2005, Discussion 48 Miyamoto, 1997, p. 21, Miyamoto, 1997, p. 21,615

14 14 Boeck, Lena ments into the olivine during grain boundary diffusion. 50 A large number of papers (e.g. Huss and Tomiyama, 2005 and 2006 or Chen and Papanastassious, 2009, all Lunar and Planetary Science Conference) are dealing with different analyses of trace elements and isotopic compositions to clarify the thermal history of pallasites and with this the formation. But all authors are pointing to continue their work to clear up the issue. After all the discussion is still open and offers some future work to do. 6. Summary Pallasites are very unique stony-iron meteorites with a relatively simple mineralogy. They can be divided into main group pallasites, Eagle Station pallasites and pyroxene pallasites, all more or less similar in mineralogy but different in the composition of the metal and of the oxygen isotopes. So that the three groups appear as well defined clusters in plots of this properties. Most scientists believe that pallasites were formed at the mantle-core boundary of differentiated asteroidal parent bodies. Nowadays the formation of pallasites is a subject of discussion. Different ideas can be found in papers by Scott, Buseck, Norton, Miyamoto and much more. Because of much work currently is in progress, no firm conclusions could be drawn on the origin of these enigmatic meteorites. 50 Miyamoto, 1997, p.21,617

15 Pallasites Their origin and composition References Web 1: (January 2009) Web 2: (March 2009) Web 3: (February 2009) Web 4: (February 2009) Bevan, Alex; de Laeter, John: Meteorites. A Journey through Space and Time. Smithsonian Institution Press: Buseck, Peter B.: Pallasite meteorites Mineralogy, Petrology, and Geochemistry. Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: Davis, Andrew M.: Treatise on Geochemistry. Volume 1. Meteorites, Comets, and Planet. Elsevier Pergamon. Oxford: Huss, G. R.; Tomiyama, T.: Minor Element behavior of pallasite olivine: Understanding pallasite thermal history and chronology. Lunar and Planetary Science XXXVI: 2005 Huss, G. R.; Tomiyama, T.: Minor and trace zoning in pallasite olivine: Modeling pallaste thermal history. Lunar and Planetary Science XXXViI: 2006 Kleinschrot, Dorothée: Meteorite Steine, die vom Himmel fallen. Beringis Sonderheft 4, Würzburg: Krot, Alexander N.; Scott, Edward R. D.: Chondrites and the Protoplanetary Disk. Vol ASP Conference Series: Lauretta, Dante S.; McSween, Harry Y.: Meteorites and the early solar system II. The University of Arizona Press, Tucson and Lunar and planetary institute, Houston: Miyamoto, M.: Chemical zoning of olivine in several pallasites. Journal of geophysical research. Vol. 102, No. E9: 1997 Norton, O. Richard: The Cambridge Encyclopedia of Meteorites. Cambridge University Press, Cambridge: Norton, O. Richard; Chitwood, Lawrence A.: Field Guide to Meteors and Meteorites. Patrick Moore s Practical Astronomy Series. Springer-Verlag, Lodon: Sears, Derek: The Origin of Chondrules and Chondrites. Cambridge University Press, Cambridge: 2004.

16 16 Boeck, Lena Stöffler, Dieter; Keil, Klaus; Scott, Edward R. D.: Shock metamorphism of ordinary chondrites. Geochimica et Cosmochimica Acta, Volume 55. Pergamon Press: Scott, Edward R. D.: Pallasites metal composition, classification and relationship with iron meteorites. Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977a. Scott, Edward R. D.: Formation of olivine-metal textures in pallasite meteorites. Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977b. Wlotzka, F.: A weathering scale for the ordinary chondrites. Meteoritics 28: (abstract)

17 8. Appendix A Glossary CAIs chondrules dark inclusions calcium-aluminium-rich inclusions 1 containing also titanium, which are highly refractory and suggested to be the first mineral formed in the solar nebular 2 approximately spherical elements of meteorites, which show evidence of partial or complete melting 1 also dark clasts are rock fragments of a size of 100 to 1000 µm, which contain aqueously altered material 3 HEDs howardite-eucrite-diogenite clan of meteorites, which belong to the asteroidal achondrites 4 and trought to have formed on the asteroid 4 Vesta 5 planetesimals bodies made of rocks or ice which formed in the primordial solar nebular 6 UOCs unequilibrate ordinary chondrites 7 Widmannstätten structure structure found on etched regions of iron meteorites showing large bars of kamacite surrounded by small fields of taenite 8 1 Lauretta and McSween, 2006, p Norton and Chitwood, 2008, p Krot and Scott, 2005, p Lauretta and McSween, 2006, p Norton and Chitwood, 2008, p Lauretta and McSween, 2006, p Web 8 Lauretta and McSween, 2006, p. 917

18 18 Boeck, Lena B List of meteorite groups and their main properties Undifferentiated meteorites Enstatite chondrites EH high total iron, highly reduced, minichondrule-bearing EL lower total iron, highly reduced, moderately size chondrules Ungrouped e.g. LEW Ordinary chondrites O Olivine-Bronzite H high total iron Olivine- L low total iron Hypersthene Amphoterite LL low total iron, low metallic iron Rumuruti chondrites R high total iron, highly oxidized, 17 δ O- rich Carbonaceous chondrites C CI CM CR CH primitive chondrites, contains phyllosilicates, chondrules-free, volatilerich, aqueously altered contains little chondrules (< 0.5 mm) and phyllosilicates, aqueously altered contains metal, mm-sized chondrules and phyllosilicates, aqueously altered similar CR, contains smallest chondrules, volatile-poor CO contains metal and chondrules ( mm) CV contains bigger chondrules (0.5 2 mm) and CAIs, partially aqueously altered CK highly oxidized, large chondrulebearing, darkened silicates Ungrouped e.g. Coolidge, LEW IAB/IIICD silicates Ungrouped chondrites e.g. Deakin 001 K-chondrites Subchondritic composition, chondrule-free, planetary-gas-bearing

19 Pallasites Their origin and composition 19 Differentiated meteorites Primitive Achondrites Acapulcoites ACAP chondritic abundance of plagioclase and troilite, medium-grained Lodranites LOD subchondritic abundance of plagioclase and troilite, coarse-grained, higher olivine content than ACAP Winonaites WIN IAB-silicate-related, highly reduced recrystallized silicates Ungrouped e.g. Divnoe Asteroidal achondrites Eucrites EU basalts Diogenites DIOG magnesium-pyroxenites Howardites HOW brecciated mixture of basalts and orthopyroxenites Angrites ANGR fassaitic-pyroxene-bearing basalt Aubrites AUB enstatie achondrites Ureilites UR olivine-pyroxene-carbonaceous matrix-bearing Brachinites BRACH equigranualar matrix of olivine, pyroxenes and plagioclase Martian meteorites SNC Shergottites SHE basalts and lherzolites Nakhlites NAK cumulus-augite-bearing pyroxenites Chassigny CHS dunite ALH ALH orthopyroxenites Lunar meteorites Mare basalts basalts, gabbros, anorthosites Impact breccias anorthositic- and mare-dominated regolith and fragmented breccias Stony irons Pallasites PAL metal plus olivine main-group pallasites (MG) Eagle-station pallasites (ES) Pyroxene pallasites Mesosiderites MES metal plus basaltic, gabbroic and orthopyroxenitic silicates Ungrouped e.g. Enon, Mt. Egerton Irons Chemical classification Magmatic groups IC, IIAB, IIC, IID, IIF, IIIAB, IIIF, IVA, IVB Nonmagmatic IAB/IIICD, IIE

20 20 Boeck, Lena groups Ungrouped IRUNGR e.g. Britstown, Denver City, Guin, Sombrerete Structural classification Hexahedrites HEX Neumann lines (IIAB, IIG) Octahedrites O Widmanstätten lines Coarsest octahedrites Ogg mm Kamacite bandwidth (IIAB, IIG) Coarsest octahedrites Ogg mm Kamacite bandwidth (IAB, IC, IIE, IIIAB, IIIE) Medium octahedrites Om mm Kamacite bandwidth (IAB, IID, IIE, IIIAB, IIIF) Fine octahedrites Of mm Kamacite bandwidth (IID, IIICD, IIIF, IVA) Finest octahedrites Off < 0.2 mm Kamacite bandwidth (IIC, IIICD) Plessitic Opl < 0.2 mm Kamacite bandwidth, spindles (IIC, IIF) Ataxite D > 16.0 % Ni (IIF, IVB) Table II: List of classes, groupes and subgroups of meteorites 9 9 Kleinschrot; 2003; p. 19 and Norton; 2002; p. 307f.

21 Pallasites Their origin and composition 21 C Petrologic type Criterion Homogenity of olivine and pyroxene compositions Structural state of low-ca pyroxene Feldspar - Chondrule glass - Metal: max. bulk Ni, wt% Sulfides: mean Ni content Matrix Chondrule-matrix integration Petrologic type All finegrained, opaque No chondrules > 5 % mean deviation Predominantly monoclinic Minor primary grains only Altered, mostly absent a < 20 %; taenite minor or absent > 0.5 wt% Mostly fine, opaque Clear, isotropic, variable abundance < 5 % homogeneous > 20 % monoclinic Secondary, < 2 µm grains < 20 % monoclinic Secondary, 2 50 µm grains Devitrified, absent Orthorhombic Secondary, > 50 µm grain Kamacite and taenite in exsolution relationship > 20 % Clastic and minor opaque Chondrules very sharply defined Chondrules well defined < 0.5 wt% Transparent, recrystallized, coarsing from 4 to 6 Chonrules readily delineated 0.8 Carbon, wt% 3 5 < Water, wt% < 1.5 Table III: Petrologic type after Van Schmus and Wood (1967), with modifications from Sears and Dodd (1988) and Brearley and Jones (1998) a Chondrule glass is rare in CM2 chondrites, but is preserved in many CR2 chondrites. Chondrules poorly defined

22 22 Boeck, Lena D Shock stage Shock stage unshocked S1 very weakly shocked S2 weakly shocked S3 moderately shocked S4 strongly shocked S5 very strongly shocked S6 Shock melting Effects from equilibration peak shock pressure Orthopyroxene Olivine Plagioclase Sharp optical extinction, irregular fractures Undulatory extinction, irregular fractures Planar fractures, undulatory extinction, irregular fractures Mosaicism (weak), planar fractures Mosaicism (strong), planar fractures + planar deformation features Undulatory extinction Undulatory extinction, partically isotropic, planar deformation features Maskelynite Restricted to local regions in or near melt zones Solid state recrystallization shock melted and staining, (normal glass) ringwoodite, melting Undulatory extinction, irregular and some planar fractures Clinoenstatite lamellae on (100), undulatory extinction, planar and irregular fractures Majorite, melting Whole-rock melting (impact melt rocks and melt breccias) Shock pressure GPa b < Shock effects in ordinary chondrites are characterized by effects in olivine and plagioclase; shock level in carbonaceous chondrites are characterized by effects mostly in olivine (Scott et al.,1992); shock levels in enstatie chondrites are characterized by effects mostly in orthopyroxene. The prime shock criteria for each shock stage are underscored. 10 b Shock pressure can be used for ordinary chondrites only. 10 Davis, 2004, p. 93

23 Pallasites Their origin and composition 23 E Grade of weathering WO no visible oxidation of metal or sulphide usually fresh falls W1 minor oxidation rims aroud metal and troilite same fresh falls are already in this stage minor oxidation veins W2 moderate oxidation of metal % of metal grains are affected 5,000 15,000 a W3 heavy oxidation of metal and troilite % replacement 15,000 30,000 a W4 complete (>95 %) oxidation of metal and troilite 20,000 35,000 a no silicate alteration W5 beginning alteration of mafic silicates (mainly along cracks) 30,000 - >45,000 a W6 massive replacement of silicates by clay minerals and oxides terrestrial ages correlated with meteorites found in Roosevelt county (New Mexico) and caused by that only useable for meteorites found in regions with similar climate conditions Wlotzka, 1993, p. 460

24 24 F Boeck, Lena Figures Figure 8: A piece of the Springwater Pallasite from the collection of the UH, University of Hawaii in Manoa, Honolulu (picture by Lena Boeck)

25 Pallasites Their origin and composition 25 Figure 9: Angular olivine crystals in the Eagle station pallasite. (a) Scale bar: 5 mm (b) and (c) enlarged areas marked in (a), reflected light microscope picture, Scale bar: 200 µm. Olivine (O) dark grey, plessite (P) light grey, Kamacite - white Scott, 1977b, p. 698

26 26 Boeck, Lena a) b) c) Figure 10: Logarithmic plots of different pallasites. Pallasitic fields are compared to iron meteorite fields. a) Au-Ni-plot, b) Ga-Ni-plot, c) Ge-Ni-plot Scott, 1977a, p. 352f.

27 Pallasites Their origin and composition 27 G References Web: (February 2009) Kleinschrot, Dorothée: Meteorite Steine, die vom Himmel fallen. Beringis Sonderheft 4, Würzburg: Krot, Alexander N.; Scott, Edward R. D.: Chondrites and the Protoplanetary Disk. Vol ASP Conference Series: Lauretta, Dante S.; McSween, Harry Y.: Meteorites and the early solar system II. The University of Arizona Press, Tucson and Lunar and planetary institute, Houston: Norton, O. Richard; Chitwood, Lawrence A.: Field Guide to Meteors and Meteorites. Patrick Moore s Practical Astronomy Series. Springer-Verlag, Lodon: Scott, Edward R. D.: Pallasites metal composition, classification and relationship with iron meteorites. Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977a. Scott, Edward R. D.: Formation of olivine-metal textures in pallasite meteorites. Geochimica et Cosmochimica Acta. Volume 41. Pergamon Press: 1977b. Wlotzka, F.: A weathering scale for the ordinary chondrites. Meteoritics 28: (abstract)