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Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M'hira (L6) chondrite
Accepted Manuscript Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M'hira (L6) chondrite Ramakant R. Mahajan, Laridhi Ouazaa Nejia, Dwijesh Ray, Sekhar Naik PII:
S0032-0633(17)30148-4
DOI:
10.1016/j.pss.2017.12.018
Reference:
PSS 4452
To appear in:
Planetary and Space Science
Received Date: 3 May 2017 Revised Date:
28 December 2017
Accepted Date: 31 December 2017
Please cite this article as: Mahajan, R.R., Nejia, L.O., Ray, D., Naik, S., Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M'hira (L6) chondrite, Planetary and Space Science (2018), doi: 10.1016/j.pss.2017.12.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M’hira (L6) chondrite Ramakant R. Mahajan1*, Laridhi Ouazaa Nejia2 , Dwijesh Ray1 and Sekhar Naik1 Physical Research Laboratory, Ahmedabad, 380009, India.
2
Départment de Géologie, Faculté des Sciences de Tunis, Université Tunis El Manar,
Campus Universitaire, 2092 Tunis, Tunisia.
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*corresponding author email : [email protected]
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Abstract
The concentrations and isotopic composition of noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon( Xe) and nitrogen were measured in the Beni M’hira L6 chondrite. The cosmic ray exposure age of Beni M’hira is estimated of 15.6 ± 3.7 (Ma). The radiogenic age, of around 485 ± 64 Ma, derived from 4He, and of around 504 ± 51 Ma from
40
Ar,
suggests an age resetting indicating the event impact. The heavy noble gases (Ar, Kr and Xe)
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concentrations imply that the gas is a mixture of trapped component Q and solar wind. The measured nitrogen abundance of 0.74 ppm and the isotopic signature of δ15N = 14.6 ‰ are within the range of ordinary chondrites. The homogeneous chemical composition of olivine (Fa:26±0.25) and low-Ca pyroxene (Fs:22.4±0.29) suggest that the Beni M’hira meteorite is
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an equilibrated chondrite. This is further corroborated by strong chondrule-matrix textural integration (lack of chondrules, except a few relict clast). Shock metamorphism generally
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corresponds to S5 (>45 GPa), however, locally disequilibrium melting (shock-melt veins) suggests, that the peak shock metamorphism was at ~75 GPa, 950°C.
Keywords : Noble gases, ordinary chondrite, cosmic ray exposure age, mineralogy
Introduction Meteorites are objects predominantly derived from asteroids or comets, which are most likely remnants of the condensates formed during the formation of the solar system. Meteorites 1
ACCEPTED MANUSCRIPT provide insight into several issues related to solid matter formation and evolution, formed during early evolution stages of the solar system. Meteorites that have fallen on Earth are classified into various classes based upon their composition (e.g. Krot et al., 2014 for most recent updates). The most common meteorite class is ordinary chondrites which comprise ~80% of all meteorites found on Earth (Meteoritical Society Bulletin Database). Among
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them, the H, L and LL chondrites represent approximately 34, 38 and 8 % of all ordinary chondritic meteorites. The ordinary chondrites listed are 46026 (Meteoritical Society Bulletin accessed on 11 Apr 2016), out of which only 894 are observed falls. Meteorites found in hot and cold deserts show signs of weathering or contamination (Gooding, 1986; Schwenzer et
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al., 2007). Hence, it is important to recover and preserve the newly fallen meteorite immediately after the fall. However, the parent bodies of this most abundant class of meteorites are uncertain and largely unknown. Thus, the trapped noble gases, nitrogen and
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cosmic ray exposure ages are an important datum within this context.
Since noble gases are chemically inert, their rare abundance in solid matter formed in solar system makes them a sensitive tool in understanding the formation and evolution of material in the solar system. The noble gases could be modified as a result of diverse processes such as radiogenic decay products, cosmogenic effects, fissionogenic product, degassing, diffusion
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etc, which modify the initial isotopic composition of cosmic bodies in the solar system. The cosmic ray exposure age is the duration of a meteoroid from its ejection until it falls on Earth. Cosmic-ray exposure ages are derived using the abundance of a nuclear reaction producing stable noble gas isotopes which allows to empirically or theoretically measure the production
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rate (Dalcher et al. 2013). Noble gases have been measured in many meteorites since 1950. These measurements are used to determine trapped radiogenic and fissionogenic components,
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cosmic ray exposure ages and gas retention ages etc. The data base has been continuously expanded with further new measurements using new sophisticated instruments. The compilation of noble gas data by Schultz and Franke (2004) contains more than 6000 analyses for meteorites from all classes. There are many specific trapped gases, isotopic composition and elemental abundance patterns that are now known. If the mechanisms of noble gases trapping in meteorites and their origin were resolved, the information would impose many crucial constraints on the physiochemical conditions in the early solar nebula. Neon in meteorites is a complex mixture of several isotopically different trapped components, but these components are commonly masked in most meteorites by cosmic-ray component and also terrestrial trapped air. Therefore, it is always difficult to verify the 2
ACCEPTED MANUSCRIPT trapped components in bulk measurements. It is believed that xenon has heterogeneous isotopic composition with mixing of nucleosynthetic presolar components in proto-planetary disk (Marti and Mathew, 2015) which is incorporated in solar system bodies during their formation. Analyses of bulk meteorites in stepwise gas extraction experiments may provide evidence of
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many primitive noble gas components, but residues separation (obtained after acid dissolution) reveal the presence of a single component only (Huss and Lewis, 1994). Among the volatile elements, trapped nitrogen in meteorites is unique in showing large isotopic variations and results are rigorously discussed in various publications (Kothari and Goel,
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1974; Kung and Clayton, 1978; Sugiura and Hashizume, 1992; Hashizume and Sugiura, 1995, Maurette et al. 2000, Marty et al. 2003, Marty et al. 2005, Füri et al. 2012). However,
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the interpretation of this observation has proved to be difficult because of the presence of many components. The simultaneous measurements of stable isotopes of all five noble gases and nitrogen in same aliquot of sample are rare. Therefore, it seems worthwhile to take another look at noble gases and nitrogen measured simultaneously, to assess if they provide information on meteoritic parent bodies and nebular conditions. Often, nitrogen is correlated with noble gases, but the true extent of this correlation is hard to judge, because noble gases
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and nitrogen are rarely measured in the same sample.
The Beni M’hira meteorite is classified as an L6 chondrite (Russell et al. 2003, Meteoritical Society Bulletin no. 87) and fell on January 8, 2001 in southeastern Tunisia (Laridhi Ouazaa
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et al., 2004). Several pieces weighing more than 16 kg, were recovered from the field. Other fragments (192 in total), weighing 12558.95 g, were collected between 2012 and 2013 (Pelé et al., 2013). The specimen lacks well defined chondrules, indicating its association with
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increasing temperature and/or increasing time spent at higher temperature in a parent body. We studied the noble gases in the bulk sample of Beni M'hira. Our approach to determine exposure ages is to compare all the stable noble gas nuclides (He, Ne, Ar, Kr, Xe) using cosmic-ray production rates. This method has the advantage to eliminate the uncertainties which may arise due to various factors such as production rates, target element chemistry, noble gas measurement uncertainties and diffusive loss of lighter elements from rocks. Mineralogy of Beni M’hira also studied to see the correlation of shock effect from petrographic observations and noble gas impact history.
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ACCEPTED MANUSCRIPT Experimental details Noble gas isotopic and petrographic measurements were performed separately on Beni M’hira sample. Two aliquots were made, one for noble gas measurements and one for electron-probe micro analysis. The samples were observed under microscope and look fresh. However, the exact location of the samples in the main mass is not known. Both
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measurements on Beni M’hira sample were carried out at the Physical Research Laboratory (PRL), Ahmedabad, India. The isotopic studies of noble gas (all five noble gases) and nitrogen content were performed using a ‘Noblesse’ multi-collector noble gas mass spectrometer (Nu Instruments U. K.) at PRL, following the procedures described elsewhere
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(Mahajan, 2015; Mahajan et al., 2016; Mahajan, 2017). The sample weighting 409.36 mg was wrapped in aluminum foil, loaded into the sample tree and preheated at 150oC under high
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vacuum to desorb terrestrial atmospheric gases. Gases (noble gases and nitrogen) were extracted in a low blank vacuum furnace at temperatures of 600, 900, 1200 and 1700oC. Gases extracted from each extraction were splitted into two parts, one for noble gases and another for nitrogen measurements. The noble gas fraction was exposed to getters (SAES NP10 getters) for cleaning of reactive gases. The getters were hot at the time of exposure and cooled subsequently to room temperature. Heavy noble gases (Ar, Kr and Xe as mixture)
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were separated from He and Ne using liquid nitrogen trap at charcoal for ten minutes. To release Ar-Kr-Xe from charcoal, it was heated at 250oC for ten minutes. The gas mixture ArKr-Xe was introduced into the mass spectrometer for measurement without separation to avoid possible separation loss. The measured concentrations and isotopic ratios of noble
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gases and nitrogen after blanks and interference corrections are given in Table 1 (He, Ne, Ar, Kr and N) and Table 2 (xenon). Cosmogenic abundances of noble gases (He, Ne, Ar, Kr, Xe)
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were calculated by assuming a two-component mixture of cosmogenic and a trapped component having terrestrial composition. Petrographic observations were carried out using a Zeiss Polarising Microscope. Mineral compositions were determined using a Cameca SX 100 electron microprobe with three wavelength dispersive spectrometers (WDS) installed at PRL. Counting times for the elements were kept generally 10 to 20 s except for Na, which was 7 s to reduce the volatilisation effect. Operating conditions were 15 kV accelerating voltage, sample current 15 nA and 1 µm beam diameter. Natural mineral and metal standards were used and the data were corrected for absorption, fluorescence and atomic number effects using routine PAP procedure (Pouchou and Pichoir, 1988). Synthetic glass NIST 610 was run during certain 4
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Results and discussion
Neon is purely cosmogenic, because
20
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Noble gases and Nitrogen
Ne/22Ne is below 1 in all temperature measurements
(Table 1 and supplementary Fig. S1). The cosmogenic ratio (22Ne/21Ne)c calculated for Beni M’hira
is extremely low (<1.05). This suggests that cosmogenic gases were produced
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primarily under high shielding conditions (Graf et al. 1990). Moreover, such a low 22Ne/21Ne ratio indicates large pre-atmospheric size of the meteoroid (Graf et al. 1990). Since the
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recovered mass of Beni M’hira is around 16.2 kg (Laridhi Ouazaa et al., 2004), there could be more fragments yet to be recovered from the field or most of the meteoroid mass was lost due to ablation during atmospheric entry.
We used the concentrations of 21Ne and 38Ar, and the 22Ne/21Ne ratio as a shielding parameter and the L chondrite gas production rates given by Dalcher et al. (2013), to calculate cosmic83
Kr and
126
Xe production rates were calculated using the L
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ray exposure ages. The 3He,
chondrite systematic from Eugster (1988). Calculated concentrations of cosmogenic noble gases 3Hec,
21
Nec,
38
Arc, 83Krc and
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Xec are summarized in Table 3. Cosmic-ray exposure
ages obtained for noble gases T3, T21, T38, T83, T126 are listed in Table 3. Because 22Ne/21Ne
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of Beni M’hira is less than 1.05, the cosmogenic production rates for noble gases could be overestimated. The mean exposure age of all noble gases (T3, T21, T38, T83, T126) of 15.6 ± 3.7
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(Ma) is chosen as the cosmic-ray exposure age of Beni M’hira. The CRE age of ordinary chondrites (L type) lie mainly within the range of 1-50 Ma (Alexeev, 2005), rarely up to 70 Ma, but commonly cluster between 5 to 40 Ma. The exposure age of Beni M’hira estimated at 15.6 ± 3.7 (Ma) coincides with the 15 Ma peak in the L chondrite exposure age histogram (Wieler, 2002).
The gas retention ages based on U-Th-He and K-Ar are of special interest. The ages are less than the solar system formation age, so they provide an upper limit to an impact event age. Gas retention ages (Table 3) based on 4He and 40Ar were calculated as follows. In the absence of trapped neon,
20
Ne/22Ne < 1, helium is assumed to be a mixture of radiogenic and
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ACCEPTED MANUSCRIPT cosmogenic products. We used (4He/3He)c = 6.2 ± 0.2 (Welten et al., 2003) for cosmogenic correction. The U = 0.008 and Th = 0.049 ppm (Laridhi Ouazaa et al., 2004) yield the U-Th4
He radiogenic age of 485 ± 64 Ma for Beni M’hira chondrite. Therefore, this ordinary
chondrite belongs to the group of meteoroids involved in the impact event of the L chondrite parent asteroid which lead to enhanced flux during early Ordovician (Schmitz et al. 2001, 40
Ar/36Ar in L chondrites is different from
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Korochantseva et al., 2007). The trapped
primordial gas or terrestrial air (Korocchaneseva et al., 2007; Weirich et al., 2012). The ratio 28.7 of 40Ar/36Ar at 1700oC release is considered as trapped ratio for assessing the radiogenic 40
Ar. The radiogenic
40
Ar is 2.25 x10-6 cm3STP/g and 996 ppm of
40
K, yields the
40
Ar
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radiogenic age as 504 ± 51 Ma. The K-Ar age is similar to radiogenic age obtained from 4He within uncertainty. Therefore, the radiogenic age of 485 Ma indicates an impact event on the ordinary chondrite parent body, which reset the chronometer. The gas retention ages of
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earlier studied ordinary chondrites of L type Kaprada L(5/6) (Bhandari et al., 2009), regolith breccia Itawa Bhopji (L3-5) (Bhandari et al., 2002), Ararki (L5) (Bhandari et al., 2008) Devari-Khera (L6) (Murty et al., 2010), Jodiya (L5) (Murty et al., 2009) are between 4.5 to 1 Ga. In contrast, the recently fallen meteorite Kamargaon (L6) (Ray et al., 2017a) have 684 Ma K-40Ar age and 160 Ma 4He-U-Th age, respectively (Ray et al., 2016, Ray et al., 2017a).
40
38
Ar/36Ar ratios are between 0.38 and 0.85 implying a trapped component, while
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The
Ar/36Ar in temperature steps range 28 to 744 suggesting for a mixture of trapped radiogenic
components (Supplementary Fig. S2). The trapped 36Ar, 84Kr and 132Xe concentrations are (in 10-12 cm3STP/g) 11972, 37.2 and 37.2 for Beni M’hira. The trapped ratios (36Ar/132Xe)t and
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(84Kr/132Xe)t are 382 and 1.19 respectively (Supplementary Fig. S3), which fall between Q (Busemann et al., 2000) and solar (Meshik et al 2014) therefore suggest that the gas is a
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mixture of Q and solar. Xenon data are shown in a three isotopic diagram of versus
136
130
Xe/132Xe
Xe/132Xe in Fig. 1. The Beni M’hira occurs around Xe-Q (Busemann et al. 2000)
and solar wind, while the contribution of Xe-HL is minute. Thus, the trapped gas in this chondrite is a mixture of Q gas and solar wind. The presence of solar wind indicated that the meteorite could be a regolith-rich rock from its parent asteroid. Krypton data are shown in a three isotopic diagram of
86
Kr/84Kr versus
83
Kr/84Kr in Fig. 2. Beni M’hira higher
temperature steps (900oC, 1200oC and 1700oC) plot on a mixing line between Kr-Q (Busemann et al. 2000) and cosmogenic, indicating that Q is the main trapped component and supported by the release of gases in these temperature steps. The 400oC data point falls between solar wind and terrestrial, and hence could be mixture of them. 6
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isotopic
composition
is
reported
as
δ15N
(‰)
where
:
δ15N
=
[{(15N/14N)sample/(15N/14N)standard}-1] x 1000, where the standard is atmospheric nitrogen with 15
N/14N = 0.003676. Isotopic ratios of trapped nitrogen in ordinary chondrites are variable,
and differ more than 100‰ (Hashizume and Sugiura, 1995; Sugiura and Hashizume, 1992). Hashizume and Sugiura (1995) discussed that the nitrogen abundances diverge from 25 ppm
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to less than 1 ppm with increasing petrographic subtype. In Beni M’hira 0.74 ppm of nitrogen was released and the measured isotopic signature have δ15N = 14.6 ‰. These values are in the observed range of ordinary chondrites (Hashizume and Sugiura, 1995; Sugiura and Hashizume, 1992). Cosmogenic
15
N is produced in meteorites through spallation reactions
cosmogenic
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(Mathew and Murty 1993). The trapped nitrogen after cosmogenic correction using Nec (Mathew and Murty 1993) is equal to δ15Ntr = -92 ‰. The possible
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candidate for the light nitrogen signature could be solar wind (Solar δ15N < -240‰; Hashizume et al. 2000) however it is masked due to cosmic ray exposure in space. We calculated a 14N/36Art as 1.01 x 105 for Beni M’hira, which is largely different as compared to solar wind (14N/36Art = 37, Marty et al. 2010) and terrestrial atmosphere. The elemental ratio 14
N/132Xet = 3.82x107 in Beni M’hira is different than the lunar samples, 7x106 (Frick et al.,
1988) and solar gas-rich aubrite Pesyanoe, 4x108 (Mathew and Marti, 2003).
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Petrography and mineral chemistry
In the Beni M’hira chondrite, olivine is the most dominant mineral irrespective of chondrule and matrix population and occupies more than 50 vol%, followed by low-Ca pyroxene (30
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vol%) and feldspar (~15 vol %), respectively. Chondules with distinct outlines are typically absent due to extensive re-crystallization resulting chondrule-matrix textural integration,
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typical for the equlibrated type 6 chondrites. However, a few Barred Olivine (BO) clasts are observed (Fig. 3a). Additionally, the olivine is found to occur as large porphyritic grains (up to 200µ) within the integrated low-Ca pyroxene-feldspar matrix (Fig. 3b). The matrix is characterised by highly recrystallised texture and is comprised of olivine, low-Ca pyroxene, feldspar, Fe,Ni metal (kamacite and taenite) and troilite. Merillite is also present locally. Fe, Ni metal and troilite often occur as conjugate grains. High Ni taenite are occasionally studded within the BO chondrite clast. Average mineral chemistry with compositional range is presented in Table 4. Mineral composition of olivine (Fa: 26.04±0.25, n=19) suggests that the Beni M'hira chondrite is highly equilibrated. Low-Ca pyroxene also falls under the highly equilibrated petrologic type (Fs: 22.49 ± 0.29, n = 27) (Table 4). Feldspar appears to be
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ACCEPTED MANUSCRIPT transformed into either solid state maskelynite or shocked glass (Fig. 3c) and is generally characterised by Si-rich (71-72 wt%) composition with uniform Ca content (CaO ~ 2.2-2.6 wt%) and is Na-poor variety (Na2O<1.4 wt%). Troilite composition is almost uniform (Fe: 64 wt%; S: 36 wt%). Kamacite (Ni: 6.5 wt%) and Taenite (Ni : 34-36 wt%) also fall under the usual ordinary chondrite range. Both olivine and low-Ca pyroxene grains are highly
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fractured. Locally incipient shock melt veins with metal globules (Fig. 3d) suggest a disequilibrium melting.
Ubiquitous evidence for the equilibrium shock effect in the Beni M'hira meteorite is offered by well developed planar fractures (PF) within olivine and pyroxene grains. Optical
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evidences of weak mosaicism in olivine and presence of maskelynite and shocked plagioclase glass further suggests that the shock stage belongs to at least S4 or S5, corresponding to
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shock pressure > 45GPa (Stöffler et al., 1991). Following Bennet and McSween (1996), the presence of opaque melt veins, polycrystalline troilite and immiscible metal droplets also account for localised disequilibrium and confirm a shock stage up to S5 for Beni M'hira. Rapidly solidified metal-sulphide intergrowth (similar to quenched metal sulphide melt, Ray et al., 2017b) further suggests that the peak shock temperature must have had attained the Fe-
Conclusions
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Ni-S eutectic (~988oC or higher).
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In this study, simultaneous measurements of noble gases (He, Ne Ar, Kr and Xe, and nitrogen) have been carried out in a bulk sample of Beni M’hira L6 chondrite. Helium is released in low temperature steps while neon, argon, krypton and xenon were released in
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higher temperature steps. Neon appears to be purely cosmogenic. The trapped ratio of heavy noble gases (Ar, Kr and Xe) indicates that the gas is mixture of Q and solar wind. The cosmic ray exposure age of Beni M’hira is 15.6 ± 3.7 (Ma). Nitrogen abundance of 0.74 ppm and the measured isotopic signature of δ15N = 14.6 ‰ in Beni M’hira are within the range of ordinary chondrites. The U-Th-4He radiogenic age 485 Ma for Beni M’hira indicates that it belongs to the group of meteorites that were ejected during the disruption of L chondritic parent asteroid that lead to enhanced flux during early Ordovician period. The evidences of impact induced shock features have also been well supported with petrographic observations. A shock stage up to S5 for Beni M'hira is reasonably assessed by observation of shock-induced silicate
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Acknowledgements
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Department of Space, Government of India is thanked for financial support for this work.
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Maurette M., Duprat J., Engrand C., Gounelle M., Kurat G., Matrajt G. and Toppani A. 2000. Accretion of neon, organics, CO2, nitrogen and water from large internplanetary dust
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particles on the early Earth. Planetary and Space Science 48, 1117-1137. Meshik A., Hohenberg C. Pravdivtseva O. and Burnett D. 2014. Heavy noble gases in solar wind delivered by Genesis mission. Geochimica et Cosmochimica Acta 127, 326-347. Murty, S. V. S., Mahajan, R. R., Shukla, A. D., Mazumdar, A. C., Shukla, P. N., Durga
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Prasad, K., Rai ,V. K., Panda, D., Ghevaria, Z. G., Goswami J. N., 2009. Jodiya (L5) and Mahadevpur (H4/5) two recent ordinary chondrite falls in India. 72nd Annual
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Meteoritical society meeting 5058.pdf Murty, S. V. S., Mahajan, R. R., Chattopadhyay, B., Shukla, A. D., 2010. Cosmic ray effects in Devari-khera (L6) and Lohawat (how): two meteorites that fell in close proximity, in space and time. 73rd Annual Meteoritical society meeting 5099.pdf Pelé, M. P., Kuntz, F., Gerbet, M., 2013. Beni M'hira, the forgotten meteorite. 70 pp Pouchou, I. L., Pichoir, F., 1988. A simplified version of the 'PAP' model for matrix corrections in EPMA, in Microbeam Analysis - 1988 (ed. D.E. Newbury), San Francisco Press, San Francisco . 315-18.
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ACCEPTED MANUSCRIPT Ray, D., Mahajan, R. R., Shukla, A. D., Goswami T. K., 2016. Fall, petrology, classification, noble gas and cosmogenic records of Komar Gaon meteorite, the latest fall in India. 79th Annual meeting of Meteoritical society #6071. Ray, D., Mahajan, R.R., Shukla, A.D., Goswami, T. and Chakrabarty, S. 2017a Petrography, classification, oxygen isotopes, noble gases and cosmogenic records of Karagaon (L6)
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meteorite : the latest fall in India. Meteoritics and Planetary Science, doi : 10.111/maps.12875.
Ray, D., Ghosh, S. and Murty, S.V.S., 2017b. On the possible origin of troilite metal nodules in the Katol chondrite. Meteoritics and Planetary Science, 52, 72-88, doi :
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10.111/maps.12742.
Russell S. S., Zipfel J., Folco L., Jones R., Grady M. M., McCoy T. and Grossman J. N., 2003. The meteoritical bulletin no 87, 2003 july. Meteoritics and Planetary Science, 38,
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A189-A248.
Schmitz, B., Tassinari, M., Bernhard Peucker-Ehrenbrink, B., 2001. A rain of ordinary chondritic meteorites in the early Ordovician. Earth and Planetary Science Letters 194, 1-15.
Schultz, L., Franke, L., 2004. Helium, neon, and argon in meteorites: A data collection
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Meteoritics & Planetary Science 39, 1889–1890.
Schwenzer, S.P., Colindes, M., Herrmann, S., Ott U., 2007. Cold desert’s fingerprints: terrestrial nitrogen and noble gas signatures, which might be confused with (martian) meteorites signatures. Lunar and Planetary Science Conference XXXVIII, 1150.pdf.
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Sugiura, N., Hashizume, K., 1992. Nitrogen isotope anomalies in primitive ordinary chondrites. Earth and Planetary Science Letters111, 441-454.
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Stöffler, D., Kiel, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochimica Cosmochimica Acta 55, 3845-3867. Weirich J. R., Swindle T. D. and Isachsen C. E., 2012. 40Ar-39Ar age of Northwest Africa 091 : more evidence for a link between L chondrites and fossil meteorites. Meteoritics and Planetary Science 47:1324-1335. Welten, K. C., Caffee, M. W., Leya, I., Masark, J., Nishiizumi, K., Wieler, R., 2003. Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history, exposure history, and pre-atmospheric size. Meteoritics and Planetary Science. 38, 157-173.
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ACCEPTED MANUSCRIPT Wieler, R., 2002. Cosmic-ray produced noble gases in meteorites. In: Noble gases in Geochemistry and Cosmochemistry, editors Porcelli D., Ballentine C. J. and Wieler R.,
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Mineralogical Society of America, Washington DC, pp. 125-170.
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 Three isotope plot of
130
Xe/132Xe versus
136
Xe/132Xe for Beni M’hira. Trapped
components solar wind (Meshik et al. 2014) , Q (Busemann et al., 2000) and Xe-HL (Huss and Lewis, 1994) are shown.
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Fig. 2 Three isotope plot of 86Kr/84Kr versus 83Kr/84Kr for Beni M’hira. Trapped components solar wind (Meshik et al. 2014), Q (Busemann et al., 2000) and HL (Huss and Lewis, 1994) are shown.
Fig. 3a Clast of Barred Olivine (BO) chondrule with interstitial feldspar (Felds). Small
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metallic (Fe,Ni) spherules occur within chondrule while large Kamacite (Kam) grains occur outside within the matrix.
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Fig. 3b Porphyritic olivine clasts embedded within matrix comprising the integrated low-Ca pyroxene and feldspar grains. Note extensive fracturing of olivine grains. Olv: Olivine, Kam: Kamacite.
Fig. 3c Irregular shaped Maskelynites (Mask) apparently fill the fractures intruding the surrounding silicate grains. Oliv :Olivine, Kam :Kamacite, Low-Ca Px : Low-Ca pyroxene.
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Fig. 3d Occurrences of localised shock-melt veins. Note presence of troilite and metallic
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globules within the melt veins. Oliv:Olivine, Felds :Feldspar, Low-Ca Px : Low-Ca pyroxene.
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T (oC)
4
He
22
Ne
36
Ar
3
Kr
He/4He
20
Ne/22Ne
21
Ne/22Ne
10-12cm3STP/g
38
Ar/36Ar
40
Ar/36Ar
N2
δ15N
82
Kr/84Kr
83
Kr/84Kr
86
Kr/84Kr
(ppm) (‰)
55.2
0.15
0.02
1.79
0.0764 ±0.0033
0.8450 ±0.0007
0.9388 ±0.0007
0.822 ±0.001
656 ±1
0.06
2.7 ±0.3
0.2114 0.2043 ±0.0002 ±0.0016
0.3031 ±0.0002
900
158
1.42
0.27
4.75
0.0986 ±0.0043
0.7941 ±0.0009
0.9380 ±0.0002
0.740 ±0.001
744 ±1
0.56
14.6 ±0.2
0.2569 0.2757 ±0.0013 ±0.0001
0.2942 ±0.0002
1200
53.2
4.42
0.85
7.14
0.0793 ±0.0034
0.8228 ±0.0001
0.9534 ±0.0004
0.381 ±0.001
55.4 ±0.1
0.07
10.5 ±0.6
1700
1.9
4.88
0.61
25.2
0.1497 ±0.0065
0.8349 ±0.0002
0.9693 ±0.0003
0.853 ±0.001
28.7 ±0.1
0.06
30.1 ± 0.1
0.2971 0.3221 ±0.0026 ±0.0029 0.2388 0.2489 ±0.0002 ±0.0008
0.2863 ±0.0017 0.2950 ±0.0012
Total
268
10.9
1.73
38.9
0.0905 ±0.0039
0.8247 ±0.0003
0.9583 ±0.0003
0.606 ±0.001
158 ±1
0.74
14.6 ± 0.2
0.2505 0.2635 ±0.0008 ±0.0011
0.2937 ±0.0011
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600
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10-8 cm3STP/g
84
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Table 1. Isotopic ratio and concentrations of measured noble gases (He, Ne Ar) and nitrogen data in Beni M’hira. , T = temperature.
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Table 2. Isotopic ratio and concentrations of xenon data in Beni M’hira Xe -12 10 cm3STP/g
128
Xe/132Xe
129
Xe/132Xe
130
600
1.15
0.0056 ±0.0003
0.0046 ±0.0006
0.0875 ±0.0007
1.128 ±0.006
0.1628 ±0.0019
0.8219 ±0.0057
0.3823 ±0.0080
0.3083 ±0.0053
900
2.04
0.0052 ±0.0004
0.0041 ±0.0006
0.0835 ±0.0015
1.166 ±0.004
0.1666 ±0.0018
0.8208 ±0.0037
0.3750 ±0.0055
0.3081 ±0.0081
1200
6.81
0.0069 ±0.0001
0.0088 ±0.0002
0.0875 ±0.0005
1.852 ±0.006
0.1619 ±0.0017
0.8278 ±0.0033
0.3965 ±0.0024
0.3345 ±0.0027
1600
21.4
0.0059 ±0.0002
0.0060 ±0.0001
0.0897 ±0.0003
2.023 ±0.007
0.1665 ±0.0012
0.8266 ±0.0011
0.3829 ±0.0003
0.3189 ±0.0010
Total
31.4
0.0061 ±0.0002
0.0065 ±0.0001
0.0887 ±0.0005
1.898 ±0.007
0.1653 ±0.0014
0.8263 ±0.0019
0.3853 ±0.0013
0.3211 ±0.0021
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Xe/132Xe
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Xe/132Xe
Xe/132Xe
131
Xe/132Xe
134
Xe/132Xe
136
Xe/132Xe
EP
124
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132
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Temperature (oC)
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Table 3 Cosmogenic, radiogenic and trapped gases in Beni M’hira, Tavarage is average of T3, T21, T38, T83 and T126 CRE ages. Cosmogenic Xec
22
21
( Ne/ Ne)c
Radiogenic T3
T21
T38
10-14 cm3STP/g 9.94 ±1.02
Ma 1.041 ±0.29
14.6 ±0.3
T126 Taverage
Her
40
Arr
10-6 cm3STP/g
20.4 13.6 16.3 13.0 ±2.2 ±0.7 ±2.3 ±1.8
15.6 ±3.7
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24.2 10.43 0.83 277 ±1.6 ±1.04 ±0.12 ±28
T83
4
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Krc
126
1.4 2.25 ±0.1 ±0.25
T4
Trapped 36
T40
132
Krtr
X
10-12 cm3STP/g
Ma 485 ±64
84
Artr
599 ±51
11972
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Arc
83
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Nec
38
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Hec
21
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37.2
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Table 4 Representative electron microprobe anayses (wt%) of major phases in Beni M’hira
46.22 (0.69) 45.28 (0.13) 8.50 (0.56)
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Merrillite
N=7 69.61 (0.47) 0.07 (0.03) 22.75 (0.42) 0.75 (0.13) 0.14 0.11 2.24 (0.33) 1.37 (0.29) 1.05 (0.06)
N = 11 0.22 0.03 0.53 0.84 (0.20) 0.03 3.50 (0.16) 46.48 (1.50) 2.69 (0.18) 0.05
-
-
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1.53 (0.24) 76.06 (0.34) 22.41 (0.29)
Feldspar glass
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26.04 (0.25) -
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SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O End member Fa (mol%) Wo En Fs
N = 19 38.38 (0.37) n.d. n.d. 23.66 (0.51) n.d. 37.72 (0.51) n.d. n.d. n.d.
Low-Ca Pyroxene High-Ca Pyroxene N = 27 N=3 55.40 (0.45) 53.09 (0.21) 0.17 (0.04) 0.47 (0.02) 0.14 (0.02) 0.47 (0.02) 14.77 (0.22) 5.32 (0.37) n.d. n.d. 28.14 (0.21) 15.91 (0.12) 0.79 (0.12) 22.60 (0.24) n.d. 0.56 (0.02) n.d. n.d.
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Olivine
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n.d. not determined, N = No. of analyses, number in parenthesis is error.
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Beni M’hira belongs to Ordovician impact event, 4He gas retention age is 485 Ma. Cosmic ray exposure age of Beni M’hira is 15.6 Ma Trapped noble gas composition (Ar, Kr, Xe) is Q type Nitrogen isotopic composition in Beni M’hira is 14.6 ‰.
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• • • •
S0032-0633(17)30148-4
DOI:
10.1016/j.pss.2017.12.018
Reference:
PSS 4452
To appear in:
Planetary and Space Science
Received Date: 3 May 2017 Revised Date:
28 December 2017
Accepted Date: 31 December 2017
Please cite this article as: Mahajan, R.R., Nejia, L.O., Ray, D., Naik, S., Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M'hira (L6) chondrite, Planetary and Space Science (2018), doi: 10.1016/j.pss.2017.12.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M’hira (L6) chondrite Ramakant R. Mahajan1*, Laridhi Ouazaa Nejia2 , Dwijesh Ray1 and Sekhar Naik1 Physical Research Laboratory, Ahmedabad, 380009, India.
2
Départment de Géologie, Faculté des Sciences de Tunis, Université Tunis El Manar,
Campus Universitaire, 2092 Tunis, Tunisia.
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*corresponding author email : [email protected]
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Abstract
The concentrations and isotopic composition of noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon( Xe) and nitrogen were measured in the Beni M’hira L6 chondrite. The cosmic ray exposure age of Beni M’hira is estimated of 15.6 ± 3.7 (Ma). The radiogenic age, of around 485 ± 64 Ma, derived from 4He, and of around 504 ± 51 Ma from
40
Ar,
suggests an age resetting indicating the event impact. The heavy noble gases (Ar, Kr and Xe)
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concentrations imply that the gas is a mixture of trapped component Q and solar wind. The measured nitrogen abundance of 0.74 ppm and the isotopic signature of δ15N = 14.6 ‰ are within the range of ordinary chondrites. The homogeneous chemical composition of olivine (Fa:26±0.25) and low-Ca pyroxene (Fs:22.4±0.29) suggest that the Beni M’hira meteorite is
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an equilibrated chondrite. This is further corroborated by strong chondrule-matrix textural integration (lack of chondrules, except a few relict clast). Shock metamorphism generally
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corresponds to S5 (>45 GPa), however, locally disequilibrium melting (shock-melt veins) suggests, that the peak shock metamorphism was at ~75 GPa, 950°C.
Keywords : Noble gases, ordinary chondrite, cosmic ray exposure age, mineralogy
Introduction Meteorites are objects predominantly derived from asteroids or comets, which are most likely remnants of the condensates formed during the formation of the solar system. Meteorites 1
ACCEPTED MANUSCRIPT provide insight into several issues related to solid matter formation and evolution, formed during early evolution stages of the solar system. Meteorites that have fallen on Earth are classified into various classes based upon their composition (e.g. Krot et al., 2014 for most recent updates). The most common meteorite class is ordinary chondrites which comprise ~80% of all meteorites found on Earth (Meteoritical Society Bulletin Database). Among
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them, the H, L and LL chondrites represent approximately 34, 38 and 8 % of all ordinary chondritic meteorites. The ordinary chondrites listed are 46026 (Meteoritical Society Bulletin accessed on 11 Apr 2016), out of which only 894 are observed falls. Meteorites found in hot and cold deserts show signs of weathering or contamination (Gooding, 1986; Schwenzer et
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al., 2007). Hence, it is important to recover and preserve the newly fallen meteorite immediately after the fall. However, the parent bodies of this most abundant class of meteorites are uncertain and largely unknown. Thus, the trapped noble gases, nitrogen and
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cosmic ray exposure ages are an important datum within this context.
Since noble gases are chemically inert, their rare abundance in solid matter formed in solar system makes them a sensitive tool in understanding the formation and evolution of material in the solar system. The noble gases could be modified as a result of diverse processes such as radiogenic decay products, cosmogenic effects, fissionogenic product, degassing, diffusion
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etc, which modify the initial isotopic composition of cosmic bodies in the solar system. The cosmic ray exposure age is the duration of a meteoroid from its ejection until it falls on Earth. Cosmic-ray exposure ages are derived using the abundance of a nuclear reaction producing stable noble gas isotopes which allows to empirically or theoretically measure the production
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rate (Dalcher et al. 2013). Noble gases have been measured in many meteorites since 1950. These measurements are used to determine trapped radiogenic and fissionogenic components,
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cosmic ray exposure ages and gas retention ages etc. The data base has been continuously expanded with further new measurements using new sophisticated instruments. The compilation of noble gas data by Schultz and Franke (2004) contains more than 6000 analyses for meteorites from all classes. There are many specific trapped gases, isotopic composition and elemental abundance patterns that are now known. If the mechanisms of noble gases trapping in meteorites and their origin were resolved, the information would impose many crucial constraints on the physiochemical conditions in the early solar nebula. Neon in meteorites is a complex mixture of several isotopically different trapped components, but these components are commonly masked in most meteorites by cosmic-ray component and also terrestrial trapped air. Therefore, it is always difficult to verify the 2
ACCEPTED MANUSCRIPT trapped components in bulk measurements. It is believed that xenon has heterogeneous isotopic composition with mixing of nucleosynthetic presolar components in proto-planetary disk (Marti and Mathew, 2015) which is incorporated in solar system bodies during their formation. Analyses of bulk meteorites in stepwise gas extraction experiments may provide evidence of
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many primitive noble gas components, but residues separation (obtained after acid dissolution) reveal the presence of a single component only (Huss and Lewis, 1994). Among the volatile elements, trapped nitrogen in meteorites is unique in showing large isotopic variations and results are rigorously discussed in various publications (Kothari and Goel,
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1974; Kung and Clayton, 1978; Sugiura and Hashizume, 1992; Hashizume and Sugiura, 1995, Maurette et al. 2000, Marty et al. 2003, Marty et al. 2005, Füri et al. 2012). However,
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the interpretation of this observation has proved to be difficult because of the presence of many components. The simultaneous measurements of stable isotopes of all five noble gases and nitrogen in same aliquot of sample are rare. Therefore, it seems worthwhile to take another look at noble gases and nitrogen measured simultaneously, to assess if they provide information on meteoritic parent bodies and nebular conditions. Often, nitrogen is correlated with noble gases, but the true extent of this correlation is hard to judge, because noble gases
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and nitrogen are rarely measured in the same sample.
The Beni M’hira meteorite is classified as an L6 chondrite (Russell et al. 2003, Meteoritical Society Bulletin no. 87) and fell on January 8, 2001 in southeastern Tunisia (Laridhi Ouazaa
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et al., 2004). Several pieces weighing more than 16 kg, were recovered from the field. Other fragments (192 in total), weighing 12558.95 g, were collected between 2012 and 2013 (Pelé et al., 2013). The specimen lacks well defined chondrules, indicating its association with
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increasing temperature and/or increasing time spent at higher temperature in a parent body. We studied the noble gases in the bulk sample of Beni M'hira. Our approach to determine exposure ages is to compare all the stable noble gas nuclides (He, Ne, Ar, Kr, Xe) using cosmic-ray production rates. This method has the advantage to eliminate the uncertainties which may arise due to various factors such as production rates, target element chemistry, noble gas measurement uncertainties and diffusive loss of lighter elements from rocks. Mineralogy of Beni M’hira also studied to see the correlation of shock effect from petrographic observations and noble gas impact history.
3
ACCEPTED MANUSCRIPT Experimental details Noble gas isotopic and petrographic measurements were performed separately on Beni M’hira sample. Two aliquots were made, one for noble gas measurements and one for electron-probe micro analysis. The samples were observed under microscope and look fresh. However, the exact location of the samples in the main mass is not known. Both
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measurements on Beni M’hira sample were carried out at the Physical Research Laboratory (PRL), Ahmedabad, India. The isotopic studies of noble gas (all five noble gases) and nitrogen content were performed using a ‘Noblesse’ multi-collector noble gas mass spectrometer (Nu Instruments U. K.) at PRL, following the procedures described elsewhere
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(Mahajan, 2015; Mahajan et al., 2016; Mahajan, 2017). The sample weighting 409.36 mg was wrapped in aluminum foil, loaded into the sample tree and preheated at 150oC under high
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vacuum to desorb terrestrial atmospheric gases. Gases (noble gases and nitrogen) were extracted in a low blank vacuum furnace at temperatures of 600, 900, 1200 and 1700oC. Gases extracted from each extraction were splitted into two parts, one for noble gases and another for nitrogen measurements. The noble gas fraction was exposed to getters (SAES NP10 getters) for cleaning of reactive gases. The getters were hot at the time of exposure and cooled subsequently to room temperature. Heavy noble gases (Ar, Kr and Xe as mixture)
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were separated from He and Ne using liquid nitrogen trap at charcoal for ten minutes. To release Ar-Kr-Xe from charcoal, it was heated at 250oC for ten minutes. The gas mixture ArKr-Xe was introduced into the mass spectrometer for measurement without separation to avoid possible separation loss. The measured concentrations and isotopic ratios of noble
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gases and nitrogen after blanks and interference corrections are given in Table 1 (He, Ne, Ar, Kr and N) and Table 2 (xenon). Cosmogenic abundances of noble gases (He, Ne, Ar, Kr, Xe)
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were calculated by assuming a two-component mixture of cosmogenic and a trapped component having terrestrial composition. Petrographic observations were carried out using a Zeiss Polarising Microscope. Mineral compositions were determined using a Cameca SX 100 electron microprobe with three wavelength dispersive spectrometers (WDS) installed at PRL. Counting times for the elements were kept generally 10 to 20 s except for Na, which was 7 s to reduce the volatilisation effect. Operating conditions were 15 kV accelerating voltage, sample current 15 nA and 1 µm beam diameter. Natural mineral and metal standards were used and the data were corrected for absorption, fluorescence and atomic number effects using routine PAP procedure (Pouchou and Pichoir, 1988). Synthetic glass NIST 610 was run during certain 4
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Results and discussion
Neon is purely cosmogenic, because
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Noble gases and Nitrogen
Ne/22Ne is below 1 in all temperature measurements
(Table 1 and supplementary Fig. S1). The cosmogenic ratio (22Ne/21Ne)c calculated for Beni M’hira
is extremely low (<1.05). This suggests that cosmogenic gases were produced
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primarily under high shielding conditions (Graf et al. 1990). Moreover, such a low 22Ne/21Ne ratio indicates large pre-atmospheric size of the meteoroid (Graf et al. 1990). Since the
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recovered mass of Beni M’hira is around 16.2 kg (Laridhi Ouazaa et al., 2004), there could be more fragments yet to be recovered from the field or most of the meteoroid mass was lost due to ablation during atmospheric entry.
We used the concentrations of 21Ne and 38Ar, and the 22Ne/21Ne ratio as a shielding parameter and the L chondrite gas production rates given by Dalcher et al. (2013), to calculate cosmic83
Kr and
126
Xe production rates were calculated using the L
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ray exposure ages. The 3He,
chondrite systematic from Eugster (1988). Calculated concentrations of cosmogenic noble gases 3Hec,
21
Nec,
38
Arc, 83Krc and
126
Xec are summarized in Table 3. Cosmic-ray exposure
ages obtained for noble gases T3, T21, T38, T83, T126 are listed in Table 3. Because 22Ne/21Ne
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of Beni M’hira is less than 1.05, the cosmogenic production rates for noble gases could be overestimated. The mean exposure age of all noble gases (T3, T21, T38, T83, T126) of 15.6 ± 3.7
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(Ma) is chosen as the cosmic-ray exposure age of Beni M’hira. The CRE age of ordinary chondrites (L type) lie mainly within the range of 1-50 Ma (Alexeev, 2005), rarely up to 70 Ma, but commonly cluster between 5 to 40 Ma. The exposure age of Beni M’hira estimated at 15.6 ± 3.7 (Ma) coincides with the 15 Ma peak in the L chondrite exposure age histogram (Wieler, 2002).
The gas retention ages based on U-Th-He and K-Ar are of special interest. The ages are less than the solar system formation age, so they provide an upper limit to an impact event age. Gas retention ages (Table 3) based on 4He and 40Ar were calculated as follows. In the absence of trapped neon,
20
Ne/22Ne < 1, helium is assumed to be a mixture of radiogenic and
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He radiogenic age of 485 ± 64 Ma for Beni M’hira chondrite. Therefore, this ordinary
chondrite belongs to the group of meteoroids involved in the impact event of the L chondrite parent asteroid which lead to enhanced flux during early Ordovician (Schmitz et al. 2001, 40
Ar/36Ar in L chondrites is different from
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Korochantseva et al., 2007). The trapped
primordial gas or terrestrial air (Korocchaneseva et al., 2007; Weirich et al., 2012). The ratio 28.7 of 40Ar/36Ar at 1700oC release is considered as trapped ratio for assessing the radiogenic 40
Ar. The radiogenic
40
Ar is 2.25 x10-6 cm3STP/g and 996 ppm of
40
K, yields the
40
Ar
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radiogenic age as 504 ± 51 Ma. The K-Ar age is similar to radiogenic age obtained from 4He within uncertainty. Therefore, the radiogenic age of 485 Ma indicates an impact event on the ordinary chondrite parent body, which reset the chronometer. The gas retention ages of
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earlier studied ordinary chondrites of L type Kaprada L(5/6) (Bhandari et al., 2009), regolith breccia Itawa Bhopji (L3-5) (Bhandari et al., 2002), Ararki (L5) (Bhandari et al., 2008) Devari-Khera (L6) (Murty et al., 2010), Jodiya (L5) (Murty et al., 2009) are between 4.5 to 1 Ga. In contrast, the recently fallen meteorite Kamargaon (L6) (Ray et al., 2017a) have 684 Ma K-40Ar age and 160 Ma 4He-U-Th age, respectively (Ray et al., 2016, Ray et al., 2017a).
40
38
Ar/36Ar ratios are between 0.38 and 0.85 implying a trapped component, while
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The
Ar/36Ar in temperature steps range 28 to 744 suggesting for a mixture of trapped radiogenic
components (Supplementary Fig. S2). The trapped 36Ar, 84Kr and 132Xe concentrations are (in 10-12 cm3STP/g) 11972, 37.2 and 37.2 for Beni M’hira. The trapped ratios (36Ar/132Xe)t and
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(84Kr/132Xe)t are 382 and 1.19 respectively (Supplementary Fig. S3), which fall between Q (Busemann et al., 2000) and solar (Meshik et al 2014) therefore suggest that the gas is a
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mixture of Q and solar. Xenon data are shown in a three isotopic diagram of versus
136
130
Xe/132Xe
Xe/132Xe in Fig. 1. The Beni M’hira occurs around Xe-Q (Busemann et al. 2000)
and solar wind, while the contribution of Xe-HL is minute. Thus, the trapped gas in this chondrite is a mixture of Q gas and solar wind. The presence of solar wind indicated that the meteorite could be a regolith-rich rock from its parent asteroid. Krypton data are shown in a three isotopic diagram of
86
Kr/84Kr versus
83
Kr/84Kr in Fig. 2. Beni M’hira higher
temperature steps (900oC, 1200oC and 1700oC) plot on a mixing line between Kr-Q (Busemann et al. 2000) and cosmogenic, indicating that Q is the main trapped component and supported by the release of gases in these temperature steps. The 400oC data point falls between solar wind and terrestrial, and hence could be mixture of them. 6
ACCEPTED MANUSCRIPT Nitrogen
isotopic
composition
is
reported
as
δ15N
(‰)
where
:
δ15N
=
[{(15N/14N)sample/(15N/14N)standard}-1] x 1000, where the standard is atmospheric nitrogen with 15
N/14N = 0.003676. Isotopic ratios of trapped nitrogen in ordinary chondrites are variable,
and differ more than 100‰ (Hashizume and Sugiura, 1995; Sugiura and Hashizume, 1992). Hashizume and Sugiura (1995) discussed that the nitrogen abundances diverge from 25 ppm
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to less than 1 ppm with increasing petrographic subtype. In Beni M’hira 0.74 ppm of nitrogen was released and the measured isotopic signature have δ15N = 14.6 ‰. These values are in the observed range of ordinary chondrites (Hashizume and Sugiura, 1995; Sugiura and Hashizume, 1992). Cosmogenic
15
N is produced in meteorites through spallation reactions
cosmogenic
21
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(Mathew and Murty 1993). The trapped nitrogen after cosmogenic correction using Nec (Mathew and Murty 1993) is equal to δ15Ntr = -92 ‰. The possible
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candidate for the light nitrogen signature could be solar wind (Solar δ15N < -240‰; Hashizume et al. 2000) however it is masked due to cosmic ray exposure in space. We calculated a 14N/36Art as 1.01 x 105 for Beni M’hira, which is largely different as compared to solar wind (14N/36Art = 37, Marty et al. 2010) and terrestrial atmosphere. The elemental ratio 14
N/132Xet = 3.82x107 in Beni M’hira is different than the lunar samples, 7x106 (Frick et al.,
1988) and solar gas-rich aubrite Pesyanoe, 4x108 (Mathew and Marti, 2003).
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Petrography and mineral chemistry
In the Beni M’hira chondrite, olivine is the most dominant mineral irrespective of chondrule and matrix population and occupies more than 50 vol%, followed by low-Ca pyroxene (30
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vol%) and feldspar (~15 vol %), respectively. Chondules with distinct outlines are typically absent due to extensive re-crystallization resulting chondrule-matrix textural integration,
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typical for the equlibrated type 6 chondrites. However, a few Barred Olivine (BO) clasts are observed (Fig. 3a). Additionally, the olivine is found to occur as large porphyritic grains (up to 200µ) within the integrated low-Ca pyroxene-feldspar matrix (Fig. 3b). The matrix is characterised by highly recrystallised texture and is comprised of olivine, low-Ca pyroxene, feldspar, Fe,Ni metal (kamacite and taenite) and troilite. Merillite is also present locally. Fe, Ni metal and troilite often occur as conjugate grains. High Ni taenite are occasionally studded within the BO chondrite clast. Average mineral chemistry with compositional range is presented in Table 4. Mineral composition of olivine (Fa: 26.04±0.25, n=19) suggests that the Beni M'hira chondrite is highly equilibrated. Low-Ca pyroxene also falls under the highly equilibrated petrologic type (Fs: 22.49 ± 0.29, n = 27) (Table 4). Feldspar appears to be
7
ACCEPTED MANUSCRIPT transformed into either solid state maskelynite or shocked glass (Fig. 3c) and is generally characterised by Si-rich (71-72 wt%) composition with uniform Ca content (CaO ~ 2.2-2.6 wt%) and is Na-poor variety (Na2O<1.4 wt%). Troilite composition is almost uniform (Fe: 64 wt%; S: 36 wt%). Kamacite (Ni: 6.5 wt%) and Taenite (Ni : 34-36 wt%) also fall under the usual ordinary chondrite range. Both olivine and low-Ca pyroxene grains are highly
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fractured. Locally incipient shock melt veins with metal globules (Fig. 3d) suggest a disequilibrium melting.
Ubiquitous evidence for the equilibrium shock effect in the Beni M'hira meteorite is offered by well developed planar fractures (PF) within olivine and pyroxene grains. Optical
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evidences of weak mosaicism in olivine and presence of maskelynite and shocked plagioclase glass further suggests that the shock stage belongs to at least S4 or S5, corresponding to
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shock pressure > 45GPa (Stöffler et al., 1991). Following Bennet and McSween (1996), the presence of opaque melt veins, polycrystalline troilite and immiscible metal droplets also account for localised disequilibrium and confirm a shock stage up to S5 for Beni M'hira. Rapidly solidified metal-sulphide intergrowth (similar to quenched metal sulphide melt, Ray et al., 2017b) further suggests that the peak shock temperature must have had attained the Fe-
Conclusions
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Ni-S eutectic (~988oC or higher).
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In this study, simultaneous measurements of noble gases (He, Ne Ar, Kr and Xe, and nitrogen) have been carried out in a bulk sample of Beni M’hira L6 chondrite. Helium is released in low temperature steps while neon, argon, krypton and xenon were released in
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higher temperature steps. Neon appears to be purely cosmogenic. The trapped ratio of heavy noble gases (Ar, Kr and Xe) indicates that the gas is mixture of Q and solar wind. The cosmic ray exposure age of Beni M’hira is 15.6 ± 3.7 (Ma). Nitrogen abundance of 0.74 ppm and the measured isotopic signature of δ15N = 14.6 ‰ in Beni M’hira are within the range of ordinary chondrites. The U-Th-4He radiogenic age 485 Ma for Beni M’hira indicates that it belongs to the group of meteorites that were ejected during the disruption of L chondritic parent asteroid that lead to enhanced flux during early Ordovician period. The evidences of impact induced shock features have also been well supported with petrographic observations. A shock stage up to S5 for Beni M'hira is reasonably assessed by observation of shock-induced silicate
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Acknowledgements
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Department of Space, Government of India is thanked for financial support for this work.
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Kung, C., Clayton, R. N., 1978. Nitrogen abundances and isotopic compositions in stony meteorites. Earth and Planetary Science Letters 38, 421-435.
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Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system. Geochimica et Cosmochimica Acta 74, 340-355.
Mathew, K. J., Marti, K. 2003. Solar wind and other gases in the regoliths of the Pesyanoe
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Prasad, K., Rai ,V. K., Panda, D., Ghevaria, Z. G., Goswami J. N., 2009. Jodiya (L5) and Mahadevpur (H4/5) two recent ordinary chondrite falls in India. 72nd Annual
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Meteoritical society meeting 5058.pdf Murty, S. V. S., Mahajan, R. R., Chattopadhyay, B., Shukla, A. D., 2010. Cosmic ray effects in Devari-khera (L6) and Lohawat (how): two meteorites that fell in close proximity, in space and time. 73rd Annual Meteoritical society meeting 5099.pdf Pelé, M. P., Kuntz, F., Gerbet, M., 2013. Beni M'hira, the forgotten meteorite. 70 pp Pouchou, I. L., Pichoir, F., 1988. A simplified version of the 'PAP' model for matrix corrections in EPMA, in Microbeam Analysis - 1988 (ed. D.E. Newbury), San Francisco Press, San Francisco . 315-18.
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ACCEPTED MANUSCRIPT Ray, D., Mahajan, R. R., Shukla, A. D., Goswami T. K., 2016. Fall, petrology, classification, noble gas and cosmogenic records of Komar Gaon meteorite, the latest fall in India. 79th Annual meeting of Meteoritical society #6071. Ray, D., Mahajan, R.R., Shukla, A.D., Goswami, T. and Chakrabarty, S. 2017a Petrography, classification, oxygen isotopes, noble gases and cosmogenic records of Karagaon (L6)
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meteorite : the latest fall in India. Meteoritics and Planetary Science, doi : 10.111/maps.12875.
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10.111/maps.12742.
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Schultz, L., Franke, L., 2004. Helium, neon, and argon in meteorites: A data collection
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Meteoritics & Planetary Science 39, 1889–1890.
Schwenzer, S.P., Colindes, M., Herrmann, S., Ott U., 2007. Cold desert’s fingerprints: terrestrial nitrogen and noble gas signatures, which might be confused with (martian) meteorites signatures. Lunar and Planetary Science Conference XXXVIII, 1150.pdf.
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ACCEPTED MANUSCRIPT Wieler, R., 2002. Cosmic-ray produced noble gases in meteorites. In: Noble gases in Geochemistry and Cosmochemistry, editors Porcelli D., Ballentine C. J. and Wieler R.,
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 Three isotope plot of
130
Xe/132Xe versus
136
Xe/132Xe for Beni M’hira. Trapped
components solar wind (Meshik et al. 2014) , Q (Busemann et al., 2000) and Xe-HL (Huss and Lewis, 1994) are shown.
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Fig. 2 Three isotope plot of 86Kr/84Kr versus 83Kr/84Kr for Beni M’hira. Trapped components solar wind (Meshik et al. 2014), Q (Busemann et al., 2000) and HL (Huss and Lewis, 1994) are shown.
Fig. 3a Clast of Barred Olivine (BO) chondrule with interstitial feldspar (Felds). Small
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metallic (Fe,Ni) spherules occur within chondrule while large Kamacite (Kam) grains occur outside within the matrix.
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Fig. 3b Porphyritic olivine clasts embedded within matrix comprising the integrated low-Ca pyroxene and feldspar grains. Note extensive fracturing of olivine grains. Olv: Olivine, Kam: Kamacite.
Fig. 3c Irregular shaped Maskelynites (Mask) apparently fill the fractures intruding the surrounding silicate grains. Oliv :Olivine, Kam :Kamacite, Low-Ca Px : Low-Ca pyroxene.
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Fig. 3d Occurrences of localised shock-melt veins. Note presence of troilite and metallic
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globules within the melt veins. Oliv:Olivine, Felds :Feldspar, Low-Ca Px : Low-Ca pyroxene.
15
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T (oC)
4
He
22
Ne
36
Ar
3
Kr
He/4He
20
Ne/22Ne
21
Ne/22Ne
10-12cm3STP/g
38
Ar/36Ar
40
Ar/36Ar
N2
δ15N
82
Kr/84Kr
83
Kr/84Kr
86
Kr/84Kr
(ppm) (‰)
55.2
0.15
0.02
1.79
0.0764 ±0.0033
0.8450 ±0.0007
0.9388 ±0.0007
0.822 ±0.001
656 ±1
0.06
2.7 ±0.3
0.2114 0.2043 ±0.0002 ±0.0016
0.3031 ±0.0002
900
158
1.42
0.27
4.75
0.0986 ±0.0043
0.7941 ±0.0009
0.9380 ±0.0002
0.740 ±0.001
744 ±1
0.56
14.6 ±0.2
0.2569 0.2757 ±0.0013 ±0.0001
0.2942 ±0.0002
1200
53.2
4.42
0.85
7.14
0.0793 ±0.0034
0.8228 ±0.0001
0.9534 ±0.0004
0.381 ±0.001
55.4 ±0.1
0.07
10.5 ±0.6
1700
1.9
4.88
0.61
25.2
0.1497 ±0.0065
0.8349 ±0.0002
0.9693 ±0.0003
0.853 ±0.001
28.7 ±0.1
0.06
30.1 ± 0.1
0.2971 0.3221 ±0.0026 ±0.0029 0.2388 0.2489 ±0.0002 ±0.0008
0.2863 ±0.0017 0.2950 ±0.0012
Total
268
10.9
1.73
38.9
0.0905 ±0.0039
0.8247 ±0.0003
0.9583 ±0.0003
0.606 ±0.001
158 ±1
0.74
14.6 ± 0.2
0.2505 0.2635 ±0.0008 ±0.0011
0.2937 ±0.0011
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600
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10-8 cm3STP/g
84
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Table 1. Isotopic ratio and concentrations of measured noble gases (He, Ne Ar) and nitrogen data in Beni M’hira. , T = temperature.
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Table 2. Isotopic ratio and concentrations of xenon data in Beni M’hira Xe -12 10 cm3STP/g
128
Xe/132Xe
129
Xe/132Xe
130
600
1.15
0.0056 ±0.0003
0.0046 ±0.0006
0.0875 ±0.0007
1.128 ±0.006
0.1628 ±0.0019
0.8219 ±0.0057
0.3823 ±0.0080
0.3083 ±0.0053
900
2.04
0.0052 ±0.0004
0.0041 ±0.0006
0.0835 ±0.0015
1.166 ±0.004
0.1666 ±0.0018
0.8208 ±0.0037
0.3750 ±0.0055
0.3081 ±0.0081
1200
6.81
0.0069 ±0.0001
0.0088 ±0.0002
0.0875 ±0.0005
1.852 ±0.006
0.1619 ±0.0017
0.8278 ±0.0033
0.3965 ±0.0024
0.3345 ±0.0027
1600
21.4
0.0059 ±0.0002
0.0060 ±0.0001
0.0897 ±0.0003
2.023 ±0.007
0.1665 ±0.0012
0.8266 ±0.0011
0.3829 ±0.0003
0.3189 ±0.0010
Total
31.4
0.0061 ±0.0002
0.0065 ±0.0001
0.0887 ±0.0005
1.898 ±0.007
0.1653 ±0.0014
0.8263 ±0.0019
0.3853 ±0.0013
0.3211 ±0.0021
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Xe/132Xe
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Xe/132Xe
Xe/132Xe
131
Xe/132Xe
134
Xe/132Xe
136
Xe/132Xe
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124
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Temperature (oC)
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Table 3 Cosmogenic, radiogenic and trapped gases in Beni M’hira, Tavarage is average of T3, T21, T38, T83 and T126 CRE ages. Cosmogenic Xec
22
21
( Ne/ Ne)c
Radiogenic T3
T21
T38
10-14 cm3STP/g 9.94 ±1.02
Ma 1.041 ±0.29
14.6 ±0.3
T126 Taverage
Her
40
Arr
10-6 cm3STP/g
20.4 13.6 16.3 13.0 ±2.2 ±0.7 ±2.3 ±1.8
15.6 ±3.7
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24.2 10.43 0.83 277 ±1.6 ±1.04 ±0.12 ±28
T83
4
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Krc
126
1.4 2.25 ±0.1 ±0.25
T4
Trapped 36
T40
132
Krtr
X
10-12 cm3STP/g
Ma 485 ±64
84
Artr
599 ±51
11972
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Arc
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38
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Hec
21
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18
37.2
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Table 4 Representative electron microprobe anayses (wt%) of major phases in Beni M’hira
46.22 (0.69) 45.28 (0.13) 8.50 (0.56)
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Merrillite
N=7 69.61 (0.47) 0.07 (0.03) 22.75 (0.42) 0.75 (0.13) 0.14 0.11 2.24 (0.33) 1.37 (0.29) 1.05 (0.06)
N = 11 0.22 0.03 0.53 0.84 (0.20) 0.03 3.50 (0.16) 46.48 (1.50) 2.69 (0.18) 0.05
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1.53 (0.24) 76.06 (0.34) 22.41 (0.29)
Feldspar glass
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26.04 (0.25) -
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SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O End member Fa (mol%) Wo En Fs
N = 19 38.38 (0.37) n.d. n.d. 23.66 (0.51) n.d. 37.72 (0.51) n.d. n.d. n.d.
Low-Ca Pyroxene High-Ca Pyroxene N = 27 N=3 55.40 (0.45) 53.09 (0.21) 0.17 (0.04) 0.47 (0.02) 0.14 (0.02) 0.47 (0.02) 14.77 (0.22) 5.32 (0.37) n.d. n.d. 28.14 (0.21) 15.91 (0.12) 0.79 (0.12) 22.60 (0.24) n.d. 0.56 (0.02) n.d. n.d.
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n.d. not determined, N = No. of analyses, number in parenthesis is error.
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SC
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Beni M’hira belongs to Ordovician impact event, 4He gas retention age is 485 Ma. Cosmic ray exposure age of Beni M’hira is 15.6 Ma Trapped noble gas composition (Ar, Kr, Xe) is Q type Nitrogen isotopic composition in Beni M’hira is 14.6 ‰.
AC C
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