Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise,Gulf of California期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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

Authors:	AM Kudo SE Barker K Keil Marvin H Beeson

Institution:	Department of Geology and Institute of Meteoritics University of New Mexico, Albuquerque, NM 87131, USA;Division of Earth Sciences, Portland State University, Portland, OR 97207, USA

Abstract:	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 TiO₂ content (0.88%), $FeO^{1} MgO$ 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 SiO₂, 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 (An₈₈), 6.7% olivine (Fo₈₆), and 4.7% clinopyroxene (Wo₄₁En₄₉Fs₁₀) 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 TiO₂ and P₂O₅ 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.

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