Tf. Yui et al., STABLE-ISOTOPE CHARACTERISTICS OF ECLOGITES FROM THE ULTRA-HIGH-PRESSURE METAMORPHIC TERRAIN, EAST-CENTRAL CHINA, Chemical geology, 137(1-2), 1997, pp. 135-147
A stable isotope study was made for the ultra-high-pressure (UHP) meta
morphic rocks in the Dabie and Sulu terrains, east-central China. delt
a(18)O values of minerals in eclogites cover a wide range, from -4 to
+6 parts per thousand. The O-isotope fractionations between minerals (
i.e, omphacite, phengite and/or amphibole) and garnet in eclogites are
mostly not in isotopic equilibrium. The large range of O-isotope comp
osition of garnet may approximate the delta(18)O variations of eclogit
es during UHP metamorphism and must have been largely inherited from t
heir pre-subduction basaltic precursors which might have interacted wi
th meteoric waters under variable temperature conditions and water/roc
k ratios before UHP metamorphism. The O-isotope disequilibrium between
minerals and garnet in eclogites was caused by retrograde hydration r
eactions. The fluids required for these hydration reactions must have
low or even negative O-isotope compositions. It is postulated that suc
h fluids were either originated from meteoric deep-infiltration or wer
e derived from low-delta(18)O (para)gneisses.