Petrogenesis of ferromanganese nodules from East of the Chagos Archipelago, Central Indian Basin, Indian Ocean

Deep-sea ferromanganese nodules occur over a large area and on many different sediment types of the Central Indian Basin, Indian Ocean. Selected samples were studied to determine their chemical and mineralogical compositions and microstructural features. Repeated laminations of variable thickness, alternately dominated by todorokite and vernadite, are characteristic of these nodules. These laminae show, on electron microprobe line scans, corresponding interlaminar partitioning of Mn-Cu-Ni and Fe-Co. The bulk chemical compositions of these nodules plot in both the hydrogenetic and early diagenetic fields on the Fe–Mn–(Ni + Cu + Co) × 10 ternary diagram. The binary diagram depicting the covariation of Mn + Ni + Cu against Fe + Co shows two distinct parallel regression lines, one delineated by nodules from terrigenous, siliceous ooze and siliceous ooze–terrigenous sediments and the other by nodules from red clay, siliceous ooze–red clay and calcareous ooze–red clay. An increasing diagenetic influence in the nodules with the nature of the host sediment types was observed in the sequence: terrigenous → siliceous ooze and red clay → siliceous/calcareous ooze–red clay. A negative correlation between Mn/Fe ratio and Co and a positive correlation between Mn/Fe ratio and (Ni + Cu) was established. The nodules show dendritic, laminated, and globular microstructures formed by primary growth of Fe–Mn oxide laminae. Depositional hiatuses in the primary microstructures indicate that the growth of these nodules was episodic. The oxide laminations show extremely complex growth patterns. Scattered biogenic remains and mineral grains acted as accessory 'seeds' for growth of oxide layers in addition to the main nuclei. None of the primary microstructures can be uniquely linked to a particular growth process or growth rate. Radial cracks, cutting across primary microstructures, are often filled by todorokite of a later generation. Post-depositional modifications of the nodules were largely controlled by accreted biogenic remains as indicated by their progressive dissolution with increasing depth from nodule surfaces, their pseudomorphic replacement by todorokite and the later growth of phillipsite and todorokite in the microfossil molds. The growth patterns of the in-filled oxides are often controlled entirely by the cavity-walls and are discordant with the primary growth fabric. Primary todorokite was recrystallized to coarser grains of different chemical composition. Later generation veins of todorokite cut across and chaotically disrupted primary laminae

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