Nirmala Jain

Jarosite occurrence in the Deccan Volcanic Province of Kachchh, western India: Spectroscopic studies on a Martian analog locality

The sulfate mineral jarosite is considered a key indicator of hydrous, acidic, and oxidizing conditions on the surface of early Mars. Here we report an analog terrestrial locality hosting jarosite from Matanumadh, Kachchh, western India, using detailed spectroscopic studies on weathered basalts of the Deccan Volcanic Province and overlying tuffaceous shales and sandstones of the Matanumadh Formation. Hyperspectral data in the visible/near-infrared (350–2500 nm) to midinfrared (4000–400 cm−1) region of the electromagnetic spectrum and X-ray diffraction patterns have been acquired on samples collected from the field to detect and characterize the hydrous sulfate and phyllosilicate phases present at the studied site. Hydrous sulfates occur in association with Al-rich phyllosilicates (kaolinite) that overlie a zone of Fe/Mg smectites in altered basalts. Jarosite is found within both saprolitic clay horizons altered from the basalt and within variegated sandstone and shale/clay units overlying the saprolite; it mostly occurs as secondary veins with or without gypsum. Jarosite is also seen as coatings on kaolinite clasts of varying shapes and sizes within the tuffaceous variegated sandstone unit. We argue that the overall geological setting of the Matanumadh area, with this unusual mineral assemblage developing within altered basalts and in the overlying sedimentary sequence, mimics the geological environment of many of the identified jarosite localities on Mars and can be considered as a Martian analog from this perspective.

Alteration and submergence of basalts in Kachchh, Gujarat, India: implications for the role of the Deccan Traps in the India–Seychelles break-up

Abstract The Deccan Trap flood basalt volcanism has commonly been considered to have initiated the break-up of India from the Seychelles (c. 62.5 Ma). In Kachchh, Gujarat, western India, the sedimentary succession in the Paleocene Matanomadh Formation was deposited on highly weathered Deccan Trap basalts that were altered to kaolinite before basin formation. This contrasts with the weathering pattern on flat-topped hills of the Deccan Traps outside the Cenozoic rift basins in the Kachchh region and other parts of India, where basalt is dominantly altered to smectitic minerals. As basalts that are altered to smectite and kaolinite occur just across the faults that bound the Matanomadh Basin, the differential weathering cannot be attributed to climate. Geochemical modelling shows that kaolinite stabilizes in preference to smectite if CO2- and O2-buffered rainwater interacts with well-drained basaltic rock at high water:rock ratios. Such conditions can be accomplished by rainfall on a slope created by Cenozoic rifting that exposes the graben flank and basin floor basalts to continuously flowing water, the composition of which is buffered by equilibration with the atmosphere. As the rift post-dates both the eruption of the basalts and subsequent smectite formation, the associated extensional tectonics must be unrelated to flood basalt volcanism, and is most likely to correspond to the India–Seychelles break-up.

Mapping of hydrothermally altered zones in Aravalli Supergroup of rocks around Dungarpur and Udaipur, India, using Landsat-8 OLI and spectroscopy

We explored the utilization of Landsat-8 Operational Land Imager (OLI) data for mapping of hydrothermal alteration zones. The region in and around the cities of Dungarpur and Udaipur of Rajasthan state in India was selected for this study. The rock types of Dungarpur and Udaipur are serpentinites, talc-carbonate, talc-schist, and quartzite of the Aravalli Supergroup. Hydrothermally altered zones and resultant hydrous minerals play an important role in the genesis of these rocks. We aimed to identify possible locations of hydrothermally altered zones in regional context around Dungarpur and Udaipur using Landsat-8 OLI data. False-color composite maps and band ratios were prepared from Landsat-8 bands. Band ratios such as band 6/band 7 (short-wave infrared 1 (SWIR1)/short wave infrared 2 (SWIR2)), band 4/band 3 (red/green), and band 5/band 6 (near infrared (NIR)/SWIR1) and visual interpretation techniques were used to identify the hydrothermally altered zones. Spectroscopic analyses of field rock samples were done to validate the hydrothermal alteration zones delineated from the analysis of Landsat-8 data. We present the combined results of Landsat-8 and field spectroradiometer analysis which brings out the hydrothermal alteration zones associated with hydrous minerals (antigorite, lizardite, montmorillonite, vermiculite, talc, and saponite). The study demonstrates the utility Landsat-8 OLI (with field spectroradiometer data) in the mapping of hydrothermally altered zones as a key in understanding geological processes.