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Signer, P.

Publications and source records attributed to Signer, P..

17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

On the Bur Gheluai H5 chondrite and other meteorites with complex exposure histories

Isotopic concentrations and track densities measured in 13 samples of the Bur Gheluai meteorite fall are presented. Experimental methods are described and results are presented for isotopic ratios of noble gases and cosmogenic radionuclide contents. Evidence for complex irradiation is discussed and a model for two-stage exposure histories is presented. The duration of each irradiation stage and possible effects on isotope production rates are considered. Explanations are suggested for the discrepant Ne production rates.

Vogt, S. K.↗
NASA NTRS1993

Solar noble gases revealed by closed system stepped etching of a metal separate from Fayetteville

Solar He, Ne, and Ar in a Fe-Ni separate from the chondrite Fayetteville are analyzed by closed system stepped oxidation. We report here data of the first 15 steps comprising 55 percent of the total solar gases. He-4/Ar-36 and Ne-20/Ar-36 are quite constant at values about 20 percent below those of present day solar wind (SWC). In this, Fe-Ni differs from lunar ilmenites where He-4/Ar-36 and Ne-20/Ar-36 in the first steps are several times below SWC. Thus, metal retains SW-noble gases even better than ilmenite, almost without element fractionation. Nevertheless, the isotopic composition of SW-He, -Ne, and -Ar in the first steps of the metal sample are identical to those found in a recently irradiated lunar ilmenite, indicating that ilmenites and chondritic metal both contain isotopically unfractionated SW noble gases. A preliminary analysis of a smaller Fayetteville metal separate shows Ne from solar energetic particles (SEP-Ne) with Ne-20/Ne-22 less than or equal to 11.5.

Murer, CH.↗
NASA NTRS1993

A long-term change of the AR/KR/XE fractionation in the solar corpuscular radiation

Solar noble gases in an ilmenite separate from breccia 79035 (antiquity greater than 1 Ga) were analyzed by closed system stepped etching (CSSE). All five gases show the familiar two-component structure: first solar-wind (SW) gases are released, followed by gases from solar energetic particles (SEP). Element patterns in 79035 are similar to those of 71501 ilmenite. SW-He-Ne were partly lost, but SEP-He-Ne-Ar are retained (nearly) unfractionated. Constant Ar/Kr/Xe ratios indicate that ilmenites contain an unfractionated sample of the heavy SW-SEP noble gases. Ar/Kr/Xe ratios in the solar corpuscular radiation are, however, different from 'solar system' values, whereby the Kr/Xe difference in 79035 is about twice as large as in 71501. We propose that Xe is less fractionated than Kr and Ar, though its first ionization potential (FIP) is higher than the 'cutoff' at approximately 11.5 eV, above which all elements in SEP are usually assumed to be depleted by a roughly constant factor. SW-Ne may be isotopically slightly heavier in the ancient SW trapped by 79035, as proposed earlier. In this work we extend our previous CSSE studies of solar noble gases including Kr and Xe to a lunar sample irradiated at least 1 Ga ago (breccia 79035, ilmenite separate, 42-64 microns). This sample was particularly gently etched in the first steps. Surprisingly, the first three steps, each releasing less than or equal to 0.5% of the total 36-Ar, showed an SEP-like trapped component plus relatively large concentrations of cosmogenic gases. Steps 4ff contain much less cosmogenic and more solar gas with a SW-like isotope pattern. Thus, a very minor easily etchable phase that has completely lost its SW-gases must be responsible for steps 1-3. We will not discuss these steps here and refer to the actual step 4 as the 'initial' etching step.

Wieler, R.↗
NASA NTRS1991

Long-term changes in composition of solar particles implanted in extraterrestrial materials

Analysis of lunar surface samples for elements implanted therein by solar corpuscular radiation reveals evidence for the following compositional changes over a time period between 1.5 and 3 Gyr: 50-percent decreases in the ratios He-4/Ar-36 and Xe/Ar-36; a 20-percent increase in the ratio He-3/He-4; a 3-percent increase in the ratio Ne-20/Ne-22; and a 50-percent increase in the ratio N-15/N-14. The causes of these changes are not resolved at this time but may include (1) a change in acceleration conditions of the solar wind, (2) a change in flux of solar energetic particles relative to that of the solar wind, and (3) a change in composition of the solar convective zone. There is good evidence for a long-term decrease in the solar-wind flux.

Kerridge, J. F.↗
NASA NTRS1991

Do meteorites contain irradiation records from exposure to an enhanced-activity sun?

The meteoritic evidence for a T Tauri phase in the sun's evolution is reviewed. Emphasis is given to effects recorded in meteoritic grains before final compaction of the meteorite parent body and the evidence that these precompaction irradiation effects do or do not require an active early sun. Several other effects attributed to such early activity are also reviewed.

Caffee, M. W.↗
NASA NTRS1990

A model for the production of cosmogenic nuclides in chondrites

A model is presented for calculating the production rates of cosmic-ray-produced He, Ne, and Ar as well as Be-10, Al-26, and Mn-53 in chondrites of variable size and shape. The predictions of this model are compared with published data for the meteorites ALH78084, St. Severin, and Keyes. The agreement is found to be about 5 percent for the concentrations of Be-10, Ne-21, Ne-22, Ar-38, and Mn-53, and to be better than 1 percent for the Ne-22/Ne-21 ratios. The correlation between P(Be-10)/P(Ne-21) and Ne-22/Ne-21 ratios is experimentally verified over a wide range of irradiation conditions.

Graf, TH.↗
NASA NTRS1986

SCR and GCR exposure ages of plagioclase grains from lunar soil

The concentrations of solar wind implanted Ar-36 in mineral grains extracted from lunar soils show that they were exposed to the solar wind on the lunar surface for an integrated time of 10E4 to 10E5 years. From the bulk soil 61501 plagioclase separates of 8 grain size ranges was prepared. The depletion of the implanted gases was achieved by etching aliquot samples of 4 grain sizes to various degrees. The experimental results pertinent to the present discussion are: The spallogenic Ne is, as in most plagioclases from lunar soils, affected by diffusive losses and of no use. The Ar-36 of solar wind origin amounts to (2030 + or - 100) x 10E-8 ccSTP/g in the 150 to 200 mm size fraction and shows that these grains were exposed to the solar wind for at least 10,000 years. The Ne-21/Ne-22 ratio of the spallogenic Ne is 0.75 + or - 0.01 and in very good agreement with the value of this ratio in a plagioclase separate from rock 76535. This rock has had a simple exposure history and its plagioclases have a chemical composition quite similar to those studied. In addition to the noble gases, the heavy particle tracks in an aliquot of the 150 to 200 mm plagioclase separate were investigated and found 92% of the grains to contain more than 10E8 tracks/sq cm. This corresponds to a mean track density of (5 + or - 1) x 10E8 tracks/sq cm. The exploration of the exposure history of the plagioclase separates from the soil 61501 do not contradict the model for the regolith dynamics but also fail to prove it.

Etique, P.↗
NASA NTRS1986

Nitrogen and noble gases in the 71501 bulk soil and ilmenite as records of the solar wind exposure: Which is correct?

The N determination in mg sized mineral separates from lunar soils by static mass spectrometry is an experimental break-through likely to contribute to the deciphering of the records left in the mineral grains by the exposure to the solar wind. In this discussion some comparisons of the results of N and noble gas analyses of the 71501 bulk soil and an ilmenite separate thereof are focussed on. Conclusions from noble gas data obtained on mineral separates from some 20 soils are summarized in a companion paper and are also discussed herein.

Signer, P.↗
NASA NTRS1986

Studies of light noble gases in mineral grains from lunar soils: A status report

Among the lunar soil constituents, monomineralic grains deserve special attention. Noble gases of carefully prepared mineral separates from lunar bulk soils were studied. The major results and conclusions of these investigations are summarized, in the context of both the regolith evolution and the history of the solar corpuscular radiation. With regard to the most abundant noble gas component in the regolith samples (the solar gases) the mineral grains have mainly two properties giving these particles among all soil constituents the best characteristics as sensors for solar gases, despite the fact, that the noble gas concentrations in a mineral separate are 10 to 60 times lower that those in a bulk sample of the same grain size. The first of these properties is the mineral dependent retentivity of the light gases He and Ne, the second property concerns the relatively short time during which a mineral grain acquires it solar gases. These two points are briefly discussed.

Wieler, R.↗
NASA NTRS1986

Cosmic ray records in Antarctic meteorites

The cosmogenic radionuclides Be(10), Al(26), and Mn(53) and noble gases were determined in more than 28 meteorites from Antarctica by nuclear analytical techniques and static mass spectrometry, respectively. The summarized results are listed. The concentrations of Al(26) and Mn(53) are normalized to the repective main target elements and given in dpm/kg Si sub eq and dpm/kg Fe. The errors stated include statistical as well as systematical errors. For noble gas concentrations estimated errors are 5% and for isotopic ratios 1.5%. Cosmic ray exposure ages T sub 21 were calculated by the noble gas concentrations and the terrestrial residence time (T) on the basis of the spallogenic nuclide Al(26). The suggested pairing of the LL6 chondrite RKPA 80238 and RKPA 80248 and the eucrites ALHA 76005 and ALHA 79017 is confirmed not only by the noble gas data but also by the concentrations of the spallation produced radionuclides. Futhermore, ALHA 80122, clasified as an H6 chondrite, has a noble gas pattern which suggest that this meteorite belongs to the ALHA 80111 shower.

Vogt, S.↗
NASA NTRS1986

Studies on cosmogenic nuclides in meteorites with regard to an application as potential depth indicators

Measurements of stable and radioactive spallation products in meteorites allow to investigate their histories, especially with respect to the exposure to galactic cosmic ray particles and the pre-atmospheric size of the object. While the concentrations of spallation products lead to the determination of exposure and terrestrial ages, production rate ratios are characteristic for the location of the sample in the meteorite. So, one of the aims of this investigation on meteorites is to obtain depth indicators from suitable pairs of cosmogenic nuclides. Because of the different depth profiles for nuclide productions it is necessary to determine the concentrations of a larger number of spallation products in aliquots of a single small sample. Such same sample measurements of Be-10 and light noble gases were performed on 15 ordinary chondrites (7 H- and 8 L-chondrites. Be-10 was determined by accelerator mass spectrometry and the noble gases were measured by static mass spectrometry. The results are summarized and discussed.

Sarafin, R.↗
NASA NTRS1986

Cosmogenic rare gases and 10-Be in a cross section of Knyahinya

The concentrations of cosmogenic nuclides were studied as a function of shielding on samples from a cross section of the 293 kg main fragment of the L5 chondrite Knyahinya. The stone broke into two nearly symmetrical parts upon its fall in 1866. The planar cross section has diameters between 40 and 55 cm. He, Ne, and Ar were measured on about 20 samples by mass spectrometry and the 10-Be activities on aliquots of 10 selected samples were determined by AMS. The 10-Be data are presented and the abundances of spallogenic nuclides are compared with the model calculations reported by Reedy for spherical L chondrites. The 10-Be production rates in Knyahinya are shown versus the shielding parameter 22-Ne/21-Ne.

Wieler, R.↗
NASA NTRS1973

He, Ne and Ar in chondritic Ni-Fe as irradiation hardness sensors.

Noble gas analyses of the Ni-Fe of 9 L, 5 H, and 2 LL chondrites quantitatively support previous suggestions of radiogenic He-4 recoil and He-3 deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic He-4 and in some cases for recoil loss of Ar-38 and gain of Ne-21. The ratio of spallogenic Ar-38 and Ne-21 in the metal phase is found to correlate strongly with He-3/Ne-21 and Ne-22/Ne-21 in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. 'Hardness indices' n equals 1.9-2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic Ne-21/Ar-38 ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic He-3/Ne-21 and Ne-21/Ar-38 in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses.

Nyquist, L.↗
NASA NTRS1972

Thermal release characteristics of spallogenic He, Ne, and Ar from the Carbo iron meteorite.

Analysis of the thermal release patterns of He, Ne, and Ar from samples of the Carbo iron meteorite, showing that virtually no fractionation of He 3, He 4, Ne 21, and Ar 38 occurs. Thus conclusions about iron meteorites based on measured noble gas ratios will be unaffected by gas loss, and measurement of these ratios cannot yield information about possible loss. Further, noble gas loss cannot explain the abnormal elemental and isotopic patterns observed in some iron meteorites, notably hexahedrites. The release of He and Ne was continuously observed throughout the stepwise heating sequence, and ratios of diffusion coefficients at two consecutive temperatures have been determined from measurements of the relative degassing rates at these temperatures at the time of temperature increase. Effective activation energies of 100 plus or minus 20 kcal/mol for both He 3 and Ne 21 are calculated from the diffusion coefficient ratios at temperatures above 1050 C. A sharp maximum at 750 C in both the effective activation energy and the gas release is correlated with the alpha, gamma-phase transition.

Nyquist, L. E.↗