GEOCHEMICAL EVOLUTION OF THE BURFELL, SOUTHERN ICELAND

The Burfell is a 670m-high ridge located in southern Iceland, 12 km northwest of Hekla volcano. It consists of interlayered sub-glacial pillow lavas and thick, columnar lava flows, and is intruded by dikes and sills. The thickness of the individual sub-aerial lava flows varies between 5 and 20 meters, and most lavas are bound above and below by hyaloclastite. There is also a 3 to 10 m-thick picrite sill near the top of the structure, which exhibits a diabase texture and variable accumulation of clinopyroxene and olivine. The ridge is capped by a layer of pillow lavas and pillow breccia. Chemical variations in samples can be sub-divided into two major suites. The pillow basalts, located intermittently throughout the stratigraphic column, are geochemically depleted, and result from large extents of melting of a depleted mantle source (Suite I). This suite exhibits chemical variations similar to the extensive Pleistocene and Holocene basalts that blanket this part of south-central Iceland and are likely derived from the same or a similar depleted mantle source. The second suite (Suite II), constituted primarily of the lava flows, is derived from a more enriched parental magma. These appear to be related via variable extents of fractional crystallization to magmas similar to those that make up the sub-glacial Pleistocene ridges (mobergs) surrounding Hekla, and potentially to historical Hekla lavas. The parental magmas of the two suites may have been derived from similar sources via different extents of melting. Suite II is the product of lower degrees of melting of the mantle, or it might be derived from a less-depleted part of the Iceland plume. Given the interfingering of Suites I and II, it appears as though two distinct magma sources that experienced different evolutionary histories were active during the same period.