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Trace element partitioning in the Morasko meteorite from Poznan, Poland
ARTICLE IN PRESS
Chemie der Erde 66 (2006) 315–318 www.elsevier.de/chemer
SHORT NOTE
Trace element partitioning in the Morasko meteorite from Poznan, Poland W. Lueckea,, A. Muszynskib, Z. Bernerc a
Institut fu¨r Chemische Technik, University of Karlsruhe, Germany Institute of Geology, Adam Mickiewicz University, Makow Polnych 16, 61-606 Poznan, Poland c Institut fu¨r Mineralogie und Geochemie, University of Karlsruhe, Kaiserstrasse 12, 76 126 Karlsruhe, Germany b
Received 20 November 2005; accepted 3 January 2006
Abstract The nickel–iron meteorite of Morasko shows isolated inclusions of troilite in the bulk mass of a Fe,Ni-alloy. During a segregation of the FeS phase chalcophile trace elements were collected from the melt. The solidification of the Fe,Niphase occurred probably later, incorporating thereby mainly siderophile trace elements. To prove this general assumption selected trace elements were determined and reveal in the two phases of the present meteorite a characteristic distribution pattern. The meteorite of Morasko is in close conformity to the iron meteorite group IAB with rounded dark FeS inclusions. r 2006 Elsevier GmbH. All rights reserved. Keywords: Trace elements; Morasko meteorite
1. Introduction The iron meteorite of Morasko near Poznan was discovered in 1914. Detailed descriptions with respect to the geomorphology and geology of the impact site and comparisons with other similar meteorites were given recently (e.g. Czegka, 1996; Stankowski, 2001; Stankowski et al., 2002). It is assumed that the meteorite orginated by separation processes from a nearly oxygenfree Fe–Ni–S melt forming a small FeS phase with all the available sulfur (i.e. dark, coarse-grained troilite globules, very low in Ni) and a main metallic iron-rich phase, including the whole available Ni. These phases together with further trace minerals were examined mineralogically by electron microprobe and geochemiCorresponding author. Present address: Institute of Geology, Adam Mickiewicz University, Makow Polnych 16, 61-606 Poznan, Poland. E-mail addresses: [email protected] (W. Luecke), [email protected] (A. Muszynski), [email protected] (Z. Berner).
0009-2819/$ - see front matter r 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2006.01.005
cally using inductively coupled plasma mass spectrometry (ICP-MS) and isotope ratio mass spectrometry (IRMS) for sulfur. The aim of this study was to evaluate the distribution mode of the trace elements according to their chemical behavior in the two main phases of the meteorite.
2. Experimental About 50 g from the meteoritic material of the Poznan Geological Institute was carefully broken between Teflon slabs. The two phases (Fe,Ni-alloy matrix and troilite with its incorporated trace minerals) were separated by hand picking under a binocular. Two samples from the Fe,Ni-alloy (c ¼ 2:7460 and 2.6302 g, each in 50 ml) as well as one sample of the FeS phase (c ¼ 1:5097 g in 50 ml) were decomposed with hot nitric acid of sub-boiled quality. The solutions (including procedural blanks) were examined mass-spectrometrically for their trace element contents (Luecke et al., 2002). The analyses were carried out by means of a
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double focussing high-resolution ICP-MS (Axiom, VG Elemental) using external calibration and Be, Tm, and Bi as internal standards. Instrumental precision was better than 2% (1s) for most of the elements analyzed. Accuracy of the measurements on the digested samples was checked by the certified reference Material TMDW from High-purity Standards, USA, and was found to be better than 10% for all the reported elements. The analytical results of the present paper (Tables 1 and 2) are in a good agreement with the INAA data given by Choi et al. (1995) from other samples of the Morasko iron meteorite. This allows two conclusions: (1) Results are accurate within a few percent because of the similarity between the results yielded by the two different analytical methods (INAA and ICP-MS). (2) There is a high chemical homogeneity in the composition of the meteorite as the samples analyzed by Choi et al. (1995) and in this study are from two different occurrences of the Morasko meteorite field. The isotope composition of sulfur from the FeS phase was measured with an ‘‘Optima’’ isotope ratio mass spectrometer (Micromass, UK) coupled on-line with an element analyser. Results are reported in the usual d34S
Table 1. Trace elements (wt%) of the Morasko iron meteorite enriched in the Fe,Ni-alloy phase, opposite to data of lower concentration in the minor FeS phase Fe,Ni-alloy phase
FeS phase
Fe490%a Ni 6.5%
b
FeSo2%a Ni 0.22%
Ge 450 ppm Gab 110 ppm Znb 20–50 ppm Asb 15 ppm Mo 10–15 ppm W 4 ppm
4 4 4 4 4 4
Ge 30 ppm Ga 7 ppm Zn 4 ppm As 5 ppm Mo 7 ppm W 2 ppm
Pt 14 ppm Ru 11 ppm Rh 4 ppm Ir 2 ppm Pd 2 ppm La 420–460 ppb Ce 1600–1900 ppb Pr 40 ppb Nd 110 ppb Sm20 ppb Euodet.lim. Gd 60 ppb
4 4 4 4 4
Pt 2 ppm Ru 2 ppm Rh 3 ppm Ir 1 ppm Pd 1 ppm Laodet.lim.c Ceodet.lim. Prodet.lim. Ndodet.lim. Smodet.lim. Euodet.lim. Gdodet.lim.
a
Meteorite data by INAA from Choi et al. (1995) Ni 6.85% Ge 496 ppm Ga 104 ppm As 11 ppm W 1.7 ppm
Ir 1.1 ppm
Estimated by sample examination. In terrestrial material of rather chalcophile character. c REE concentrations in the FeS phase range below the detection limit of the analytical method applied. b
Table 2. Trace elements (wt%) enriched in the FeS phase (troilite), opposite to data of lower concentration in the Fe,Nialloy phase Fe,Ni-alloy phase
FeS phase
Cu 120–160 ppm o
Cu Cu 158 ppm 350–360 ppm Pb 10–20 ppm Ag 5 ppm Cra Cr 25 ppm 0.95–1.23%
Pb1 ppm Ag 2 ppm Cr 20–40 ppm a
o o 5
Morasko meteorite data by INAA from Choi et al. (1995)
In terrestrial material of lithophile character.
notation relative to the Vienna Canon Diablo Troilite international standard (V-CDT). The isotope measurements were calibrated against the internationally certified standards AGV1, AGV2, and AGV3. The reported value is the mean of three individual measurements from one sample of about 50 mg troilite. The precision of the method is better than 0.3%. For analytical details see Berner et al. (2002).
3. Results 3.1. Mineralogical investigations The meteorite is composed to about 98 wt% of an Fe,Ni-alloy and about 2 wt% of dark FeS nodules, up to 20 mm in diameter (Fig. 1). The latter forms the host of different trace minerals, which are likewise the carriers of many trace elements. The principal Fe–Ni minerals are kamacite with generally about 6% Ni (comp. Table 1) and taenite with up to 30% Ni. Both phases were not investigated separately. FeS occurs as troilite, being often wrapped by rounded flakes of graphite. The trace minerals in the troilite phase are represented by schreibersite, cohenite, sphalerite, and graphite (Dominik, 1976). Additionally, tiny grains of daubreelite, altaite, and silicate minerals were found, the latter being identified as the green Na-pyroxene cosmochlore (theoretically: NaCrSi2O6; Muszynski et al., 2002) and as plagioclase (Karwowski, unpublished data). Most of these trace minerals occur at the margins, but some within the troilite nodules.
3.2. Geochemical investigations The element contents in the metallic phase and in troilite are presented in Tables 1,2, and 3. Corresponding to their geochemical behaviour the trace elements show an enrichment or a decrease in concentration in one of these two phases. Because of the low oxygen
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Table 3. Trace element contents (wt%) in the FeS phase (right side, first block) and of silicates forming observed trace minerals in the FeS phase (right side, second block), opposite to data of lower concentration in the Ni,Fe-alloy phase Fe,Ni-alloy phase
Fig. 1. Photograph of a portion of the Morasko meteorite (Poznan, Poland) showing typically rounded, dark troilite inclusions (the slab is ca. 20 cm in maximum dimension).
fugacity in the extraterrestrial environment none of the elements were able to act lithophile. In the present case this is much evident for elements like Cr, C, and P. According to the geochemical affinity, which defines the siderophile or chalcophile character, trace elements are collected either by the iron–nickel or by the sulfidic melt (Jana and Walter, 1997). Those with typically chalcophile character, like Cu, Pb, and Ag are concentrated in troilite (Table 2). Because of the low oxygen partial pressure in the melt, Cr behaves as a chalcophile element under these conditions, forming the terrestrially unknown sulfide mineral daubreelite (FeCr2S4). However, the association of elements like Ti, V, Nb, Th (Table 3 right side, first block) with troilite is remarkable, because under terrestrial conditions (together with oxygen) they typically show a lithophile character. As documented by the electron microprobe analyses and supported also by ICP-MS data it can be assumed that traces of oxygen present in the sulfidic melt were used up to generate the above reported silicate minerals with the elements of Table 3 right side, second block and maybe even oxides of some elements of Table 3 right side, first block during the stage of troilite formation. The iron melt together with Ni (the present Fe,Nialloy phase) selectively collected the platinum group elements (mainly Pt and Ru, but also Rh, Ir, Pd) and further siderophile elements like Ge, Ga, and Mo (Table 1). Rare earth elements behave in presence of oxygen lithophile, under oxygen-free forming conditions this element group shows a siderophile character. Being trapped by the Fe,Ni-alloy the REE concentrations in the FeS phase range even below the detection limit of the analytical procedure applied here (det.lim.o20 ppb, comp. Table 1). As, Ga, and Zn geochemically acting generally as chalcophile elements, are enriched to some degree in the metallic phase, the latter forms, however, also tiny sphalerite [(Zn,Fe)S] inclusions in the troilite, too. The absence of oxygen in the Fe,Ni-melt ultimately forced P and C to behave siderophile as expressed by the
FeS phase
Ti 6 ppm V1 ppm Zr 0.4 ppm Y 40–70 ppb Nb 10–20 ppb Hf 10–20 ppb Sc 2–4 ppb Tho1 ppb
o o o o o o o o
Ti 50 ppm V 50–70 ppm Zr 0.7–1.0 ppm Y 100–200 ppb Nb 600–800 ppb Hf 40–50 ppb Sc 6–9 ppb Th 20 ppb
Na 90 ppm Rb 0.1–0.3 ppm Mg 12 ppm Ca 15–25 ppm Sr 5 ppm Ba 3 ppm Si 30–60 ppm
o o o o o o o
Na 130–160 ppm Rb 0.5–0.7 ppm Mg 14 ppm Ca 50 ppm Sr 8 ppm Ba 6 ppm Si 80–90 ppm
presence of trace amounts of minerals like schreibersite [(Fe,Ni)3P] and cohenite [(Fe,Ni)3C]. The isotopic composition of the sulfur in troilite is slightly enriched in 34S (d34S ¼ 0.78% vs. V-CDT), but still lies in the typical range of extraterrestrial material.
4. Geochemical typisation of the Morasko meteorite The major mineral phases of the Morasko meteorite show characteristic trace element patterns which may act as clues in deciphering the solidification history of the melt. The analytical data (Tables 1–3) suggest that the FeS globules solidified first, collecting and enriching trace elements with chalcophile character. During further cooling, the Fe,Ni-rich and sulfur-depleted melt collected siderophile elements, concentrating them in the Fe,Ni-alloy. Nevertheless, it cannot be excluded that the main phases of the Fe,Ni–FeS system were formed simultaneously by exsolution from a parent melt, during which specific trace elements were fractionated in a different way. However, it is difficult to explain why only FeS was formed during the chalcophile stage, despite the excess of Ni in the melt. Traces of Ni found in the troilite are only attributed to Ni-bearing schreibersite and cohenite. Using the wide diversity of data on iron meteorites (Wasson, 1970; Wasson et al., 1980; Kracher and Wasson, 1982; McCoy et al., 1996) a subdivision into 13 clusters as chemical groups (Mittlefehldt et al., 1998; Krot et al., 2004; Haack and McCoy, 2004) was
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established on the basis of their Ni, Ga, Ge, and Ir contents. The average element compositions of these groups and the chemical data presented in the latter three studies allow to ascribe the Morasko meteorite to group IAB (Kracher et al., 1980). For this iron group (the ‘‘irons’’) a great mineralogical and chemical information pattern (modes, mineralogies, mineral compositions, etc.) admits to achieve a five-fold classification scheme, too. According to this the Morasko meteorite may be classified close to the ‘‘IAB irons with rounded inclusions’’, well represented by e.g. the Odessa meteorite (Bunch et al., 1970).
5. Discussion The origin of the IAB group Fe-meteorites and their genetic conditions is controversially discussed in the literature (e.g. Mittlefehldt et al., 1998). The condensation of extraterrestrial matter to solid metallic or silicate phases from nebula is not consistent with exsolution processes and fractionations of the elements in the meteoric material. Similarly, molten pools generated by impact processes cannot adequately explain the formation of these irons. Because of their reduced dimension such pools must have been quickly quenched and therefore would not allow fractional crystallization and element fractionation, as it is documented by the chemistry of the Morasko meteorite. According to early genetic hypotheses the multitude of different meteoritic samples vagabonding through the space preferably were interpreted as result of fragments of bursting parental masses from asteroidal bodies. Obviously, in the course of their history this material must have been submitted to thermal processes which allowed element differentiation. Heating and subsequent cooling associated with partial melting and fractional crystallization may have generated zones of metallic (cores ?) and of basaltic (‘‘crust-like’’) composition. However, the thermal source, the circumstances of cooling and the cause of the bursting of the parent body into fragments are questions still open for debate. While heating and cooling the differentiation processes in the parent body need geological times. The possibly impact triggered bursting and the hurling of the resulted fragments back into the space is only a short event.
Acknowledgements This study was financially supported by the Stiftungsinitiative Johann Gottfried Herder under the patronage of the HRK and the DAAD, Bonn (Germany). The authors thank Mrs. Claudia Mo¨ssner (University of Karlsruhe) for her support in carrying out the ICP-MS analyses.
References Berner, Z.A., Stu¨ben, D., Leosson, M.A., Klinge, H., 2002. Sand O-isotopic character of dissolved sulphate in the cover rock aquifers of a Zechstein salt dome. Appl. Geochem. 17, 1515–1528. Bunch, T.E., Keil, K., Olsen, E., 1970. Mineralogy and petrology of silicate inclusions in iron meteorites. Contr. Mineral. Petrol. 25, 297–340. Choi, B., Ouyang, X., Wasson, J.T., 1995. Classification and origin of IAB and IIICD iron meteorites. Geochim. Cosmochim. Acta 59, 593–612. Czegka, W., 1996. Das holozaene Meteoritenkraterfeld von Morasko bei Posen (Polen). Aufschluss 47, 165–185. Dominik, B., 1976. Mineralogical and chemical study of coarse octahedrite Morasko (Poland). Prace Mineral. 47, 61. Haack, H., McCoy, T.J., 2004. Iron and stony-iron meteorites. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 1, Meteorites, Comets, and Planets by Davis, A.E. (Ed.), pp. 325–345 (Chapter 1.12). Jana, D., Walker, D., 1997. The influence of sulfur on partitioning of siderophile elements. Geochim. Cosmochim. Acta 61, 5255–5277. Kracher, A., Wasson, J.T., 1982. The role of S in the evolution of the parental cores of the iron meteorites. Geochim. Cosmochim. Acta 46, 2419–2426. Kracher, A., Willis, J., Wasson, J.T., 1980. Chemical classification of iron meteorites – IX. Geochim. Cosmochim. Acta 44, 773–787. Krot, A.N., Keil, K., Goodrich, C.A., Scott, E.R.D., Weisberg, M.K., 2004. Classification of meteorites. In: Holland, H.D., Turekian, K.K., (Eds.), Treatise on Geochemistry, vol. 1, Meteorites, Comets, and Planets by Davis, A.E. (Ed.), pp. 83–128 (Chapter 1.05). Luecke, W., Muszynski, A., Berner, Z., Moessner, C., 2002. The trace element patterns of kamacite and troilite in the Morasko meteorite from Poznan, Poland (poster presentation). In: Seventh Rio Symposium, Florianopolis, Brazil. McCoy, T.J., Ehlmann, A.J., Benedix, G.K., Keil, K., Wasson, J.T., 1996. The Lueders, Texas, IAB iron meteorite with silicate inclusion. Meteor. & Planet. Sci. 31, 419–422. Mittlefehldt, D.W., McCoy, T.J., Goodrich, C.A., Kracher, A., 1998. Non-chondritic meteorites from asteroidal bodies. In: Papike, J.J. (Ed.), Planetary Materials. Mineralogical Society of America: Reviews in Mineralogy, vol. 36, pp. 1–195 (Chapter 4). Muszynski, A., Stankowski, W., Dzierzanowski, L., Karwowski, L., 2002. New data about the Morasko Meteorite. Polish Mineral. Soc. Spec. Papers 18, 134–137. Stankowski, W., 2001. The geology and morphology of the natural reserve ‘‘Meteoryt Morasko’’. Planet. Space Sci. 49, 749–753. Stankowski, W., Muszynski, A., Klimm, K., Schliestedt, M., 2002. Mineralogy of Morasko meteorite and the structure of the craters. Proc. Estonian Acad. Sci. Geol. 51, 227–240. Wasson, J.T., 1970. The chemical classification of iron meteorites. IV. Irons with Ge concentrations greater than 190 ppm. Icarus 12, 407–423. Wasson, J.T., Willis, J., Wai, C.M., Kracher, A., 1980. Origin of iron meteorite Groups IAB and IIICD. Z. Naturforsch. 35a, 781–795.
Chemie der Erde 66 (2006) 315–318 www.elsevier.de/chemer
SHORT NOTE
Trace element partitioning in the Morasko meteorite from Poznan, Poland W. Lueckea,, A. Muszynskib, Z. Bernerc a
Institut fu¨r Chemische Technik, University of Karlsruhe, Germany Institute of Geology, Adam Mickiewicz University, Makow Polnych 16, 61-606 Poznan, Poland c Institut fu¨r Mineralogie und Geochemie, University of Karlsruhe, Kaiserstrasse 12, 76 126 Karlsruhe, Germany b
Received 20 November 2005; accepted 3 January 2006
Abstract The nickel–iron meteorite of Morasko shows isolated inclusions of troilite in the bulk mass of a Fe,Ni-alloy. During a segregation of the FeS phase chalcophile trace elements were collected from the melt. The solidification of the Fe,Niphase occurred probably later, incorporating thereby mainly siderophile trace elements. To prove this general assumption selected trace elements were determined and reveal in the two phases of the present meteorite a characteristic distribution pattern. The meteorite of Morasko is in close conformity to the iron meteorite group IAB with rounded dark FeS inclusions. r 2006 Elsevier GmbH. All rights reserved. Keywords: Trace elements; Morasko meteorite
1. Introduction The iron meteorite of Morasko near Poznan was discovered in 1914. Detailed descriptions with respect to the geomorphology and geology of the impact site and comparisons with other similar meteorites were given recently (e.g. Czegka, 1996; Stankowski, 2001; Stankowski et al., 2002). It is assumed that the meteorite orginated by separation processes from a nearly oxygenfree Fe–Ni–S melt forming a small FeS phase with all the available sulfur (i.e. dark, coarse-grained troilite globules, very low in Ni) and a main metallic iron-rich phase, including the whole available Ni. These phases together with further trace minerals were examined mineralogically by electron microprobe and geochemiCorresponding author. Present address: Institute of Geology, Adam Mickiewicz University, Makow Polnych 16, 61-606 Poznan, Poland. E-mail addresses: [email protected] (W. Luecke), [email protected] (A. Muszynski), [email protected] (Z. Berner).
0009-2819/$ - see front matter r 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2006.01.005
cally using inductively coupled plasma mass spectrometry (ICP-MS) and isotope ratio mass spectrometry (IRMS) for sulfur. The aim of this study was to evaluate the distribution mode of the trace elements according to their chemical behavior in the two main phases of the meteorite.
2. Experimental About 50 g from the meteoritic material of the Poznan Geological Institute was carefully broken between Teflon slabs. The two phases (Fe,Ni-alloy matrix and troilite with its incorporated trace minerals) were separated by hand picking under a binocular. Two samples from the Fe,Ni-alloy (c ¼ 2:7460 and 2.6302 g, each in 50 ml) as well as one sample of the FeS phase (c ¼ 1:5097 g in 50 ml) were decomposed with hot nitric acid of sub-boiled quality. The solutions (including procedural blanks) were examined mass-spectrometrically for their trace element contents (Luecke et al., 2002). The analyses were carried out by means of a
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double focussing high-resolution ICP-MS (Axiom, VG Elemental) using external calibration and Be, Tm, and Bi as internal standards. Instrumental precision was better than 2% (1s) for most of the elements analyzed. Accuracy of the measurements on the digested samples was checked by the certified reference Material TMDW from High-purity Standards, USA, and was found to be better than 10% for all the reported elements. The analytical results of the present paper (Tables 1 and 2) are in a good agreement with the INAA data given by Choi et al. (1995) from other samples of the Morasko iron meteorite. This allows two conclusions: (1) Results are accurate within a few percent because of the similarity between the results yielded by the two different analytical methods (INAA and ICP-MS). (2) There is a high chemical homogeneity in the composition of the meteorite as the samples analyzed by Choi et al. (1995) and in this study are from two different occurrences of the Morasko meteorite field. The isotope composition of sulfur from the FeS phase was measured with an ‘‘Optima’’ isotope ratio mass spectrometer (Micromass, UK) coupled on-line with an element analyser. Results are reported in the usual d34S
Table 1. Trace elements (wt%) of the Morasko iron meteorite enriched in the Fe,Ni-alloy phase, opposite to data of lower concentration in the minor FeS phase Fe,Ni-alloy phase
FeS phase
Fe490%a Ni 6.5%
b
FeSo2%a Ni 0.22%
Ge 450 ppm Gab 110 ppm Znb 20–50 ppm Asb 15 ppm Mo 10–15 ppm W 4 ppm
4 4 4 4 4 4
Ge 30 ppm Ga 7 ppm Zn 4 ppm As 5 ppm Mo 7 ppm W 2 ppm
Pt 14 ppm Ru 11 ppm Rh 4 ppm Ir 2 ppm Pd 2 ppm La 420–460 ppb Ce 1600–1900 ppb Pr 40 ppb Nd 110 ppb Sm20 ppb Euodet.lim. Gd 60 ppb
4 4 4 4 4
Pt 2 ppm Ru 2 ppm Rh 3 ppm Ir 1 ppm Pd 1 ppm Laodet.lim.c Ceodet.lim. Prodet.lim. Ndodet.lim. Smodet.lim. Euodet.lim. Gdodet.lim.
a
Meteorite data by INAA from Choi et al. (1995) Ni 6.85% Ge 496 ppm Ga 104 ppm As 11 ppm W 1.7 ppm
Ir 1.1 ppm
Estimated by sample examination. In terrestrial material of rather chalcophile character. c REE concentrations in the FeS phase range below the detection limit of the analytical method applied. b
Table 2. Trace elements (wt%) enriched in the FeS phase (troilite), opposite to data of lower concentration in the Fe,Nialloy phase Fe,Ni-alloy phase
FeS phase
Cu 120–160 ppm o
Cu Cu 158 ppm 350–360 ppm Pb 10–20 ppm Ag 5 ppm Cra Cr 25 ppm 0.95–1.23%
Pb1 ppm Ag 2 ppm Cr 20–40 ppm a
o o 5
Morasko meteorite data by INAA from Choi et al. (1995)
In terrestrial material of lithophile character.
notation relative to the Vienna Canon Diablo Troilite international standard (V-CDT). The isotope measurements were calibrated against the internationally certified standards AGV1, AGV2, and AGV3. The reported value is the mean of three individual measurements from one sample of about 50 mg troilite. The precision of the method is better than 0.3%. For analytical details see Berner et al. (2002).
3. Results 3.1. Mineralogical investigations The meteorite is composed to about 98 wt% of an Fe,Ni-alloy and about 2 wt% of dark FeS nodules, up to 20 mm in diameter (Fig. 1). The latter forms the host of different trace minerals, which are likewise the carriers of many trace elements. The principal Fe–Ni minerals are kamacite with generally about 6% Ni (comp. Table 1) and taenite with up to 30% Ni. Both phases were not investigated separately. FeS occurs as troilite, being often wrapped by rounded flakes of graphite. The trace minerals in the troilite phase are represented by schreibersite, cohenite, sphalerite, and graphite (Dominik, 1976). Additionally, tiny grains of daubreelite, altaite, and silicate minerals were found, the latter being identified as the green Na-pyroxene cosmochlore (theoretically: NaCrSi2O6; Muszynski et al., 2002) and as plagioclase (Karwowski, unpublished data). Most of these trace minerals occur at the margins, but some within the troilite nodules.
3.2. Geochemical investigations The element contents in the metallic phase and in troilite are presented in Tables 1,2, and 3. Corresponding to their geochemical behaviour the trace elements show an enrichment or a decrease in concentration in one of these two phases. Because of the low oxygen
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Table 3. Trace element contents (wt%) in the FeS phase (right side, first block) and of silicates forming observed trace minerals in the FeS phase (right side, second block), opposite to data of lower concentration in the Ni,Fe-alloy phase Fe,Ni-alloy phase
Fig. 1. Photograph of a portion of the Morasko meteorite (Poznan, Poland) showing typically rounded, dark troilite inclusions (the slab is ca. 20 cm in maximum dimension).
fugacity in the extraterrestrial environment none of the elements were able to act lithophile. In the present case this is much evident for elements like Cr, C, and P. According to the geochemical affinity, which defines the siderophile or chalcophile character, trace elements are collected either by the iron–nickel or by the sulfidic melt (Jana and Walter, 1997). Those with typically chalcophile character, like Cu, Pb, and Ag are concentrated in troilite (Table 2). Because of the low oxygen partial pressure in the melt, Cr behaves as a chalcophile element under these conditions, forming the terrestrially unknown sulfide mineral daubreelite (FeCr2S4). However, the association of elements like Ti, V, Nb, Th (Table 3 right side, first block) with troilite is remarkable, because under terrestrial conditions (together with oxygen) they typically show a lithophile character. As documented by the electron microprobe analyses and supported also by ICP-MS data it can be assumed that traces of oxygen present in the sulfidic melt were used up to generate the above reported silicate minerals with the elements of Table 3 right side, second block and maybe even oxides of some elements of Table 3 right side, first block during the stage of troilite formation. The iron melt together with Ni (the present Fe,Nialloy phase) selectively collected the platinum group elements (mainly Pt and Ru, but also Rh, Ir, Pd) and further siderophile elements like Ge, Ga, and Mo (Table 1). Rare earth elements behave in presence of oxygen lithophile, under oxygen-free forming conditions this element group shows a siderophile character. Being trapped by the Fe,Ni-alloy the REE concentrations in the FeS phase range even below the detection limit of the analytical procedure applied here (det.lim.o20 ppb, comp. Table 1). As, Ga, and Zn geochemically acting generally as chalcophile elements, are enriched to some degree in the metallic phase, the latter forms, however, also tiny sphalerite [(Zn,Fe)S] inclusions in the troilite, too. The absence of oxygen in the Fe,Ni-melt ultimately forced P and C to behave siderophile as expressed by the
FeS phase
Ti 6 ppm V1 ppm Zr 0.4 ppm Y 40–70 ppb Nb 10–20 ppb Hf 10–20 ppb Sc 2–4 ppb Tho1 ppb
o o o o o o o o
Ti 50 ppm V 50–70 ppm Zr 0.7–1.0 ppm Y 100–200 ppb Nb 600–800 ppb Hf 40–50 ppb Sc 6–9 ppb Th 20 ppb
Na 90 ppm Rb 0.1–0.3 ppm Mg 12 ppm Ca 15–25 ppm Sr 5 ppm Ba 3 ppm Si 30–60 ppm
o o o o o o o
Na 130–160 ppm Rb 0.5–0.7 ppm Mg 14 ppm Ca 50 ppm Sr 8 ppm Ba 6 ppm Si 80–90 ppm
presence of trace amounts of minerals like schreibersite [(Fe,Ni)3P] and cohenite [(Fe,Ni)3C]. The isotopic composition of the sulfur in troilite is slightly enriched in 34S (d34S ¼ 0.78% vs. V-CDT), but still lies in the typical range of extraterrestrial material.
4. Geochemical typisation of the Morasko meteorite The major mineral phases of the Morasko meteorite show characteristic trace element patterns which may act as clues in deciphering the solidification history of the melt. The analytical data (Tables 1–3) suggest that the FeS globules solidified first, collecting and enriching trace elements with chalcophile character. During further cooling, the Fe,Ni-rich and sulfur-depleted melt collected siderophile elements, concentrating them in the Fe,Ni-alloy. Nevertheless, it cannot be excluded that the main phases of the Fe,Ni–FeS system were formed simultaneously by exsolution from a parent melt, during which specific trace elements were fractionated in a different way. However, it is difficult to explain why only FeS was formed during the chalcophile stage, despite the excess of Ni in the melt. Traces of Ni found in the troilite are only attributed to Ni-bearing schreibersite and cohenite. Using the wide diversity of data on iron meteorites (Wasson, 1970; Wasson et al., 1980; Kracher and Wasson, 1982; McCoy et al., 1996) a subdivision into 13 clusters as chemical groups (Mittlefehldt et al., 1998; Krot et al., 2004; Haack and McCoy, 2004) was
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established on the basis of their Ni, Ga, Ge, and Ir contents. The average element compositions of these groups and the chemical data presented in the latter three studies allow to ascribe the Morasko meteorite to group IAB (Kracher et al., 1980). For this iron group (the ‘‘irons’’) a great mineralogical and chemical information pattern (modes, mineralogies, mineral compositions, etc.) admits to achieve a five-fold classification scheme, too. According to this the Morasko meteorite may be classified close to the ‘‘IAB irons with rounded inclusions’’, well represented by e.g. the Odessa meteorite (Bunch et al., 1970).
5. Discussion The origin of the IAB group Fe-meteorites and their genetic conditions is controversially discussed in the literature (e.g. Mittlefehldt et al., 1998). The condensation of extraterrestrial matter to solid metallic or silicate phases from nebula is not consistent with exsolution processes and fractionations of the elements in the meteoric material. Similarly, molten pools generated by impact processes cannot adequately explain the formation of these irons. Because of their reduced dimension such pools must have been quickly quenched and therefore would not allow fractional crystallization and element fractionation, as it is documented by the chemistry of the Morasko meteorite. According to early genetic hypotheses the multitude of different meteoritic samples vagabonding through the space preferably were interpreted as result of fragments of bursting parental masses from asteroidal bodies. Obviously, in the course of their history this material must have been submitted to thermal processes which allowed element differentiation. Heating and subsequent cooling associated with partial melting and fractional crystallization may have generated zones of metallic (cores ?) and of basaltic (‘‘crust-like’’) composition. However, the thermal source, the circumstances of cooling and the cause of the bursting of the parent body into fragments are questions still open for debate. While heating and cooling the differentiation processes in the parent body need geological times. The possibly impact triggered bursting and the hurling of the resulted fragments back into the space is only a short event.
Acknowledgements This study was financially supported by the Stiftungsinitiative Johann Gottfried Herder under the patronage of the HRK and the DAAD, Bonn (Germany). The authors thank Mrs. Claudia Mo¨ssner (University of Karlsruhe) for her support in carrying out the ICP-MS analyses.
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