Crystalline rock recovered from the Iceland Plateau at 69°14.989'N, 12°41.894'W in August 1993, is tholeiitic basalt. Pillow structures are well defined by glassy rims grading into vesicular rinds and aphyric interiors. Some plagioclase and augite microphenocrysts are present, and olivine may have been. Pigeonite is found in cores of some augite. Major- and trace-element abundances vary indicating magma-mixing, but the nature of the magma-mixing is not known. Trace-element ratios also vary, sometimes greatly, indicating that the “mixed magmas" may be magmas derived from different depths in a melting column, as has been proposed by other workers. The rocks differ from many typical MORB samples in that they are relatively evolved (mg = 46); however, they are strikingly similar to the majority of basalt glasses from the Kolbeinsey Ridge. Iceland Plateau basalts are T-MORBs, transitional to N-MORB and E-MORB with (La/Sm)n 0.68-0.72, Zr/Nb <20, and a range of major- and trace-element abundances that also clearly span the gap between average N-MORB and average E-MORB. An 40Ar/39Ar age of 13.2 ± 0.3 Ma from groundmass concentrates is an accurate but possibly imprecise age.
Date of initial receipt: 1 July 1995
Date of acceptance: 11 January 1996
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