Mobilization of REE during crustal aging in the Troodos Ophiolite, Cyprus

REE data for secondary minerals from low- and high-temperature alteration environments in the Troodos Ophiolite have been determined using a combined mass spectrometry-neutron activation technique. Smectite- and analcime-REE mimic fresh rock profiles and have concentrations ranging from < 1–5 times chondritic values. Celadonite is depleted in LREE and total REE relative to the rocks. Groundmass-replacing chlorite and epidote are similar to, or enriched in, REE relative to fresh rocks. In contrast, fault-related chlorite and void-filling epidote are LREE-enriched with large positive Eu anomalies and yield comparable REE profiles to those of hydrothermal fluids venting at mid-ocean ridges. The zeolites, clinoptilolite and laumontite, are also LREE-enriched with REE contents significantly lower than rock values. Carbonate is enriched in HREE relative to LREE and concentrations vary from 0.2 to 18 times chondritic values. REE data for these minerals indicate that seawater-REE contents are modified by seawater-rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging. The absolute and relative abundances of REE in seawater-derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water/rock ratios. Rocks altered at low temperatures will show no change or a decrease in REE content; the LREE/HREE ratio will reflect the proportion of smectite, celadonite and carbonate in the groundmass assemblage. In contrast, rocks altered at high temperatures will show no change or an increase in REE; the LREE/HREE ratio will reflect the proportion of void-filling and groundmass-replacing chlorite and epidote in the rock.