OLIVINE AND SPINEL SYSTEMATICS OF BORING VOLCANIC FIELD (BVF) BASALTS: EVIDENCE FOR MAGMA SOURCE VARIATIONS AND INTERACTION AMONG BASALTIC MAGMAS

Basalts of the Plio-Pleistocene BVF of the Portland Basin fall into two principal categories (Leeman et al., 2005). Group 1 lavas include low-K tholeiites (LKT) and various OIB-type lavas, both with a trace element geochemistry more typical for intraplate or extensional settings. Group 2 lavas include basalts with arc-characteristic trace element patterns and range from calc-alkalic (CAB) to high-K (HKCA, absarokite). Olivine phenocrysts and included spinels were analyzed from basalts representing the above compositional spectrum to evaluate possible distinctions in source characteristics for the different basalt groups. Only primitive basalts and basaltic andesites (Mg# of 56-70, 170-320 ppm Cr, 100-190 ppm Ni) were considered. The most forsteritic olivines in each sample are mostly consistent with expected equilibrium compositions: ~Fo83 in LKT’s; ~Fo87 in CAB and HKCA. However, many samples also contain more Fe-rich olivines (as low as ~Fo74). Only a few Group 2 lavas seem to lack such evolved olivines. Spinel compositions vary widely. The most primitive spinels (TiO2<2 wt%, Mg#>40) have the following properties. LKT spinels have the lowest Cr# (~30) but highest Fe2+/Fe* (~0.7) and Al2O3 (~35 %). HKCA spinels have higher Cr# (~70), Fe2+/Fe* (0.3), and Al2O3 (~6 %). CAB and BA spinels fall in between these end member compositions. Our findings are consistent with Smith and Leeman (2005). Most samples also yield spinel inclusions with more evolved compositions with Mg# as low as 10 and TiO2 wt.% as high as 12 wt.%. Lower Mg# of spinel inclusions is well correlated with more Fe-rich olivine composition. Other parameters remain nearly constant, e.g., yielding horizontal arrays in a plot Fe2+/Fe* vs. Mg#. Compositions of primitive spinel inclusions allow discrimination between Group 1 and Group 2 lavas and suggest that mantle sources for these basalt types are distinct, with the Group 2 source being more refractory. This suggests Group 1 and 2 magmas are unlikely to be related to a common source affected by different degrees of metasomatism by slab-derived fluids. Furthermore, evolved olivine and spinel compositions suggest that individual lava flows are commonly composites of magmas with slight differences in their degree of evolution.

Leeman et al., 2005, JVGR, v.140, p.67; Smith and Leeman, 2005, JVGR v. 140, p. 49.