• Dwijesh Ray

      Articles written in Journal of Earth System Science

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

      Dwijesh Ray Catherine Mevel Ranadip Banerjee

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      Mylonitic 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 (32–35mm/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.

    • Microtextural and mineral chemical analyses of andesite–dacite from Barren and Narcondam islands: Evidences for magma mixing and petrological implications

      Dwijesh Ray S Rajan Rasik Ravindra Ashim Jana

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      Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.

    • Iron-nickel metallic components bearing silicate-melts and coesite from Ramgarh impact structure, west-central India: Possible identification of the impactor


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      The Ramgarh structure (rim-to-rim diameter $\sim$2.4 km) in the Vindhyan Supergroup of sedimentary rocks (including sandstone, shale and minor limestone) of the Mesoproterozoic age in the west-central India, is India’s third confirmed asteroid impact crater. This eroded structure is roughly rectangular in shape and resembles to the Barringer Crater, USA. The presence of central peak and its current crater diameter/depth ratio of $\sim$12 well corroborate the range (10–20) of terrestrial complex asteroid impact craters. The mm-sized, iron-rich (FeO $\sim$50 wt.% in average), spherule-like particles, recovered from the alluvium inside the Ramgarh structure, have internal morphology similar to those of the accretionary lapilli described in known impact craters. The in-situ LA-ICP-MS analyses also suggested high Co–Ni (up to 13,000 and 2500 ppm, respectively)-rich areas locally within these spherules/lapilli. A few non-in-situ, mm-sized particles, recovered from the rim of the structure show the presence of coesite, one of the diagnostic indicators of shock metamorphism. A few fragments of iron-rich, Ca–Al–silicate glasses recovered from the soil inside the structure and outside of the western crater rim include the presence of dendritic magnetite with occasional inclusions of relict native iron. Our microprobe analyses confirm that these metallic irons contain high proportions of Co ($\sim$350–3000 ppm), Ni ($\sim$200–4000 ppm) and Cu ($\sim$2200–7000 ppm) and possibly could be the relict component of a Cu-rich iron meteorite impactor. The field observation and relative enrichment of compatible and incompatible trace elements in the spherule-like substance (recovered from the alluvium inside the Ramgarh structure) as compared to target rocks suggests that hydrothermal activity played an important role in the evolution of the crater.

    • Understanding genesis of iron oxide concretions present in Dhandraul (Vindhyan) Sandstone: Implications in formation of Martian hematite spherules


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      The iron oxide concretions of Shankargarh (Allahabad), India belongs to Dhandraul Sandstone of Vindhyan Supergroup. Petrography of concretions shows abundant quartz grains embedded within the iron oxide cementation. XRD analysis of the concretion shows diagnostic peaks for quartz, hematite, and goethite. The A–CN–K and A–CNK–FM ternary diagrams drawn for concretion and host rock bulk composition clearly indicate the interaction of concretions rock with iron-bearing diagenetic fluids. A negative Ce anomaly, lower Th/U ratio, and enrichment of redox responsive trace element (e.g., vanadium) indicate concretion formation is redox-controlled. The concretions show Fe enrichment and Si depletion as compared to the host sandstone. The mass balance calculations indicate that the total Fe$_{2}$O$_{3}$ in the ferruginous sandstone system is 17.63 wt%. The iron mobilization and recycling in the sandstone pore spaces have formed concretions with Fe$_{2}$O$_{3}$ (25–35% by volume). The sandstone volume required to produce a 6 mm diameter iron oxide concretion is 1807.83 mm$^{3}$. The Fe laminae and random red colouration patterns in Dhandraul sandstone are consistent with the movement of iron-enriched fluid through pores and spaces. These iron oxide concretions have similarities with the hematite spherules discovered in the Burn Formation, Meridiani Planum, Mars.


      $\bullet$ Random red colouration pattern, iron laminae, the role of advective and diffusive processes during the formation of the iron oxide concretions present in Dhandraul sandstone are discussed in detail.

      $\bullet$ Mineralogical and textural study of the Shankargarh iron oxide concretions using petrography and XRD.

      $\bullet$ Fe bearing paleofluid circulation, redox processes, and elemental mobility (enrichment and depletion of elements) are discussed in detail using whole-rock geochemistry of Shankargarh iron oxide concretions and associated sandstone.

      $\bullet$ The similarities and difference between Shankargarh iron oxide concretions and Martian hematite spherules.

    • Lunar feldspathic meteorite Dhofar 081: Petrochemical constraints on petrogenesis


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      Two alternative petrogenetic models, plagioclase flotation and serial magmatism have been proposed to explain the origin of the lunar anorthositic crust, covering ${\sim}$80% of the lunar highland. In this study, we re-examine the possible relict igneous texture present in an inferred lunar highland breccia clast (area ${\sim}$1 mm$^{2}$) in the Dhofar 081 meteorite. Our new petrographic and in-situ mineral microprobe chemical data on this clast show this coarse grained (average grain size ${\sim}$0.5 mm) clast preserves relict igneous texture where subhedral, prismatic low-Ca pyroxene has intergrown with anhedral anorthitic plagioclase, suggesting its eutectic crystallization from its parent silicate magma. Absence of maskelynite and similarity of Na, K contents of plagioclase with the FAN assemblages negate the possibility of crystallization of the studied relict clast from an impact melt. The mineral-chemical data of Dhofar 081 suggest it is FAN (Ferroan anorthosite) in composition (after Warren in Annu. Rev. Earth Planet. Sci. 13:201–240, 1985). Hence, intergrown crystallization of minerals in the present relict igneous clasts and other reported FAN samples argues against a cumulate origin of the lunar anorthosite. The orthopyroxenes present in the unbrecciated portion of this meteoritic clast include bimodal low- and high-iron geochemical sub-groups. The application of orthopyroxene and plagioclase thermobarometry (after Gasparik in Contrib. Mineral. Petrol. 96:357–370, 1987) on our new microprobe data, and also two-pyroxene thermometry (after Lindsley in Am. Mineral. 68:477–493, 1983; Putirka in Rev. Mineral. Geochem. 69(1):61–120, 2008) on our new microprobe data and synthesis of literature data constrain the pressure and temperature of crystallization of lunar anorthosite parent magma close to 8 kbar and 1050$^{\circ}$C, respectively. Application of Fo–An–Q experimental phase diagram at high pressure (up to 20 kbar) negates the possibility of generation of lunar anorthosite from a lherzolite source, the parent magma of these anorthosites probably lie on or close to Fo–An join of this phase diagram close to the spinel field.


      $\bullet$ Lunar anorthositic meteorite represents the global highland crust of Moon.

      $\bullet$ Relict igneous clast of lunar anorthositic meteorite shows intergrown texture vis-a-vis eutectic crystallisation.

      $\bullet$ Pressure and temperature of crystallisation of parent magma close to 8 kbar and 1050$^{\circ}$C.

      $\bullet$ Serial magmatism is consistent to explain the textural and mineral-chemical characters and vis-à-vis petrogenesis of lunar anorthosite.

    • The unusual low Mg rock clasts from Lohawat Howardite, India: Petrogenetic implications


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      We report here a few, unusual, low Mg rock clasts from Lohawat howardite, a megaregolith of matrix-dominated polymict breccia comprising clasts of ${\sim}$16% diogenites, ${\sim}$9% basaltic eucrites and ${\sim}$3% cumulate eucrites. Low Mg rock clasts are largely characterised by silica–fayalitic olivine (Fa$_{73–95}$)– ferroaugite (En$_{3–29}$Fs$_{32–55}$Wo$_{34–41}$) bearing symplectites along with ferroan pyroxenes, calcic feldspars (An 89.3), low Mg ilmenite (${\le}$0.3 wt.% MgO), troilite and SiO$_2$ polymorph. A few low Mg clasts without symplectites contain ferro-pigeonite (En$_{30}$Fs$_{55.8}$Wo$_{14}$), ternary calcic feldspar (An$_{81.8}$Ab$_{17.4}$Or$_{0.8}$), low Mg ilmenite (${\ge}$0.3 wt.% MgO) and a quartz-ilmenite intergrowth phase. Two tiny melt inclusions include K-rich granite and Ca-rich granitic composition, respectively. These low Mg rock clasts appeared ballistically transported regolith fines of granite from some unidentified, distant areas where high-energy impact on eucrite crust could have generated limited parental melt (via impact-induced remelting) and ultimately produced the low Mg rocks and acidic melts, respectively.

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