Contrib Mineral Petrol (1987) 96:121-139 Contributions to Mineralogy and Petrology 9 Springer-Verlag 1987 Experimental petrology of normal MORB near the Kane Fracture Zone: 220-25 ~ N, mid-Atlantic ridge D.R. Tormey 1, T.L. Grove 1, and W.B. Bryan 2 1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Abstract. Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid- Atlantic ridge near the Kane Fracture Zone (KFZ, 22 ~ to 25 ~ N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in "normal" mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-arm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid composi- tions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of ba- salt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv- plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and pla- gioclase as phenocrysts, and are well removed from clinopy- roxene saturation at low pressure, The compositional varia- tion can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Ma- jor and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set. Offprint requests to: T.L. Grove Introduction Early investigations of mid-ocean ridge basalts (MORB) were carried out on samples dredged from the mid-Atlantic ridge in the vicinity of the Kane Fracture Zone (22o-30 ~ N, Muir and Tilley 1966; Melson et al. 1968; Miyashiro et al. 1969) and at 45~ (Muir et al. 1964). As a result, the chemical and petrographic characteristics of these collec- tions, and of a smaller sample suite from the eastern Pacific Ocean (Engel et al. 1965), are the basis for the current defi- nition of "normal MORB". These basalts were called nor- real, because they were the first to be analyzed and de- scribed in detail, and they were thought to be characteristic of topographically normal sections of mid-ocean ridge. The chemical characteristics of normal MORB are a depletion in large, low valency cations (Cs, Rb, K, Ba, Pb, Sr) relative to ocean island and continental tholeiites, chondrite-nor- realized La/Sm< 1, low 87/86 Sr and high 143/144 Nd. Re- cent geochemical investigations have found that substantial chemical variability exists, even among "normal" mid ocean ridge basaIts (Bryan et al. 1981; Langmuir and Bender 1984; Thompson et al. 1985), and the recognized chemical diversity of the "normal" basalt group is contin- ually expanding. Although the basalts adjacent to the Kane Fracture Zone remain an excellent sample of "normal MORB", more recent geochemical and petrologic studies of the mid-Atlantic ridge from 22 ~ to 25 ~ N in the vicinity of the Kane Fracture Zone (Bryan et al. 1981 ; O'Donnell and PresnalI 1980) and young oceanic crust at DSDP sites 395 and 396 (Dungan and Rhodes 1978; Dungan etal. 1978; Rhodes et al. 1978) have greatly expanded the data set and have revealed increasingly subtle complexities with- in these "typical" basalts. This study describes the results of low pressure phase equilibrium experiments carried out on a suite of least evolved normal MORBs from the mid-Atlantic ridge north and south of the Kane Fracture Zone. These experiments provide a framework for assessing the imprint of low pres- sure fractional crystallization on normal MORB chemistry. The experiments quantify the liquid line of descent, the major element mineral/melt distribution coefficients, the phase appearance sequence and the phase proportions ex- pected from low pressure crystallization of normal MORB. Calculated major and trace element models of low pressure fractional crystallization provide the necessary information to evaluate the more complex open system magmatic pro-