Nitrogen in IDPs and Renazzo

NanoSIMS measurements of nitrogen isotopic distributions in IDPs and Renazzo: uniform ¹⁵N enrichment in a chondritic IDP

Floss C. and Stadermann F. J. (2002) Lunar and Planetary Science XXXIII, Abstract #1350, Lunar and Planetary Institute, Houston (CD-ROM). This text as PDF File

Introduction

Interplanetary dust particles (IDPs) and primitive meteorites are well known to exhibit enrichments of deuterium and 15N [1], although the two types of isotopic anomalies are generally not directly related. The carrier phases of these anomalies are poorly characterized. Organic compounds are clearly the source of many of the D and 15N enrichments [2-4], but other carriers are also implicated [5-7]. Furthermore, it remains unclear whether or not the carriers are the same in both types of extraterrestrial materials. To address this question, we have been carrying out heating experiments on matrix material from Renazzo [8, 9], a CR chondrite which contains enrichments of both D and 15N [10]. These experiments will later be extended to IDPs and other primitive meteorites.

As a part of this effort, we are also using the high spatial resolution of the new NanoSIMS at Washington University [11] to survey and compare the microdistributions of D, C and N isotopic compositions in various IDPs and fragments of Renazzo matrix material. Here we report the first results of this phase of our work.

Experimental Procedures

We measured the C and N isotopic compositions of 15 IDPs and five Renazzo matrix fragments. The IDPs come from stratospheric collectors L2009, L2011 and L2036. Most of the IDPs have previously been measured for H isotopes in our ims 3f ion microprobe using standard procedures [12], and have delta-D SMOW values ranging between -512 permil and +3600 permil (unpubl. data). All samples are mounted on high-purity Au foil together with various isotopic standards.

The NanoSIMS measurements were made in a raster imaging mode in which a Cs+ beam (~100 nm in diameter) is scanned over the sample surface. Details of the procedure are given by [13]. The acquired secondary ion images ranged in size from 5 x 5 micrometer^2 to 35 x 35 micrometer^2, depending on the size of the particle being analyzed. In each measurement five different species (12C-, 13C-, 12C14N-, 12C15N- and 28Si-) were collected simultaneously at a mass resolution high enough to resolve interferences from neighboring peaks. Secondary electron images of the particles were also acquired during each analysis. Results are calibrated to isotopic standards measured along with the samples.

Each measurement was made as a series of 15 to 40 scans or layers which were subsequently added together to constitute a single image measurement. This procedure avoids potential data loss problems caused by the artificial limit of 216 counts/pixel set by the counting system of the NanoSIMS [13]. It has the additional advantage that observed isotopic 'hotspots' can be traced through the different layers to verify that their presence is not simply due to statistical variations.

Results and Discussion

Consistent with previous results [1, 10, 13] we find that both IDPs and Renazzo matrix fragments have essentially normal C isotopic compositions. Nitrogen isotopic compositions exhibit greater variability.

IDPs:
Among the IDPs measured, several contain fragments or discrete sub-grains that are enriched in 15N. The grains range in size from less than 0.5 micrometer to 3 micrometer and have delta-15N values between +230 permil and +600 permil, similar to earlier results [1, 13]. One IDP (L2036-c17-E2), nicknamed Eucken, which contains the most deuterium-depleted H isotopic composition recorded to date (delta-D SMOW = -512 permil) unfortunately did not contain enough C or N to make meaningful isotopic measurements. The most interesting result from the N isotopic measurements on IDPs comes from a particle labeled Kipling (L2011-R12).

Figure 1. Image (35 x 35 micrometer^2) of delta-15N in Kipling.

Kipling is a chondritic IDP, originally 12 micrometer in diameter. After crushing into the Au foil, its dimensions measured approximately 30 x 30 micrometer^2. In contrast to the very localized 15N enrichments noted in some of the IDPs discussed here, Kipling is dominated by heavy N. Figure 1 shows the N isotopic composition of Kipling in terms of delta-15N. The particle appears non-contiguous in the image because some portions have either been sputtered away or have very low CN- signals.

Much of the IDP is uniformly enriched in 15N, shown by the green areas in Fig. 1. An overall average of these regions has a delta-15N value of +510 permil. In contrast, the small region in the upper right portion of Fig. 1 shows two particles with normal N (average delta-15N = +23 permil). These grains appear to be some type of contaminant in the original BSE image of the area. Finally, two discrete sub-grains in the center of Kipling have strongly enriched 15N, with delta-15N values of +1090 permil and +1250 permil, respectively. The 'hotspots' can be traced through all 20 layers of the image. These are the highest 15N enrichments observed in IDPs to date. Both sub-grains, and the whole IDP, have normal C isotopic compositions.

Figure 2. Image (25 x 25 micrometer^2) of delta-15N in Renazzo.

Renazzo:
We measured five fragments of matrix material from Renazzo for comparison with the IDP data. In most of the fragments, N appears to be concentrated in discrete regions or sub-grains, in contrast to C and Si abundances which are more uniform over the fragments' areas. Many of these N-rich regions have light enrichments of 15N on the order of +100 permil to +400 permil, consistent with previous results [10]. One fragment also contains a grain with a distinctly larger 15N enrichment of +940 permil, similar to the highest delta-15N value measured by [10]. Finally, one matrix fragment contains two distinct regions with different N isotopic compositions (Fig. 2). Areas shown in light green in Fig. 2 have high N abundances and normal N isotopic compositions (average delta-15N = +20 permil). In contrast, a large region in the center of the fragment (darker green to yellow in Fig. 2) has lower N abundances and isotopically heavy N (average delta-15N = +230 permil). Preliminary examination of the fragment suggests compositional differences between the two areas, but detailed study will be required to determine their exact nature.

Origin of N Anomalies:
Deuterium enrichments in IDPs are generally attributed to low-temperature chemical processes occurring in the interstellar medium [14]. Similar processes have been invoked to account for the 15N enrichments in IDPs and primitive meteorites [e.g., 1] although much smaller fractionation effects are expected. Indeed, a recent study by [15], investigating the possibility of producing N isotopic fractionations from ion-molecule reactions involving various N-bearing species in interstellar clouds found that fractionations are generally small and cannot account for 15N enrichments on the order of 500 permil or more, such as those observed here in Kipling, and in other IDPs [13, 16]. Most recently, however, calculations of gas-grain fractionations suggest that much higher 15N enrichments may be obtained in dense molecular clouds where O/C-bearing molecules are removed from the gas [17]. Such a mechanism may account for the high d15N values observed in the Kipling 'hotspots' (Fig. 1).

Conclusions

In contrast to most 15N enrichments in IDPs which appear to occur as discrete 'hotspots', we find that the IDP Kipling is dominated by heavy N. It also contains two sub-grains with the highest 15N enrichments observed to date in an IDP. Renazzo matrix fragments appear to have smaller 15N enrichments than those in IDPs, but they seem to be more consistently enriched. Different carrier phases or more extensive parent body processing in Renazzo than in the IDPs may account for the differences. We are presently investigating the phases associated with the N enrichments in both types of materials.

References

[1] Messenger S. and Walker R. M. (1997) In Astrophys. Impl. Lab. Study Presolar Mat., 545. [2] Cronin J. R. et al. (1993) GCA 57, 4745. [3] Guan Y. et al. (1997) LPS XXVIII, 489. [4] Keller L. P. et al. (1997) LPS XXVIII, 707. [5] Deloule E. and Robert F. (1995) GCA 59, 4695. [6] Guan Y. and Zolensky M. E. (1997) LPS XXVIII, 487. [7] Russell S. S. et al. (1995) Met. 30, 399. [8] Floss C. et al. (2000) LPS XXXI, #1359. [9] Stadermann F. J. and Floss C. (2001) MAPS 36, A196. [10] Guan Y. (1997) Ph.D. Thesis, Washington Univ. [11] Stadermann F. J. et al. (1999) LPS XXX, #1407. [12] McKeegan K. D. (1987) Ph.D. Thesis, Washington Univ. [13] Stadermann F. J. (2001) LPSXXXII, #1792. [14] Sandford S. A. et al. (2001) MAPS 36, 1117. [15] Terzieva R. and Herbst E. (2000) Mon. Not. R. Astron. Soc. 317, 563. [16] Messenger S. et al. (1996) MAPS 31, A88. [17] Charnley S. B. and Rodgers S. D. (2001) LPS XXXII, #1437.

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NanoSIMS measurements of nitrogen isotopic distributions in IDPs and Renazzo: uniform ¹⁵N enrichment in a chondritic IDP Floss C. and Stadermann F. J. (2002) Lunar and Planetary Science XXXIII, Abstract #1350, Lunar and Planetary Institute, Houston (CD-ROM). This text as PDF File

Introduction	Interplanetary dust particles (IDPs) and primitive meteorites are well known to exhibit enrichments of deuterium and 15N [1], although the two types of isotopic anomalies are generally not directly related. The carrier phases of these anomalies are poorly characterized. Organic compounds are clearly the source of many of the D and 15N enrichments [2-4], but other carriers are also implicated [5-7]. Furthermore, it remains unclear whether or not the carriers are the same in both types of extraterrestrial materials. To address this question, we have been carrying out heating experiments on matrix material from Renazzo [8, 9], a CR chondrite which contains enrichments of both D and 15N [10]. These experiments will later be extended to IDPs and other primitive meteorites. As a part of this effort, we are also using the high spatial resolution of the new NanoSIMS at Washington University [11] to survey and compare the microdistributions of D, C and N isotopic compositions in various IDPs and fragments of Renazzo matrix material. Here we report the first results of this phase of our work.
Experimental Procedures	We measured the C and N isotopic compositions of 15 IDPs and five Renazzo matrix fragments. The IDPs come from stratospheric collectors L2009, L2011 and L2036. Most of the IDPs have previously been measured for H isotopes in our ims 3f ion microprobe using standard procedures [12], and have delta-D SMOW values ranging between -512 permil and +3600 permil (unpubl. data). All samples are mounted on high-purity Au foil together with various isotopic standards. The NanoSIMS measurements were made in a raster imaging mode in which a Cs+ beam (~100 nm in diameter) is scanned over the sample surface. Details of the procedure are given by [13]. The acquired secondary ion images ranged in size from 5 x 5 micrometer^2 to 35 x 35 micrometer^2, depending on the size of the particle being analyzed. In each measurement five different species (12C-, 13C-, 12C14N-, 12C15N- and 28Si-) were collected simultaneously at a mass resolution high enough to resolve interferences from neighboring peaks. Secondary electron images of the particles were also acquired during each analysis. Results are calibrated to isotopic standards measured along with the samples. Each measurement was made as a series of 15 to 40 scans or layers which were subsequently added together to constitute a single image measurement. This procedure avoids potential data loss problems caused by the artificial limit of 216 counts/pixel set by the counting system of the NanoSIMS [13]. It has the additional advantage that observed isotopic 'hotspots' can be traced through the different layers to verify that their presence is not simply due to statistical variations.
Results and Discussion	Consistent with previous results [1, 10, 13] we find that both IDPs and Renazzo matrix fragments have essentially normal C isotopic compositions. Nitrogen isotopic compositions exhibit greater variability. IDPs: Among the IDPs measured, several contain fragments or discrete sub-grains that are enriched in 15N. The grains range in size from less than 0.5 micrometer to 3 micrometer and have delta-15N values between +230 permil and +600 permil, similar to earlier results [1, 13]. One IDP (L2036-c17-E2), nicknamed Eucken, which contains the most deuterium-depleted H isotopic composition recorded to date (delta-D SMOW = -512 permil) unfortunately did not contain enough C or N to make meaningful isotopic measurements. The most interesting result from the N isotopic measurements on IDPs comes from a particle labeled Kipling (L2011-R12).
Figure 1. Image (35 x 35 micrometer^2) of delta-15N in Kipling.
	Kipling is a chondritic IDP, originally 12 micrometer in diameter. After crushing into the Au foil, its dimensions measured approximately 30 x 30 micrometer^2. In contrast to the very localized 15N enrichments noted in some of the IDPs discussed here, Kipling is dominated by heavy N. Figure 1 shows the N isotopic composition of Kipling in terms of delta-15N. The particle appears non-contiguous in the image because some portions have either been sputtered away or have very low CN- signals. Much of the IDP is uniformly enriched in 15N, shown by the green areas in Fig. 1. An overall average of these regions has a delta-15N value of +510 permil. In contrast, the small region in the upper right portion of Fig. 1 shows two particles with normal N (average delta-15N = +23 permil). These grains appear to be some type of contaminant in the original BSE image of the area. Finally, two discrete sub-grains in the center of Kipling have strongly enriched 15N, with delta-15N values of +1090 permil and +1250 permil, respectively. The 'hotspots' can be traced through all 20 layers of the image. These are the highest 15N enrichments observed in IDPs to date. Both sub-grains, and the whole IDP, have normal C isotopic compositions.
Figure 2. Image (25 x 25 micrometer^2) of delta-15N in Renazzo.
	Renazzo: We measured five fragments of matrix material from Renazzo for comparison with the IDP data. In most of the fragments, N appears to be concentrated in discrete regions or sub-grains, in contrast to C and Si abundances which are more uniform over the fragments' areas. Many of these N-rich regions have light enrichments of 15N on the order of +100 permil to +400 permil, consistent with previous results [10]. One fragment also contains a grain with a distinctly larger 15N enrichment of +940 permil, similar to the highest delta-15N value measured by [10]. Finally, one matrix fragment contains two distinct regions with different N isotopic compositions (Fig. 2). Areas shown in light green in Fig. 2 have high N abundances and normal N isotopic compositions (average delta-15N = +20 permil). In contrast, a large region in the center of the fragment (darker green to yellow in Fig. 2) has lower N abundances and isotopically heavy N (average delta-15N = +230 permil). Preliminary examination of the fragment suggests compositional differences between the two areas, but detailed study will be required to determine their exact nature. Origin of N Anomalies: Deuterium enrichments in IDPs are generally attributed to low-temperature chemical processes occurring in the interstellar medium [14]. Similar processes have been invoked to account for the 15N enrichments in IDPs and primitive meteorites [e.g., 1] although much smaller fractionation effects are expected. Indeed, a recent study by [15], investigating the possibility of producing N isotopic fractionations from ion-molecule reactions involving various N-bearing species in interstellar clouds found that fractionations are generally small and cannot account for 15N enrichments on the order of 500 permil or more, such as those observed here in Kipling, and in other IDPs [13, 16]. Most recently, however, calculations of gas-grain fractionations suggest that much higher 15N enrichments may be obtained in dense molecular clouds where O/C-bearing molecules are removed from the gas [17]. Such a mechanism may account for the high d15N values observed in the Kipling 'hotspots' (Fig. 1).
Conclusions	In contrast to most 15N enrichments in IDPs which appear to occur as discrete 'hotspots', we find that the IDP Kipling is dominated by heavy N. It also contains two sub-grains with the highest 15N enrichments observed to date in an IDP. Renazzo matrix fragments appear to have smaller 15N enrichments than those in IDPs, but they seem to be more consistently enriched. Different carrier phases or more extensive parent body processing in Renazzo than in the IDPs may account for the differences. We are presently investigating the phases associated with the N enrichments in both types of materials.
References	[1] Messenger S. and Walker R. M. (1997) In Astrophys. Impl. Lab. Study Presolar Mat., 545. [2] Cronin J. R. et al. (1993) GCA 57, 4745. [3] Guan Y. et al. (1997) LPS XXVIII, 489. [4] Keller L. P. et al. (1997) LPS XXVIII, 707. [5] Deloule E. and Robert F. (1995) GCA 59, 4695. [6] Guan Y. and Zolensky M. E. (1997) LPS XXVIII, 487. [7] Russell S. S. et al. (1995) Met. 30, 399. [8] Floss C. et al. (2000) LPS XXXI, #1359. [9] Stadermann F. J. and Floss C. (2001) MAPS 36, A196. [10] Guan Y. (1997) Ph.D. Thesis, Washington Univ. [11] Stadermann F. J. et al. (1999) LPS XXX, #1407. [12] McKeegan K. D. (1987) Ph.D. Thesis, Washington Univ. [13] Stadermann F. J. (2001) LPSXXXII, #1792. [14] Sandford S. A. et al. (2001) MAPS 36, 1117. [15] Terzieva R. and Herbst E. (2000) Mon. Not. R. Astron. Soc. 317, 563. [16] Messenger S. et al. (1996) MAPS 31, A88. [17] Charnley S. B. and Rodgers S. D. (2001) LPS XXXII, #1437.

	NanoSIMS Home Page

NanoSIMS measurements of nitrogen isotopic distributions in IDPs and Renazzo: uniform 15N enrichment in a chondritic IDP

Introduction

Experimental Procedures

Results and Discussion

Conclusions

References

NanoSIMS measurements of nitrogen isotopic distributions in IDPs and Renazzo: uniform ¹⁵N enrichment in a chondritic IDP