June 2017 LIP of the Month

Mars on Earth – a perspective from weathering of the Deccan Trap basalts in India

Souvik Mitra¹, Saibal Gupta^1*, Kaushik Mitra¹, Satadru Bhattacharya², Prakash Chauhan² & G. Parthasarathy³

¹Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur – 721 302, India

²Space Applications Centre, Indian Space Research Organization, Ahmedabad – 380 015, India

³National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad – 500 007, India

(* corresponding author; email:saibl2008@gmail.com)

The following is compiled from a series of recent publications that include, but are not limited to, Bhattacharya et al. (2016), Mitra et al. (2016) and Mitra et al. (2017). For full details, please refer to these papers.

1. Introduction

Recent advances in planetary science have re-ignited interest in Large Igneous Provinces in general, and flood basalt provinces in particular. For instance, the planet Mars is known to have surface lithologies that are dominantly basaltic, suggesting parallels with terrestrial mafic volcanic provinces. A particularly interesting discovery on the Martian surface has been the evidence for the presence of liquid water, mineralogically manifested in the form of hydrous clay minerals and sulfates, similar to basalt weathering products on Earth. In particular, one of the most curious findings has been that of the hydrous sulfate mineral jarosite, [(K,Na)Fe₃(SO₄)₂ (OH)₆)], that occurs in relative abundance on the surface (Christensen et al., 2004; Herkenhoff et al., 2004; Klingelhofer et al., 2004; Rieder et al., 2004; Squyres et al., 2004). Jarosite comprises ~10% in outcrop at the Meridiani Planum, the landing site of the Opportunity rover (Christensen et al., 2004), while smaller amounts have also been recently discovered in parts of Mawrth Vallis (Farrand et al., 2009), Valles Marineris (Milliken et al., 2008), Noctis Labyrinthus (Weitz et al., 2011; Thollot et al., 2012), NE Syrtis Major (Ehlmann and Mustard, 2012) and Gale Crater (Leveille et al., 2015). Identifying and understanding how jarosite formed naturally in localities on Earth can therefore serve to constrain possible environmental conditions on early Mars. However, apart from being a mineralogical indicator of aqueous activity on the Martian surface, jarosite also has a limited stability field, and can only be precipitated from highly acidic (pH<3), oxidizing aqueous fluids in a sulfur-bearing system (e.g. Baron and Palmer, 1996; Bigham et al., 1996). These extreme conditions are not easily realized in natural terrestrial situations, thereby limiting the range of environments in which jarosite can be formed and preserved on Earth, particularly on basaltic surfaces.

The Deccan basalts constitute a Large Igneous Province (LIP) that cover much of central and western India and erupted at ~65 Ma (Figure 1, inset; Baksi, 2014; Courtillot et al., 1986; Pande, 2002). The composition of the basalts is predominantly tholeiitic, although alkaline, acidic and carbonate lavas occasionally exist (Shukla et al., 2001). The general absence of sedimentary deposits over a vast area of the basaltic province suggests that this high altitude plateau may have been exposed to weathering since eruption. A number of studies have been conducted on the weathering of these basalts in several parts of the central and western India (Salil et al., 1997, Greenberger et al., 2012, Bhattacharyya et al., 2006), and show similarities in the weathered derivatives. Most commonly, the weathered rocks are characterized by a dominance of smectite along with zeolite, montmorillonite, illite and gibbsite.

In the Matanomadh area of Kachchh district, Gujarat state, western India, hydrous sulfates such as natroalunite [NaAl₃ (SO₄)₂(OH)₆] and clay minerals like kaolinite have previously been reported from the lateritic horizons of the Deccan Volcanic Province (DVP) (Chitale and Guven, 1987). These occurrences are unique, as hydrous sulfates have not been reported from the extensive trap sections elsewhere in the DVP of India (e.g. Greenberger et al., 2012; Widdowson and Cox, 1996). In the present study, spectral characterization of the samples from a vertical laterite profile and a tuffaceous shale/sandstone sequence of Matanomadh Formation developed over the Deccan basalts are presented in the Visible/Near-infrared (VNIR) and mid-IR domains We supplement these data with X-ray diffraction patterns, and argue that the positive identification of jarosite in this section (Jain et al., 2011; Bhattacharya et al., 2012; Mitra et al., 2014), in addition to the other hydrous sulfates (Chitale and Guven, 1987; Siddaiah and Kumar, 2009) considerably strengthen the case for this locality to be considered as a terrestrial Martian analog site for studying the formation of jarosite and other hydrous sulfates on the Red Planet.

2. Geological background

The study area is located at Matanomadh (Kachchh), situated ~86 km northwest of Bhuj, Gujarat, India (Figure 1). Biswas (1992) described Kachchh as a pericratonic rift basin characterized by a near complete sequence from Triassic to Recent, with a few stratigraphic breaks punctuating transgressive cycles. Rifting was initiated during the break-up of Gondwanaland in the Mesozoic (Biswas, 1999, 2005), and was followed by widespread volcanism associated with the Deccan Traps in the late Cretaceous-Paleocene time. In the waning stages of Deccan volcanism, volcaniclastic, clastic and reworked volcanic debris were deposited on weathered and lateritised trap basalts in fluvial and lacustrine settings to form the Matanomadh Formation (Biswas, 1992). The Matanomadh Formation is overlain by lignites of the late Paleocene-Early Eocene Naredi Formation, suggesting that the basin was restricted and isolated at this time (Biswas, 1992; Mathew et al., 2013). In the middle Eocene (Bartonian) time, there was a rapid rise in sea-level in the basin leading to the deposition of shelf carbonates (Banerjee et al., 2012; Saraswati et al., 2014). Uplift of the entire basin is supposed to have occurred in post-Pliocene times (Biswas, 1971, 1992).

Chitale and Guven (1987) reported natroalunite from the low-lying hills of Matanomadh that were capped by laterites. In addition to natroalunite, Siddaiah and Kumar (2009) reported minamiite, a Ca-bearing hydrous aluminium sulfate, from a variegated Paleocene sandstone unit of the Matanomadh Formation. The presence of jarosite, the key Martian mineral, in the Paleocene sedimentary succession of Kachchh has been reported recently by Jain et al. (2011), Bhattacharya et al. (2012) and Mitra et al. (2014).

3. Geological setting of the sampling locality

The sampling sites are located near the village of Matanomadh, in the Kachchh district of Gujarat (Figure 1), where unaltered basalts are overlain by spheroidally weathered equivalents, that ultimately disintegrate into an overlying zone of colourful clay boulders (see Bhattacharya et al. 2016). At the first section (Site 1; Figure 2), the lowermost part of the sequence comprises basalts of the Deccan Traps that show evidence of spheroidal weathering. Above this zone, the basalt spheroids appear to be more intensely weathered, and disintegrate into separate boulders. These individual clay boulders are characterised by concentric color bands, with the color varying from red, brick red, orange to yellow to white. Systematic sampling of a basalt spheroid with well developed “onion shells” has been carried out in order to study the degree of weathering from rim to core of the spheroid where the outermost rim represents the most altered part. The zone of spheroidally weathered basalts grades upward into a zone of saprolite clays. Hydrous sulfate samples have been collected from the cream yellow layers within this stratified saprolite horizon dominated mostly by Al-rich phyllosilicates. The saprolite horizon is overlain with a sharp contact by a hard crust laterite unit. The second vertical section (Site 2; Figure 3) is exclusively that of the Paleocene sedimentary package belonging to the Matanomadh Formation, comprising altered ash/black shale horizons and tuffacecous sandstones. The hydrous sulfate layers and/or lenses within the Paleocene sandstones and ash/clay horizons (Figure 4) are cream-yellow to white in color, and occur mostly along vertical, horizontal and diagonal fractures within gray tuffaceous clay horizon that are overlain by purple-colored, coarse grained, cross-bedded sandstones (Figure 4). In the cross-bedded sandstone unit, the hydrous sulfates primarily occur as clasts of varying sizes.

4. Spectroscopic methods and results

Visible and Near-infrared (VNIR) Spectroscopy was conducted to measure spectra within 350-2500 nm wavelength range, while Fourier Transform Infrared (FTIR) spectra have been obtained for characterizing samples in the Mid-IR range (4000 to 400 cm^–1). Details of the methodology and instruments used are given in Bhattacharya et al. (2016) and Mitra et al. (2016). Spectral features in the VNIR range were collected in situ. Subtle variations in the VNIR and mid-IR spectral features and XRD data over a number of samples collected from different levels of the near continuous, well-developed weathering profile at Matanomadh suggest that a strong correlation exists between the degree of leaching and alteration that the parent rock has undergone and the observed spectral signatures. Systematic spectral study of basalt spheroids with concentric “onion shells” (Figure 5a) show that the cores of the spheroids are the least altered, and still preserve the characteristic crystal field absorptions of pyroxene. The effects of leaching and alteration increase progressively towards the rims of the spheroids resulting in complete disappearance of the pyroxene signature. Instead, three new diagnostic absorption features appear near 1400, 1900 and 2200-2300 nm in the rim samples; these result from overtones and combination tones of OH/H₂O and metal-OH stretching and bending, respectively. These spectral features indicate the presence of hydrous Fe/Mg smectite clays together with some water (Figure 5b). Samples from the weathered part of each basalt spheroid do not retain the signature of any mineral phase that was present in the original basalt. Each shell is dominated by smectite, which is the most prominent among all the other alteration products. In the overlying weathered clay boulder samples (Figure 5c), spectral features of each concentric rim contain almost identical absorption bands signifying the dominance of a single clay mineral, kaolinite or halloysite (Figure 5d). In the mid-IR range, the smectitic alteration is seen to be restricted to a surficial layer a few millimeters thick on basalt samples, while in powdered samples of basalts, the spectra are dominated by the pristine mineralogy of the basalt. Confirming the VNIR spectroscopic features, FTIR spectroscopy on samples from the overlying clay boulders show that kaolinite occurs even in the interior of the basalt spheroids. Importantly, the kaolinitized clay boulders are restricted to the base of the overlying Cenozoic sedimentary sequence. Basalts outside the Cenozoic basins, along the rift flanks, are only altered to smectite and do not show extensive leaching (Figure 6).

The key Martian mineral, jarosite is extensively developed in Paleocene sediments of the overlying Matanomadh formation. Jarosite occurs as cream yellow layers within stratified saprolite horizon occurring between the unstratified natroalunite-dominated saprolitic clay zone and a hard-crust laterite horizon, and also within veins that cut across the tuffaceous shale and sandstone layers overlying the saprolite/laterite zone, suggesting precipitation of jarosite from downward percolating fluids (Figure 7). In the overlying cross-bedded sandstone layer, yellow clasts of the underlying tuffaceous layers are partially and/or completely altered to jarosite/jarosite-gypsum-mixture/gypsum-kaolinite/halloysite-mixture; jarosite is also present in the sandstone matrix. On the other hand, the white clasts within the tuffaceous sandstone unit are primarily mixtures of alunite and kaolinite/halloysite. This indicates that jarosite formation is not primary, and possibly post-dates the formation of alunite / kaolinite-bearing white vein and deposition of the cross-bedded sandstone layer.

The alteration sequence in the basalts at Matanomadh therefore, is characterized by the igneous mineralogy of the unaltered basalt being replaced by smectite; this grades upward into a zone of kaolinite/halloysite, similar to that described by Greenberger et al. (2012) and Widdowson and Cox (1996) from elsewhere in the Deccan Traps LIP. Indeed, there has been speculation that this low temperature aqueous alteration pattern may even be relevant to the alteration pattern on the Martian surface (Greenberger et al., 2012). What distinguishes the Matanomadh section from other sections of altered Deccan basalts, however, are the overlying sediments that host the hydrous sulfates natroalunite and jarosite. These two minerals, and in particular jarosite, are very important in the context of Martian analog studies.

5. Geochemical modeling

The two most important parameters controlling the water chemistry, and thus the weathering itself, are: (a) the type of system i.e. open or closed (b) the water : rock ratio. These two factors primarily determine the availability of elements during the alteration and therefore ultimately determine the weathered mineral assemblages produced (Ehlmann, 2011). The aim of the calculation was to predict basalt weathering products in different geological settings representative of open or closed systems and different water : rock ratios. In closed systems, both water and rock are taken to be isolated from the atmosphere and there is no equilibrium between atmospheric gases and the system. Therefore, an un-buffered CO₂ and O₂ condition would be good representation of a closed system. On the other hand, in an open system atmospheric gases are in constant equilibrium with the water and thus would be represented by fixed fugacities of CO₂ and O₂. With these two cases, the calculations were performed for different water : rock ratios by weight, starting from W:R = 1:1 to W:R = 10⁵ :1. A high water : rock ratio represents a small quantity of water in the transient state (Catalano, 2013). The low water-rock ratios promote water to interact with the rock in minute fractures or pore spaces within the rock and the redox, pH and ion activities are controlled locally (Franzson et al., 2008; Meunier, 2005).

Thermodynamic equilibrium reaction-path modeling was carried out on The Geochemists’ Workbench (GWB) 10.0.4 (Bethke, 2007) with the help of V8 R6+ database, an expanded variant of Lawrence Livermore National Laboratory database. Using the numerical thermodynamic reaction path model REACT, a software included in the GWB, the alteration mineral assemblages produced from basalt weathering and alteration were calculated using a range of water : rock ratios (1:1 to 10⁵:1). The basalt composition used is from the uppermost Deccan basalt flow, Flow F9 at Anjar, Kachchh, about 100 km from Matanomadh, measured by XRF and ICP-MS analyses (Shukla et al., 2001). All weathering calculations were carried out at ambient surface temperature. (25° C) and pressure (1 atm). The basalt was then titrated against an aqueous solution with composition similar to pure rainwater (after Garrells, 1971). The acidity of the water was calculated by assuming equilibrium with CO₂ in the atmosphere (Bethke, 2007).

Results

Under buffered (open system) conditions, and at lower water-rock ratios, montmorillonite and dolomite are the major alteration products comprising almost 77% of the altered mineral assemblage. Soluble species like Na oversaturate and are incorporated in smectites like montmorillonite and nontronite. As a result, the pH of the system rises rapidly initially to moderate alkalinity (pH = 9.42) after about 20% weathering of basalt. At intermediate water-rock ratios (i.e. water : rock = 10:1 to 1000:1), the weight percent of smectites and kaolinite does not change noticeably, but a substantial amount (~17%) of montmorillonite-Na is replaced by montmorillonite-Ca, and there is complete replacement of nontronite-Na by nontronite-Ca. Dolomite decreases with increasing water-rock ratio owing to the decrease in the equilibrium pH conditions. At these ratios, a new smectite group mineral, saponite, begins to form along with monrmorillonite and nontronite. At higher water-rock ratios (i.e. 10⁴:1 and 10⁵:1), kaolinite becomes the dominant alteration mineral. Leaching of soluble cations during extensive weathering of basalt produces kaolinite rather than montmorillonite or nontronite. During the early stages of weathering, when the pH is 5-7, dissolved Al precipitates as gibbsite (Al(OH)₃). The dissolved silica (H₄SiO₄) concentration increases, and subsequently reacts with Al(OH)₃ to form kaolinite after the basalt has undergone about 35% weathering. There is preferential formation of saponite-Mg over saponite-Ca, and montmorillonite is replaced by nontronite and saponite. However, the relative weight % of these smectites makes up less than a quarter of the total phyllosilicates produced after complete basalt weathering.

The type of smectite formed during basalt weathering in a closed system is strikingly different from that formed in an open system. Saponite is the dominant smectite at all water-rock ratios. Montmorillonite is absent in all simulations while nontronite makes up only a small fraction of the total alteration assemblage, reaching a maximum of 15% in high rainfall conditions. Gibbsite also forms in high water-rock ratio scenarios. Hematite forms during partial weathering of basalt in intermediate water conditions and dissolves completely during very high rainfall.

To investigate the effect of varying CO₂ content in the atmosphere, Mitra et al. (2017) performed a range of calculations with f_CO2 values increasing from the present day value of 10^-3.5 to extreme values of 10^-2.0 (Figure 8). Increasing concentrations of atmospheric CO₂ help maintain the acidity of the system, with the lowest values (pH ~6.12) obtained at the highest f_CO2 value of 10^-2.0, thereby facilitating the formation and stabilization of kaolinite as the major alteration product. At these values, kaolinite comprises 82% of the altered assemblage formed in an open system (Figure 8, left panel), with only minor amounts of associated smectite, haematite and rutile. Under closed system conditions, however, the results differ. For low f_CO2 values varying from 10^-3.5- 10^-3.0 (Figure 8, right panel), the altered mineral assemblage is dominated by smectitic group minerals. Kaolinite starts appearing in the assemblage at an f_CO2 value of 10^-2.98. However, as f_CO2 values increase from 10^-2.5 - 10^-2.0, kaolinite begins to dominate the weathered products even under closed system conditions, comprising ~79% of the assemblage at the highest f_CO2 value. This demonstrates that kaolinite can be formed as the major alteration product of basalt weathering in high pCO₂ conditions even if it is not buffered to the system continuously.

The implications of these results are important in the context of the question being addressed in this section. In high rainfall, tropical conditions, and atmospheric pCO₂ less than 10^-2.5, basalts will be altered to smectites on flow tops (closed system conditions), and kaolinite on slopes (open system conditions). However, if pCO₂ exceeds 10^-2.5, all basalts whether on flow tops or slopes, will tend to be altered to kaolinite in preference to smectite. Thus, correlating the alteration of basalts with topography under the high pCO₂ conditions that may have existed following Deccan Traps extrusion is critically dependent on the precise atmospheric CO₂ concentration at the time.

6. Discussion and conclusions

Implications of basalt weathering to smecitite and kaolinite

Our envisaged model is schematically depicted in Figure 9. Following eruption in the late Cretaceous to Early Palaeocene, the Deccan lava flows in Kachchh initially also form flat-topped hills whose tops would be altered to smectite in the course of heavy rainfall in the end-Cretaceous as is the case elsewhere in the Deccan Traps. Thereafter, in the Early Palaeocene, normal faulting related to a renewed phase of rifting in the Kachchh region would result in the development of a topographic slope in this region This slope facilitates run-off, and therefore, the rainwater no longer stagnates on flow tops, but is channeled away along the rift. Basalts exposed along the rift flanks are therefore effectively exposed to large amounts of rainwater buffered by atmospheric CO₂ and O₂, whose chemistry is not substantially altered by prolonged interaction with the basalt. These are precisely the conditions that stabilize kaolinite on the rift floor and flanks, on which the continental to marine Cenozoic succession is subsequently deposited Thus, in contrast to the Deccan Trap lava flows in most other parts of peninsular India, the traps that form the basement to the Cenozoic rift basins of Kachchh (in the study area) are composed of kaolinite, rather than smectitic basalt.

Implications for Matanomadh as a Martian analog locality

A stratigraphy comprising altered, smectite-rich basalts over unaltered basalts, overlain by kaolinite/alunite/jarosite bearing-horizons within confined or restricted basins, as seen in the Matanomadh Formation, has parallels with some of the above Martian occurrences. The Meridiani Planum region on Mars is characterized by evaporite deposits within a thick sequence of sedimentary rocks called the “Burns Formation” (Grotzinger et al., 2005) that overlies the basaltic basement (McLennan et al., 2005). The evaporites include hydrous Ca-Mg sulfates as well as jarosite (Klingelhöfer et al., 2004). Phyllosilicates, dominantly smectites, have been detected by the Opportunity rover in the rim of the Endeavour Crater (Arvidson et al., 2014), and clays in association with sulfates have been extensively identified in many parts of the Meridiani Planum by the Mars Reconnaissance Orbiter (MRO) (Flahaut et al., 2015). In the Mawrth Vallis region of Mars, jarosite has also been interpreted to occur in layers associated with Fe-Mg smectites within a topographic depression; this layer is overlain by an Al-phyllosilicate bearing layer (Farrand et al., 2009). Jarosite-phyllosilicate associations have also been reported from Syrtis Major (Ehlmann and Mustard, 2012) and Columbus crater (Wray et al., 2011).

In comparison, the association documented in Matanomadh appears to be very appropriate as a Martian analog locality. Repeated transgressive cycles in the Cenozoic of Kachchh provide saline water required for sulfate precipitation (Biswas, 1992). Activity of acidic water is evidenced from the stabilization of jarosite and alunite that occur in association with phyllosilicates (kaolinite and possibly smectite). In addition, the Matanomadh occurrence is on basaltic basement, and like Mars, requires an alternative mechanism to maintain low pH levels in the water, since longer flow paths through or over mafic rocks would tend to neutralize acidity (Baldridge et al., 2009; Hurowitz et al., 2010; McHenry et al., 2011). An additional observation in Matanomadh that tallies with the record in Syrtis Major is the presence of jarosite in veins and fractures, again indicating precipitation from, or interaction with, fluids and the host rock, similar to the jarosite documented in this study in the tuffaceous shale layer.

There are, of course, many obvious points of difference between Matanomadh and the Martian jarosite occurrences. For instance, there is as yet no reported pyrite-bearing layer in the succession on Mars, unlike the pyrite-bearing lignites overlying the Matanomadh Formation, that was suggested by Chitale and Guven (1987) as the source for the acidic waters. A second point of difference is that jarosite formation in this study is commonly observed within the alunite and kaolinite bearing layer, and not with the smectites, unlike what has been reported from Martian localities such as the Mawrth Vallis (Farrand et al., 2009). A third difference between Matanomadh and the hydrous sulfate-bearing Martian localities is that alunite has not yet been reported by any of the rovers from the Martian surface; the only reports of the presence of Martian alunite so far are based on spectral data collected from orbit in the Terra Sirenum (Swayze et al., 2008) and Iani Chaos (Sefton-Nash et al., 2012) regions.

However, in spite of these differences, the Matanomadh jarosite occurrence appears reasonably similar to many of the Martian localities, and is also relatively less modified by later surface processes as it is Cenozoic in age. Thus, as a potential model locality for studying Martian surficial processes that resulted in jarosite formation, we conclude that Matanomadh in Kachchh, Gujarat, India appears to be particularly appropriate.