Cassata, W.S.

Cohen, B.E.

Griffin, S.

Lee, M.R.

Mark, D.F.

Smith, C.L.

Tomkinson, T.

English

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THE NAKHLITES SAMPLE MULTIPLE IGNEOUS UNITS: EVIDENCE FROM 40Ar/39Ar 
CHRONOLOGY AND GEOCHRMISTRY.  B. E. Cohen1,2, D. F. Mark1, M. R. Lee2, W. S. Cassata3, S. Griffin2, 
C. L. Smith4, and T. Tomkinson1, 1Scottish Universities Environmental Research Centre, UK 
(ben.cohen@glasgow.ac.uk), 2School of Geographical and Earth Sciences, University of Glasgow, UK, 3Lawrence 
Livermore National Laboratory, California, USA 4Natural History Museum, London, UK. 
 
 
Introduction:  Mars hosts the solar system’s larg-
est volcanoes. Here we present 40Ar/39Ar ages of six 
nakhlites, meteorites that were ejected from Mars by a 
single impact event. These chronologic results were 
recently published in Nature Communications [1]. 
These age data can also be used to inform the search 
for the nakhlite source crater, and through the geo-
chemistry of the nakhlite meteorites, to show how the 
magmatic system evolved through time. If the location 
of the nakhlite source crater can be confirmed, the 
chronological data for the nakhlites (cosmogenic expo-
sure age for the crater, and age of the source terrain 
lavas) will also provide an ‘absolute’ temporal anchor 
for the surface of Mars that could underpin a Martian 
crater counting calibration model, while the chemical 
data could provide a useful ground-truth point for re-
mote-sensing studies. 
Methods:  High-resolution (n43–45 steps) laser 
step-heating 40Ar/39Ar dating was undertaken on multi-
ple aliquots (n2–5) of six nakhlites [1]. Cosmogenic 
(38Ar) exposure ages were also obtained [1]. The cos-
mogenic exposure ages allow for accurate correction of 
cosmogenic and chlorine-derived contributions to the 
bulk isotope measurements and for determination of 
accurate 40Ar/39Ar age data [2]. The REE data were 
compiled from published sources.  
Results:  Analyses of unirradiated fragments of 
each of the six nakhlites yielded concordant cosmogen-
ic exposure ages, and a weighted mean of 10.7 ± 0.8 
Ma (2σ). These indistinguishable cosmogenic exposure 
ages confirm that the nakhlites are launch-paired, i.e., 
ejected from Mars in a single impact event.  
40Ar/39Ar ages reveal that the nakhlites sample a 
stratigraphically layered volcanic sequence, and tem-
porally constrain at least four discrete eruptive events 
spanning 93 ± 11 Ma (1,416 ± 7 Ma to 1,322 ± 9 Ma 
[2σ], Fig. 1a). Our robust 40Ar/39Ar results are a prod-
uct of the high-resolution approach to the step-heating 
experiments, and employment of procedures to make 
appropriate corrections for cosmogenic and chlorine-
derived 36Ar and 38Ar [2] – which is important given 
the abundance of chlorine in the nakhlites (often 
>1,000 ppm Cl [3]). The highly reproducible age data 
(Fig. 1a) are consistent with the near-pristine character 
of the meteorites and associated low degrees of shock 
metamorphism [4]. 
Discussion:  Our stratigraphic model is consistent 
with the geology of Martian volcanoes. High-
resolution satellite imagery has revealed sequences of 
lava flows, with individual layers typically 4–26 m 
thick [5]. Our model of a layered volcanic sequence 
(Fig. 1) differs from a previous interpretation of the 
nakhlites that invoked sampling from a single thick 
flow/intrusive unit [e.g., 6]. Such a model would re-
quire all of the nakhlites to have the same cooling age, 
which is inconsistent with our 40Ar/39Ar data (Fig. 1). 
 
 
Figure 1. Stratigraphic model for the nakhlite me-
teorites, from [1]. (a) Summary of 40Ar/39Ar age data. 
Each meteorite has multiple aliquots with highly re-
producible ages (red squares). Bold black squares and 
horizontal grey bars represent weighted mean ages. 
(b) Schematic cross-section for a layered lava flow 
sequence, with nakhlite stratigraphic relationships and 
outline of post-impact structure. 
 
Our 40Ar/39Ar and cosmogenic exposure ages con-
strain the provenance of the nakhlites on the surface of 
Mars, including their ejection crater, and properties of 
1892.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
their source volcano. More than seven different Mar-
tian craters have been suggested as potential sources 
for the nakhlites [7-9]. However, when we use the re-
cent re-mapping of the Martian surface by NASA [10], 
we find that only one of these craters is situated in a 
mid-Amazonian volcanic terrain compatible with our 
40Ar/39Ar results (Fig. 2). This crater has preserved 
ejecta rays that are indicative of a recent impact event. 
It has a diameter of 6.5 km, which is large enough for 
the impact to have had sufficient energy to have exca-
vated and ejected material beyond Mars’ orbit [11].  
 
Figure 2. A potential nakhlite source crater, locat-
ed on the Elysium lava plains at 130.799°E, 29.674°N. 
(a) Detail of the crater rim, showing numerous sub-
horizontal layers (arrows), which are interpreted as 
lava flows. Part of HiRISE image ESP 017997_2100, 
NASA/JPL/University of Arizona. (b) Overview of the 
6.5 km wide crater. THEMIS image V13713007, Band 
3, NASA/ASU. 
 
A layered volcanic sequence (Fig. 1b) is consistent 
with the mineralogical, petrological, and geochemical 
differences between nakhlites (e.g., Fig. 3), which we 
interpret as due to changes in magma composition be-
tween related but temporally distinct units. This sce-
nario is unsurprising if one considers an analogy of a 
moderate-sized bolide hitting a volcano on Earth, and 
the number of chemically similar yet temporally dis-
tinct igneous units that could be ejected. An important 
requirement in interpreting nakhlite chemistry is to 
untangle the effects of crystal settling on the data, and 
we plan further investigation of the REE and other 
nakhlite chemical data in order to understand the evo-
lution of their source volcano through time. 
 
 
Figure 3. (a) Compilation of published REE data 
demonstrating the nakhlites have a considerable com-
positional range. (b) The REE slope (i.e., La/Lu) also 
differs significantly between nakhlites, which is signif-
icant as the ratio is sensitive to variations in degree of 
melting, and is not substantially effected by crystal 
fractionation. Ages are from [1]. Note that NWA 817 
and Gov. Valadares lack precise ages. 
 
References: [1] Cohen B. E. et al. (2017) Nat. 
Comm., 8, 1–9. [2] Cassata W. S. and Borg L. E. 
(2016) GCA, 187, 279-293. [3] Cartwright J. A. et al. 
(2013) GCA, 105, 255-293. [4] Treiman A. H. (2005) 
Chemie der Erde – Geochem., 65, 203-270. 
[5] Keszthelyi L. et al. (2008) JGR, 113, E04005. 
[6] Mikouchi T. (2012) LPS XLIII, Abstract #2363. 
[7] Harvey R. P. and Hamilton V. E. (2005) LPS 
XXXVI, Abstract #1019. [8] Tornabene L. L. et al. 
(2006) JGR, 111, E10006. [9] Kereszturi A. and Chat-
zitheodoridis E. (2016) Orig. Life Evol. Biosph., 46, 
455-471. [10] Tanaka K. L. (2014) USGS Map, 3292. 
[11] Head J. N. et al. (2017) Science, 298, 1752–1756. 
Acknowledgements:  We thank the U.K. STFC for 
funding, and the Smithsonian, NASA Meteorite Work-
ing Group, NHM London, Macovich Collection and 
the Japanese Antarctic Meteorite Research Centre for 
providing samples. 
1892.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


The Nakhlites Sample Multiple Igneous Units: Evidence From 40Ar/39Ar Chronology And Geochemistry

Enlighten

THE NAKHLITES SAMPLE MULTIPLE IGNEOUS UNITS: EVIDENCE FROM 40Ar/39Ar CHRONOLOGY AND GEOCHRMISTRY.  B. E. Cohen1,2, D. F. Mark1, M. R. Lee2, W. S. Cassata3, S. Griffin2, C. L. Smith4, and T. Tomkinson1, 1Scottish Universities Environmental Research Centre, UK (ben.cohen@glasgow.ac.uk), 2School of Geographical and Earth Sciences, University of Glasgow, UK, 3Lawrence Livermore National Laboratory, California, USA 4Natural History Museum, London, UK.   Introduction:  Mars hosts the solar system’s larg-est volcanoes. Here we present 40Ar/39Ar ages of six nakhlites, meteorites that were ejected from Mars by a single impact event. These chronologic results were recently published in Nature Communications [1]. These age data can also be used to inform the search for the nakhlite source crater, and through the geo-chemistry of the nakhlite meteorites, to show how the magmatic system evolved through time. If the location of the nakhlite source crater can be confirmed, the chronological data for the nakhlites (cosmogenic expo-sure age for the crater, and age of the source terrain lavas) will also provide an ‘absolute’ temporal anchor for the surface of Mars that could underpin a Martian crater counting calibration model, while the chemical data could provide a useful ground-truth point for re-mote-sensing studies. Methods:  High-resolution (n43–45 steps) laser step-heating 40Ar/39Ar dating was undertaken on multi-ple aliquots (n2–5) of six nakhlites [1]. Cosmogenic (38Ar) exposure ages were also obtained [1]. The cos-mogenic exposure ages allow for accurate correction of cosmogenic and chlorine-derived contributions to the bulk isotope measurements and for determination of accurate 40Ar/39Ar age data [2]. The REE data were compiled from published sources.  Results:  Analyses of unirradiated fragments of each of the six nakhlites yielded concordant cosmogen-ic exposure ages, and a weighted mean of 10.7 ± 0.8 Ma (2σ). These indistinguishable cosmogenic exposure ages confirm that the nakhlites are launch-paired, i.e., ejected from Mars in a single impact event.  40Ar/39Ar ages reveal that the nakhlites sample a stratigraphically layered volcanic sequence, and tem-porally constrain at least four discrete eruptive events spanning 93 ± 11 Ma (1,416 ± 7 Ma to 1,322 ± 9 Ma [2σ], Fig. 1a). Our robust 40Ar/39Ar results are a prod-uct of the high-resolution approach to the step-heating experiments, and employment of procedures to make appropriate corrections for cosmogenic and chlorine-derived 36Ar and 38Ar [2] – which is important given the abundance of chlorine in the nakhlites (often >1,000 ppm Cl [3]). The highly reproducible age data (Fig. 1a) are consistent with the near-pristine character of the meteorites and associated low degrees of shock metamorphism [4]. Discussion:  Our stratigraphic model is consistent with the geology of Martian volcanoes. High-resolution satellite imagery has revealed sequences of lava flows, with individual layers typically 4–26 m thick [5]. Our model of a layered volcanic sequence (Fig. 1) differs from a previous interpretation of the nakhlites that invoked sampling from a single thick flow/intrusive unit [e.g., 6]. Such a model would re-quire all of the nakhlites to have the same cooling age, which is inconsistent with our 40Ar/39Ar data (Fig. 1).   Figure 1. Stratigraphic model for the nakhlite me-teorites, from [1]. (a) Summary of 40Ar/39Ar age data. Each meteorite has multiple aliquots with highly re-producible ages (red squares). Bold black squares and horizontal grey bars represent weighted mean ages. (b) Schematic cross-section for a layered lava flow sequence, with nakhlite stratigraphic relationships and outline of post-impact structure.  Our 40Ar/39Ar and cosmogenic exposure ages con-strain the provenance of the nakhlites on the surface of Mars, including their ejection crater, and properties of 1892.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)their source volcano. More than seven different Mar-tian craters have been suggested as potential sources for the nakhlites [7-9]. However, when we use the re-cent re-mapping of the Martian surface by NASA [10], we find that only one of these craters is situated in a mid-Amazonian volcanic terrain compatible with our 40Ar/39Ar results (Fig. 2). This crater has preserved ejecta rays that are indicative of a recent impact event. It has a diameter of 6.5 km, which is large enough for the impact to have had sufficient energy to have exca-vated and ejected material beyond Mars’ orbit [11].   Figure 2. A potential nakhlite source crater, locat-ed on the Elysium lava plains at 130.799°E, 29.674°N. (a) Detail of the crater rim, showing numerous sub-horizontal layers (arrows), which are interpreted as lava flows. Part of HiRISE image ESP 017997_2100, NASA/JPL/University of Arizona. (b) Overview of the 6.5 km wide crater. THEMIS image V13713007, Band 3, NASA/ASU.  A layered volcanic sequence (Fig. 1b) is consistent with the mineralogical, petrological, and geochemical differences between nakhlites (e.g., Fig. 3), which we interpret as due to changes in magma composition be-tween related but temporally distinct units. This sce-nario is unsurprising if one considers an analogy of a moderate-sized bolide hitting a volcano on Earth, and the number of chemically similar yet temporally dis-tinct igneous units that could be ejected. An important requirement in interpreting nakhlite chemistry is to untangle the effects of crystal settling on the data, and we plan further investigation of the REE and other nakhlite chemical data in order to understand the evo-lution of their source volcano through time.   Figure 3. (a) Compilation of published REE data demonstrating the nakhlites have a considerable com-positional range. (b) The REE slope (i.e., La/Lu) also differs significantly between nakhlites, which is signif-icant as the ratio is sensitive to variations in degree of melting, and is not substantially effected by crystal fractionation. Ages are from [1]. Note that NWA 817 and Gov. Valadares lack precise ages.  References: [1] Cohen B. E. et al. (2017) Nat. Comm., 8, 1–9. [2] Cassata W. S. and Borg L. E. (2016) GCA, 187, 279-293. [3] Cartwright J. A. et al. (2013) GCA, 105, 255-293. [4] Treiman A. H. (2005) Chemie der Erde – Geochem., 65, 203-270. [5] Keszthelyi L. et al. (2008) JGR, 113, E04005. [6] Mikouchi T. (2012) LPS XLIII, Abstract #2363. [7] Harvey R. P. and Hamilton V. E. (2005) LPS XXXVI, Abstract #1019. [8] Tornabene L. L. et al. (2006) JGR, 111, E10006. [9] Kereszturi A. and Chat-zitheodoridis E. (2016) Orig. Life Evol. Biosph., 46, 455-471. [10] Tanaka K. L. (2014) USGS Map, 3292. [11] Head J. N. et al. (2017) Science, 298, 1752–1756. Acknowledgements:  We thank the U.K. STFC for funding, and the Smithsonian, NASA Meteorite Work-ing Group, NHM London, Macovich Collection and the Japanese Antarctic Meteorite Research Centre for providing samples. 1892.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

40Ar/39Ar ages of six nakhlites yield ages spanning 93 ± 11 Ma, and show these meteorites sample at least four discrete eruptive events

Cohen, B. E.

Mark, Darren F.

Lee, M. R.

Cassata, W. S.

Griffin, S

Smith, C. L.

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The Nakhlites Sample Multiple Igneous Units:Evidence from 40Ar/39Ar Chronology and Geochemistry

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The Nakhlites Sample Multiple Igneous Units: Evidence From 40Ar/39Ar Chronology And Geochemistry

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