Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise, Gulf of California

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Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO 2 content (0.88%), FeO ∗ MgO ratio (0.95 with total Fe as FeO), and Mg# (100 Mg Mg + Fe ″ = 70 ), sample 483-17-2-(78–83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a La Sm cn (chondrite-normalized) = 0.36, and Eu Sm cn = 1.05. Because other samples analyzed have higher SiO 2, lower Mg#, and a negative Eu anomaly ( Eu Sm cn as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An 88), 6.7% olivine (Fo 86), and 4.7% clinopyroxene (Wo 41En 49Fs 10) from 483-17-2-(78–83) to form the least differentiated sample with Mg# = 63. The La Sm cn of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher La Sm cn (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO 2 and P 2O 5 are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower La Sm cn and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78–83) is required.