[BCG06-P11] Petrography of magnesite-orthopyroxene-olivine veins in the Ujaragssuit Nunât layered ultramafic body terrane, southern West Greenland
Keywords:Ancient Carbonation, Ultramafic rocks, West Greenland
Ultramafic rocks are transformed into serpentinite and carbonated rocks when they react with H2O-CO2 fluids under low temperature conditions. Carbonated ultramafic rocks from the Archean geological unit might provide information on impact of carbonatization of peridotite on atmospheric and water chemistry in Archean.
The studied magnesite-bearing veins occur in amphibole harzburgite. The magnesite-bearing veins contain coarse grained olivine and orthopyroxene, anthophyllite and talc, with minor amounts of phlogopite, chlorite, Cr-spinel and Fe-Ni sulfide. Magnesite contain anthophyllite and olivine. Anthophyllite is radial in shape. The Mg# (Mg/(Mg+Fe) atomic ratio) is 0.927-0.941 for magnesite, 0.862-0.882 for anthophyllite, 0.893-0.901 for coarse-grained orthopyroxene, 0.974-0.984 for talc, respectively. Orthopyroxene sometimes contains magnesite as inclusion, and micro-veins mainly composed of talc. Fine-grained olivine with minor amount of magnetite occurs in the micro-veins. The Mg# is 0.893-0.901 for coarse-grained orthopyroxene, 0.974-0.984 for talc, and 0.872-0.889 for fine-grained olivine. The Mg# of fine-grained olivine in talc micro-veins (0.872-0.889) are distinctively lower than coarse-grained olivine and olivine inclusions in magnesite (0.882-0.905).
The carbonate veins were formed as follows: 1) coarse-grained orthopyroxene was formed by the reaction between olivine in the host and CO2-bearing fluid
Mg2SiO4 (P1) + CO2 (fluid) = MgCO3 + MgSiO3 (1),
whereas coarse-grained olivine and anthophyllite were formed by reaction with low-CO2 activity fluids, such as
5MgSiO3 + H2O = Mg2SiO4 (secondary) + Mg3Si4O10(OH)2 (2),
and the fine-grained olivine in talc veins in large grained orthopyroxene was formed by the reaction
9MgSiO3 + H2O (fluid) = Mg7Si8O22(OH)2 + Mg2SiO4 (3).