Remote sensing based approach for mapping of CO2

Earth-Science Reviews 135 (2014) 122–140

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Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev

Remote sensing based approach for mapping of CO2 sequestered regions in Samail ophiolite massifs of the Sultanate of Oman Sankaran Rajendran a,⁎, Sobhi Nasir a, Timothy M. Kusky b, Salah al-Khirbash a a b

Department of Earth Sciences, Sultan Qaboos University, Al-Khod, 123 Muscat, Oman State Key Lab for Geological Processes and Mineral Resources, Three Gorges Research Center for Geohazards, China University of Geosciences, Wuhan, China

a r t i c l e

i n f o

Article history: Received 17 January 2012 Accepted 8 April 2014 Available online 23 April 2014 Keywords: Remote sensing CO2 sequestration ASTER Samail ophiolite massifs Oman

a b s t r a c t Documentation of chemical weathering and CO2 sequestration in the Samail ophiolite massifs of the Sultanate of Oman represents an important case study for Geological Carbon Capture and Storage System (GCCSS). The present research study demonstrates the capability of remote sensing technique for mapping of weathered zones and potential CO2 sequestration area abundances at different scales within peridotites in the northern mountain region of the Samail ophiolite massifs. The carbonate mineral index (CI) applied with other mineral indices to the TIR wavelength region of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) TIR spectral bands 13 and 14 mapped CO2 sequestered minerals along the structural- and wadi-controlled CO2 flowing regions. Peridotites, the source rocks of CO2 sequestration in the study area, were mapped using an ASTER 8, 3 and 1 band combinations. The decorrelated Landsat TM image discriminated the rock types associated with peridotites of ophiolite sequences and delineated the region of weathered and altered serpentinized peridotites in the zone of CO2 sequestration. CO2 sequestration mapping was carried out using Landsat TM satellite data that span 20 years (1986, 1998, 2000, 2003 and 2006) to assess the present status of CO2 sequestration in this region. The image interpretations are verified with existing geological maps and through field and laboratory studies. The spectral measurements of carbonate minerals at 1300 to 2500 nm with the spectral resolution of ~7 nm using a PIMA SP infrared spectrometer in the field and laboratory show the presence of hydroxyl-bearing minerals and carbonates that have spectral absorption features around 1.4 μm, 1.9 μm and 2.35 μm. The strong absorptions around 2.35 μm are mainly due to C\O bonds in carbonate minerals such as calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), aragonite (CaCO3) and siderite (FeCO3), which form 15 to 57%, 12 to 53%, 9 to 38%, 11 to 21% and 3 to 8% respectively in the samples. The absorptions around 1.4 μm and 1.9 μm are caused by hydration effects of hydroxyl minerals including antigorite and montmorillonite present at 10 to 21% and 37 to 81% respectively in the samples. The alterations of serpentinite are evidenced by the presence of antigorite and lizardite minerals. X-ray powder diffraction analyses further confirms the occurrence of CO2 sequestered major carbonate minerals such as aragonite, calcite and dolomite in the samples. The study demonstrates that the ASTER and Landsat TM satellite multispectral sensors are useful to detect the carbonate minerals, to delineate the peridotites and to discriminate the areal abundance of potential CO2 sequestration. This technique is a useful tool to map and monitor the region of CO2 sequestration in well exposed arid and semi-arid regions and to analyze and understand this aspect of the world geological carbon capture and storage system. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . CO2 sequestration rationale . . . . . . . . . . . . . . . . . Peridotites and associated geology of the Samail ophiolite massifs Spectral characteristics of peridotite minerals . . . . . . . . . Data processing and methods . . . . . . . . . . . . . . . . 5.1. Satellite data . . . . . . . . . . . . . . . . . . . . 5.2. Image processing . . . . . . . . . . . . . . . . . . 5.3. Field work and laboratory analyses . . . . . . . . . .

⁎ Corresponding author. Tel.: +968 24142284. E-mail address: [email protected] (S. Rajendran).

http://dx.doi.org/10.1016/j.earscirev.2014.04.004 0012-8252/© 2014 Elsevier B.V. All rights reserved.

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6.

Results and discussion . . . . . . . . . . . . . . . . . . . . . 6.1. Satellite data image interpretation . . . . . . . . . . . . 6.1.1. Mapping of CO2 sequestered minerals . . . . . . . 6.1.2. Mapping of peridotites . . . . . . . . . . . . . 6.1.3. Mapping of CO2 sequestered regions . . . . . . . 6.2. Field study . . . . . . . . . . . . . . . . . . . . . . . 6.3. Laboratory study . . . . . . . . . . . . . . . . . . . . 6.3.1. Spectral characteristics of CO2 sequestered minerals 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

The continued increase in CO2 emissions and the related changes in the concentration of the greenhouse gas in the atmosphere and ocean is a global issue, and much consideration is being given to how CO2 can be removed from the atmosphere. Rutt Bridges (2011) stated that the Carbon Capture and Storage (CCS) system faces storage challenges in site selection, CO2 transportation costs, pipeline and site permits, and uncertainties in monitoring CO2 sequestration. But, in Earth history, nature is performing silicate weathering at the earth's surface which is estimated to remove CO2 from the atmosphere equivalent to approximately 150 Mt C per year (Gaillardet et al., 1999), over an order of magnitude smaller than the 5.5 Gt C emitted by fossil fuel combustion (IPCC, 2007). If weathering rates can be increased over large areas even within the range observed today, the acceleration of weathering processes could play a meaningful role in an integrated strategy for atmospheric removal of CO2. CO2 is a major weathering agent in silicate rocks such as peridotites by the conversion of nearly all of the CaO and part of the MgO to carbonate minerals. Over the past several years many papers have been published that discuss the advantages of CO2 injection into mafic and ultramafic rock formations, including deep-sea basalts (Goldberg et al., 2008; Oelkers et al., 2008) and peridotites (Kelemen and Matter, 2008; Andreani et al., 2009; Matter and Kelemen, 2009; Kelemen et al., 2011). The two recent workshops report on geological carbon capture and storage in mafic and ultramafic rocks (Godard et al., 2011), and the scientific drilling in the Samail ophiolite of Oman (Kelemen et al., 2013) emphasizes the importance of research studies on the geological carbon capture and storage system and the need of scientific drilling in the Samail ophiolite of Oman. Thus, the study of CO2 sequestration in ultramafic peridotites of ophiolite regions is of great worldwide importance. CO2 sequestration due to weathering in peridotites in ophiolite sequences of the Samail massifs of the Sultanate of Oman is a major and typical example and needs to be studied in more detail. Due to the 1169 km2 extent of the Samail ophiolite massifs and extremely rugged topography with elevations between 0 and 2500 m, exhaustive sampling and detailed mapping are impossible to assess the potential of CO2 sequestration in this region. Remote sensing is able to provide more continuous mineralogical and lithological mapping in the almost continuous barren rock exposures in an arid climate like the region of the Samail massifs. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat sensors cover the wavelengths of absorption regions of peridotites and CO2 bearing minerals. Several works on mapping of ophiolite sequences using remote sensing have been carried out (Abrams et al., 1983; Abrams and Hook, 1995; Sabins, 1997; Abdeen et al., 2001; Kusky and Ramadan, 2002; Rowan and Mars, 2003; Rowan et al., 2003; Combe et al., 2006; Mars and Rowan, 2006; Gad and Kusky, 2007; Rajendran et al., 2012, 2013; Rajendran and Nasir, 2013, 2014), but few studies have focused on the application of remote sensing to mapping of CO2 sequestration worldwide. This study suggests that the detection of CO2 sequestered carbonate minerals and mapping of peridotite (the source rock) occurrences

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are of great importance in the GCCSS and global monitoring of CO2 which will increase the knowledge among scientists to improve and create a sustainable GCCSS in the future. Thus, the present study is undertaken to demonstrate the capability of remote sensing techniques to detect the occurrences of CO2 sequestered carbonate minerals, to discriminate the peridotites (the source rock of CO2 sequestration) and to map the region of CO2 sequestration ongoing in the ophiolite sequences of northern Samail massifs of the Sultanate of Oman (Fig. 1) as a part of research work on the GCCSS. 2. CO2 sequestration rationale In brief, the removal of carbon dioxide from the atmosphere and ocean by the natural weathering processes of silicate rocks like peridotites is one of the long-term mechanisms of balancing the CO2 concentration in the atmosphere. Peridotite is composed of N40% of the mineral olivine (Le Maitre, 1989). The typical residual mantle peridotite exposed on the seafloor and in ophiolites is composed of 70–85% of olivine, together with dunite composed of more than 90% olivine. During the weathering processes of peridotites, the minerals such as olivine (particularly the Mg-rich end member), pyroxenes and serpentine remove CO2 from the atmosphere (O'Connor et al., 2005; Gerdemann et al., 2007). In detail, the atmospheric CO2 reacts with rainwater to form carbonic acid. At the end, this carbonic acid chemically attacks the olivine on its surface and dissolves it to produce hydrates and carbonates such as serpentine, talc, magnesite, dolomite and calcite. The reaction series of the predominant minerals can be expressed as Mg2 SiO4 ½olivine þ 2CO2 →2MgCO3 ½magnesite þ SiO2 ½silica

ð1Þ

Mg3 SiO3 ðOHÞ4 ½serpentine þ 3CO2 →3MgCO3 ½magnesite þ SiO2 ½silica þ 2H2 O

ð2Þ

where, CO2 represents CO2 in either gas or supercritical fluid form, depending on the pressure and temperature conditions under which the reaction takes place. The second and third most abundant minerals in peridotite are orthopyroxene and clinopyroxene, participating in considerable amounts of reduction in CO2. The peridotite carbonation in these minerals can be simplified by the reactions of 4Mg2 SiO4 þ CaMgSi2 O6 þ 6H2 O þ CO2 ¼ 3Mg3 Si2 O5 ðOHÞ4 þ CaCO3 ðcalciteÞ:

ð3Þ

This reaction often takes place in stages, e.g., 2þ ðaqÞ

4Mg2 SiO4 þ CaMgSi2 O6 þ 7H2 O ¼ 3Mg3 Si2 O5 ðOHÞ4 þ Ca

þ 2OH

− ðaqÞ

ð4Þ in the subsurface, and then Ca

2þ ðaqÞ

− ðaqÞ

þ 2OH

þ CO2ðaq or gasÞ ¼ CaCO3 þ H2 O

ð5Þ

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A B

C

Fig. 1. The figure shows (A) MODIS image draped over digital elevation data showing the distribution of ophiolites (red in color) in parts of the Tethyan region (after Khan and Mahmood, 2008), (B) the study area location in the Samail ophiolite massifs of Oman mountain region (after Robertson and Searle, 1990) and (C) the sequence of Oman ophiolites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

when fluids modified by reaction with peridotite form travertine at or near the surface. It is stated that 140 g of olivine will sequester 176 g of CO2 (1 ton olivine/1.25 ton CO2), with the help of 72 g of water, i.e. rain or seawater (Schuiling and Krijgsman, 2006; Olsen, 2007). The increasing of weathering and subsequent sequestration of Ca- and Mg-carbonates in peridotites are the most important processes in the CO2 sequestration that can significantly control the CO2 concentration in the atmosphere. Thus, the study of occurrence of CO2 sequestered minerals and the spatial distribution of peridotites are very important. 3. Peridotites and associated geology of the Samail ophiolite massifs The study region, part of the northern mountain region of the Samail ophiolite massifs (Fig. 1B) is located parallel to the east coast of the Gulf of Oman on the Arabian plate, and includes rocks formed during the Late Cretaceous (Glennie et al., 1973, 1974; Searle and Malpas, 1980; Coleman, 1981; Boudier et al., 1985; Lippard et al., 1986). The Samail massifs represent a 600 km long by up to 150 km wide slab of oceanic crust and mantle emplaced due to the closure of the Tethyan Ocean. It constitutes a complete ophiolite sequence which includes (from base to top), basal shear zone and metamorphic sole, ultramafic peridotites, cumulate gabbros, plutonic rocks, sheeted dikes and a volcanic complex (Fig. 1C). The basal peridotites are distinguishable by prominent occurrences of harzburgite and dunite with compositional banding which are highly serpentinized. The well-developed mafic–ultramafic transition zone is located in between the top of the strongly sheared harzburgite (with dunite) tectonic unit and the base of the continuous layered gabbro unit (representing repetition of the petrological Moho). The seismic Moho is in the upper horizon of the transition zone where dunites become interlayered with lenses of gabbro, grading themselves into the continuous layered gabbro unit (Nicolas and Prinzhofer, 1983). The harzburgites occur as irregular bodies over a hundred meters in thickness in the peridotite section of the ophiolite. Studies of petrography of dunite of this region show that the rock consists of more than 90% olivine and small amounts of interstitial pyroxenes and plagioclase (Tamura and Arai, 2006). The gabbroic–basaltic rocks form the overlying crustal section. At the top, diabase dyke swarms (sheeted dykes) and basic extrusives — mostly spilites with pillow lavas or hyaloclastites are distributed in a sequence below the pelagic sequences (Fig. 1C). 4. Spectral characteristics of peridotite minerals The reflectance spectrum of a rock depends on the mineralogical composition of its surface, which is usually a mixture of the whole rock mineralogy and weathering minerals. The absorption bands in the visible and short wavelength infrared are from either electronic or vibrational processes in these minerals (Hunt and Salisbury, 1970). Comprehensive spectral absorption–compositional studies can provide important insights into the causes of spectral variations and quantitative data for use in the interpretation of optical remote sensing data (Cloutis and Gaffey, 1991; Cloutis et al., 2004). The offset spectral plots of major and altered minerals of peridotite rock stacked from the USGS Mineral Spectral Library (Envi. 4.8) which are involved in CO2 sequestration are given in Fig. 2. Olivine shows a broad absorption feature centered near 1.0 μm due to the presence of ferrous iron (Abrams et al., 1988). The typical spectra of serpentine show a nearly flat plot with a shallow absorption feature at 0.45 μm due to ferric iron. The sharp band absorption at 1.4 μm shown by hydration effects and the absorptions at 2.3 μm are due to vibrational processes of Mg\OH (Abrams et al., 1988). The CO2 sequestered carbonate minerals of peridotites such as calcite (CaCO3), dolomite (MgCO3), magnesite (CaMg (CO3)2) and siderite (FeCO3) show significant useful narrow absorption features around 2.35 μm due to C\O bonds (Hunt, 1977; Mars and Rowan, 2010). The calcite and dolomite (end members of series) can be distinguished and identified by variations in their absorptions (shown by the vertical line in

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Value (Offset for clarity)

S. Rajendran et al. / Earth-Science Reviews 135 (2014) 122–140

0.5

1.0

1.5

2.0

2.5

Wavelength Fig. 2. Spectral plot of peridotite bearing minerals stacked from the USGS Mineral Spectral Library (Envi. 4.7).

Fig. 2) between 2.44 and 2.45 μm and the magnesite between 2.30 and 2.33 μm (Kuosmanen et al., 2000; Combe et al., 2006; Mars and Rowan, 2010). The iron carbonate, siderite, shows absorption near 2.3 μm due to C\O bonds. The other hydroxyl minerals associated with peridotites such as montmorillonite ((Na, Ca)0,3(Al,Mg)2Si4O10(OH)2·n(H2O)), antigorite ((Mg, Fe++)3 Si2O5(OH)4), anhydrite (CaSO4) and tremolite (Ca2Mg5Si8O22(OH)2) can be easily distinguished by their absorptions at 1.4 and 1.9 μm and the series of features between 2.0 and 2.4 μm (vertical dashed lines in Fig. 2) which are due to hydroxyl bands (Combe et al., 2006; Mars and Rowan, 2010). Montmorillonite has a 2.2 μm AlOH absorption feature. More detailed information on spectral absorption characters of rocks and minerals of ophiolite complex can be found in Combe et al. (2006). Their study describes in detail the Serpentine Index and mapping of lithology and region of alteration of parts of Samail massif using the cost-effective high spectral resolution (hyperspectral) space born HyMap imager and ground truth GER3700 spectrometer in the visible infrared wavelengths.

5. Data processing and methods 5.1. Satellite data The ASTER sensor onboard the Earth Observing System (EOS) TERRA platform is a multispectral imaging system, launched in December 1999, which travels in a near circular, sun-synchronous orbit with an inclination of approximately 98.2°, an altitude of 705 km and a repeat cycle of 16 days. It measures visible reflected radiation in three spectral bands (VNIR between 0.52 and 0.86 μm, with 15-m spatial resolution) and infrared reflected radiation in six spectral bands (SWIR between 1.6 and 2.43 μm, with 30-m spatial resolution). ASTER records the data in band 3B (0.76–0.86 μm) with a backward looking angle that enables the calculation of digital elevation models (DEM). In addition, it receives emitted radiation in five spectral bands in the thermal infra-red region

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between 8.125 and 11.65 μm, with 90-m spatial resolution (Fujisada, 1995). The increase of spectral bands in the SWIR region (one spectral band for Landsat versus six spectral bands for ASTER) enhances the surface mineralogical and lithological mapping capability. In this contribution, we used 14-band ASTER Level 1B data acquired on April 18, 2006 and supplied in terms of scaled radiance at-sensor data with radiometric and geometric corrections applied. It is georeferenced in the UTM projection and for the WGS-84 ellipsoid. We chose the absorption bands of carbonate minerals and peridotites among the 14 ASTER bands and processed and interpreted to the region of interest to detect the carbonate minerals and discriminate the peridotites, which are responsible for the CO2 sequestration as discussed above. To assess and find changes in the CO2 sequestration region, we mapped specific ophiolite massifs using the Landsat TM and ETM satellite data. These have seven multispectral bands including six channels in the visible and reflected infrared regions, from 0.45 to 2.35 μm and one channel in the thermal infrared region of the electromagnetic spectrum from 10.4 to 12.5 μm. Data were acquired on May 14, 1986, May 15, 1998, July 31, 2000, March 4, 2003 and March 4, 2006 for twenty years. The chosen data all have 0% cloud cover of the selected study area. A regional analysis of CO2 sequestration is carried out using an entire scene covering an area of 180 km × 180 km acquired on March 4, 2006. Analysis of time series Landsat satellite data required systematic pre-processing which makes it possible to compare the acquired images. Initially, all images were resampled to a 30-m resolution and coregistered using topographic maps (used to identify ground control points) and UTM projection (Zone 40, Clarke 1980 spheroid, PSD93 datum) coordinate system with a root mean square error of less than 0.5 pixels per image. Then, these were processed systematically by

converting DN values (recorded by sensor) to spectral radiance (at the sensor), to surface reflectance (at the sensor) and removal of atmospheric effects that occurred due to absorption and scattering (atmospheric correction). The DN values of images were converted to surface reflectance by the method consisting of sensor calibration and full radiative transfer modeling, including a correction of topography-induced illumination variations (Schroeder et al., 2006; Röder et al., 2008a,b). The impact of sensor degradation on the gain parameter was accounted using the data published by K. Thome et al. (1997), K.J. Thome et al. (1997) and Teillet et al. (2001) for Landsat TM data. The calibration of ETM+ images were based on parameters published by the Chander et al. (2009). A radiative transfer model was performed for each scene based on the Acronym 6S and 5S Codes as described by Vermote et al. (1997) and Tanre et al. (1990). The atmospheric transmission factors were calculated using the Modtran-4 code (Berk et al., 1999). 5.2. Image processing The mapping of CO2 sequestration in the Samail ophiolite massifs is carried out using image band ratioing and decorrelation stretching digital image processing techniques. The occurrences of CO2 sequestered minerals in the peridotite regions are carried out using the mineral indices such as Carbonate Index (CI = (band13 / band14)), Quartz Index (QI = (band 11 × band 11) / (band 10 × band 12)) and Mafic Index (MI = (band 12 / band 13)) in ASTER thermal infrared bands (Ninomiya et al., 2005; Corrie et al., 2010). The discrimination of peridotites is carried out by ASTER 8, 3 and 1 RGB band combinations which developed based on the spectral absorptions of peridotite minerals.

Fig. 3. Mineral Indices RGB image (R: CI; G: QI; B: MI) shows the distribution of the CO2 sequestered minerals in cyan color along the wadis (drainages) and structural zones (the regions better exposed and have interaction with atmosphere and water), altered serpentinites associated peridotites in purple color and the associated rocks layered gabbro, dykes, basalts and pelagic sequences in yellow brown to light yellow colors (based on the presence of silica contents). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S. Rajendran et al. / Earth-Science Reviews 135 (2014) 122–140

Further, we carried out the decorrelation stretching image processing classification over Landsat (TM and ETM) satellite data to show the region of weathering, alteration and CO2 sequestration in the Samail ophiolite massifs at different scales. The classification is discussed by Gillespie et al. (1986), Rothery (1987a,b) and Abrams et al. (1988), which is based on a principal component transformation of the acquired data. The transformed channels here are themselves contrast stretched and arbitrarily assigned to primary colors for display as a color composite image. The processing of the above data follows the methods of earlier workers (Rothery, 1987a,b; Abrams et al., 1988; Ninomiya, 2002). The satellite data of the selected regions were processed, analyzed and interpreted using ENVI (4.8) and ArcGIS (10) software's and evaluated with field checking and laboratory studies. In this study, the processed images for the occurrences of CO2 sequestered minerals, the discrimination of source rock and the CO2 sequestered regions are given as such in relevant sections in order to have better visual interpretations of such features. The regional geological map (KSEPL, 1974) was used to verify the processed remote sensing data.

127

5.3. Field work and laboratory analyses Systematic field work was carried out during September 2010 and March, 2011 in the selected regions to verify the distribution of CO2 sequestered minerals and the source rock. During the field work, collection of samples of minerals, rocks and water was carried out. The spectral properties of field samples were analyzed using a PIMA SP infrared spectrometer at the Department of Earth Sciences, Sultan Qaboos University. The instrument is fabricated for field spectroscopy by Integrated Spectronics Pty Ltd., Australia. It identifies and analyzes the spectral signal of minerals in the wavelength ranges from 1300 to 2500 nm with PIMA VIEW software (version 3.1). The spectral resolution is ~7 nm. It has a built in wavelength calibration target plate and is capable to measure spectra from 10 s to around 5 min speed. Selected samples were analyzed in X-ray powder diffraction method using X'Pert PRO (Panalytical Company, The Netherlands) working based on PW1710 (Cu: 1.54) in the department for the identification of CO2 sequestered minerals.

Fig. 4. ASTER RGB image shows the regional distribution of the CO2 sequestered minerals in parts of the Samail ophiolite massifs.

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6. Results and discussion 6.1. Satellite data image interpretation 6.1.1. Mapping of CO2 sequestered minerals As described above, the five TIR spectral bands of the ASTER satellite data were processed using the mineral indices for mapping of CO2 sequestered minerals to the selected area in a large scale and the result in RGB color composite image (R: CI; G: QI; B: MI) is given in Fig. 3. The occurrences of CO2 sequestered minerals are well detected on the image due to a unique absorption of C\O bonding (near 11.4 μm for calcite and 11.2 μm for dolomite) in cyan color along the Wadis (drainages) and structural zones (Abrams et al., 1988) where these are better exposed and have more interaction with atmosphere and water (see the expressions of reaction series in Section 2). The lithological interpretations agree well with published geological data and field observations. The regional analysis for occurrences of carbonate minerals was carried out over the entire scene of ASTER

data covering 60 × 60 sq. km using the mineral indices and the result in RGB color composite is given in Fig. 4. The processed regional image highlights similar interpretations and shows the occurrences and distribution of CO2 sequestered minerals in the region. The mineral indices applied over ASTER–TIR bands proved to be an effective unified method of approach for detecting CO2 sequestered mineral identification in the arid region (Ninomiya et al., 2005). The regions of CO2 sequestration are compared with the structural map of the Eastern Mountain regions of Oman (Fig. 5) compiled within a geographical information system (GIS) database using Landsat Thematic Mapper (TM) images and Shuttle Imaging Radar (SIR-C) data by Kusky et al. (2005). The map shows the occurrences of high numbers of fault sets in this region which are accelerating the CO2 sequestrations in this region (Figs. 3, 4). The analyses show the capability of ASTER sensor to detect occurrences and distributions of CO2 sequestered minerals at a regional scale for better assessment of CO2 sequestration in this study region. Further, the result demonstrates the utility of high spectral resolution and the development of appropriate processing methods for mineralogical identifications.

Fig. 5. Structural map of the Eastern Mountain regions of Oman. Numbers 1–7 index the localities of seven areas with the evidence of Tertiary–Quaternary faulting. After Kusky et al. (2005).

S. Rajendran et al. / Earth-Science Reviews 135 (2014) 122–140

6.1.2. Mapping of peridotites Rothery (1987a) and Abrams et al. (1988) have previously outlined the level of apparent lithological discrimination possible with TM images in parts of Oman. Band ratio image enhancement techniques and the supervised classification technique (Sabins, 1997, 1999) to map the serpentinites and associated lithologies have been carried out by several researchers (Sultan et al., 1986, 1987; Rothery, 1987a; Crippen, 1989; Frei and Jutz, 1989; Kusky et al., 2005). The study of spectral absorptions of MgO- and OH-bearing minerals of peridotites leads us to develop ASTER 8, 3 and 1 spectral band combinations to map the peridotites in this work. The resulting RGB image to map the source rock of CO2 sequestration is given in Fig. 6. The peridotites show the sharp discrimination with associated rocks. It appears in dark green to pale green colors which depend on the concentration of serpentine in harzburgite and dunite. Such variations may also indicate the peridotites that are deeply affected by hydrothermal processes and serpentinite formations. The presence of olivine in dunite and harzburgite of the peridotite region is confirmed by the detection of its common product of hydrothermal transformation: serpentine (narrow Mg\OH absorption feature). The distinctive dark green reflectance of serpentinites is caused by the abundance of antigorite, lizardite, clinopyroxenite and magnetite mineral compositions. The associated rocks such as layered gabbro, dykes, basalts and pelagic sequences

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appear in shades of purple color on the image. The gabbros and other rocks of the crustal sequence are relatively poor in serpentine (Fig. 6). 6.1.3. Mapping of CO2 sequestered regions To assess the CO2 sequestered parts of Samail ophiolite massifs, mapping was carried out by a decorrelation stretching image processing technique (Abrams et al., 1988) besides the RGB band combination images of ASTER data on the detection of CO2 sequestered minerals and discrimination of the peridotites. The calibrated three infrared spectral bands 7, 5 and 4 of Landsat ETM satellite data of March 4, 2006 were chosen to process based on spectral absorption characters of minerals of peridotite. Here, band 7 responds to the presence of hydroxyl and carbonate bearing minerals in peridotite by a reduction in reflectance due to various hydroxyls, H2O, and carbonate related features. Band 5 serves to characterize the general albedo of the materials to highlight certain weathered or altered minerals in peridotite rocks which have particularly high reflectances near 1.6 μm, whereas ferrous iron can cause depression of the reflectance curve in this region. Band 4 contains information relating to the presence of iron minerals in peridotite (Abrams et al., 1988). The resulting image is given in Fig. 7A. The image shows excellent discrimination of lithological boundaries and structures existing on the geological map (Fig. 7B; KSEPL, 1974). The geology of area shows the occurrences of major rock types such as strongly sheared

Fig. 6. ASTER 8, 3, and 1 RGB image show the discrimination of peridotites of Samail ophiolite massifs. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

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Fig. 8. The figure shows the Landsat TM 7, 5, and 4 decorrelated RGB images of years (A) 1986, (B) 1998, (C) 2000 and (D) 2003 (legend as in Fig. 7b).

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harzburgite with dunite (P), cumulate layered gabbro (PG), gabbroic hypabyssal rocks (HG), diabase dyke swarms (D) and basic extrusives (mostly spilites with pillow lavas or conglomerate) (E) of Middle Cretaceous age. The exposures of the Oman Melange (OM), Mayhah formation (Mh), Wahrah formation (Wa) and Oman exotics (Ex) occur in the NW and NE parts of the area (Fig. 7B) as post-nappe units belonging to late Tertiary to Quaternary age. The ophiolitic metagabbros in the area are closely associated with other ophiolitic members i.e., peridotite and metabasalts (Fig. 1C). Almost all the rock types are distinct in color and separable on the image (Fig. 7A). We can interpret the contacts of peridotites with associated rocks with their spatial distributions. The peridotite appears in two colors viz. the upper part, for about 1 km below the Moho, the weathered or altered peridotites (serpentinized) rest directly on the mantle sequence (tectonized peridotite) appear in purple colors and the mantle sequence appears dark green in color. The cumulate layered gabbro exhibits cyan through greenish-yellow colors, reflecting differences in mafic content. The correlation of purple color of the region on the image may be related to weathering, hydrothermal alteration or serpentinization in the region rich in serpentine which can be mapped as the region of CO2 sequestration within the areas of ophiolite sequences (Abrams et al., 1988). The hydrothermal alteration extends at

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least as deep as the base of the crust through fracture-controlled tectonized peridotites (Figs. 3, 7A). The detection of hydrothermal transformations of olivine in serpentines and their alteration to carbonates along wadis and fracture networks are characterized by narrow spectral signatures and are validated through the spectral absorptions of mineral identifications. Further, to see the changes of CO2 sequestration for the past twenty years, the Landsat TM and ETM data of May 14, 1986; May 15, 1998; July 31, 2000 and March 4, 2003 are processed by the methods described above and given in Fig. 8. The processed images are well correlatable with the image results of March 4, 2006 (Fig. 7A) and confirm the occurrences of CO2 sequestered regions. On the other hand, the sequences of images do not show any significant changes in the weathering and CO2 sequestrations in the regions that may be due to the slow rate of weathering in peridotites and/or the use of low spectral and spatial resolutions of satellite data in this study. It can be noted, however, that Kelemen and Matter (2008), and Mervine et al. (2014) stated that the rate of CO2 uptake via weathering of peridotite is poorly known and previous workers inferred that carbonate veins in mantle peridotite in the Samail ophiolite massifs of Oman have an average 14C age of 26,000 years, and are not 30–95 million years old. Their study on 14 C in samples of dolomite and magnesite shows ages from 1600 to

Fig. 9. Landsat TM 7, 5, and 4 decorrelated RGB image of 2006 show the regional distribution of CO2 sequestration (purple in color) and the discrimination of peridotites (dark green in color) in parts of the northern mountain region of the Samail ophiolite massifs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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43,000 years, similar to the previously measured range of 840 to 36,000 years. Therefore, we suggest that a time series study be performed using high spatial resolution and hyperspectral remote sensing techniques to quantitatively and qualitatively assess the capacity of the CO2 sequestration region (Combe et al., 2006). An experimental quantitative study on increase of weathering versus CO2 sequestration, coupled with hyperspectral data study in laboratory and field on peridotites, and processing of high spatial resolution remote sensing data demonstrates the potential use of peridotites and assessment of the CO2 sequestration region. We also carried out regional mapping of CO2 sequestration using an entire Landsat ETM scene covering an area of 180 km × 180 km acquired on March 4, 2006 (Fig. 9). The image shows the regional distribution of CO2 sequestration due to weathering and alteration in the extension of some processes of low-grade metamorphism. The purple color is

consistent throughout the length of the peridotite occurrences and running along late-stage fractures in the allochthonous mantle rocks which are interpreted as a post-emplacement serpentinization effect (Rothery, 1984a,b). By contrast, the northern peridotites are very dark, indicating that no such low-grade metamorphism was superimposed on the first serpentine transformation. Image interpretation shows that the region of weathering processes, tectonic environments like faulting and fracturing and the contact zone of peridotites with layered gabbros are the places of serpentinization processes and CO2 sequestration. The moderate spatial resolution pixels of Landsat images contain more mixed information and thus the accuracy of evaluation of results was done by visual comparison and qualitative assessment of the processed images and confirmation with the field checking. The field check and spectral signature measurements of samples of this region confirm that the regions of peridotites are weathered, altered and CO2

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Fig. 10. Field photograph shows (A) the panoramic view of the ophiolite occurrences in the Wadi Fizz (see Fig. 1 for location), (B) the fresh exposures of peridotites exposed to CO2 sequestration in the open cast chromite mine occurred in ophiolite sequences (inset shows the peridotite in hand specimen), (C) the mesoscopic observation of olivine and pyroxene minerals in the serpentinized peridotites, (D) the weathering of serpentinites and (E) the growth and development of cauliflower carbonates (magnesite) over the serpentinites and (F) the CO2 sequestration in the fractured peridotites.

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sequestered and evidenced through the occurrences of hydroxyl and carbonate bearing minerals which are confirmed by laboratory analyses and discussed in following section in detail. 6.2. Field study We carried out field work in several places including some of the places not covered on the image scenes situated across from the occurrences of peridotites parallel to the Samail massifs between September, 2010 and March 2011 in order to evaluate our satellite image interpretation and investigate the occurrences of CO2 sequestrations. We discuss here with more details about two areas namely Wadi Fizh and Wadi Mistal of Samail ohiolite region, which show field evidences for the CO2 sequestration interpreted from the imagery (Fig. 9). In the field, the occurrences of different rock types are identified and the regions of peridotites that were weathered, altered and CO2 sequestered are verified. There is no obvious and consistent difference between the

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two types of mantle sequences (tectonized peridotite and serpentinized peridotite) revealed as green and purple on the enhanced imagery (Fig. 7A). Classic examples of the geologic features observed in the peridotite region of Wadi Fizh (Fig. 10A) show weathering and CO2 sequestrations in well exposed outcrops, cross-cuts, and underground workings, providing an excellent opportunity to characterize the weathering and interaction of geochemical and structural processes. In the field, the ultramafic rocks of the Samail massifs consist mainly of highly fractured tectonized peridotite (Fig. 10B and C) and intensely serpentinized dunites. The ophiolitic metagabbros are closely associated with serpentinites and metabasalts. In well preserved ophiolite sections, the layering in the basal layered gabbros is parallel to the surface separating them from the peridotite tectonites. The serpentinite rock sequences were developed in a supra-subduction zone setting (EIMezayen, 1983). There is no thermal metamorphic effect between the serpentinite and the adjacent rocks and the contact between the serpentinite and the underlying mélange rocks is sharp and marked

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Fig. 11. Field photograph evidences the occurrences of CO2 sequestration in the Samail ophiolite massifs of the northeastern mountain region of Oman (A) and (B) CO2 sequestrations in fractures of peridotites result in the piling of carbonates, (C) CO2 sequestrations in major fractures occurred perpendicular to downstream in ophiolite sequences at the Wadi Fizz region of the Samail massifs, (D) occurrences of CO2 sequestration as coating over the country rock along the valley in the Wadi Fizz and (E) at Wadi Mistal and (F) CO2 sequestration as coating over river flowing surfaces in the Wadi Fizz (inset photograph shows the field measurements of temperature, Ec, pH and TDS).

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by a thrust fault striking NS and NE–SE. The oxidized serpentinized dunite exhibits reddish to brownish color (Fig. 10D) with a gradational contact with the fresh serpentinite. The weathered and altered serpentinites are accompanied by the formation of a network of numerous magnesite and dolomite veinlets providing evidence for the development of CO2 sequestration. Large magnesite–dolomite veins along major tectonic structures are observed. In hand specimens, fresh serpentinite is fine to medium-grained, massive and greenish gray in color (inset photograph in Fig. 10D). Also, cauliflower, fine-grained and massive magnesite is recorded cementing over serpentinites in the fracture system and observed in mining sites (Fig. 10E). The magnesites are formed by the weathering of serpentinite under the action of CO2-rich surface and ground water. The field verifications show that the peridotites are highly fractured and the CO2 sequestered carbonate minerals are locally abundant alteration products of serpentine in the area due to water circulation that was concentrated along faults and wadis. In several places, the

carbonates occur as nodular bodies, fracture filling stockwork and as cemented breccias (Nasir et al., 2007). The CO2 sequestration is developed due to the weathering of peridotites (Fig. 10F) in the fracture system and is measured to be a few millimeters to less than a meter in thickness. The CO2 sequestered carbonate veins extend in the fracture system from a few centimeters in the hand specimens to a few kilometers in the field, and are correlated with the length and width of the fracture systems developed in the region. The CO2 sequestration in fractures observed in the weathered peridotites in the mining area of Wadi Fizz is shown in Fig. 11A and B. The CO2 sequestration in major fracture systems found across downstream and parallel to the peridotite hill formations in the ophiolite sequences (Fig. 11C) was observed in the foot hill region of Wadi Fizz. Here, we can see that the serpentinites were converted to carbonates along the contact with the metavolcanics particularly fault zones. The tectonic shearing, fracturing and brecciation of the peridotite rocks provide the necessary permeability for CO2 sequestration in this region.

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Fig. 12. Field photograph evidences (A) the natural absorption of CO2 and precipitation of carbonates at Ibra (B) typical weathering of peridotites in the contacts of atmosphere CO2 and water at Ibra (C) the near surface CO2 sequestrations in the fractures and joints of peridotites near Helwa, Al Mudaibi (D) massive CO2 sequestration in the major fractures of peridotites at Wadi Fizz. (E) CO2 sequestration in the surface of the Wadi Mistal and (F) dashed line separates the region of weathering and CO2 sequestration at Ibra, correlatable to the satellite data mapping as in Fig. 8.

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Further, in the field, the CO2 sequestered minerals are noticed along the wadis on the eastern border of the sedimentary rocks coming from the Samail massifs, identified by its large concretions, where the water table is close to the surface. Here, the CO2 dissolves in near surface water and having rock–water interaction increases the water salinity and precipitates on surfaces. The low temperature reaction between near surface ground waters and peridotite takes place in equilibrium with the atmosphere, increasing dissolved Mg++ and HCO− 3 concentrations to form Mg\HCO3 waters which lead to the precipitation of Mg carbonates such as magnesite and dolomite on the surface. Also, the dissolution and concentration of Ca++ raise the pH to very high levels in waters and precipitate dolomite and calcite minerals (Barnes and O'Neil, 1969; Bruni et al., 2002; Evans, 2008). The thin surface CO2 sequestrations are abundant on the surface of the rocks of Wadi Fizz (Fig. 11D) and on the banks of Wadi Mistal (Figs. 1B and 11E) from precipitates (Fig. 11F) of highly saline carbonated water. The presences of high pH and TDS in near surface ground waters have been documented during the field work. The field measurements of Ec, pH and TDS on near surface ground waters using portable laboratory water kits showed results ranging in between 101.4 and 117.3, 8.2 and 8.3 and 67 and 77 respectively and the water temperatures range between 25.7 and 27.2 °C. The highly dense brines for CO2 sequestration and the precipitates are observed at Ibra (Figs. 1B; 12A and B) along the wadis. The occurrences of travertine as precipitates (like stalagtites) in the fracture systems are evidenced at Helwa near Almudaibi (Figs. 1B and 12C). Maximum thickness of fully sequestered peridotite as magnesite is noticed at the junction of water flow in Wadi Fizz (Fig. 12D). The CO2 sequestration is also evidenced as precipitated, secreted and concreted carbonates on the river sediments over large areas of Wadi Mistal (Fig. 12E). The region of weathering and CO2 sequestration is correlatable with image interpretations (Figs. 7A, 8 and 9) which are evidenced at Ibra (Fig. 12F). Overall, the field verification revealed that all these locations are observed with the occurrences of CO2 sequestered minerals due to weathering of olivine and serpentine minerals. The image interpretations agree well with published geological data and field observations. 6.3. Laboratory study More than one hundred spectral measurements have been taken using a PIMA SP infrared spectrometer at different locations in the

13% Aragonite + 42% Calcite + 45% Montmorillonite

58% Water + 25% Calcite+ 17% Antigorite

36% Calcite + 64% Water 30%Water + 66%Montmorillonite + 4% Siderite

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field and over the samples collected and measured in the laboratory to identify the minerals of CO2 sequestration. The results are obtained as spectral signature plots with the quantitative measurement of percentage of mineral types in the samples. The study shows the occurrences of major carbonate minerals namely calcite, dolomite, magnesite, aragonite and siderite in various percentages in this region. The hydroxyl minerals such as antigorite, and montmorillonite associated with carbonate minerals are also detected in the samples. The spectral plots of selected samples representing the occurrence of carbonate and hydroxyl minerals in different proportions in the CO2 sequestration region are given in Fig. 13. Carbonate minerals such as calcite, dolomite, magnesite, aragonite and siderite occur in the ranges of 15 to 57%; 12 to 53%, 9 to 38%; 11 to 21% and 3 to 8% respectively. The major hydroxyl minerals such as antigorite and montmorillonite are present in the ranges of 10 to 21% and 37 to 81% respectively. The presence of minor amounts of tremolite and anhydrite in the samples are also noticed in this study. 6.3.1. Spectral characteristics of CO2 sequestered minerals In the spectral plot, the hydroxyl-bearing and carbonate minerals show absorption features around 1.4 μm, 1.9 μm and 2.35 μm in the infrared wavelength regions (Fig. 13). The absorptions at 1.4 μm and 1.9 μm (vertical dashed lines in Fig. 13) are due to hydration effects caused by the hydroxyl minerals namely antigorite and montmorillonite. The absorptions around 2.3 μm (vertical line in Fig. 13) are mainly due to CO3. The samples consist of CO2 sequestered carbonate minerals such as calcite (CaCO3), dolomite (CaMg(CO3)2) and aragonite (CaCO3) showing strong absorptions at 2.35 μm due to C\O bonds (Hunt, 1977) as discussed in Section 4. The other magnesite (MgCO3) and siderite (FeCO3) carbonates do not show any significant useful narrow absorption features around 2.35 μm. The serpentinite alteration is evidenced by the presence of the serpentine mineral antigorite. The selected samples collected from the field were further analyzed by the X-ray powder diffraction (XRD) method to evaluate the occurrence of carbonate minerals of the CO2 sequestered region. The results of XRD analyses are given in Fig. 14, which confirmed the presences of CO2 sequestered carbonate minerals such as aragonite, calcite and dolomite in the samples. Calcium carbonates occur as calcite and aragonite minerals. The serpentinite alterations are evidenced by the presence of the serpentinite mineral lizardite.

15% Aragonite + 42% Calcite + 43% Montmorillonite

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76% Montmorillonite + 24% Magnesite

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7. Conclusions The present study is carried out to map the areal abundance of CO2 sequestration in peridotite rocks in the northern mountain region of the Samail ophiolite massifs of the Sultanate of Oman using the capability of multi-spectral remote sensing data. The improved spatial and spectral resolutions in VNIR, SWIR and TIR bands of ASTER spectral regions detected the areal abundance of CO2 sequestered minerals in the study area. The developed ASTER 8, 3 and 1 visible and near infra-red spectral band combinations discriminated the peridotite rocks of the Samail massifs. The decorrelated infra-red bands of the Landsat (TM and ETM) sensor show the regions of CO2 sequestrations i.e., the highly weathered and altered serpentinized peridotites within the mantle sequence. The interpreted image results agree well with the existing geological maps and field evidence. The field checking and laboratory spectral measurements of carbonate minerals in 1300 to 2500 μm wavelengths with the spectral resolution of ~ 7 nm using the PIMA SP infrared spectrometer show the occurrences of hydroxyl-bearing and carbonate minerals with the absorption features around 1.4 μm, 1.9 μm and 2.35 μm. The identified carbonate minerals are calcite, dolomite, magnesite, aragonite and siderite and the hydroxyl minerals are antigorite, and montmorillonite. The results of X-ray powder diffraction analyses on field samples confirm the occurrence of major carbonate minerals such as aragonite, calcite and dolomite in this CO2 sequestration region. The serpentinite alterations are evidenced by the presence of serpentinite minerals such as antigorite and lizardite. In this context, we suggest that the identified space technique can be used as a powerful tool to detect the CO2 sequestered minerals, to discriminate the regions of peridotites and the source rock of CO2 sequestered minerals, and to monitor the region of weathering and CO2 sequestration. Mapping of CO2 sequestration in the northern mountain region of the Samail massifs of Oman may be applicable to well-exposed arid and semi-arid regions worldwide. This study expands knowledge of the geological carbon capture system using an effective cost, time and environmental safety technique. Acknowledgments The authors are thankful to NASA Land Processes Distributed Active Archive Center User Services, USGS Earth Resources Observation and Science (EROS) Center (https://LPDAAC.usgs.gov) for providing the ASTER and Landsat data. The PIMA SP infrared spectrometer used in this study is supported by Sultan Qaboos University. The study is partially supported by the National Natural Science Foundation of China (Grants 91014002, and 40821061) and the Ministry of Education of China (B07039) to Prof. Timothy M. Kusky. The XRD analytical helps are extended by Mr. Saif Amer Al-Maamari and the laboratory helps extended by Mr. Abdulla Al-Fahdi and Mr. Hilal Said Al-Zidi, the Technicians, Department of Earth Sciences, SQU are thankfully acknowledged. The transport facility extended by SQU to carry out field check in rugged Samail ophiolite massifs is thankfully acknowledged. Sincere thanks are given to Dr. John C. Mars, USGS, who read earlier versions of the manuscript and provided several constructive comments and suggestions. The discussion extended by Dr. Bernhard Pracejus is thankfully acknowledged. The authors are very much thankful to the anonymous reviewers and editor of the journal for their valuable reviews and providing constructive comments and suggestions that have helped to present the work lucidly. References Abdeen, M.M., Allison, T.K., Abdelsalam, M.G., Stern, R.J., 2001. Application of ASTER bandratio images for geological mapping in arid regions; the Neoproterozoic Allaqi Suture, Egypt. Abstr. Program Geol. Soc. Am. 3 (3), 289. Abrams, M.J., Hook, S.J., 1995. Simulated ASTER data for geologic studies. IEEE Trans. Geosci. Remote Sens. 33, 692–699.

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