Origin of Strong Positive Zr Anomalies in Clinopyroxenes from Ophiolitic Dunite

Guenter Suhr Mineral. Petrogr. Inst., Uni Koeln, Zuelpicher Str. 49b, Germany


Hans A. Seck Mineral. Petrogr. Inst., Uni Koeln, Zuelpicher Str. 49b, Germany

Up to 2500 m, monotonous dunite is exposed above mantle harzburgite and below crustal gabbro in the Blow Me Down Mtn. Massif of the Bay of Islands Ophiolite (Canada). A typical dunite would have 98% olivine, 2% spinel, and may have traces (0.05%) of cpx. Only in the upper part of the dunite sequence is cpx more abundant. Analysis of REE, Zr, and Ti in the cpx by SIMS and ICP-MS-LAM revealed three groups of patterns: (i) MORB type cpx patterns; (ii) cpx patterns typical for strongly depleted mantle peridotites except for Zr; (iii) cpx-patterns which would be in equilibrium with boninitic melts.

The group (ii) cpx patterns are characterized by high HREEN concentrations (4-8), low LREEN (0.04-0.2) and slopes in
the intermediate REE section which may be even steeper than in strongly depleted residual peridotites. Contrary to residual
peridotites which tend to have negative Zr anomalies, they display a strong positive Zr or Ti anomaly. We suggest that the high Zr and high HREEs are an inherited feature of a precursor dunite formed from a MORB type melt. Percolation of extremely refractory melts through this existing dunite sequence has lowered, even for very small melt fluxes, the concentration of all elements with low mineral-melt distribution coefficients (chromatographic effect). Relatively speaking, the dunite has "high" bulk distribution coefficient for HREE, Ti, and Zr so that these elements are less affected by migrating melts. As a result, only light and some middle REE approached equilibrium with the migrating melt.

The proposed model is consistent with the fact that dunites with more abundant cpx display MORB-type cpx patterns,
since such dunites are better buffered against reaction with migrating melts. Only very small quantities of refractory (near incremental) melts are required to imprint their LREE pattern on pre-exsiting dunites. Calculations show that this process is effective over the scale of the dunite sequence (2000 m). In essence, the dunite formed as a cumulate from MORB, but the REE pattern of cpx records the migration of small quantities of refractory, near-incremental melts through the cumulate pile.