Crust–primitive magma interaction beneath neovolcanic rift zone of Iceland recorded in gabbro xenoliths from Midfell, SW Iceland

Abstract

We studied glass-bearing gabbro xenoliths from picritic pillow lavas erupted in the Midfell area, SW Iceland. The aim of this study is to investigate the processes affecting chemical composition of primitive Icelandic magmas at crustal depths. The xenoliths formed at 340±160 MPa (2σ) lithostatic pressure are cumulates of earlier crystallized magmas. Later interaction of the xenoliths with the transporting magmas occurred at 1,190–1,230°C and shallower depths (40±120 MPa). The xenoliths consist of reversely zoned, slightly resorbed, subhedral plagioclase (PL, An81-90, 50–60%), anhedral, strongly resorbed clinopyroxene (CPX, mg-number=83–90, 25–32%), trace amounts (<1%) of small, up to 500 μm, euhedral olivine (OL, Fo85-89) and spinel (SP, mg-number=66–73, cr-number=29–51), and interstitial glass (9–22%). No significant difference in chemical composition between surrounding host pillow rim glasses (K2O/TiO2=0.02−0.04, CaO/Al2O3=0.93−1.04, [La/Sm] n =0.38–0.54, where subscript n points on element concentrations normalized to those in chondrite) and interstitial glasses (K2O/TiO2=0.03–0.06, CaO/Al2O3=1.00–1.07, [La/Sm] n =0.41–0.74) was recognized. Deep glassy embayments in CPX (K2O/TiO2=0.02–0.06, CaO/Al2O3=1.00–1.06, [La/Sm] n =0.66–0.99) have also chemical compositions very close to those of interstitial and host glasses. Glass embayments in PL (K2O/TiO2=0.03–0.11, CaO/Al2O3=0.87–1.06, [La/Sm] n =0.70–0.97) vary more significantly in CaO/Al2O3 ratios than those in CPX likely due to their stronger modification by post-entrapment crystallization. All glass types, regardless of whether they are enclosed in minerals or represent interstitial or host matrix glasses, are characterized by significant excess of Sr ([Sr/Sr*] n =1.7–2.7) but not of Ba relative to Nb ([Ba/Nb] n =0.45–0.96). Partial melting of a gabbro with [La/Sm] n =0.54, [Sr/Sr*] n =10.5 and [Ba/Nb] n =4.5 inferred from the composition of the xenoliths studied cannot account for the observed trace element concentrations and ratios in the associated glasses. In fact, the compositions of melts calculated for 1–40% of partial melting match neither compositions of the interstitial glasses nor CPX- and PL-hosted glassy embayments with respect to [Ba/Nb]n ratios. Mixing of these partial melts with presumably uncontaminated magma composition does not explain chemical variations observed in the glasses as well. In contrast, the model of melt percolation through gabbroic matrix reproduces in a satisfactory way the compositions of glasses. We believe that the pronounced enrichment in Sr relative to other trace elements of similar incompatibility observed in many Icelandic tholeiites and probably in many MORB-type magmas worldwide could result from such interaction of the ascending magmas with gabbroic cumulates at crustal depths. We emphasize that the percolation model discussed here results in hybrid, Sr-enriched magmas which are saturated with CPX and PL. The enrichment in Sr of PL-undersaturated melts (so-called “ghost plagioclase signature‘’; Sobolev et al. in Nature 404:986–990, 2000) could not be produced by such percolation process.