The reflectance spectroscopy of ultramafic rocks from the Nagaland Ophiolite Complex, NE India: Convergence of spectroscopic and petrological analyses

The mafic-ultramafic cumulate rocks and mantle peridotites of an ophiolite are excellent recorders of the geodynamic evolution of a fossil subduction zone. The spectro-radiometric absorption features of olivine, orthopyroxene, clinopyroxene and serpentine, which are the dominant constituents of these ultramafic rocks, have emerged as essential tools for their identification. This study uses six different kinds of ultramafic rock samples - spinel lherzolite, spinel harzburgite, wehrlite, orthopyroxenite, olivine websterite, and dunite compositions from the Nagaland Ophiolite Complex, NE India. These are investigated with integrated petrographic, mineral and bulk rock compositional and reflectance spectroscopic studies to identify the constituent minerals. The study indicates that the most dominant olivine absorption features vary from 1038 to 1049 nm in Ni-poor ultramafic cumulates of wehrlite (Mg-# = 89, Ni = 355 ppm) and olivine websterite (Mg-# = 90.5, Ni = 361 ppm) to 1066-1097 nm in nickeliferous mantle peridotites of dunite (Mg-# = 87, Ni = 3658 ppm) and spinel lherzolite (Mg-# = 87, Ni = 2708 ppm) compositions. Reversal of the 625 nm reflectance maximum in olivine occurs for Ni concentrations in the mineral between 2200 and 2955 ppm. The low-Ca pyroxene (cf. orthopyroxene) species show characteristic absorption features at 902-926 nm in lherzolite and harzburgite samples and 918 nm in monomineralic orthopyroxenite. High-Ca pyroxene (cf. clinopyroxene) in most rocks records absorption features at 643-666 and 709-811 nm. The study also recognises major absorption bands at 1382-1400, 1913-1948, and 2304-2326 nm as indicative of secondary hydrous minerals as a product of alteration in the investigated rocks. The significance of major (Mg-Fe) and minor (Mn, Ni and Cr) element compositions of these ultramafic rocks to explain specific spectral signatures in olivine, orthopyroxene and clinopyroxene is discussed. The findings are expected to aid the interpretation of remotely sensed data from terrestrial and extra-terrestrial bodies, mapping ultramafic cumulate and mantle peridotite rocks in remote, covered, and unmapped terranes of ophiolitic origin, and in the process, identification of fossil oceanic subduction systems.

Automated summary

This study investigates the constituent minerals of different kinds of ultramafic rock samples through petrographic, mineral, and compositional studies. It identifies the dominant absorption features of olivine and different pyroxenes in these rocks. The study also recognizes absorption bands indicative of secondary hydrous minerals associated with alteration. The findings have significance in interpreting remotely sensed data and identifying fossil subduction systems.