Hydrothermal alteration studies of gabbros from Northern Central Indian Ridge and their geodynamic implications

International audienceMylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32-35 mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼700-800°C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼300°C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro

A petrogenetic model of basalts from the Northern Central Indian Ridge: 3-11° S

Mid-Ocean Ridge Basalts (MORB) from the Northern Central Indian Ridge (NCIR) were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typical transitional MORB (T-MORB) variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having #Mg > 0.68 are the product of partial melting at an estimated pressure of ~ 10 kbar. It is inferred that the magma was subsequently modified at a pressure > 10 kbar by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure (between 10–5 GPa), the bulk composition of the magma became systematically depleted in MgO, and enriched in total FeOt, TiO2, P2O5 and Na2O. There was, however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K2O. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cr. In addition, the total REE of the magma also increased with fractionation, without any change in (La/Yb)N value

Global documentation of overlapping lobate deposits in Martian gullies

International audienceGullies on Mars are kilometre-scale sediment transport systems comprising an alcove, channel and debris apron. In some gullies lobate deposits have been observed, implying a debris-flow-like process, but uncertainty remains as to the importance of this process in the gully-population and whether lobate deposits are related to a specific context. Our study utilized the HiRISE image archive to document evidence of overlapping lobate deposits on gully-fan surfaces within craters emplaced between 30˚-75˚ in both hemispheres. We have identified 26 craters in which lobate deposits occur in gullies, of which 6 were previously reported. This corresponds to 3.39% of the 765 gullied craters studied using 1004 HiRISE images. We show that gullies with lobate deposits (1) do not show any location preference, (2) are poleward-facing, (3) are found in craters with and without latitude dependent mantle (LDM) and/or glacier-like-forms, (4) are emplaced at slopes at the foot of the fans that range from 18-25° in the northern and 7-19° in the southern hemisphere, and (5) form in craters of all ages. From our observations, we infer that preservation of lobate deposits in gullies is the main factor leading to their relative paucity within the gully-population. Further, we observe morphological features associated with terrestrial wet-debris flows including: overlapping tongue-shaped terminal lobes, levees, channel backfilling, plug formation and avulsion. We have not observed any significant present-day changes in the morphology and topography of gullies and/or lobes. We conclude that a debris-flow-like process is likely responsible for the majority of sediment transport in gully-landforms

Geomorphic evidence of lobate deposits in gullies on Mars - global survey using HiRISE images

International audienceGullies on Mars, found between 30˚-75˚ in both the hemispheres, represent steep slope drainage systems comprising an alcove, a channel and a depositional apron [1]. Lobate deposits similar to those found in terrestrial debris-flow gullies have been observed in martian gullies, consequently researchers suggested that water-bearing debris-flows likely formed martian gullies. However, since there are only few sites on Mars where the evidence of lobate deposits is preserved, and recent observations have revealed present-day modifications in martian gullies that result in features akin to water-bearing debris-flows [2], sublimation of seasonal CO2-frost has been suggested to drive gully formation [2]. Thus, water has been suggested to never have been involved in gully formation [2], thereby raising a question whether lobate deposits are from wet (water-bearing) or dry (CO2-frost) flows. To decipher whether the CO2-frost driven mechanism has formed the entire suite of gullies, we have conducted geomorphic investigation utilizing the High Resolution Imaging Science Experiment (HiRISE) images of the craters emplaced between 30˚-75˚ in both the northern and southern hemispheres to document evidence of overlapping lobate deposits. We primarily aim to infer whether the geomorphic evidence of overlapping lobate deposits are widespread and whether their presence reflects a global climate signal. Furthermore, we aim to understand the orientation preferences and slope ranges of crater walls on which lobate deposits are found and examine whether the age of the craters containing evidence of lobate deposits influence their formation and distribution. We have found 20 new sites (on crater walls) on Mars (6 in the north and 14 in the south) in which unambiguous lobate deposits are preserved. Overlapping lobes are evident at all the sites, such as stacking of relatively small-sized individual lobes or laterally elongated lobes at the fan termini or at the fan surface. We use the most up-to-date global data on gully orientation [3] to compare the orientation of sites with gullies and those with lobate deposits. We measure the slope of surfaces dominated with overlapping lobes using digital terrain models (DTM) from Context camera (CTX) stereo images and compare them with previously reported values of slopes estimated for lobate deposits [4-5]. The crater age will be used to infer if there is a preference for a time-period during which the lobate deposits are emplaced. Together, we will use our observations to decipher the origin of lobate deposits, and examine the possible role of water to critically assess the hypothesis that present-day CO2-frost driven processes can explain the formation and evolution of entire suite of martian gullies. [1] M.C. Malin, K.S. Edgett, Science 288 (2000) 2330-2335. [2] C.M. Dundas, A.S. McEwen, S. Diniega, C.J. Hansen, S. Byrne, J.N. McElwaine, Geol. Soc. Lond. Spec. Publ. 467 (2017). [3] S.J. Conway, T.N. Harrison, R.J. Soare, A.W. Britton, L.J. Steele, Geol. Soc. Lond. Spec. Publ. 467 (2017). [4] A. Johnsson, D. Reiss, E. Hauber, H. Hiesinger, M. Zanetti, Icarus 235 (2014) 37-54. [5] T. De Haas, D. Ventra, E. Hauber, S.J. Conway, M.G. Kleinhans, Icarus 258 (2015) 92-108

Glass and mineral chemistry of northern central Indian ridge basalts: compositional diversity and petrogenetic significance

The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst An(60-90)] and groundmass An(35-79)]), olivine (Fo(81-88)), diopside (Wo(45-51), En(25-37), Fs(14-24)), and titanomagnetite (FeOt similar to 63.75 wt% and TiO2 similar to 22.69 wt%). The whole-rock composition of these basalts has similar Mg# mole Mg/mole(Mg+Fe2+)] (VT basalt: similar to 0.56-0.58; VM basalt: similar to 0.57), but differ in their total alkali content (VT basalt: similar to 2.65; VM basalt: similar to 3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the Sigma REE of magma also increased with fractionation, without any change in the (La/Yb)(N) value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56-0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63-0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure > 10 kbar, followed by Fe-rich olivine at < 10 kbar pressure

GLOBAL DOCUMENTATION OF OVERLAPPING LOBATE DEPOSITS IN MARTIAN GULLIES: IMPLICATIONS FOR THE ROLE OF DEBRIS-FLOW PROCESS IN GULLY FORMATION

International audienceFigure 2a. Subframe of HiRISE image ESP_020774_1445, which shows a part of the pole-facing wall of Los crater (dia.: ~7.8 km; best-fit age: ~7.9±3 Ma) cantered on 35.08˚ S, 76.22˚ W. These gullies have slope gradients at the top, middle and bottom of ~35˚, 28˚, and 14˚, respectively. 2b. Detail of the gully fan surface within Los with overlapping lobate deposits, including convex-up and tongue shaped terminal lobes with lateral levees

Spectral characteristics of banded iron formations in Singhbhum craton, eastern India: Implications for hematite deposits on Mars

Banded iron formations (BIFs) are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior (Canada) and Carajas (Brazil) have been studied by planetary scientists to trace the evolution of hematite deposits on Mars. Hematite deposits are extensively identified in Meridiani region on Mars. Many hypotheses have been proposed to decipher the mechanism for the formation of these deposits. On the basis of geomorphological and mineralogical studies, aqueous environment of deposition is found to be the most supportive mechanism for its secondary iron rich deposits. In the present study, we examined the spectral characteristics of banded iron formations of Joda and Daitari located in Singhbhum craton in eastern India to check its potentiality as an analog to the aqueous/marine environment on Mars. The prominent banding feature of banded iron formations is in the range of few millimeters to few centimeters in thickness. Fe rich bands are darker (gray) in color compared to the light reddish jaspilitic chert bands. Thin quartz veins (<4 mm) are occasionally observed in the hand-specimens of banded iron formations. Spectral investigations have been conducted in VIS/NIR region of electromagnetic spectrum in the laboratory conditions. Optimum absorption bands identified include 0.65, 0.86, 1.4 and 1.9 μm, in which 0.56 and 0.86 μm absorption bands are due to ferric iron and 1.4 and 1.9 μm bands are due to OH/H2O. To validate the mineralogical results obtained from VIS/NIR spectral radiometry, laser Raman and Fourier transform infrared spectroscopic techniques were utilized and the results were found to be similar. Goethite-hematite association in banded iron formation in Singhbhum craton suggests dehydration activity, which has altered the primary iron oxide phases into the secondary iron oxide phases. The optimum bands identified for the minerals using various spectroscopic techniques can be used as reference for similar mineral deposits on any remote area on Earth or on other hydrated planetary surfaces like Mars

Morphologic and morphometric differences between gullies formed in different substrates on Mars: new insights into the gully formation processes

International audienceMartian gullies are kilometer-scale, geologically young features with a source alcove, transportation channel, and depositional fan. On the walls of impact craters, these gullies typically incise into bedrock or surfaces modified by the latitude-dependent mantle (LDM; inferred as consisting of ice and admixed dust) and glaciation. To better understand the differences in the alcoves and fans of gullies formed in different substrates and infer the flow types that led to their formation, we have analyzed the morphology and morphometry of 167 gully systems in 29 craters distributed between 30 and 75°S. Specifically we measured length, width, gradient, area, relief, and relief ratio of the gully alcoves and fans; Melton ratio, relative concavity index, and perimeter; and form factor, elongation ratio, and circularity ratio of the gully alcoves. Our study reveals that gully alcoves formed in LDM/glacial deposits are more elongated than the gully alcoves formed in bedrock, and they possess a distinctive V-shaped cross section. We have found that the mean gradient of fans formed by gullies sourced in bedrock is steeper than the mean gradient of fans of gullies sourced in LDM/glacial deposits. These differences between gullies were found to be statistically significant and discriminant analysis has confirmed that alcove perimeter, alcove relief, and fan gradient are the most important variables for differentiating gullies according to their source substrates. The comparison between the Melton ratio, alcove length, and fan gradient of Martian and terrestrial gullies reveals that Martian gully systems were likely formed by terrestrial debris-flowlike processes. Present-day sublimation of CO 2 ice on Mars may have provided the adequate flow fluidization for the formation of deposits akin to terrestrial debris-flow-like deposits