Geochemical characteristics of mid-ocean ridge basalts

Abstract

Major and trace element (Rb, Sr, Y, Zr, Nb, Ba, Sc, Ni, Co, Cr, V) data are presented on ten mid-ocean ridge basalts (MORB), together with a basalt sample from the offshore slope of the Yap Trench and a basalt from the Tertiary ophiolite complex of eastern Taiwan. Rare earth element (REE) and Sr isotope data are presented on eight of the samples, together with REE data on U.S.G.S. standard rock, BHVO-1. A strong correlation exists between percent TiO 2 (proportional to amount of melting) and Al 2 O 3 /TiO 2 , CaO/TiO 2 ratios of these close to primary MORB. These ratios increase up to a maximum (about 20 and 17 respectively) as TiO 2 decreases, indicating a progressive release of Al and Ca from the mantle source. The limiting values of the ratios are close to chondritic and are reached at about 0.8% TiO 2 . At this high degree of melting, the major Al- and Ca-bearing mineral phases are eliminated from the mantle residue. Model calculations on the major element data indicate that MORB with 0.7% and 1.5% TiO 2 could represent about 25 and 15% melting (respectively). This modelling is consistent with the abundances of the first transition series metals (Sc to Ni), which are also interpretable in terms of degree of partial melting and residual mineralogy. Based on the REE data, the samples can be divided into depleted types (typical of “normal” MORB), and enriched types (the so-called “plume” and “transitional” types). Despite the range of (La/Sm) N in these samples (0.38–1.97) the Ti/Zr ratios remain close to chondritic (about 110) indicating that these two elements have a similar degree of incompatibility at this level of melting. Chondritic normalized patterns for typical “normal” (N)- and “plume” (P)-type MORB are presented for the REE and K, Nb, U, Th, Ba, Rb and Cs and it is suggested that this represents an increasing order of element incompatibility in MORB. The behaviour of Y, Ti, Zr, P and Sr relative to the REE is also discussed and it is shown that for most primitive MORB, Y = Ho, Ti = Eu, Zr = Sm, P = Nd and Sr is between Ce and Nd. In addition the Zr/Nb ratio correlates with the La/Sm ratio. This element correlation can be used to predict the general shape of REE patterns. The Sr isotope data are discussed in terms of already published data from the three oceans. It is suggested that P-type MORB originate from depleted mantle sources which only recently (say ≤300 m.y.) were added to by an incompatible-element-rich phase. Various models proposed to explain both the depletion in N-type, enrichment in the P-type MORB sources and the proposed mantle heterogeneity are discussed. It is concluded that mantle depletion is due to a continuing process rather than an episodic event in the early Archaean or 1.6 b.y. ago.