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Iodine overabundances measured in the surface layers of an antarctic stony and iron meteorite
Geochimica
et Cosmochimica
0016-7037/90/$3.00 + .I0
Acta Vol. 54, pp. 2503-2506
Copyright@ 1990Pefgamon Press pk. Printed in U.S.A.
Iodine overabundances measured in the surface layers of an Antarctic stony and iron meteorite K. G. HEUMANN,* J. NEUBAUER,and W. REIFENH~~USER Institut fdr Anorganische Chemie, UniversitltsstraDe 3 I, 8400 Regensburg, Federal Republic of Germany (Received November 13, 1989; acceptedin revisedform May 29, 1990)
Abstract-The surface layer and an interior sample of a large Antarctic H5 chrondite and of a IIIA iron meteorite were analysed with isotope dilution mass spectrometry for chlorine, bromine, and iodine. In addition, the evaporites of the stony meteorite and the surface layer of two different cracks in the iron meteorite were investigated. An evident iodine overabundance was found in the surface layer and in the evaporites of the chondrite specimen (enrichment factors of 11 and 6, respectively, compared with the iodine concentration in the interior). An even much higher iodine enrichment (factor 60-130) was observed in the corroded surface layers of the exterior and the two cracks of the iron meteorite. This result clearly confirms our previous hypothesis that an iodine concentration profile with depth has to be expected in Antarctic meteorites since we had also found it in Antarctic rocks. In some of the analysed meteorite surface layers low bromine enrichment could also be detected, but no significant chlorine enrichment was found. In the marine atmosphere near the Antarctic Peninsula an average methyl iodide concentration of 2.4 pptv was determined with gas chromatography using a halogen-sensitive electron capture detector. The corresponding surface sea water samples showed a mean of 2.6 rig/l of this biogenic iodine compound. The existence of methyl iodide in the Antarctic atmosphere confirms our previous hypothesis that this compound influences the iodine content of Antarctic meteorites and rocks. INTRODUCTION
Our two different specimens of this meteorite were one surface sample of 0.47 16 g and one interior sample of 0.4452 g from a depth of about 5 cm. An evaporite layer of about 2 mm covered the underlying surface chip (approximately 3 mm thickness), which was mechanically separated.
IN 1985 DREIBUS ET AL. REPORTED a mysterious iodine overabundance in Antarctic meteorites. We could confirm these results analysing different specimens of Antarctic eucrites, high-iron and low-iron chondrites ( HEUMANNet al., 1987). A similarly high enrichment effect was not found for chlorine and bromine. By analysing different types of Antarctic rocks, a significant decrease of the iodine concentration was determined from the surface to the interior of the rocks. We interpreted from this observation that atmospheric iodine interacts with the surface of Antarctic rocks leading to iodine overabundances in the surface layers. We postulated that methyl iodide is the probable atmospheric compound which, after photolytic decomposition, is responsible for the iodine overabundances in Antarctic rocks and meteorites. The two most essential topics in proof of this previous hypothesis were the investigation of the iodine concentration at different depths of an Antarctic meteorite and the evidence of methyl iodide in the south polar atmosphere. An interesting question also was whether similar iodine effects can occur for iron meteorites
Iron Meteorite The Mt. Wegener iron meteorite (group IIIA) was discovered in 1988 in the southeastern part of the Sheckleton Range on top of Mt. Wegener ( 80042’S, 23’35’W) during a German expedition. The exterior of the meteorite is corroded. The exposure age was determined to be about 650 X lo6 a, and the upper limit for the terrestrial age was determined to be 4 15 X lo3 a. A more detailed description of this meteorite is given by SCHULTZet al. ( 1989). A 2.054 g portion of this meteorite specimen with a total weight of 3480 g was analysed. This portion was divided into four sections, as shown in Fig. 1. As a result a partly corroded layer of the exterior with a thickness of 2-4 mm, two corroded surface layers (about 2 mm) of two different cracks, and the uncorroded interior of the meteorite specimen have been analysed.
as well.
SAMPLE DESCRIWION Stony Meteorite
Atmospheric Samples
The meteorite specimen LEW 85320 with complete fusion crust and a few cracks (total weight 110.224 kg) is an H5 chondrite and was found during the 1985 186 ANSMET expedition on the Lewis Cliff Ice Tongue. A more detailed description of this meteorite is given elsewhere (SCOREet al., 1986).
Methyl iodide was analysed in the marine atmosphere near the Antarctic Peninsula aboard the German polar research ship FS Polarstern during the expedition ANT VI / 2 from the beginning of November until the middle of December 1987. The sampling positions lay between 60-67”s and 5469”W. The samples were collected by sucking the air from the ship’s bow through a 9 m long V4A steel pipeline directly
* To whom correspondence should be addressed. 2503
K. G. Heumann, J. Neubauer. and W. Reifenhsuser
2504
Table 1. Halogen concentrations in different layers of the H5 chondrite LEW 85320, enrichment factors (related to the iodine concentration in the interiorof the mateorite) and comparison with non-Antarctic chondrites and an Antarctic lamprophyre
uncorroded
Sample
crack
surface
LEW 85320 Evaporrtes
Concentration I Br
o-o.2
0.62 +.02 1.12 +.02 0.10 i.01
0.2-0.5
Underlying chip Interior
exterior (section
Depth [cm]
5
0.48 +.oz 2.58 r.05 <0.3
[ug/gl Cl
133 217 152 t 4 80
6.2
>1.6
1.7
11.2
>8.5
1.9
1
1
1
f4
Non-Antarctic chandrites '
total
0.040.11
0.18680.59 180
Antarctic lamprophyre'
o-1.5
0.49 t.02
0.31 t.02
LDREIBUS ~HE~MANN
Enrichment factor I l3r Cl
764 i20
-
-
-
7.0
1.6
1.1
et al. (1979) et al. (1987)
1I
FIG, 1. Analysed sections of a fragment of the iron meteorite Mt. Wegener.
RESULTS AND DISCUSSION Halogen Concentrations
into an adsorption system. Contamination by the ship is no problem in the case of the dete~ination of the biogenic compound methyl iodide. Surface sea water samples at the same geographic positions were taken for comparison. EXPERIMENTAL Halogen Analyses in Meteorites The halogen concentrations were analysed with isotope dilution mass s~ctromet~ (IDES) using the formation of negative thermal ions for isotope ratio m~suremen~. Detailed info~ation about the production of negative thermal halide ions is given elsewhere ( HEUMANN, 1988; HELJMANNet al., 1985). The analytical procedure for
the IDMS analyses of halides in silicate rocks and stony meteorites was previously published ( HEUMANNand WEISS, 1986; HEUMANN et al., 1985, 1987). The decomposition of the iron meteorite was carried out similar to that of stony meteorites with hydrofluoric acid under pressure in a Teflon vessel. The only difference was the decomposition time which was about 8 h for the stony meteorite and approximately 30 h for the iron meteorite.
Methyl Iodide Analyses with GC/ECD The measurements of methyl iodide were carried out with a gas chromatograph, type GC-9 APE (Shimadzu), using a halogen-sensitive electron capture detector (ECD) and two coupled 50 m long capillary columns (inner diameter 0.32 mm), type SE 54 (film thickness of stationary phase 5 am) and type OV 1701 (film thickness i pm). The temperature program was as follows: starting temperature 20°C L%~T = 3”C/min, tinal temperature 180°C. The samples were collected by pumping air through a cooled ( -44OC) adsorption tube which was filled with Tenax GC. First, the air was dried by passing it through another tube filled with magnesium perchlorate. After sampling, the adsorption tube was heated up to 220°C for desorption of the compounds. These compounds were then condensed in a capillary cold trap cooled with liquid nitrogen before introduction into the GC. Such a sampling process is normally used for the gas chromatographic determination of volatife halogen compounds in the air (BAL~CH~ITER et al., 1986; BAUMANN and HEUMANN, 1987). Methyl iodide was ejected from the sea water by a He gas flow. After drying and cryotrapping, the sample was analysed in the GC/ECD system. A detailed description of the analytical procedure is given elsewhere ( REIFENHAUSER and HEUMANN, 1990).
in a Stony and an Iron Meteorite
The halogen results of the two different Antarctic meteorites are summarized in Tables 1 and 2. In cases where an
error in the analytical result is given for the Antarctic meteorite samples, this is not the standard deviation but the deviation from the mean of two independent analyses. Due to the small sample amounts available, only one or two independent halogen determinations could be carried out. The results clearly show an iodine enrichment in the surface layers of the Antarctic meteorites. The interior of the H5 chrondrite LEW 85320 shows, for all halogens, concentrations which are within the known range of these elements for non-Antarctic chrondrites ( DREIBUS et al., 1979; see Table I). Recently, SHINONAGA et al. ( 1989) found a significant decrease of iodine concentration from the surface to the interior of a L6 Antarctic chrondrite (ALHA 7723 1) by radiochemical neutron activation analysis. In the chrondrite LEW 85320 we found the highest iodine enrichment in the surface chip which lay directly under the evaporites. The evaporites were still enriched in iodine compared with the interior of the meteorite, but the enrichment factor was a little lower than for the underlying chip. This can be attributed to different leaching effects. It was found that the evaporitic products of Antarctic meteorites predominantly consist of magnesium and calcium salts (VELBEL, 1988). JULL et al. ( 1988) have shown that one weathering product on the surface of the specimen LEW 85320 is nes-
Table
2. Halogen concentrations in the differentanalysed sections of the Mt. Wegener iron meteoriteand enrxhment factors
(related to the lodine cuncentratlon interior of the meteorite)
Sample
No? Depth lcml
Exterior
3
o-o.4
Crack
2
o-o.2
Crack Interior ~
3 4
O-D.2 0.4-Z
Concentration Br I 2.9 2.9 6.4 +_6 3.1 0.05
1 Section No. in Figure 1
[lg/g] Cl
Enrichment I Br
in
the
factor Cl
0.12
124.6
58
>1.2
0.7
0.65
70.3
128
>6.5
0.4
0.61 (0.1
404.7 182.5
>6.1
2.2 1
62 11
Overabundance of iodine in Antarctic meteorites quehonite, a hydrous magnesium carbonate. If the highly soluble magnesium or calcium iodide salts are formed in the evaporitic products, then these salts could be easily leached by water. HEUMANN et al. ( 1987) also found an evident iodine enrichment only in the first surface layers (down to 1.5 cm) of different Antarctic rocks (rhyolite, granite, quartz, lamprophyre). From the mineralogical point of view an H chondrite can be compared best with a lamprophyre. It is therefore interesting to notice that the iodine enrichment factor of 7.0 for the surface layer of the lamprophyre is in the same range as for the evaporites (factor 6.2) and the underlying chip (factor 11.2) of our Antarctic HS chondrite (see Table 1). This suggests a similar absorption mechanism for iodine by the surface layers of these mineralogical comparable samples. Consequently, the other above-mentioned Antarctic rocks significantly showed other enrichment factors of 1.7,5.2, and 1.6, respectively. There also seems to be bromine enrichment in the surface layer of our Antarctic H5 chondrite. However, the bromine enrichment factor could not be determined exactly because the detection limit of our method was reached for this element in the case of the interior sample. Nevertheless, the bromine enrichment factor of the surface layer of the Antarctic lamprophyre and of the evaporites of the chondrite are comparable (see Table 1). No significantly high enrichment of chlorine could be observed in the evaporites and in the underlying chip of this ordinary chondrite. Very high iodine overabundances were found in the corroded exterior and the two different corroded crack surface layers of the iron meteorite “Mt. Wegener” compared with the uncorroded interior of this specimen. It is remarkable that the effect of an iodine overabundance, which had been detected previously only in Antarctic stony meteorites, could also be observed for an Antarctic iron meteorite. Because of the detection limit for the bromine analysis, the enrichment factors of this element could not be exactly determined. However, these enrichment factors are evidently lower than those found for iodine. Again, no significant trend was observed for chlorine. The remarkably high iodine enrichment factors in the range of 60- 130 (see Table 2 )-compared with the corresponding factors for the H5 chondrite-of the corroded exterior and crack surface layers of the iron meteorite must be due to different absorption effects of iodine in the different surface compounds of both types of meteorites. It is known that the corrosion products of the Mt. Wegener iron meteorite are akaganeite, maghemite, and geothite ( BUCHWALD and CLARKE, 1989; SCHULTZ et al., 1989). We therefore must assume that these basic corrosion products are able to preferably absorb iodine from the Antarctic environment. Sources of Iodine Overabundances
Even if our investigations have clearly shown that there is a decreasing concentration profile with depth, the following question still remains: what are the sources of the iodine overabundances in the surface layers of Antarctic meteorites? VELBEL( 1988) discussed three different hypotheses for the element distribution in evaporites of Antarctic meteorites:
2505
1) The evaporite formation causes element depletion in the interior of the meteorite, which means that this element is leached in the interior and deposited in the evaporitic weathering products. Our results, obtained from the H5 chondrite LEW 85320, show that it is not the evaporites that contain the highest iodine overabundance but the underlying surface chip. This does not contradict the above given hypothesis, but it also does not strongly support this assumption. On the other hand, the iodine concentration of 0.1 Fg/g in the interior of the Antarctic chondrite agrees well with the highest levels measured in non-Antarctic chrondrites (see Table 1). This result contradicts a leaching effect for iodine. The extremly high iodine enrichment in the exterior and crack surface layers of the iron meteorite “Mt. Wegener” also contradicts this hypothesis, because the leaching effect in the interior of an iron meteoriteif there is one at all-should normally be lower compared with the corresponding effect in a stony meteorite. 2) The evaporite formation and low element concentrations in the interior are results of the pre-terrestrial history of the meteorite, e.g., the shock history. Our measurements of iodine overabundances in the surface layers of Antarctic rocks ( HEUMANNet al., 1987) contradict this assumption. 3) The element distribution of the evaporites is due to external contaminations and is unrelated to element depletions in the interior. We believe that external contamination by a reactive iodine compound is in all probability the major source of iodine overabundances in the surface layers, including the evaporitic weathering products of Antarctic meteorites and rocks. Methyl Iodide as a Contamination Compound In a previous paper ( HEUMANN et al., 1987), we stated that methyl iodide emitted from the polar sea is the most probable compound for external iodine contamination of Antarctic meteorites and rocks. Methyl iodide (CH31) has an atmospheric lifetime of a few days ( LOVELOCK, 1982) which permits the atmospheric transport from the polar sea into inner Antarctica. The existence of CH31 in the atmosphere is above all limited by photolysis. The reactive iodine radicals produced by this photolytic reaction can increase the iodine concentration in the surface layers of Antarctic meteorites and rocks. The contamination must be caused by such a reactive iodine compound and not by iodide or iodate, for example, which are still present in the Antarctic atmosphere by the sea spray effect. If sea spray iodide or iodate would affect the iodine concentration in Antarctic rocks and meteorites, a similar effect should be all the more measurable for chlorine, and this effect should be dependent on the distance from the ice edge. To prove the assumption that CH31 could be the compound responsible for iodine contamination of meteorites and Antarctic rocks, we carried out GC measurements of halogen hydrocarbons in the marine atmosphere near the Antarctic Peninsula. Besides the anthropogenic fluorochlorohydrocarbons (e.g., CC13F), we measured CH31 as an organic iodine compound in all samples of the Antarctic atmosphere. The analysed average as well as the range of conare listed in Table 3. Corresponding centrations
K. G. Heumann, J. Neubauer, and W. Reifenhluser
2506
Table 3. Methyl iodide analysed in the marine atmosphere and I” surface sea water with GCIECD in a region near the Antarctic Peninsula
Concentration Atmosphere sea-water
Average
[pptvl [tq/ll
’
2.4 2.6
‘Rel. standard deviation for single sample is 2-5% >Number of analysed samples
Range 0.6-7.9 0.2-7.5
Samples ’ 21 34
measurements of a
concentrations in the surface water of the polar sea are given for comparison. Our results clearly show that the polar sea is the source of biogenically produced CHsI. An average concentration of 2.4 pptv in the atmosphere near the Antarctic Peninsula confirms our previous hypothesis that methyl iodide can cause the iodine contaminations in Antarctic meteorites and rocks. The evidence of methyl iodide in the Antarctic atmosphere also confirms our first hypothesis of a geochemical iodine cycle in Antarctica, which was discussed in detail elsewhere (HEUMANNet al., 1987). Acknowledgmenls-This work was financially supported within the Schwerpunktprogramm Antarktisforschung by the Deutsche Forschungsgemeinschaft. We are grateful to the Alfred-Wegener-Institut hir Polar- und Meeresforschung, Bremerhaven, and to the crew of the polar research ship FS Polarstern for their continuous help. We thank the Meteorite Working Group of the Lunar and Planetary Institute in Houston for providing samples from the specimen LEW 85320; L. Schultz, Max-Planck-Institut in Maim, and P. Englert, San Jose State University, for highly cooperative work; and G. Dreibus, Max-Planck-Institut in Mainz, for helpful comments. Editorial handling: E. J. Olsen REFERENCES K., MAYER P., and CLASST. ( 1986) Chemistry of organic traces in air: IV. Analysis of C,- and Cz-halocarbons in ambient air by cold trap injection and wide bore glass capillary gas chromatography. Fresenius Z. Anal. Chem. 323, 334-339. BAUMANNH. and HEUMANNK. G. ( 1987) Analysis of organobromine compounds and HBr in motor car exhaust gases with a GC/
BALLSCHMITER
microwave plasma system. Fresenius Z. Anal. Chem. 327, 186192. BUCHWALDV. F. and CLARKE R. S. (1989) Corrosion of Fe-Ni alloys by Cl-containing akaganeite: The Antarctic meteorite case. Amer. Mineral. 79, 656. DREIBUSG., SPETTELB., and WANKE H. ( 1979) Halogens in meteorites and their primordial abundances. In Origin and Distribution ofthe Elements (ed. L. H. AHRENS), pp. 33-38, Pergamon Press. DREIBUSG., W,&NKEH., and SCHULTZL. ( 1985) Mysterious iodineoverabundances in Antarctic meteorites (abstr.) . Workshop on Antarctic Meteorites, Mainz, 1I - 13. HEUMANNK. G. ( 1988) Isotope dilution mass spectrometry. In Inorganic Mass Spectrometry (eds. F. ADAMSet al.), pp. 30 l-376, J. Wiley & Sons. HEUMANNK. G. and WEISSH. ( 1986) Mass spectrometric iodine trace determinations in geochemical standard reference materials and in meteorites. Fresenius Z. Anal. Chem. 323, 852-858. HEUMANNK. G., SCHINDLMEIER W., ZEININGERH., and SCHMIDT M. ( 1985) Application of an economical and small thermal ionization mass spectrometer for accurate anion trace analyses. Fresenius Z. Anal. Chem. 320,457-462. HEUMANNK. G., GALL M., and WEISS H. (1987) Geochemical investigations to explain iodine-overabundances in Antarctic meteorites. Geochim. Cosmochim. Acta S&2541-2547. JULL A. J. T., CHENC S., GILDING J. L., and VELBELM. A. ( 1988) Rapid growth of magnesium-carbonate weathering products in a stony meteorite from Antarctica. Science 242, 4 17-4 19. LOVEL~CK J. E. ( 1982) The production and fate of reduced volatile species. In Atmospheric Chemistry (ed. E. D. GOLDBERG), pp. 199-2 13. Springer-Verlag. REIFENH~USER W. and HEUMANNK. G. ( 1990) Gas chromatographic determination of methyl iodide in the Antarctic atmosphere and the south polar sea. Atmos. Environ. (submitted). SCHULTZL., SPETTEL B., WEBER H. W., HOFLEH. C., BUCHWALD V., BREMERR., HERPERS U., NEUBAUERJ., and HEUMANNK. G. ( 1989) Mt. Wegener, a new Antarctic iron meteorite (abstr.). Meteoritical Society Meeting, Vienna. SCORER., G~ODING J. L., and MASON B. ( 1986) Macroscopic and thin section description of LEW 85320. AntarcticMeteorite Newslett. 9, 22-23. SHINONAGA T., EBIHARA M., NAKAHARA H., KONDOHA., HONDA M., MIYAMOTOM., and KOJIMAH. ( 1989) Alteration of Antarctic meteorites and their anomalous abundances of halogens (abstr.). 14th Symp. Antarctic Meteorites, Tokyo. VELBELM. A. ( 1988) The distribution and significance of evaporitic weathering products on Antarctic meteorites. Meteoritics 23, 15 l159.
et Cosmochimica
0016-7037/90/$3.00 + .I0
Acta Vol. 54, pp. 2503-2506
Copyright@ 1990Pefgamon Press pk. Printed in U.S.A.
Iodine overabundances measured in the surface layers of an Antarctic stony and iron meteorite K. G. HEUMANN,* J. NEUBAUER,and W. REIFENH~~USER Institut fdr Anorganische Chemie, UniversitltsstraDe 3 I, 8400 Regensburg, Federal Republic of Germany (Received November 13, 1989; acceptedin revisedform May 29, 1990)
Abstract-The surface layer and an interior sample of a large Antarctic H5 chrondite and of a IIIA iron meteorite were analysed with isotope dilution mass spectrometry for chlorine, bromine, and iodine. In addition, the evaporites of the stony meteorite and the surface layer of two different cracks in the iron meteorite were investigated. An evident iodine overabundance was found in the surface layer and in the evaporites of the chondrite specimen (enrichment factors of 11 and 6, respectively, compared with the iodine concentration in the interior). An even much higher iodine enrichment (factor 60-130) was observed in the corroded surface layers of the exterior and the two cracks of the iron meteorite. This result clearly confirms our previous hypothesis that an iodine concentration profile with depth has to be expected in Antarctic meteorites since we had also found it in Antarctic rocks. In some of the analysed meteorite surface layers low bromine enrichment could also be detected, but no significant chlorine enrichment was found. In the marine atmosphere near the Antarctic Peninsula an average methyl iodide concentration of 2.4 pptv was determined with gas chromatography using a halogen-sensitive electron capture detector. The corresponding surface sea water samples showed a mean of 2.6 rig/l of this biogenic iodine compound. The existence of methyl iodide in the Antarctic atmosphere confirms our previous hypothesis that this compound influences the iodine content of Antarctic meteorites and rocks. INTRODUCTION
Our two different specimens of this meteorite were one surface sample of 0.47 16 g and one interior sample of 0.4452 g from a depth of about 5 cm. An evaporite layer of about 2 mm covered the underlying surface chip (approximately 3 mm thickness), which was mechanically separated.
IN 1985 DREIBUS ET AL. REPORTED a mysterious iodine overabundance in Antarctic meteorites. We could confirm these results analysing different specimens of Antarctic eucrites, high-iron and low-iron chondrites ( HEUMANNet al., 1987). A similarly high enrichment effect was not found for chlorine and bromine. By analysing different types of Antarctic rocks, a significant decrease of the iodine concentration was determined from the surface to the interior of the rocks. We interpreted from this observation that atmospheric iodine interacts with the surface of Antarctic rocks leading to iodine overabundances in the surface layers. We postulated that methyl iodide is the probable atmospheric compound which, after photolytic decomposition, is responsible for the iodine overabundances in Antarctic rocks and meteorites. The two most essential topics in proof of this previous hypothesis were the investigation of the iodine concentration at different depths of an Antarctic meteorite and the evidence of methyl iodide in the south polar atmosphere. An interesting question also was whether similar iodine effects can occur for iron meteorites
Iron Meteorite The Mt. Wegener iron meteorite (group IIIA) was discovered in 1988 in the southeastern part of the Sheckleton Range on top of Mt. Wegener ( 80042’S, 23’35’W) during a German expedition. The exterior of the meteorite is corroded. The exposure age was determined to be about 650 X lo6 a, and the upper limit for the terrestrial age was determined to be 4 15 X lo3 a. A more detailed description of this meteorite is given by SCHULTZet al. ( 1989). A 2.054 g portion of this meteorite specimen with a total weight of 3480 g was analysed. This portion was divided into four sections, as shown in Fig. 1. As a result a partly corroded layer of the exterior with a thickness of 2-4 mm, two corroded surface layers (about 2 mm) of two different cracks, and the uncorroded interior of the meteorite specimen have been analysed.
as well.
SAMPLE DESCRIWION Stony Meteorite
Atmospheric Samples
The meteorite specimen LEW 85320 with complete fusion crust and a few cracks (total weight 110.224 kg) is an H5 chondrite and was found during the 1985 186 ANSMET expedition on the Lewis Cliff Ice Tongue. A more detailed description of this meteorite is given elsewhere (SCOREet al., 1986).
Methyl iodide was analysed in the marine atmosphere near the Antarctic Peninsula aboard the German polar research ship FS Polarstern during the expedition ANT VI / 2 from the beginning of November until the middle of December 1987. The sampling positions lay between 60-67”s and 5469”W. The samples were collected by sucking the air from the ship’s bow through a 9 m long V4A steel pipeline directly
* To whom correspondence should be addressed. 2503
K. G. Heumann, J. Neubauer. and W. Reifenhsuser
2504
Table 1. Halogen concentrations in different layers of the H5 chondrite LEW 85320, enrichment factors (related to the iodine concentration in the interiorof the mateorite) and comparison with non-Antarctic chondrites and an Antarctic lamprophyre
uncorroded
Sample
crack
surface
LEW 85320 Evaporrtes
Concentration I Br
o-o.2
0.62 +.02 1.12 +.02 0.10 i.01
0.2-0.5
Underlying chip Interior
exterior (section
Depth [cm]
5
0.48 +.oz 2.58 r.05 <0.3
[ug/gl Cl
133 217 152 t 4 80
6.2
>1.6
1.7
11.2
>8.5
1.9
1
1
1
f4
Non-Antarctic chandrites '
total
0.040.11
0.18680.59 180
Antarctic lamprophyre'
o-1.5
0.49 t.02
0.31 t.02
LDREIBUS ~HE~MANN
Enrichment factor I l3r Cl
764 i20
-
-
-
7.0
1.6
1.1
et al. (1979) et al. (1987)
1I
FIG, 1. Analysed sections of a fragment of the iron meteorite Mt. Wegener.
RESULTS AND DISCUSSION Halogen Concentrations
into an adsorption system. Contamination by the ship is no problem in the case of the dete~ination of the biogenic compound methyl iodide. Surface sea water samples at the same geographic positions were taken for comparison. EXPERIMENTAL Halogen Analyses in Meteorites The halogen concentrations were analysed with isotope dilution mass s~ctromet~ (IDES) using the formation of negative thermal ions for isotope ratio m~suremen~. Detailed info~ation about the production of negative thermal halide ions is given elsewhere ( HEUMANN, 1988; HELJMANNet al., 1985). The analytical procedure for
the IDMS analyses of halides in silicate rocks and stony meteorites was previously published ( HEUMANNand WEISS, 1986; HEUMANN et al., 1985, 1987). The decomposition of the iron meteorite was carried out similar to that of stony meteorites with hydrofluoric acid under pressure in a Teflon vessel. The only difference was the decomposition time which was about 8 h for the stony meteorite and approximately 30 h for the iron meteorite.
Methyl Iodide Analyses with GC/ECD The measurements of methyl iodide were carried out with a gas chromatograph, type GC-9 APE (Shimadzu), using a halogen-sensitive electron capture detector (ECD) and two coupled 50 m long capillary columns (inner diameter 0.32 mm), type SE 54 (film thickness of stationary phase 5 am) and type OV 1701 (film thickness i pm). The temperature program was as follows: starting temperature 20°C L%~T = 3”C/min, tinal temperature 180°C. The samples were collected by pumping air through a cooled ( -44OC) adsorption tube which was filled with Tenax GC. First, the air was dried by passing it through another tube filled with magnesium perchlorate. After sampling, the adsorption tube was heated up to 220°C for desorption of the compounds. These compounds were then condensed in a capillary cold trap cooled with liquid nitrogen before introduction into the GC. Such a sampling process is normally used for the gas chromatographic determination of volatife halogen compounds in the air (BAL~CH~ITER et al., 1986; BAUMANN and HEUMANN, 1987). Methyl iodide was ejected from the sea water by a He gas flow. After drying and cryotrapping, the sample was analysed in the GC/ECD system. A detailed description of the analytical procedure is given elsewhere ( REIFENHAUSER and HEUMANN, 1990).
in a Stony and an Iron Meteorite
The halogen results of the two different Antarctic meteorites are summarized in Tables 1 and 2. In cases where an
error in the analytical result is given for the Antarctic meteorite samples, this is not the standard deviation but the deviation from the mean of two independent analyses. Due to the small sample amounts available, only one or two independent halogen determinations could be carried out. The results clearly show an iodine enrichment in the surface layers of the Antarctic meteorites. The interior of the H5 chrondrite LEW 85320 shows, for all halogens, concentrations which are within the known range of these elements for non-Antarctic chrondrites ( DREIBUS et al., 1979; see Table I). Recently, SHINONAGA et al. ( 1989) found a significant decrease of iodine concentration from the surface to the interior of a L6 Antarctic chrondrite (ALHA 7723 1) by radiochemical neutron activation analysis. In the chrondrite LEW 85320 we found the highest iodine enrichment in the surface chip which lay directly under the evaporites. The evaporites were still enriched in iodine compared with the interior of the meteorite, but the enrichment factor was a little lower than for the underlying chip. This can be attributed to different leaching effects. It was found that the evaporitic products of Antarctic meteorites predominantly consist of magnesium and calcium salts (VELBEL, 1988). JULL et al. ( 1988) have shown that one weathering product on the surface of the specimen LEW 85320 is nes-
Table
2. Halogen concentrations in the differentanalysed sections of the Mt. Wegener iron meteoriteand enrxhment factors
(related to the lodine cuncentratlon interior of the meteorite)
Sample
No? Depth lcml
Exterior
3
o-o.4
Crack
2
o-o.2
Crack Interior ~
3 4
O-D.2 0.4-Z
Concentration Br I 2.9 2.9 6.4 +_6 3.1 0.05
1 Section No. in Figure 1
[lg/g] Cl
Enrichment I Br
in
the
factor Cl
0.12
124.6
58
>1.2
0.7
0.65
70.3
128
>6.5
0.4
0.61 (0.1
404.7 182.5
>6.1
2.2 1
62 11
Overabundance of iodine in Antarctic meteorites quehonite, a hydrous magnesium carbonate. If the highly soluble magnesium or calcium iodide salts are formed in the evaporitic products, then these salts could be easily leached by water. HEUMANN et al. ( 1987) also found an evident iodine enrichment only in the first surface layers (down to 1.5 cm) of different Antarctic rocks (rhyolite, granite, quartz, lamprophyre). From the mineralogical point of view an H chondrite can be compared best with a lamprophyre. It is therefore interesting to notice that the iodine enrichment factor of 7.0 for the surface layer of the lamprophyre is in the same range as for the evaporites (factor 6.2) and the underlying chip (factor 11.2) of our Antarctic HS chondrite (see Table 1). This suggests a similar absorption mechanism for iodine by the surface layers of these mineralogical comparable samples. Consequently, the other above-mentioned Antarctic rocks significantly showed other enrichment factors of 1.7,5.2, and 1.6, respectively. There also seems to be bromine enrichment in the surface layer of our Antarctic H5 chondrite. However, the bromine enrichment factor could not be determined exactly because the detection limit of our method was reached for this element in the case of the interior sample. Nevertheless, the bromine enrichment factor of the surface layer of the Antarctic lamprophyre and of the evaporites of the chondrite are comparable (see Table 1). No significantly high enrichment of chlorine could be observed in the evaporites and in the underlying chip of this ordinary chondrite. Very high iodine overabundances were found in the corroded exterior and the two different corroded crack surface layers of the iron meteorite “Mt. Wegener” compared with the uncorroded interior of this specimen. It is remarkable that the effect of an iodine overabundance, which had been detected previously only in Antarctic stony meteorites, could also be observed for an Antarctic iron meteorite. Because of the detection limit for the bromine analysis, the enrichment factors of this element could not be exactly determined. However, these enrichment factors are evidently lower than those found for iodine. Again, no significant trend was observed for chlorine. The remarkably high iodine enrichment factors in the range of 60- 130 (see Table 2 )-compared with the corresponding factors for the H5 chondrite-of the corroded exterior and crack surface layers of the iron meteorite must be due to different absorption effects of iodine in the different surface compounds of both types of meteorites. It is known that the corrosion products of the Mt. Wegener iron meteorite are akaganeite, maghemite, and geothite ( BUCHWALD and CLARKE, 1989; SCHULTZ et al., 1989). We therefore must assume that these basic corrosion products are able to preferably absorb iodine from the Antarctic environment. Sources of Iodine Overabundances
Even if our investigations have clearly shown that there is a decreasing concentration profile with depth, the following question still remains: what are the sources of the iodine overabundances in the surface layers of Antarctic meteorites? VELBEL( 1988) discussed three different hypotheses for the element distribution in evaporites of Antarctic meteorites:
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1) The evaporite formation causes element depletion in the interior of the meteorite, which means that this element is leached in the interior and deposited in the evaporitic weathering products. Our results, obtained from the H5 chondrite LEW 85320, show that it is not the evaporites that contain the highest iodine overabundance but the underlying surface chip. This does not contradict the above given hypothesis, but it also does not strongly support this assumption. On the other hand, the iodine concentration of 0.1 Fg/g in the interior of the Antarctic chondrite agrees well with the highest levels measured in non-Antarctic chrondrites (see Table 1). This result contradicts a leaching effect for iodine. The extremly high iodine enrichment in the exterior and crack surface layers of the iron meteorite “Mt. Wegener” also contradicts this hypothesis, because the leaching effect in the interior of an iron meteoriteif there is one at all-should normally be lower compared with the corresponding effect in a stony meteorite. 2) The evaporite formation and low element concentrations in the interior are results of the pre-terrestrial history of the meteorite, e.g., the shock history. Our measurements of iodine overabundances in the surface layers of Antarctic rocks ( HEUMANNet al., 1987) contradict this assumption. 3) The element distribution of the evaporites is due to external contaminations and is unrelated to element depletions in the interior. We believe that external contamination by a reactive iodine compound is in all probability the major source of iodine overabundances in the surface layers, including the evaporitic weathering products of Antarctic meteorites and rocks. Methyl Iodide as a Contamination Compound In a previous paper ( HEUMANN et al., 1987), we stated that methyl iodide emitted from the polar sea is the most probable compound for external iodine contamination of Antarctic meteorites and rocks. Methyl iodide (CH31) has an atmospheric lifetime of a few days ( LOVELOCK, 1982) which permits the atmospheric transport from the polar sea into inner Antarctica. The existence of CH31 in the atmosphere is above all limited by photolysis. The reactive iodine radicals produced by this photolytic reaction can increase the iodine concentration in the surface layers of Antarctic meteorites and rocks. The contamination must be caused by such a reactive iodine compound and not by iodide or iodate, for example, which are still present in the Antarctic atmosphere by the sea spray effect. If sea spray iodide or iodate would affect the iodine concentration in Antarctic rocks and meteorites, a similar effect should be all the more measurable for chlorine, and this effect should be dependent on the distance from the ice edge. To prove the assumption that CH31 could be the compound responsible for iodine contamination of meteorites and Antarctic rocks, we carried out GC measurements of halogen hydrocarbons in the marine atmosphere near the Antarctic Peninsula. Besides the anthropogenic fluorochlorohydrocarbons (e.g., CC13F), we measured CH31 as an organic iodine compound in all samples of the Antarctic atmosphere. The analysed average as well as the range of conare listed in Table 3. Corresponding centrations
K. G. Heumann, J. Neubauer, and W. Reifenhluser
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Table 3. Methyl iodide analysed in the marine atmosphere and I” surface sea water with GCIECD in a region near the Antarctic Peninsula
Concentration Atmosphere sea-water
Average
[pptvl [tq/ll
’
2.4 2.6
‘Rel. standard deviation for single sample is 2-5% >Number of analysed samples
Range 0.6-7.9 0.2-7.5
Samples ’ 21 34
measurements of a
concentrations in the surface water of the polar sea are given for comparison. Our results clearly show that the polar sea is the source of biogenically produced CHsI. An average concentration of 2.4 pptv in the atmosphere near the Antarctic Peninsula confirms our previous hypothesis that methyl iodide can cause the iodine contaminations in Antarctic meteorites and rocks. The evidence of methyl iodide in the Antarctic atmosphere also confirms our first hypothesis of a geochemical iodine cycle in Antarctica, which was discussed in detail elsewhere (HEUMANNet al., 1987). Acknowledgmenls-This work was financially supported within the Schwerpunktprogramm Antarktisforschung by the Deutsche Forschungsgemeinschaft. We are grateful to the Alfred-Wegener-Institut hir Polar- und Meeresforschung, Bremerhaven, and to the crew of the polar research ship FS Polarstern for their continuous help. We thank the Meteorite Working Group of the Lunar and Planetary Institute in Houston for providing samples from the specimen LEW 85320; L. Schultz, Max-Planck-Institut in Maim, and P. Englert, San Jose State University, for highly cooperative work; and G. Dreibus, Max-Planck-Institut in Mainz, for helpful comments. Editorial handling: E. J. Olsen REFERENCES K., MAYER P., and CLASST. ( 1986) Chemistry of organic traces in air: IV. Analysis of C,- and Cz-halocarbons in ambient air by cold trap injection and wide bore glass capillary gas chromatography. Fresenius Z. Anal. Chem. 323, 334-339. BAUMANNH. and HEUMANNK. G. ( 1987) Analysis of organobromine compounds and HBr in motor car exhaust gases with a GC/
BALLSCHMITER
microwave plasma system. Fresenius Z. Anal. Chem. 327, 186192. BUCHWALDV. F. and CLARKE R. S. (1989) Corrosion of Fe-Ni alloys by Cl-containing akaganeite: The Antarctic meteorite case. Amer. Mineral. 79, 656. DREIBUSG., SPETTELB., and WANKE H. ( 1979) Halogens in meteorites and their primordial abundances. In Origin and Distribution ofthe Elements (ed. L. H. AHRENS), pp. 33-38, Pergamon Press. DREIBUSG., W,&NKEH., and SCHULTZL. ( 1985) Mysterious iodineoverabundances in Antarctic meteorites (abstr.) . Workshop on Antarctic Meteorites, Mainz, 1I - 13. HEUMANNK. G. ( 1988) Isotope dilution mass spectrometry. In Inorganic Mass Spectrometry (eds. F. ADAMSet al.), pp. 30 l-376, J. Wiley & Sons. HEUMANNK. G. and WEISSH. ( 1986) Mass spectrometric iodine trace determinations in geochemical standard reference materials and in meteorites. Fresenius Z. Anal. Chem. 323, 852-858. HEUMANNK. G., SCHINDLMEIER W., ZEININGERH., and SCHMIDT M. ( 1985) Application of an economical and small thermal ionization mass spectrometer for accurate anion trace analyses. Fresenius Z. Anal. Chem. 320,457-462. HEUMANNK. G., GALL M., and WEISS H. (1987) Geochemical investigations to explain iodine-overabundances in Antarctic meteorites. Geochim. Cosmochim. Acta S&2541-2547. JULL A. J. T., CHENC S., GILDING J. L., and VELBELM. A. ( 1988) Rapid growth of magnesium-carbonate weathering products in a stony meteorite from Antarctica. Science 242, 4 17-4 19. LOVEL~CK J. E. ( 1982) The production and fate of reduced volatile species. In Atmospheric Chemistry (ed. E. D. GOLDBERG), pp. 199-2 13. Springer-Verlag. REIFENH~USER W. and HEUMANNK. G. ( 1990) Gas chromatographic determination of methyl iodide in the Antarctic atmosphere and the south polar sea. Atmos. Environ. (submitted). SCHULTZL., SPETTEL B., WEBER H. W., HOFLEH. C., BUCHWALD V., BREMERR., HERPERS U., NEUBAUERJ., and HEUMANNK. G. ( 1989) Mt. Wegener, a new Antarctic iron meteorite (abstr.). Meteoritical Society Meeting, Vienna. SCORER., G~ODING J. L., and MASON B. ( 1986) Macroscopic and thin section description of LEW 85320. AntarcticMeteorite Newslett. 9, 22-23. SHINONAGA T., EBIHARA M., NAKAHARA H., KONDOHA., HONDA M., MIYAMOTOM., and KOJIMAH. ( 1989) Alteration of Antarctic meteorites and their anomalous abundances of halogens (abstr.). 14th Symp. Antarctic Meteorites, Tokyo. VELBELM. A. ( 1988) The distribution and significance of evaporitic weathering products on Antarctic meteorites. Meteoritics 23, 15 l159.