Experimental constraints on the generation of FeTi basalts, andesites, and rhyodacites at the Galapagos Spreading Center, 85°W and 95°W

One‐atmosphere experiments conducted on a synthetic glass similar to Galapagos Spreading Center (GSC) FeTi basalt (POO.82N2), (Byerly et al., 1976) define liquid lines of descent at f O2 values between the quartz‐fayalite‐magnetite (QFM) buffer and 2 log units more oxidizing than the nickle‐nickle oxide (NNO) buffer. The experiments provide a framework for understanding the development of FeTi basalts by fractionation at near‐ocean floor conditions. GSC lavas from near 85°W initially follow a compositional trend, distinguished by FeO° (= FeO + 0.9Fe 2 O 3 ) enrichment and SiO 2 depletion, which is nearly identical to the trend observed in experiments at QFM to which olivine seeds were added. This compositional trend can be produced by crystallization along an olivine → pigeonite reaction boundary in a shallow crystal‐rich magma reservoir. In contrast, GSC lavas from 95°W do not mimic the 1‐atm liquid line of descent, but appear to have fractionated at somewhat higher pressure. Basaltic liquids from 95°W underwent fractional crystallization at 1–2 kbar, did not experience FeO° enrichment along an olivine → low‐Ca pyroxene reaction boundary, and developed FeO° enrichment concomitant with SiO 2 enrichment. This compositional variation is consistent with a differentiation process in which crystals are continually removed from contact with liquid. Rhyodacites from 95°W cannot be related to the basalts and FeTi basalts recovered at 95°W by shallow‐level crystal fractionation. Instead, rhyolite liquids were formed either by fractionation of similar parents at greater depth and higher P H2O , or formed by fractionation of different parents. Andesite formed by mixing between basaltic and rhyodacitic liquids. As a consequence, mixed andesites define a trend of decreasing P 2 O 5 which has been previously interpreted to represent apatite saturation at approximately 0.22 wt% P 2 O 5 , significantly earlier than at 85°W (where P 2 O 5 decreases at approximately 0.7 wt% P 2 O 5 ). Our experiments suggest that the f O2 when titanomagnetite first saturates at the GSC was approximately at the NNO buffer. Together with the Fe 2 O 3 /FeO data of Byers et al. (1983, 1984) and Christie et al. (1986), this requires an increase in f O2 during crystallization in excess of that produced during closed‐system fractional crystallization. We suggest that this increase in f O2 results from interaction with oxidizing surroundings in an open‐system process.