Halogen and Cl isotopic systematics in Martian phosphates: Implications for the Cl
1
cycle and surface halogen reservoirs on Mars
2 3
Authors: J. J. Bellucci1, M.J. Whitehouse1, T. John2, A.A. Nemchin1,3, J. F. Snape1, P. A.
4
Bland3, and G.K. Benedix3.
5 6
Affiliations: 1Department of Geosciences, Swedish Museum of Natural History, SE-104
7
05 Stockholm, Sweden. 2Institut für Geologische Wissenschaften, Freie Universität
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Berlin, Malteser Str. 74-100, 12249 Berlin, Germany. 3Department of Applied Geology,
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Curtin University, Perth, WA 6845, Australia 10
11
*Corresponding Author’s email: jeremy.bellucci@gmail.com Phone: +46761742750 12
13 14
Abstract:
15
The Cl isotopic compositions and halogen (Cl, F, Br, and I) abundances in 16
phosphates from eight Martian meteorites, spanning most rock types and ages currently 17
available, have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). 18
Likewise, the distribution of halogens has been documented by x-ray mapping. Halogen 19
concentrations range over several orders of magnitude up to some of the largest 20
concentrations yet measured in Martian samples or on the Martian surface, and the inter-21
element ratios are highly variable. Similarly, Cl isotope compositions exhibit a larger 22
range than all pristine terrestrial igneous rocks. Phosphates in ancient (>4 Ga) meteorites 23
(orthopyroxenite ALH 84001 and breccia NWA 7533) have positive 37Cl anomalies
24
(+1.1 to +2.5 ‰). These samples also exhibit explicit whole rock and grain scale 25
evidence for hydrothermal or aqueous activity. In contrast, the phosphates in the younger 26
basaltic Shergottite meteorites (<600 Ma) have negative 37Cl anomalies (-0.2 to -5.6 ‰).
27
Phosphates with the largest negative 37Cl anomalies display zonation in which the rims
28
of the grains are enriched in all halogens and have significantly more negative 37Cl
29
anomalies suggestive of interaction with the surface of Mars during the latest stages of 30
basalt crystallization. The phosphates with no textural, major element, or halogen 31
enrichment evidence for mixing with this surface reservoir have an average 37Cl of
-32
0.6 ‰, supporting a similar Cl isotope composition for Mars, the Earth, and the Moon. 33
Oxidation and reduction of chlorine is the only process known to strongly fractionate Cl 34
isotopes, both positively and negatively, and perchlorate has been detected in weight 35
percent concentrations on the Martian surface. The age range and obvious mixing history 36
of the phosphates studied here suggest perchlorate formation and halogen cycling via 37
brines, which have been documented on the Martian surface, has been active throughout 38
Martian history. 39
40
*Manuscript
Introduction
41
The behavior of volatile species on a planetary body is key to understanding its 42
late accretionary history, low-temperature atmospheric/surface chemistry, and by 43
inference, potential for life. The halogens (Cl, F, Br, and I) are amongst the most 44
hydrophilic elements on a planetary body and, as such, quantification of their abundances 45
in, and transfer between, various geochemical reservoirs has great potential as a tracer of 46
surface processes. Specifically, halogens form ionic complexes in the presence of water, 47
which inextricably links them to the hydrological cycle. Halogen complexes (e.g., halide 48
salts (NaCl, KCl) or perchlorate (ClO4-) compounds) have significant effects, both
49
positive and potentially negative, on biology (e.g, Srinivasan and Viraraghavan, 2009). 50
Additionally, perchlorate can serve as an energy source for anaerobic reduction in 51
microorganisms (Coats and Achenbach, 2004). 52
Halogens are in the same periodic group and are strongly incompatible during 53
mantle melting but may show some overall dependence on pressure-temperature 54
conditions (Joachim et al., 2015, Kendrick et al., 2012, Saal et al., 2002). Additionally, 55
oxidized species of halogens (e.g., perchlorate and iodate) occur together on Earth in arid 56
environments, providing further evidence for similar geochemical behavior in variable 57
oxidation states (Lybrand et al., 2016). Together with abundances of the halogens, the Cl 58
isotopic system yields important insights into the interactions between surficial, low-59
temperature reservoirs because Cl isotopes do not fractionate significantly in planetary 60
accretion, magmatic processes, or hydrothermal alteration (Chiaradia et al. 2014; John et 61
al. 2010; Kusebauch et al., 2015a; Selverstone and Sharp, 2011; Sharp et al., 2007, 2010, 62
2013). This has resulted in a similar Cl isotope composition between the Earth’s mantle, 63
its crust, the Moon, and primitive meteorites (Kusebauch et al., 2015a, John et al. 2010; 64
Sharp et al., 2007, 2010, 2013). Phosphate minerals (e.g., apatite, merrillite) are 65
ubiquitous in igneous rocks across a wide range of whole rock compositions and are 66
extremely sensitive to post-crystallization fluid-driven alteration and/or metamorphic 67
processes (Engvik et al. 2009; Kusebauch et al. 2015a). Since they control the majority of 68
the halogen budget and the Cl isotopic composition of an igneous rock or meteorite (e.g., 69
Rozszjar et al., 2011), they provide an unparalleled tracer of processes affecting these 70
elements, as well as being amenable to high spatial resolution in situ investigation on a 71
sub-mineral grain scale. 72
Mars is enriched in halogens compared to Earth (Sharp and Draper, 2013) and 73
highly stratified, with orders of magnitude higher abundances of halogens in the crust 74
relative to the mantle (Filiberto et al., 2016, Hecht et al., 2009, Keller et al., 2007, 75
McCubbin et al., 2016). Significant progress has been made in understanding the surface 76
halogen and chloride chemistry of the surface of Mars via orbiters and landers (e.g., 77
Farley et al., 2016, Hecht et al., 2009, Keller et al., 2007, Martin-Torres et al., 2015), but 78
how the hydrosphere and atmosphere of Mars interact with the lithosphere remains 79
ambiguous. The only samples currently available to study directly these interactions on 80
Mars are Martian meteorites. Martian meteorites are currently categorized into six 81
groups: Shergottites, an augite-basalt (NWA 8159), Chassignites, Nakhlites, an 82
orthopyroxenite ALH 84001, and NWA 7533 and its pairs (e.g., Agee et al., 2013, Agee 83
et al., 2014, Humayun et al., 2013, Mittlefehldt, 1994, Nyquist et al., 2001 and references 84
therein). Shergottites are the most widely available, classified into basaltic and lherzolitic 85
types (e.g., Nyquist et al., 2001) and further subdivided, on the basis of light rare earth 86
element concentrations and radiogenic isotope compositions, into a spectrum from 87
depleted, through intermediate, to enriched varieties (e.g., Borg et al., 2005, Brennecka et 88
al., 2014). The enriched Shergottites generally have a younger age (~170 Ma) than the 89
depleted Shergottites (~300-580 Ma) (e.g., Borg et al., 2005, Brennecka et al., 2014). 90
Phosphates in some Martian meteorites are explicitly not in igneous equilibrium 91
with their host rocks, having widely varying compositions on a sub-grain scale, complex 92
inner structures, and a chemistry that strongly indicates interaction with the Martian crust 93
or a Cl-rich crustal fluid (Howarth et al., 2016, McCubbin et al., 2016, Shearer et al., 94
2015). In order to investigate and try to constrain the halogen and chlorine cycle on Mars, 95
the halogen concentrations and Cl isotopic composition of the phosphates of eight 96
Martian meteorites, spanning most of the available Martian lithologies, have been 97
investigated here by Secondary Ion Mass Spectrometry (SIMS) and by electron 98
microprobe x-ray mapping. The samples investigated in this study include six 99
Shergottites (4 enriched, 2 depleted), the ultramafic cumulate (ALH 84001), and the only 100
explicitly crustal sample from Mars (NWA 7533). Two additional Shergottites, Nakhla, 101
and two sections of Chassigny were also surveyed but yielded no suitable targets. This 102
sample set, containing two ancient samples (>4 Ga) that are most likely derived from the 103
Southern Hemisphere and six young (<600 Ma) samples that are suggested to originate 104
from the Northern Hemisphere, based on crystallization ages and impact crater densities 105
(e.g., Nyquist et al., 2001), therefore allows the investigation of the halogen and Cl 106
isotopic cycle on Mars from different reservoirs, surface locations, and times. 107
108
Martian phosphates
Northwest Africa (NWA) 7533 is identified as a regolith breccia (Agee et al., 110
2013, Humayun et al., 2014). NWA 7533 has evolved clasts that contain zircon and 111
phosphates with U-Pb ages of 4.42 Ga and 1.35 Ga, respectively (Bellucci et al., 2015a, 112
Humayun et al., 2013). As a regolith breccia, there is likely to be a difference between 113
clast- and matrix-hosted apatites, the former linked to formation at 4.42 Ga while the 114
latter can, in principle, have crystallized originally or could have been modified at any 115
time between 4.42 Ga and 1.35 Ga (Bellucci et al., 2015a). The presence of sulfides in 116
cracks is a critical observation (Lorrand et al., 2015) that points to likely hydrothermal 117
activity during amalgamation of the breccia at 1.35 Ga. Additionally, a single merrillite 118
grain in a pair of NWA 7533 shows replacement by Cl-rich apatite indicating interactions 119
with Cl-rich fluids (Shearer et al., 2015). 120
Allan Hills (ALH) 84001 is a coarse-grained orthopyroxenite. ALH 84001 has a 121
crystallization age of 4091 ± 30 Ma determined by 176Lu-176Hf (2 , Lapen et al., 2010).
122
ALH 84001 has experienced post-crystallization aqueous alteration that precipitated 123
carbonates at 3900 ± 40 to 4040 ± 100 Ma based on Rb-Sr and Pb-Pb isochrons, 124
respectively (Borg et al., 1999). The analyzed phosphate in ALH84001 was apatite. 125
The Shergottites studied here encompass both the enriched and depleted varieties. 126
The enriched Shergottites analyzed in this study are Roberts Massif (RBT) 04262, 127
Larkman Nunatuk (LAR) 12011, NWA 4864, and Zagami, with the depleted Shergottites 128
represented by Tissint and Sayh al Uhamymir (SaU) 005. The crystallization ages of 129
these samples are all < 600 Ma (e.g., Bellucci et al., 2015b, Brenneka et al., 2014, Borg et 130
al., 2005, Lapen et al., 2008, Shafer et al., 2010, Table 1). Based on previous literature, 131
the apatites and merrillites in RBT 04262 and LAR 12011 are explicitly not in 132
equilibrium with the host rock, are zoned in F, Cl, and OH- and likely have interacted 133
with Cl-rich fluids on the Martian surface, while Zagami has apatite in equilibrium with 134
the whole rock and has been used to calculate a magmatic water content (Howarth et al., 135
2016, McCubbin et al., 2016). Published information is unavailable for the rest of the 136 analyzed Shergottites. 137 138 Analytical methods 139 140
Halogen concentrations (Cl, F, Br, and I) and stable Cl isotopic compositions 141
(reported as 37Cl, where 37Cl = [(37
Cl/35Cl)sample/(37Cl/35Cl)standard – 1]*1000, expressed
142
relative to Standard Mean Ocean Chloride with a defined value of 0.0 ‰; Kaufman et al., 143
1984) were measured using a CAMECA IMS1280 large-geometry secondary ion mass 144
spectrometer at the NordSIMS facility, Swedish Museum of Natural History. The 145
procedures used here closely follow those of Kusebauch 2015b,c and Marks et al., 2012. 146
Samples (polished thin sections or epoxy-mounted, polished rock chips) were first 147
cleaned using ethanol and distilled water and then coated with 30 nm of Au. Prior to spot 148
analyses, the analytical positions were sputtered for 120 s over a 25 x 25 m raster area 149
to remove the Au coating and any remaining surface contamination. For both 150
concentration and isotope analysis, secondary ions of halogens species were sputtered 151
from the surface using a ~15 m, ~2 nA Cs+ Gaussian focused, raster- (10 m)
152
homogenized primary beam with an impact energy of 20 kV. An low-energy, normal-153
incidence electron beam was used in conjunction with the primary Cs+ beam to
154
counteract any charging effects on the surface of the sample. Secondary ions were 155
centered in a 2500 m field aperture, which provides a ca. 30 m field of view on the 156
sample that is smaller than the combined spot and pre-sputter area, thus precluding 157
admission of surface contaminant halogens to the mass spectrometer. For halogen 158
concentration measurements, secondary ions were filtered at high mass resolution (M/ M 159
= 5000) and species of 19F-, 37Cl-, 127I-, and a matrix peak of 40Ca31P- were then measured
160
in either an electron multiplier (<106 cps) or Faraday cup (>106 cps) in mono-collection
161
mode using a peak hopping routine. Measurement of an interference-free Br peak (at 162
mass 79 or 81) would require a mass resolution of >13000 to separate it from CaCl 163
molecular interferences, which would entail a significant loss of sensitivity. Instead, a 164
combined 81Br- + 44Ca35Cl- + 46Ca37Cl- peak was measured and stripped of its CaCl
165
interferences during data reduction using the measurement of the 40Ca37Cl- species,
166
assuming the natural isotopic abundances of Ca and Cl. Ratios of 19F-, 37Cl-, 81Br and 127I
-167
to the matrix 40Ca31P- species were used to calculate concentrations using relative
168
sensitivity factors determined by analyses of the Durango apatite standard during the 169
same session. Concentrations for F, Cl, Br, and I assumed for Durango are 33500, 4099, 170
0.84, and 0.73 g/g (values from Kusebauch et al., 2015c). When using a low 171
concentration standard, unknowns with high concentrations may be slightly inaccurate. 172
However, this inaccuracy will manifest as a systematic multiplier and not affect the 173
relative differences in concentrations between samples. External precision for two 174
analytical sessions (1 , n=32) for F, Cl, Br, and I for Durango apatite were 3%, 5%, 16%, 175
and 7%, respectively. 176
Cl isotopic compositions were measured in adjacent spots in the same grains as 177
the halogen concentrations where possible or in adjacent grains where necessary. The 178
secondary ions 35Cl- and 37Cl- were filtered at a mass resolution of 2500 (M/ M) and 179
measured simultaneously using two Faraday cups in four blocks of ten integrations 180
resulting in an overall counting time of 160 s. Magnet stability was maintained using an 181
NMR field sensor in regulation mode. Measurements of two different apatite standards 182
(Durango and a pure, synthetic Cl-apatite) of known composition ( 37Cl of 2.0‰ and
183
0.5‰, respectively from Kusebauch et al., 2015c) were interspersed with unknown 184
apatite analyses. To account for a matrix dependent variation in instrumental mass bias of 185
up to ca. 0.5 ‰ resulting from variable Cl content, a linear correction based on drift-186
corrected Cl intensities was applied to all unknown isotopic compositions relative to 187
intensity and isotopic composition of the bracketing apatite standards. External precision 188
(1 ) over two analytical sessions on Durango apatite (n=49) and the synthetic apatite 189
(n=39) were both 0.14 ‰. Corrected concentration measurements and Cl isotopic 190
compositions are presented in Table 1, while uncorrected Cl isotopic ratios for standards 191
and unknowns are presented in Supplementary Tables 1-2, respectively. 192
X-ray maps were made using a JEOL JXA 8200 Superprobe at the Freie 193
Universität, Berlin. The instrument is equipped with five WDS detectors and an EDS 194
detector for quantitative analyses. All x-ray maps were made using a 15kV, 4nA, and a 2-195
5 m wide electron beam. Species of Si, Cl, F, Ca, Fe, and P x-ray images were collected 196
simultaneously with a resolution of 2-5 m. Counts of each element on each spot were 197
then converted to a numerical matrix, which was exported and processed offline-using 198
Matlab to generate the x-ray maps (Supplementary Figures, after Kusebauch et al., 199
2015c). 200
Results
202
The halogen concentrations measured in phosphates of the eight samples span 203
several orders of magnitude, with highly variable halogen ratios (Cl/I, Cl/Br, and Cl/F), 204
and a range in 37Cl from -5.6 to +2.5‰, the latter being in general agreement with Cl
205
isotopic measurements by previous workers (Sharp et al., 2016, Figure 1, Figure 2, Table 206
1). The rims of phosphates in sample LAR12011 have the most negative 37Cl of any
207
Martian meteorite phosphate measured to date and contain orders of magnitude more 208
halogens. Phases measured in these samples are both merrillite and apatite and are listed 209
in Table 1. Given the complex intergrowth and potential secondary processes acting on 210
phosphates in Martian meteorites (Figure 3, Howarth et al., 2016, McCubbin et al., 2016, 211
Shearer et al., 2015), the mineral species listed in Table 1 are the only ones that have 212
been confidently identified post analysis. Unlisted species may be a combination of 213
apatite or merrillite as these minerals often occur inter-grown in these samples. 214
Phosphates from ancient samples ALH 84001 and NWA 7533 have positive 37Cl
215
anomalies; while the phosphates from younger basalts have negative or no 37Cl
216
anomalies relative to bulk Earth, Moon, and chondrites. Importantly, it is evident that the 217
most non-zero 37Cl compositions, both positive and negative, correlate with increased
218
concentrations in all halogens and have variable inter-element halogen ratios (Figure 1, 219
Figure 2). 220
Electron probe element x-ray maps of the phosphates in LAR 12011 indicate 221
marked zoning with crystal margins being enriched in halogens with different halogen 222
ratios (Figure 3A,B), which is in agreement with previous studies (Howarth et al., 2016, 223
McCubbin et al., 2016). As the zoned margins form a strictly defined area surrounding 224
more pristine cores (Figure 3A,B), this enrichment must have occurred via a fluid 225
mediated replacement process (Engvik et al. 2009, Kusebauch et al., 2015b, Putnis 2002) 226
during interaction with surface brines during the final stages of phosphate crystallization 227
(e.g., Figure 3B). Importantly, the zone of halogen enrichment corresponds with a more 228
pronounced negative 37Cl value (-6 ‰ vs -3 ‰). This enrichment in halogens is absent
229
from the phosphates in SaU 005, NWA 4864, Tissint, and Zagami (Figure 3C, McCubbin 230
et al., 2016), which show no chemical zonation and have the least extreme 37Cl values of
231
all of the meteorites studied. Similarly, these samples show halogen concentrations and 232
halogen ratios that group together indicating they have not experienced this process, 233
which is in agreement with previous studies (Figure 3C, McCubbin et al., 2016). 234
The phosphates from the two oldest samples, NWA 7533 and ALH 84001, have 235
highly contrasted halogen and Cl isotope characteristics, when compared to the basaltic 236
samples. Phosphates within the 4.428 Ga clasts of crustal breccia sample NWA 7533 237
display relatively constant halogen and Cl isotopic compositions ( 37Cl ~+2‰), while
238
those within the matrix have a significantly larger range in both halogen concentration 239
and Cl isotopic compositions ( 37Cl from +1.4 to +2.5‰) and display a larger variation in
240
halogen vs. halogen ratios. X-ray maps of clast phosphates show replacement-indicative 241
zoning in F and Cl (Figure 3D), with even Si-enriched material in some minerals (Figure 242
4) (e.g., Engvik et al., 2009; Kusebauch et al., 2015c). In contrast, phosphate in 243
orthopyroxenite sample ALH 84001 has a 37Cl composition of +1.1 ‰ and displays no
244
zoning in F and Cl (Figure 3E). 245
246
Discussion
Halogens in Phosphates
248
The phosphates that are not in equilibrium with the whole rock or that show 249
textures, such as replacement rims or zoning indicative of igneous activity, have non-250
zero 37Cl and varying halogen concentrations that must have been imposed after
251
eruption at or near the surface of Mars. All halogen ratios vs. halogen concentration 252
diagrams (Figure 2) illustrate that phosphates with near zero 37Cl have similar halogen
253
concentrations, while those with non-zero 37Cl have varying halogen concentrations and
254
ratios. The only exception to this trend is Zagami in Cl/F vs. F (Figure 2), which shows a 255
larger range than the other meteorites. As seen in electron microprobe maps of LAR 256
12011 (Figure 3A,B), some of the cores of the phosphates in this sample have similar 257
halogen ratios and concentrations to that of non-altered basalts. However, the rims have 258
strongly contrasting halogen concentrations and halogen ratios, with Cl/Br and Cl/I being 259
significantly elevated and close to those measured in the alteration products of the 260
Nakhlites, the Martian surface, and NWA 7533 (Figure 2, Cartwright et al., 2013, Rao et 261
al., 2005, 2009). The variability in halogen ratios vs. halogen concentrations (Figure 2) 262
indicates that several varieties of halogen-rich fluids may be responsible for the 263
compositional ranges seen here. This observation adds additional chemical evidence of 264
Cl- and halogen-rich fluid interactions on Mars, similar to what is measured in the major 265
element variability in phosphates from these and other Martian samples, as well as the 266
Martian surface (McCubbin et al., 2016, Rao et al., 2005, 2009). 267
The phosphates in NWA 7533 have the highest concentrations of halogens yet 268
measured for materials from or on Mars. The trend measured in NWA7533 in Cl/Br vs. 269
Br is greater than that measured in alteration products in Nakhla (Cartwright et al., 20013 270
and Rao et al., 2005) and is parallel to the trend observed in the Meridiani rocks 271
measured by the Opportunity rover, which was considered to be indicative of two phases 272
of groundwater recharge (Rao et al., 2009). A similar but more restricted trend is seen in 273
Cl/I vs. I (Figure 2). The large compositional ranges in these samples, the similarity to 274
mixing seen on the Martian surface, and the replacement textures already identified (e.g., 275
Figures. 3, 4, Shearer et al., 2015) are all strong evidence of surface hydrothermal 276
processes having a dominant influence on the halogen signature of the phosphates in this 277 sample. 278 279 Origin of variability in 37Cl 280
Samples that show no evidence for alteration, secondary processes, and elevation 281
in halogen concentrations can be used to estimate a Martian mantle 37Cl composition of
282
-0.6 ‰ (Table 1). In other samples, however, the spread in 37Cl measured here is larger
283
than can be generated by igneous or hydrothermal activity alone(Chiaradia et al. 2014;
284
John et al. 2010; Kusebauch et al. 2015b,c; Selverstone and Sharp, 2011; Sharp et al. 285
2007, 2010, 2013). Observations from the surface of Mars suggest aqueous activity and 286
document high concentrations of halogens (Hecht et al., 2009, Keller et al., 2006, Martín-287
Torres et al., 2015). Potential daily hydrological activity, in the form of brines, which by 288
definition have high concentrations of salts, has been suggested (Martín-Torres et al., 289
2015). The Mars Odyssey Gamma Ray Spectrometer (GRS) has measured the 290
concentration of Cl at the surface of Mars at 5000 g/g (Keller et al., 2006) and the 291
aqueous laboratory on Phoenix Mars Lander subsequently identified the likely carrier 292
species as perchlorate (ClO4-/Cl- = 4.4, Hecht et al., 2009). Similarly, perchlorate has
been directly observed in Martian meteorite EETA 79001 (Kounaves et al., 2014). Since 294
the surface of Mars is enriched by several orders of magnitude in Cl compared to 295
phosphates that are in equilibrium with the host rock (5000 g/g vs ~80 g/g, 296
respectively) even very limited mixing (<1.4%) between the crust and mantle derivatives 297
will completely overprint any mantle Cl signatures originally present in the phosphates 298
(Figure 5). 299
Formation of perchlorate on Earth fractionates Cl isotopes both positively and 300
negatively in 37Cl, resulting in values ranging from -14 to +5 ‰ (Bohlke et al., 2005,
301
Jackson et al., 2010). No specific mechanism or fractionation factor for atmospheric 302
chlorine oxidation is reported, but perchlorate formed by oxidation in arid deserts has the 303
only strong negative 37Cl measured to date. While atmospheric oxidation can explain the
304
significantly negative 37Cl values, the processes that generate the positive values are
305
currently unknown (Jackson et al., 2010). Natural perchlorate and iodate are present on 306
Earth and co-vary in arid deserts via formation by atmospheric photochemistry (e.g., the 307
Atacama, Bohlke et al., 2005, Jackson et al., 2010, Lybrand et al., 2016), which is
308
comparable to the surface environment of Mars. Extremely negative 37Cl values of up to
309
-51 ±5 ‰ have been measured at Gale Crater and are attributed to oxidative perchlorate 310
formation and reduction (Farley et al., 2016). However, the quantity of perchlorate 311
formed via the oxidation of chlorine by ozone is probably insufficient to explain the 312
concentrations observed on the surface (Smith et al., 2014). Instead, the abundance of 313
perchlorate on the Martian surface is likely a function of two formation pathways: 314
atmospheric oxidation and photocatalysis by metal oxides in the Martian soil (Carrier et 315
al., 2015, Catling et al., 2010). While atmospheric formation of perchlorate results in 316
large fractionations in 37Cl, the fractionation effects of the photocatalysis pathway on 317
37Cl in the Martian soil is unknown.
318
The formation processes of perchlorate are not well understood on Earth or on 319
Mars. Similarly, the halogen cycle and behavior of the halogens is unknown on Mars. 320
However, perchlorate has been detected at every landing site. The ratio of ClO4-/Cl- was
321
measured as 4.4 in at least one surface location on Mars (Hecht et al., 2009) and suggests 322
that the oxidized Cl species is the most prevalent with a 37Cl value of -51‰ (Farley et
323
al., 2016). Precipitation of chlorine species does not greatly fractionate Cl isotopes, if 324
there are no oxidation state changes; hence atmospheric perchlorate precipitation onto the 325
Martian surface is unlikely to affect the Cl isotopic signature (Sharp et al., 2007; Sharp et 326
al. 2013). Oxidation is the only mechanism by which to generate large negative 37Cl
327
anomalies and reduction of chlorine creates positive 37Cl anomalies. While the interplay
328
between the two processes on Mars is largely unknown, it has been used convincingly to 329
explain the Cl isotope composition measured on the Martian surface (-20 to -51 ‰, 330
Farley et al., 2016). Therefore, the most straightforward conclusion is that isotopically 331
distinct Cl, along with other halogen species present on the Martian surface, mixed with 332
mantle-derived halogens and Cl isotopes originally present in igneous phosphates to 333
produce the isotopic compositions and halogen concentrations measured here. 334
335
Incorporating surface Cl into old and young samples
336
The observations that the formation of perchlorate is capable of fractionating Cl 337
isotopes in both positive and negative directions and its presence at one location on the 338
Martian surface of 4.4 x that of Cl- imply that the extreme negative 37Cl values obtained
from the phosphates measured here likely result from a three-stage process (illustrated 340
schematically in Figure 6). This process comprises: 1) generation of the anomalous Cl 341
isotopic signature by photochemical reactions in the atmosphere or the Martian soil 342
( 37Cl = -55 to +5 ‰); 2) incorporation of that Cl into surface brine(s) via evaporation
343
and condensation ( 37Cl = -55 to +5 ‰); and 3) along with the other halogens, which
344
occur together in arid environments, infiltrated/mixed with primary basaltic phosphates 345
( 37Cl ~ -0.6 ‰, assumed mantle average, Table 1). This process created the halogen
346
enrichments in margins of zoned phosphates seen in Figure 3A,B and produced the strong 347
non-zero 37Cl values. Similar evidence for atmospheric photochemical reactions has
348
been observed in mass independent fractionation in S isotopes in some basaltic Martian 349
meteorites (Franz et al., 2014). If mixing with photochemically derived isotopically 350
anomalous perchlorates are responsible for the compositions seen here and the 351
perchlorate/chloride ratio across the planet is indeed 4.4, the chlorides on Mars must have 352
even stranger Cl isotopic signatures by mass balance. The chloride signature is absent in 353
the data presented here and hopefully, can be identified in future studies to gain a better 354
understanding of the halogen/Cl isotopic cycle on Mars. 355
356
The Cl isotopic compositions and heterogeneity observed in the ancient samples 357
examined here (NWA 7533 and ALH 84001) are likely a reflection of slightly different 358
processes. The sulfides in NWA 7533 indicate that it has a history of hydrothermal 359
alteration that took place around 1.4 Ga, after the formation of the main components 360
found in the sample (including phosphates, Lorrand et al., 2015). Similarly, one matrix 361
derived phosphate shows replacement with silica-enriched material, potentially indicating 362
hydrothermal activity (Figure 4). Additionally, Cl-rich apatites are the dominant 363
phosphate in NWA 7533 and there is evidence of merrillite being replaced with Cl-rich 364
apatite (Shearer et al., 2015). Therefore, the range observed in 37Cl and trends in
365
halogen concentration vs. halogen ratios are likely associated with hydrothermal activity, 366
which produced the sulfides, overprinting original magmatic 37Cl signatures, replacing
367
some merrillite with Cl-rich apatite and forming the replacement texture seen in the 368
phosphate in Figure 4. 369
There is no zoning in the phosphate analyzed in ALH 84001 but it has a positive 370
37Cl (Figure 3E). There are two, unfortunately indistinguishable, pathways by which this
371
could occur: 1) ALH 84001 is a cumulate and it could have assimilated anomalous crustal 372
Cl during crystallization or 2) ALH 84001 experienced aqueous alteration after initial 373
crystallization at 3.9-4.0 Ga, which precipitated carbonates (Borg et al., 1999) and the 374
current Cl isotopic composition is a reflection of that alteration, while the original Cl 375
isotopic composition was completely overprinted. Similarly, ALH 84001 is unique in I vs 376
Cl/I but similar to SaU 005, NWA 4864, Zagami and Tissint in Br vs Br/Cl and F vs F/Cl 377
(Figure 2). Regardless of mechanism, the events that affected the Cl isotope composition 378
of ALH 84001 and halogen concentration happened at ~4 Ga. Since low-temperature 379
fractionation of Cl isotopes, likely via perchlorate formation, and subsequent 380
incorporation into magmatic rocks has also affected recent lavas (~160 Ma), this process 381
seems to have been operating over nearly the entirety of Martian history. 382
383
Implications for bulk Martian 37Cl and environmental conditions on Mars
384
As a consequence of the high concentration of strongly fractionated Cl likely 385
covering the entire Martian surface, any Martian samples that have mixed, even slightly, 386
with this reservoir cannot be used to determine confidently its bulk or initial Cl isotopic 387
composition directly. However, SaU 005, NWA 4864, Tissint, and Zagami contain 388
phosphates with Cl isotopic compositions close to zero ( 37Cl ~0.2 to -1 ‰), have the
389
lowest halogen abundances, similar halogen ratios, and have no phosphate zoning or 390
other textural features indicative of alteration processes. Therefore, the mixing processes 391
invoked to explain the extreme positive and negative 37Cl compositions in all other
392
meteorites in this study likely did not affect these samples and the most straightforward 393
interpretation is that this value approximates the Cl isotopic composition of Martian 394
mantle. If the Martian mantle has a 37Cl of ~ -0.6 ‰, this implies that the Cl isotopic
395
composition for the Earth, Moon, and Mars and all inner solar system materials is similar. 396
Lastly, ancient samples and young samples have contrasting Cl isotopic 397
compositions and are derived from the different hemispheres of Mars. This supports two 398
scenarios, which are indistinguishable given the limited number of samples: 1) a shift in 399
environmental conditions over time or 2) a hemispheric difference in 37Cl compositions
400
as the crust of the northern hemisphere is significantly younger than that of the southern 401
hemisphere or a combination of both. Oxidizing conditions produce perchlorate with 402
more negative 37Cl anomalies and reducing conditions produce chloride and fractionate
403
Cl isotopes in the positive direction (Smith et al., 2014). If the Martian atmosphere was 404
more reducing in the past, it could be reflected in the positive 37Cl anomalies recorded in
405
the phosphates in ancient meteorites, while the current oxidizing conditions are 406
responsible for the extremely negative 37Cl observed in the phosphates from the recent
407
basaltic meteorites and on the Martian surface. 408
409
Conclusions
410
The halogen concentrations and Cl isotope compositions of phosphates from eight 411
Martian meteorites have been measured by SIMS. In conjunction with concentration 412
measurements and x-ray maps of major elements, some phosphates in the basaltic 413
meteorites show evidence for late stage hydrothermal additions of halogens from 414
different surface reservoirs and atmospherically altered negative 37Cl. The phosphates in
415
the four basalts that do not show evidence for this mixing process have a Cl isotopic 416
composition that is identical to the terrestrial mantle ( 37Cl of -0.6 ‰), suggesting that
417
the Earth, the Moon, and Mars could have an identical starting 37Cl. Ancient samples
418
from the southern hemisphere have a positive 37Cl and show evidence for assimilation,
419
low temperature alteration, and or hydrothermal alteration. In contrast, recent meteorites 420
have a significantly negative 37Cl potentially highlighting a shift in the Martian
421
atmosphere from reducing to oxidizing conditions and/or a difference in hemispheric Cl 422
isotopic compositions. The 4 Ga – 160 Ma age range of the sample set studied here 423
indicates that surficial enrichment in halogens is ancient and the processes of formation 424
of perchlorate by photochemical reactions and halogen transport by brines have been 425
ongoing for most of Martian history and are global in scale. 426
427
Acknowledgements
428
The authors would like to acknowledge Marianne Ahlbom for access to the SEM 429
at Stockholm University and Kerstin Lindén for preparing the samples. JJB would like to 430
thank Dr. Michael Stewart at the University of Illinois, Urbana-Champaign for the 431
introduction to Cl isotopes many years ago. Dr. Ludovic Ferrière and the Vienna 432
Museum of Natural History are thanked for loaning the sample of Tissint used in this 433
study. Drs. Addi Bischoff and Ansgar Greshake and the University of Münster and 434
Natural History Museum in Berlin are thanked for the samples of SaU 005 and NWA 435
4864. This work benefited from discussions with Dr. Patricia Clay. An anonymous 436
reviewer and Dr. Bernard Marty are thanked for constructive comments on this and an 437
earlier draft of this work. PAB acknowledges support from the Australian Research 438
Council via their Australian Laureate Fellowship scheme. GKB acknowledges support 439
from Curtin University via their Research Fellowship scheme. This work was partially 440
funded by the Deutsche Forschungsgemeinschaft (SFB-TRR 170, subproject B5-1; TJ). 441
This is TRR 170 publication No 7. This work was funded by grants from the Knut and 442
Alice Wallenberg Foundation (2012.0097) and the Swedish Research Council (VR 621-443
2012-4370) to MJW and AAN. The NordSIMS ion microprobe facility operates as a 444
Nordic infrastructure. This is NordSIMS publication #477. 445
446
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609 610 611 612
Figure 1. Halogen concentrations in g/g vs 37Cl. The box heights and widths represent 613
both range in both halogen concentrations and 37Cl, respectively. Uncertainties are
614
smaller than the boxes. The grey band represents the Earth’s mantle 37Cl compositions
615
(Sharp et al., 2007, 2013) and the red band represents measurements of Martian meteorite 616
37Cl isotopic compositions by previous workers (Sharp et al., 2016). The red and grey
617
bands do not indicate ranges in halogen concentrations for the Martian or terrestrial 618
mantle. 619
Figure 2. Halogen ratios (Cl/I, Cl/Br, and ClF) vs. halogen concentrations (in g/g). The
620
grey field is defined by SaU 005, NWA 4864, Zagami, and Tissint, which show no 621
secondary features in microprobe mapping or major element concentrations (McCubbin 622
et al., 2016). The Nakhla veins field in Cl/Br vs. Br is from Rao et al., 2005, while the 623
trend for Meridiani rocks measured by the Opportunity lander is from Rao et al., 2009. 624
The bulk nakhlite measurements are from Cartwright et al., 2013. 625
Figure 3. Electron microprobe x-ray maps of P, Cl, and F in phosphates from LAR12011
626
(A,B), Tissint (C), NWA7533 (D), and ALH84001 (E). Scale bar units are counts. Clear 627
zonation in the halogens is apparent in LAR12011 and NWA7533. This zone of 628
enrichment in LAR12011 corresponds with increasingly negative 37Cl (- 3‰ vs. -6‰).
629
Additional x-ray maps for other grains and for elements Si, Fe, and Ca available in 630
Supplementary Materials. 631
Figure 4. X-ray maps of Si, P, Cl, and Ca for a clast phosphate in NWA7533 indicating
632
hydrothermal replacement of phosphate by silica rich veins. 633
Figure 5. Standard mixing calculation with a 80 g/g initial phosphate Cl concentration
634
with an isotope composition of 37Cl of -0.6, the average of SaU 005, NWA 4864,
635
Zagami, and Tissint (Table 1) and a crustal Cl concentration of 5000 g/g (Keller et al., 636
2006). Positive mixing end member 37Cl is a hypothetical +5 ‰, which is seen as the
637
maximum perchlorate 37Cl value on Earth (Jackson et al., 2010) and negative 37Cl of –
638
14 ‰, the most negative perchlorate value on Earth (Jackson et al., 2010) and ranging to 639
-51 ‰, which has been measured in Gale crater (Farley et al., 2016). 640
Figure 6. Schematic diagram of the Cl cycle on Mars proposed here. Recent basalts erupt
641
with phosphates with a 37Cl of -0.6 ‰, which is representative of the bulk Martian
642
mantle. When on or near the surface, these phosphates interact with halogen rich brines 643
that have incorporated isotopically fractionated Cl, which occurred by photochemistry in 644
the atmosphere (a minimum of -55 to a maximum of +5 ‰, Jackson et al., 2010) or the 645
soil (which would have a similar composition). The perchlorate on the Martian surface 646
that is likely incorporated into the Martian brines has a 37Cl of -55 to -1 ‰ (Farley et al.,
647
2016). 648
649 650
10 100 1000 10000 100000 1 10 100 1000 10000 -6 -5 -4 -3 -2 -1 0 1 2 3 1 10 100 1 10 100 2.00 -6 -5 -4 -3 -2 -1 0 1 2 3
Earth’
s Mantle
Earth’
s Mantle
-6 -5 -4 -3 -2 -1 0 1 2 3Earth’
s Mantle
-6 -5 -4 -3 -2 -1 0 1 2 3Earth’
s Mantle
Cl
37
I
[
μ
g/g]
Cl
37
F
[
μ
g/g]
Cl [
μ
g/g]
Br [
μ
g/g]
NWA 7533 ALH 84001 Tissint NWA 4864 Zagami LAR 12011 core LAR 12011 rim RBT 04262 SaU 005}
Depleted Shergottites}
Enriched Shergottites Figure 1[I in μg/g]
Cl/I
1 10 100 1000 10000 100000 0.0001 0.001 0.01 0.1 1 10 100 Bulk NakhlitesA)
100 0.01 0.1 1 10Cl/F
C)
[Br in μg/g]
Cl/B
r
B)
1 10 100 1000 0.01 0.1 1 10 100 1000 Bulk Nakhlites Nakhla Veins Meridiani NWA 7533 ALH 84001 Tissint NWA 4864 Zagami LAR 12011 core LAR 12011 rim RBT 04262 SaU 005}
Depleted Shergottites}
Enriched Shergottites Figure 20 100 200 0 20 40 100 μ m 0 100 200 300 P Cl F A) 150 μ m 0 100 200 300 P B) 0 50 100 Cl 0 10 20 30 40 F 0 100 200 300 100 μ m C) P 0 10 20 30 40 50 Cl 0 10 20 30 40 F D) 0 100 200 300 300 μ m P 0 10 20 30 40 F 0 50 100 150 200 Cl E) 200 300 μm 200 40 LAR 12011 LAR 12011 Tissint NWA7533 ALH84001 Figure 3
0 400 800 200 200 400 600 200 300 Ca Si 200 μ m Figure 4
-5 -4 -3 -2 -1 0 1 2 3
Initial Martian Phosphate = 80 μg/g, δ Cl = -0.637
Range in Martian phosphate
Cl 37 37 δ Cl = +5 37 δ Cl = -14 37 δ Cl = -51 Figure 5
δ
37Cl -0.6
~~ Atmospher e Lithospher Hydrospher e UV Radiation Atmospher e ClO4 - δ37 Cl = -55 t o +5 Halogen Br ines δ37 Cl = -55 t o +5 surficial hydrothermal system magmatic hydrothermal system Co nd en sation Ev ap or ati on Regolith ClO 4 - δ37 Cl = -55 t o +5 Stage 3 Stage 3 Stage 1 Stage 2 Figure 6Table 1. Halogen concentrations and δ37Cl of phosphates in Martian meteorites
Sample Phosphate Image Phase Notes F [µg/g] 2σ Cl [µg/g] 2σ Br [µg/g] 2σ I [µg/g] 2σ Cl/F Cl/Br Cl/I δ37Cl 1σ1 Age in Ma NWA7533 Image 5 Apatite matrix 6135 265 62452 3289 802 148 23 1 10 78 2716 2.51 0.05 4450-1350a
NWA7533 Image 3 Apatite matrix 7326 310 64559 3650 231 44 53 3 9 279 1213 2.49 0.05 matrix 6436 277 60024 3153 191 35 12 1 9 315 4947 2.44 0.05 NWA7533 Image 10 Apatite matrix 8143 367 52400 3086 522 98 2 0.1 6 100 33098 1.54 0.05 matrix 7177 360 57698 3485 551 104 12 1 8 105 4975 1.46 0.05 NWA7533 Image 11 Apatite matrix 8339 385 71911 4323 410 77 7 0.4 9 175 11028 2.00 0.05 Apatite matrix 7527 330 66511 3641 279 52 12 1 9 238 5752 1.98 0.05 2.01 0.05 NWA7533 Image 9 Apatite matrix 8692 514 70418 5846 890 173 31 3 8 79 2275 1.68 0.05 1.47 0.06 NWA7533 Image 8 Apatite matrix 9244 405 63827 3627 288 54 2 0.1 7 222 30162 1.44 0.05 Apatite matrix 8088 356 61814 3336 341 63 6 2 8 181 10992 2.33 0.05 NWA7533 Image 7 Apatite clast 11639 496 50568 2773 394 73 30 2 4 128 1695 1.96 0.05 1.40 0.05 NWA7533 Image 1 Apatite clast 7223 366 67931 4144 265 49 84 6 9 257 810 2.10 0.05 Apatite clast 14686 667 50203 3016 204 38 73 3 3 246 688 1.91 0.05 NWA7533 Image 2 Apatite clast 5833 361 55364 4217 421 85 23 1 9 131 2413 2.10 0.05 Apatite clast 6873 287 59221 3128 486 90 28 3 9 122 2117 2.09 0.05
LAR12011 Merillite Core 3 0.3 16 1 5 1 0.2 0.01 5 3 79 -2.72 0.14 185b
Merillite Core 6 0.5 76 5 56 13 0.6 0.04 12 1 126 -2.78 0.13 Image 3 Merillite Core 8 0.4 76 6 36 7 1.0 0.1 10 2 76
Merillite Core 11 0.5 64 5 63 12 1.1 0.5 6 1 58 Rim 847 39 1798 107 110 21 1.3 0.1 2 16 1383
Image 3 Apatite Rim 2869 203 7256 657 391 77 1.5 0.2 3 19 4837 -5.56 0.14 Apatite Rim 10313 447 5782 320 291 54 5.2 0.3 1 20 1112 -4.10 0.14 RBT04262 Apatite 21764 972 637 33 33 6 22 1 0.03 20 29 -4.38 0.28 170b Apatite 12648 828 367 24 13 2 46 6 0.03 28 8 -4.39 0.15 Apatite 22237 1023 1052 66 29 5 22 2 0.05 36 47 -4.00 0.17 Apatite 21437 957 833 48 25 5 9 1 0.04 33 89 SaU 005* Apatite 17.3 2.9 156 24 7 1 2.8 0.6 9 22 57 -0.19 0.25 Merilite 12.5 0.9 43 4 2.6 0.4 1.0 0.1 3 16 42 Merilite 5.9 0.2 15 1 5.2 0.8 0.9 0.1 3 3 16 Merilite 12.6 1.0 48 9 12 2 1.8 0.2 4 4 27 11.8 0.6 109 6 3.0 0.5 1.4 0.2 9 36 76 NWA 4864 Merilite 10 0.6 34 3 4 1 0.32 0.03 3 9 109 -0.26 0.19 10 0.3 113 20 5 1 0.25 0.02 12 23 448 -1.01 0.22 Merilite 10 1.1 20 3 1.4 0.2 0.25 0.03 2 15 80 Merilite 12 0.5 62 10 11 4 0.37 0.04 5 6 169 Merilite 10 0.5 53 3 5 1 0.29 0.03 5 11 184 Merilite 10 0.7 22 2 4 1 0.35 0.03 2 5 64 Merilite 13 1.1 28 3 5 1 0.31 0.04 2 6 91 Merilite 16 0.6 31 2 5 1 0.44 0.04 2 6 70 344 37 26 2 9 2 0.28 0.02 0.1 3 93 -0.86 0.45 162b Zagami Apatite 42 2 16 1 7 1 0.19 0.01 0.4 2 83 -0.98 0.23 Merillite 12 1 117 6 6 1 0.38 0.02 10 19 305 Apatite 90 12 146 10 8 1 0.36 0.02 2 19 411 Apatite 41 6 289 19 15 3 0.41 0.02 7 19 708 Apatite 26 2 60 3 18 3 0.25 0.01 2 3 243 5 0 76 5 13 2 0.35 0.02 14 6 219 Merillite 11 1 65 6 17 3 0.24 0.01 6 4 273 20 1 32 2 7 1 0.38 0.02 2 5 83 81 5 10 1 7 1 0.24 0.01 0.1 1 42
Tissint* Image 1 Merillite 5 0.3 170 9 17 3 0.90 0.08 37 10 189 -0.16 0.09 574c
Image 1 Merillite 3 0.1 124 18 7 1 0.23 0.01 47 17 547 Image 2 Merillite 3 0.1 101 11 5 1 0.21 0.01 40 22 480 Image 2 Merillite 3 0.2 235 22 6 1 0.40 0.02 73 39 585 Image 3 Merillite 3 0.1 52 3 14 3 0.31 0.02 20 4 169
ALH84001* Image 1 Apatite 144 13 157 9 11 2 2.7 0.2 1 15 58 1.08 0.16 4090d
Image 1 Apatite 149 9 157 16 19 4 3.2 0.3 1 8 49
Average of Zagami, Tissint, SaU 005, and NWA 4864 30 80 8 0.6 11 12 209 -0.6 *Samples SaU 005, Tissint, and ALH84001 only had one Cl isotopic measurement that yielded enough Cl counts for accurate isotopic measurements
1. Listed uncertanties include propegated external error a) Humayun et al., 2012, Bellucci et al., 2015b b) Bellucci et al., 2015a
c) Brennecka et al., 2014 d) Lapen et al., 2010, Bellucci et al., 2015a Table 1
Table 1. Halogen concentrations and 37Cl of phosphates in Martian meteorites
Sample Phosphate Image Phase Notes F [ g/g] 2
NWA7533 Image 5 Apatite matrix 6135 265
NWA7533 Image 3 Apatite matrix 7326 310
matrix 6436 277
NWA7533 Image 10 Apatite matrix 8143 367
matrix 7177 360
NWA7533 Image 11 Apatite matrix 8339 385
Apatite matrix 7527 330
NWA7533 Image 9 Apatite matrix 8692 514
NWA7533 Image 8 Apatite matrix 9244 405
Apatite matrix 8088 356
NWA7533 Image 7 Apatite clast 11639 496
NWA7533 Image 1 Apatite clast 7223 366
Apatite clast 14686 667
NWA7533 Image 2 Apatite clast 5833 361
Apatite clast 6873 287
LAR12011 Merillite Core 3 0.3
Merillite Core 6 0.5
Image 3 Merillite Core 8 0.4
Merillite Core 11 0.5
Rim 847 39
Image 3 Apatite Rim 2869 203
Apatite Rim 10313 447
RBT04262 Apatite 21764 972
Apatite 12648 828
Apatite 22237 1023
Table 1 spreadsheet
Supplementary material for online publication only
