Oxygen and hydrogen isotope studies of serpentinization in the Troodos ophiolite complex, Cyprus

Abstract

Oxygen and hydrogen isotopic compositions were measured on 12 serpentine and 2 actinolite samples from the Troodos ophiolite complex, Cyprus. The single analyzed antigorite (δD= −60, δ 18O= 7.1) is isotopically similar to all previously analyzed antigorites from ultramafic bodies. However, although their D/H ratios are relatively “normal” (δD= −70to−92), the δ 18O values of most of the Troodos lizardite-chrysotile serpentines (+12.6 to +14.1) are much higher than the 2.0–9.3‰ range typically found in such serpentines. Such high δ 18O values have previously been found only in the serpentine-like mineraloids termed “deweylites”, which apparently formed at Earth-surface temperatures, and in a single sample from Vourinos, Greece that is in contact with high- 18O limestone. The Troodos lizardite-chrysotile samples cannot have formed by reaction with heated ocean waters, but instead must have formed in contact with large amounts of some type of meteoric, metamorphic, or formation water, either (1) at very low temperatures in a near-surface environment, or (2) at about 100°C from waters that were abnormally enriched in 18O (δ 18O ≈ +4 to +8) . The latter possibility seems most plausible inasmuch as extensive evaporites were deposited throughout the Mediterranean Sea during the late Miocene, and this would have been accompanied by strong 18O enrichments of the local meteoric waters. Heated ocean waters, however, probably were responsible for the formation of the actinolitic amphiboles (δ 18O= 4.6 to 5.5; δD= −51to−46) from the gabbro and ultramafic zones in the Troodos complex. The amphiboles must have formed at considerably higher temperatures and at an earlier stage than the lizardite-chrysotile serpentinization.