MANTLE EVOLUTION RECORDED IN CR-SPINEL FROM THE BAY OF ISLAND OPHIOLITIC COMPLEX, NORTHERN APPALACHIANS
We studied mantle harzburgite and overlying cumulate dunite in the BOIC. Cr-spinel in harzburgite is subhedral to anhedral ranging from 0.2 to 1mm in size. The cores of Cr-spinel have low YFe3+ (0.02- 0.09). They show a continuous variation in Cr# from 0.20 to 0.78 and XMg from 0.50 to 0.72. Olivine contains fosterite (Fo) contents ranging from 89 to 92 and NiO from 0.29 to 0.44 wt%, and plots in the olivine-spinel mantle array (OSMA) of Arai (1994). The wide variation in Cr# reflects a change from a moderately fertile mantle to a more refractory mantle. In Cr# vs. Mg# diagram, the former plot in an area overlapping the fields for abyssal peridotite and subarc mantle and the latter in the forearc mantle field. The contents of TiO2 vary from 0.02 to 0.16 wt% and plot in subarc field in Ti-Al diagram of Kamenetsky (2001).
In dunite Cr-spinel is subhedral to anhedral ranging from 0.5 to 1.2mm in size. They have Cr# ranging from 0.19 to 0.71, XMg from 0.44 to 0.67 and low YFe3+ (<0.09). The Fo contents (88 to 90) in olivine are lower than those of harzburgite. Most samples plot outside the OSMA and the Cr# of the primitive spinel projected to OSMA range from 0.4 to 0.8. The platinum group elements (PGE) in dunite show slightly positively sloped primitive-mantle normalized pattern with low Os and Ir, confirming that dunite is a cumulate of a mafic melt. The overall PGE contents are high, 44 ppb, suggesting the melt formation at high temperatures.
Values observed and projected for Cr# in dunite suggests that the source mantle for its parental melt was refractory, similar to forearc mantle, indicating the progressive depletion in the mantle during the evolution of the BOIC. Therefore the compositional variation of spinel reflects a change in the tectonic settings during the BOIC formation and a progressive depletion in the mantle through melt extraction in subduction setting.