The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt/rock reaction

Abstract

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al 2O 3 = 0.6–1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low ε Nd(T) values (+2 to −7), but high to very high 87Sr/ 86Sr(T) ratios (up to 0.715), whereas rocks with more fertile major element compositions (Al 2O 3 = 2–3.2%; CaO = 2–2.6%) give REE patterns showing increasing depletion from Lu to La and have high ε Nd(T) values (+6 to +8), but low 87Sr/ 86Sr(T) ratios (0.703–0.704). The low neodymium and high strontium isotopic ratios of those peridotites having U-shaped REE patterns demonstrate that the LREE enrichments observed in these rocks are the result of contamination by continental crust and not the consequence of peculiar fractionation during melting or chromatographic effects during mantle ascent. A clinopyroxene concentrate prepared from a bulk sample of lherzolite with ε Nd(T) = −7 and 87Sr/ 86Sr(T) = 0.708 gives MORB-like isotopic signatures: ε Nd(T) = +14 and 87Sr/ 86Sr(T) = 0.7035. Furthermore, two highly serpentinized samples with U-shaped REE patterns show strongly negative δD values (−95 and −110‰) much typical of peridotites altered by continental metamorphic fluids or/and meteoritic waters. From these results, we conclude that the crustal contamination corresponds to a low-temperature alteration event which took place during or after obduction of the Trinity ophiolite complex onto the continent and is not the consequence of any high-temperature invasion of the ultramafic rocks by melts/fluids derived from subducted continental materials.