JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B3, PAGES 3795-3820, MARCH 10, 1986 Gabbroic Xcnoliths and Host Ferrobasalt From the Southern Ridge Juan de Fuca JACQUELINE EABY DIXON 1 AND DAVID A. CLAGUE U.S. GeologicalSurvey,Menlo Park, California JEAN-PHILIPPE EISSEN 2 Universire Louis Pasteur, Laboratoire de Petrographie, Strasbourg, France Rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobas- alt collected along a 12-kin section of the southern Juan de Fuca Ridge. Xenoliths comprise << 1% of the dredge contents and range in sizefrom 1 cm 3 (glomerocryst) to 240 cm 3. The xenoliths have ophitic to intersertal texture with 5-50% interstitial glass of ferrobasaltic composition more evolved than the host lava. On the basisof texture and mineralogy, the xenoliths have been subdivided into three types' type I, plagioclase + olivine + glass' type II, plagioclase + augite + glass + olivine' and type III, plagioclase + augite + olivine + glass + pigeonite (partially inverted) + Fe-Ti oxides. Mineral and glass inclusion compositions suggesta sequenceof evolution for the three xenolith types in which type I is the least evolved and type III is the most evolved. Application of a graphical pyroxene geothermometer to augite in xenolith types II and III yields crystallization temperatures of 1100ø-1200øC and to host-lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100ø-1150øC. Coexisting titanomagnetite-ilmenitepairs in type III xenoliths yield temperature estimatesof 1000ø-1070øCand log f02= -9.7 to -10.8. We infer that the xenoliths represent the partiallycrystalline "mush" boundary zone of a magma chamber based on the abundanceof interstitial glass, zonation of mineral grains in the most crystalline samples, and coherence of chemicaltrendsbetweeninterstitial glass, glass inclusions, and mineral phases. The evolved composition of the xenoliths provides evidencefor the presence of melts more fractionatedthan the host ferrobasaltin the magma chamber.The erupted ferrobasaltis a hybrid lava formed by mixing these highly evolvedmelts with more primitive melts. INTRODUCTION Recent studies of oceanic crust suggest that size and lon- gevity of axial magma chambers beneath oceanic spreading centers and the petrology of the erupted igneous rocks are largely a function of spreading rate. Magma chambers beneath slow spreading centers are thought to be small and in some places transitory [Stakes et al., 1984], while beneath inter- mediate to fast spreading centers they are steady state [Mac- donald, 1982]. A natural consequence of steady state magma chambers undergoing continuous fractionation is mixing be- tween batches of primitive magma and residual differentiates. Mixing is now recognized as an important petrologic process in the generation of mid-oceanic ridge basalt (MORB) and has been invoked by various authors to explain (1) the eruption of homogeneous basalt compositions over long time periods [Usselman and Hod#e, 1978], (2) the presence of anomalous phenocrysts and melt inclusions [Dungan and Rhodes, 1978], (3) the concentrations of incompatible elements in some mod- erately evolved compositionsin excess of that predicted by simple fractional crystallizationmodels [O'Hara, 1977; Bryan and Moore, 1977; Bryan et al., 1979; Stakes et al., 1984], (4) a reversal in mineral crystallization sequences[Walker et al., 1979], (5) magma with chemical and mass balance character- istics of plagioclase accumulation even though the lavas con- tain few or no plagioclase phenocrysts [Flower, 1982], and (6) the homogeneity of isotopic compositions along moderate to fast spreading centers [Cohen and O'Nions, 1982; Allegre et al., 1983; Batiza, 1984]. The role of mixing in the production of ferrobasalt, how- •Now at Division of Geological and PlanetarySciences, California Institute of Technology, Pasadena,California. 2Now at Officede la Recherche Scientifique et Technique Outre- Mer, Noumea, New Caledonia. Copyright 1986 by the American GeophysicalUnion. Paper number 5B5733. 0148-0227/86/005B-5733 $05.00 ever, is lesscertain. Cla•tue and Bunch [1976] used linear least squares mixing models of major elements to show that ferro- basalt can be produced by shallow level fractionation of plagioclase,clinopyroxene, and minor olivine in the average proportions of 9.3:7.7:1 with up to 74% of the parental magma fractionally crystallizing. Mattey and Muir [1980], on the other hand, found that a model for mixing of periodically injected batches of primitive magma with residual differ- entiates could more closely predict variations in major and trace elements in ferrobasalt from Deep Sea Drilling Project (DSDP) sites 424 and 425 near the Galapagos spreading center 86øW. Natland [1983] found no geochemical evidence for mixing between olivine tholeiite and ferrobasalt at the East Pacific Rise near the Siqueiros Fracture Zone. He argued that ferro- basalt in this area formed due to crystal fractionation in small isolated magma bodies such as dikes or shallow intrusions above larger magma chambers.Sinton et al. [1983] proposed that ferrobasalt is generated in small isolated magma bodies behind propagating rift tips. Farther behind the propagating rift tip, extreme differentiation becomes less likely as the steady state thermal configuration of a normal ridge is ap- proached. These studies suggest that the extent of differ- entiation of MORB may be controlled by a delicate balance betweencooling and magma supply rates. The effects of differentiation and mixing are recorded in oceanic plutonic rocks, but at the present time, sampling of the plutonic rocks is severely biased toward the Atlantic and Indian oceans, where slow spreading rates result in large dis- placements along transform faults and rift valley boundaries [Engel and Fisher, 1975; Hodges and Papike, 1976; CAY- TROUGH, 1979; Fox and Stroup, 1981]. Data for gabbros from intermediate to fast spreading centers are less abundant [Vanko and Batiza, 1982; Hebert et al., 1983]. Moreover, in- terpretation of magmatic conditions is impeded by defor- mation, metamorphism, or brecciation of most oceanic gab- broic and ultramafic rocks. 3795