TY - JOUR
T1 - Oxygen isotope fractionation in decarbonation metamorphism
T2 - the Mottled Zone event
AU - Matthews, Alan
AU - Kolodny, Yehoshua
PY - 1978/3
Y1 - 1978/3
N2 - Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the "Mottled Zone Event" is a low-pressure (1-25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ 18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ 18O in silicates can be modelled as a Rayleigh distillation of CO2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.
AB - Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the "Mottled Zone Event" is a low-pressure (1-25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ 18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ 18O in silicates can be modelled as a Rayleigh distillation of CO2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.
UR - http://www.scopus.com/inward/record.url?scp=0009061026&partnerID=8YFLogxK
U2 - 10.1016/0012-821X(78)90154-1
DO - 10.1016/0012-821X(78)90154-1
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AN - SCOPUS:0009061026
SN - 0012-821X
VL - 39
SP - 179
EP - 192
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 1
ER -