The nakhlites are a group of mafic

igneous rocks that crystallized on Mars at ca. 1.3-1.4

Ga [1-5]. They are amongst the least shocked Martian

meteorites, with cumulate igneous textures (Fig. 1) and

thus provide a crucial record of igneous activity and

fluid-rock interaction on the red planet.

Crystallization of the nakhlites. Understanding the

original structure of the nakhlite source is crucial for

using these stones to explore Martian igneous processes.

In particular, are the different nakhlites from a single

thick and differentiated lava flow/sill [6; 7], or do

they instead represent magmatically related – but distinct

– flows/intrusions? If the various meteorites are

derived from separate units, then there will be differences

in crystallization ages within the nakhlite suite –

which may be identified if these differences are sufficiently

large relative to the attainable precision of radioisotopic

dating techniques. Some studies [e.g., 5] appear

to resolve age differences between different

stones, and we aim to test the single vs. multiple unit

hypotheses via application of detailed 40Ar/39Ar stepheating

of six nakhlites.&lt;p&gt;&lt;/p&gt;

Alteration of the nakhlites, and the timing of waterrock

interaction. In addition to primary magmatic

minerals and glasses, the nakhlites contain secondary

minerals including clays and carbonates that were precipitated

by Martian aqueous fluids prior to impact

ejection (Fig. 2) [8-10]. When did this alteration occur?

As the clays contain potassium, they are amenable

to K-Ar and 40Ar/39Ar dating. K-Ar data from Lafayette

suggests the alteration phases formed between 0

to 670 Ma [11]. We consider further the timing of alteration

using our 40Ar/39Ar data.&lt;p&gt;&lt;/p&gt

Cohen, B. E.

Lee, M. R.

Mark, D. F.

Smith, C. L.

Tomkinson, Tim O R

High-resolution 40Ar/39Ar chronology of six nakhlites is used to examine the igneous age and alteration history of this group of martian meteorites

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                          Cohen, B. E., Mark, D. F., Tomkinson, T. O. R., Lee, M. R., & Smith, C. L.(2015). Martian Igneous Activity and Fluid-Based Alteration: ChronologicalConstraints from 40Ar/39Ar Analyses of the Nakhlites. Abstract from 46thLunar and Planetary Science Conference, Texas, United States.Publisher's PDF, also known as Version of recordLink to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlMARTIAN IGNEOUS ACTIVITY AND FLUID-BASED ALTERATION: CHRONOLOGICAL CONSTRAINTS FROM 40Ar/39Ar ANALYSES OF THE NAKHLITES.  B. E. Cohen1, D. F. Mark1, T. Tom-kinson1, M. R. Lee2, and C. L. Smith3, 1Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride, G75 0QF, UK (ben.cohen@glasgow.ac.uk), 2School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, UK, 3Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK.  Introduction:  The nakhlites are a group of mafic igneous rocks that crystallized on Mars at ca. 1.3-1.4 Ga [1-5]. They are amongst the least shocked Martian meteorites, with cumulate igneous textures (Fig. 1) and thus provide a crucial record of igneous activity and fluid-rock interaction on the red planet.  Crystallization of the nakhlites.  Understanding the original structure of the nakhlite source is crucial for using these stones to explore Martian igneous process-es. In particular, are the different nakhlites from a sin-gle thick and differentiated lava flow/sill [6; 7], or do they instead represent magmatically related – but dis-tinct – flows/intrusions? If the various meteorites are derived from separate units, then there will be differ-ences in crystallization ages within the nakhlite suite – which may be identified if these differences are suffi-ciently large relative to the attainable precision of radi-oisotopic dating techniques. Some studies [e.g., 5] ap-pear to resolve age differences between different stones, and we aim to test the single vs. multiple unit hypotheses via application of detailed 40Ar/39Ar step-heating of six nakhlites. Alteration of the nakhlites, and the timing of water-rock interaction.  In addition to primary magmatic minerals and glasses, the nakhlites contain secondary minerals including clays and carbonates that were pre-cipitated by Martian aqueous fluids prior to impact ejection (Fig. 2) [8-10]. When did this alteration oc-cur? As the clays contain potassium, they are amenable to K-Ar and 40Ar/39Ar dating. K-Ar data from Lafa-yette suggests the alteration phases formed between 0 to 670 Ma [11]. We consider further the timing of al-teration using our 40Ar/39Ar data. Methods:  We applied 40Ar/39Ar chronology to six meteorites: Lafayette, Nakhla, Yamato 000593, Yama-to 000749, Northwest Africa 5790, and Miller Range 03346. These stones cover the mineralogical and chemical diversity within the nakhlite group, in partic-ular spanning from the top to base of the accepted stratigraphic order for the nakhlites [6; 7]. Martian meteorites can contain diverse argon res-ervoirs [4], in particular: trapped Martian atmosphere, 40Ar from radioactive decay of potassium from primary igneous phases, 40Ar from radioactive decay of potas-sium from Martian weathering products, and cosmo-genic Ar. The Ar budget of these meteorites can be further complicated by loss of Ar due to impact ejec-tion, Earth atmospheric entry, and nuclear recoil (spe-cifically 37Ar and 39Ar) due to the requirement to irra-diate samples for 40Ar/39Ar dating. In an attempt to distinguish between the different reservoirs and char-acterize potential loss of Ar, we subjected groundmass separates to high-resolution heating schedules of be-tween 43-45 steps (Fig. 3). Aliquots of clinopyroxene separates (12 steps) and olivine concentrates (10 steps) were also analyzed to understand the Ar budget of the main minerals within the nakhlites [4]. Samples were irradiated for 80 hours in the Cd-lined TRIGA reactor, Oregon State University, and analyzed at SUERC on a MAP 215-50 spectrometer. Data were regressed using the protocol outlined by [4] using the decay constants and standard values of [12], and Martian atmospheric 40Ar/36Ar of 1900 [13].  Fig. 1:  Transmitted-light image of Nakhla, ac-quired between crossed polarizers, showing fresh pla-gioclase crystals (center; dark grey laths) surrounded by pyroxene. Field of view is 0.4 mm.  Fig. 2:  Transmitted light image of Nakhla with veins of brown alteration phases (dominantly smectite and carbonates [10]) cutting across pyroxene crystals. Field of view is 0.4 mm. 1886.pdf46th Lunar and Planetary Science Conference (2015) Fig. 3:  Incremental-heating data for groundmass aliquots of the nakhlite meteorites.   Fig. 4:  Expanded view of the previous diagram, showing the low-temperature steps.   Results and discussion:  40Ar/39Ar data from the nakhlites have some broad similarities (Fig. 3). For all samples, the bulk of the gas released yields apparent ages of ~1.3 Ga, although the age spectra are not com-pletely flat, reflecting unmixing of the diverse Ar res-ervoirs contained in the meteorites. Nevertheless, indi-vidual samples yield near-concordant analyses over considerable portions (~30-50%) of the 39Ar released (Fig. 3) enabling investigation of the age of igneous crystallization for each meteorite. These preliminary results indicate that all of the analyzed nakhlites have similar igneous crystallization ages, but there are dif-ferences between the stones that we will investigate further via additional interrogation of the data. The pyroxene and olivine analyses (not shown) yield simi-lar 1.3 Ga apparent ages, albeit with lower precision due to the lower K contents of these phases. At high temperatures, all of the meteorites yield scattered results, with apparent ages that are much younger than 1.3 Ga. We interpret these high-temperature steps to represent recoil of 39Ar and 37Ar. Lafayette was most susceptible to nuclear recoil, while Yamato 000749 was least susceptible (Fig. 3).  Timing of alteration.  The low-temperature steps yield noticeable differences amongst the meteorites (Fig. 4). Nakhla has low-temperature steps of ~1.1 Ga, while Yamato 000593 and NWA 5790 yield apparent ages as young as ~700 Ma. The young signal is partic-ularly strong for NWA 5790, influencing the first ~40% of the gas released (Fig. 3). On the other hand, Lafayette and Yamato 000749 have low-temperature steps with apparent ages of ~1.3 Ga, similar to the ig-neous crystallization ages, while MIL 03346 has low-temperature steps that approach 1.5 Ga (Fig. 4).  These varied results demonstrate substantial low-temperature Ar heterogeneity amongst the nakhlite analyses. The differences are unlikely to be due to re-coil, as the samples with large amounts of recoil at high temperatures (especially Lafayette) do not have corresponding young ages at low temperatures. In-stead, by analogy with 40Ar/39Ar analyses of altered terrestrial basalts [14], we interpret the young low-temperature steps as being influenced by the Ar budget of the Martian aqueous alteration, with the detailed heating schedules allowing separation of the low-temperature alteration signal from the high-temperature igneous signal [15]. We aim to test this scenario further via 40Ar/39Ar dating of alteration mate-rials from the nakhlites that were encapsulated prior to irradiation to prevent loss of 37Ar and 39Ar due to nu-clear recoil. References:  [1] Nyquist L. E. et al. (2001) Space Sci. Rev., 96, 105-164. [2] Swindle T. D., and Olson E. K. (2004) Meteoritics & Planet. Sci., 39, 755-766. [3] Park J. et al. (2009) GCA, 73, 2177-2189. [4] Cassata W. S. et al. (2010) GCA, 74, 6900-6920. [5] Korochantseva E. V. et al. (2011) Meteoritics & Planet. Sci., 46, 1397-1417. [6] Mikouchi T. et al. (2012) LPS XLIII, Abstract #2363. [7] Mikouchi T. et al. (2006) LPS XXXVII, Abstract #1865. [8] Trieman A. H. et al. (1993) Meteoritics, 28, 86-97. [9] Gooding J. L. et al. (1991) Meteoritics, 26, 135–143. [10] Tomkinson T. et al. (2013) Nature Comm., 4:2662, 1-6. [11] Swindle T. D. et al. (2000) Meteoritics & Planet. Sci., 35, 107-115. [12] Renne P. R. et al. (2011) GCA, 75, 5097-5100. [13] Mahaffy P. R. et al. (2013) Science, 341, 263-266. [14] Foland K. A. et al. (1993) Chem. Geol., 107, 173-190. [15] VanLaningham S., and Mark D. F. (2011) GCA, 75, 2324-2335.  1886.pdf46th Lunar and Planetary Science Conference (2015)

Martian Igneous Activity and Fluid-Based Alteration:Chronological Constraints from 40Ar/39Ar Analyses of the Nakhlites

Cohen, B.E.

Mark, D.F.

Tomkinson, T.

Lee, M.R.

Smith, C.L.

Enlighten

    Cohen, B.E., Mark, D.F., Tomkinson, T., Lee, M.R., and Smith, C.L. (2015) Martian Igneous Activity and Fluid-Based Alteration: Chronological Constraints from 40Ar/39Ar Analyses of the Nakhlites. In: 46th Lunar and Planetary Science Conference, The Woodlands, TX, USA, 16-20 Mar 2015.  Copyright © 2015 The Authors   http://eprints.gla.ac.uk/102432/       Deposited on:  10 February 2015                  Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk MARTIAN IGNEOUS ACTIVITY AND FLUID-BASED ALTERATION: CHRONOLOGICAL CONSTRAINTS FROM 40Ar/39Ar ANALYSES OF THE NAKHLITES.  B. E. Cohen1, D. F. Mark1, T. Tom-kinson1, M. R. Lee2, and C. L. Smith3, 1Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride, G75 0QF, UK (ben.cohen@glasgow.ac.uk), 2School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, UK, 3Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK.  Introduction:  The nakhlites are a group of mafic igneous rocks that crystallized on Mars at ca. 1.3-1.4 Ga [1-5]. They are amongst the least shocked Martian meteorites, with cumulate igneous textures (Fig. 1) and thus provide a crucial record of igneous activity and fluid-rock interaction on the red planet.  Crystallization of the nakhlites.  Understanding the original structure of the nakhlite source is crucial for using these stones to explore Martian igneous process-es. In particular, are the different nakhlites from a sin-gle thick and differentiated lava flow/sill [6; 7], or do they instead represent magmatically related – but dis-tinct – flows/intrusions? If the various meteorites are derived from separate units, then there will be differ-ences in crystallization ages within the nakhlite suite – which may be identified if these differences are suffi-ciently large relative to the attainable precision of radi-oisotopic dating techniques. Some studies [e.g., 5] ap-pear to resolve age differences between different stones, and we aim to test the single vs. multiple unit hypotheses via application of detailed 40Ar/39Ar step-heating of six nakhlites. Alteration of the nakhlites, and the timing of water-rock interaction.  In addition to primary magmatic minerals and glasses, the nakhlites contain secondary minerals including clays and carbonates that were pre-cipitated by Martian aqueous fluids prior to impact ejection (Fig. 2) [8-10]. When did this alteration oc-cur? As the clays contain potassium, they are amenable to K-Ar and 40Ar/39Ar dating. K-Ar data from Lafa-yette suggests the alteration phases formed between 0 to 670 Ma [11]. We consider further the timing of al-teration using our 40Ar/39Ar data. Methods:  We applied 40Ar/39Ar chronology to six meteorites: Lafayette, Nakhla, Yamato 000593, Yama-to 000749, Northwest Africa 5790, and Miller Range 03346. These stones cover the mineralogical and chemical diversity within the nakhlite group, in partic-ular spanning from the top to base of the accepted stratigraphic order for the nakhlites [6; 7]. Martian meteorites can contain diverse argon res-ervoirs [4], in particular: trapped Martian atmosphere, 40Ar from radioactive decay of potassium from primary igneous phases, 40Ar from radioactive decay of potas-sium from Martian weathering products, and cosmo-genic Ar. The Ar budget of these meteorites can be further complicated by loss of Ar due to impact ejec-tion, Earth atmospheric entry, and nuclear recoil (spe-cifically 37Ar and 39Ar) due to the requirement to irra-diate samples for 40Ar/39Ar dating. In an attempt to distinguish between the different reservoirs and char-acterize potential loss of Ar, we subjected groundmass separates to high-resolution heating schedules of be-tween 43-45 steps (Fig. 3). Aliquots of clinopyroxene separates (12 steps) and olivine concentrates (10 steps) were also analyzed to understand the Ar budget of the main minerals within the nakhlites [4]. Samples were irradiated for 80 hours in the Cd-lined TRIGA reactor, Oregon State University, and analyzed at SUERC on a MAP 215-50 spectrometer. Data were regressed using the protocol outlined by [4] using the decay constants and standard values of [12], and Martian atmospheric 40Ar/36Ar of 1900 [13].  Fig. 1:  Transmitted-light image of Nakhla, ac-quired between crossed polarizers, showing fresh pla-gioclase crystals (center; dark grey laths) surrounded by pyroxene. Field of view is 0.4 mm.  Fig. 2:  Transmitted light image of Nakhla with veins of brown alteration phases (dominantly smectite and carbonates [10]) cutting across pyroxene crystals. Field of view is 0.4 mm. 1886.pdf46th Lunar and Planetary Science Conference (2015) Fig. 3:  Incremental-heating data for groundmass aliquots of the nakhlite meteorites.   Fig. 4:  Expanded view of the previous diagram, showing the low-temperature steps.   Results and discussion:  40Ar/39Ar data from the nakhlites have some broad similarities (Fig. 3). For all samples, the bulk of the gas released yields apparent ages of ~1.3 Ga, although the age spectra are not com-pletely flat, reflecting unmixing of the diverse Ar res-ervoirs contained in the meteorites. Nevertheless, indi-vidual samples yield near-concordant analyses over considerable portions (~30-50%) of the 39Ar released (Fig. 3) enabling investigation of the age of igneous crystallization for each meteorite. These preliminary results indicate that all of the analyzed nakhlites have similar igneous crystallization ages, but there are dif-ferences between the stones that we will investigate further via additional interrogation of the data. The pyroxene and olivine analyses (not shown) yield simi-lar 1.3 Ga apparent ages, albeit with lower precision due to the lower K contents of these phases. At high temperatures, all of the meteorites yield scattered results, with apparent ages that are much younger than 1.3 Ga. We interpret these high-temperature steps to represent recoil of 39Ar and 37Ar. Lafayette was most susceptible to nuclear recoil, while Yamato 000749 was least susceptible (Fig. 3).  Timing of alteration.  The low-temperature steps yield noticeable differences amongst the meteorites (Fig. 4). Nakhla has low-temperature steps of ~1.1 Ga, while Yamato 000593 and NWA 5790 yield apparent ages as young as ~700 Ma. The young signal is partic-ularly strong for NWA 5790, influencing the first ~40% of the gas released (Fig. 3). On the other hand, Lafayette and Yamato 000749 have low-temperature steps with apparent ages of ~1.3 Ga, similar to the ig-neous crystallization ages, while MIL 03346 has low-temperature steps that approach 1.5 Ga (Fig. 4).  These varied results demonstrate substantial low-temperature Ar heterogeneity amongst the nakhlite analyses. The differences are unlikely to be due to re-coil, as the samples with large amounts of recoil at high temperatures (especially Lafayette) do not have corresponding young ages at low temperatures. In-stead, by analogy with 40Ar/39Ar analyses of altered terrestrial basalts [14], we interpret the young low-temperature steps as being influenced by the Ar budget of the Martian aqueous alteration, with the detailed heating schedules allowing separation of the low-temperature alteration signal from the high-temperature igneous signal [15]. We aim to test this scenario further via 40Ar/39Ar dating of alteration mate-rials from the nakhlites that were encapsulated prior to irradiation to prevent loss of 37Ar and 39Ar due to nu-clear recoil. References:  [1] Nyquist L. E. et al. (2001) Space Sci. Rev., 96, 105-164. [2] Swindle T. D., and Olson E. K. (2004) Meteoritics & Planet. Sci., 39, 755-766. [3] Park J. et al. (2009) GCA, 73, 2177-2189. [4] Cassata W. S. et al. (2010) GCA, 74, 6900-6920. [5] Korochantseva E. V. et al. (2011) Meteoritics & Planet. Sci., 46, 1397-1417. [6] Mikouchi T. et al. (2012) LPS XLIII, Abstract #2363. [7] Mikouchi T. et al. (2006) LPS XXXVII, Abstract #1865. [8] Trieman A. H. et al. (1993) Meteoritics, 28, 86-97. [9] Gooding J. L. et al. (1991) Meteoritics, 26, 135–143. [10] Tomkinson T. et al. (2013) Nature Comm., 4:2662, 1-6. [11] Swindle T. D. et al. (2000) Meteoritics & Planet. Sci., 35, 107-115. [12] Renne P. R. et al. (2011) GCA, 75, 5097-5100. [13] Mahaffy P. R. et al. (2013) Science, 341, 263-266. [14] Foland K. A. et al. (1993) Chem. Geol., 107, 173-190. [15] VanLaningham S., and Mark D. F. (2011) GCA, 75, 2324-2335.  1886.pdf46th Lunar and Planetary Science Conference (2015)

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Martian Igneous Activity and Fluid-Based Alteration: Chronological Constraints from 40Ar/39Ar Analyses of the Nakhlites

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Martian Igneous Activity and Fluid-Based Alteration:Chronological Constraints from 40Ar/39Ar Analyses of the Nakhlites

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