A field, chemical, and stable isotope study of subseafloor metamorphism of the Josephine ophiolite, California‐Oregon

The Josephine ophiolite is a large, complete ophiolite generated in a Late Jurassic back arc basin along western North America. The ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and δ 18 O values. Characteristic mineral zonation includes mica±orthoclase and hematite in the lower extrusives, greenschist‐facies assemblages in the upper sheeted dikes, and amphibolite‐facies assemblages in the lower sheeted dikes and high‐level gabbro. Cumulate gabbros are only incipiently altered but are depleted in 18 O indicating high‐temperature alteration with fluids that evolved at low water(W)/rock(R) ratios. A second, off‐axis circulation system is recorded by metalliferous sediments that occur 8–21 m above the ophiolite. Alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy, chemistry, and δ 18 O values within individual pillows and dikes. The heterogenity is due to (1) variations in W/R ratio and starting material during alteration by downwelling seawater (e.g., pillow cores versus glassy rims) and (2) localized discharge mineralization (mostly epidosites) superimposed on the background recharge alteration. Epidosites occur largely as dike‐parallel stringers, irregular replacement of pillow and massive lava, and replacement of interpillow and pillow‐breccia matrices. The chemistry of most extrusives and sheeted dikes is characterized by loss of Ca and gain in Na and Mg, consistent with alteration by downwelling fluids. In addition, the sheeted dikes are depleted in K 2 O, and the lower sheeted dikes and high‐level gabbros are depleted in Zn and Cu. The δ 18 O and δD values for most samples from the extrusive sequence indicate alteration by seawater at <200°C and high W/R ratios, whereas alteration of the sheeted‐dike complex took place at 250°–450°C and involved fluids enriched in 18 O relative to seawater. The δ 18 O values and chemistry of the sheeted‐dike complex indicate that downwelling seawater evolved by interaction with diabase at low W/R ratios into high‐Ca, δ 18 O‐rich, metal‐rich, low pH fluids similar to those venting at modern “black smokers.” Epidosites represent extreme metasomatism and are strongly enriched in Ca and depleted in Mg, Na, Zn, and Cu. “Albite epidosites” in the extrusive sequence show similar bulk changes, but to a lesser degree. Fluids calculated to be in exchange equilibrium with epidosites and albite‐epidosites are enriched in 18 O. This feature and their chemistry imply that they represent pathways of discharging fluids. Cooling and/or mixing of fluids discharging upwards through the sheeted dikes occurred at the contact with pillow lavas, resulting in silicification and sulfide mineralization at about 350°C. Continued cooling of the discharging fluids upward in the extrusive sequence resulted in formation of albite‐epidosites, hematitic pillow lavas, and potassic alteration. At one locality (Turner‐Albright), hot discharging fluids locally vented directly onto the seafloor to form massive sulfide deposits. The 18 O enrichment in the extrusive sequence was produced during both recharge and discharge alteration involving seawater and an 18 O‐enriched fluid, respectively. Gregory and Taylor (1981) have emphasized the role of upwelling, 18 O‐enriched fluids in the off‐axis evolution of the sheeted‐dike complex at fast spreading ridges. Our data indicate that such fluids also played an important role in 18 O enrichment in the extrusive sequence of the Josephine ophiolite, which probably formed at a slow spreading ridge. There is no evidence of hydrothermal fluids significantly depleted in 18 O relative to seawater (e.g., Cathles, 1983), even during low‐temperature recharge alteration of the extrusive sequence. Mass balance considerations suggest that 18 O‐depleted fluids are unlikely to form by alteration at high W/R ratios such as those typical of the extrusive sequence of the Josephine ophiolite. Magmatic activity and hydrothermal circulation in the Josephine ophiolite were cyclic, as indicated by the presence of very primitive lavas and breccias, older sulfide‐rich pillow screens in the basal sheeted dike complex, and multiple massive sulfide deposits in one stratigraphic section (Turner‐Albright), some of which are overlain by up to 5 m of mudstone. The cyclic magmatic and hydrothermal activity, along with the high degree of extensional faulting indicated by >50° rotation of the entire crustal sequence at the spreading axis, suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic.