Trace elements and petrogenesis of DSDP 37 basalts

Abstract

All geochemical data for basalts from holes 332A. 332B. 333. 334, and 335 drilled into the west flank of the Mid-Atlantic Ridge near 37° N, are reviewed. The 785 samples analyzed, providing some 14 000 data items, are grouped into 50 units of distinct geochemical character. Holes 332A, 332B, 333, and 334 penetrated basalt piles containing 32 normal oceanic tholeiite units. 2 tholeiites with low Zr. 7 units with low Zr and high Ca, and 8 units with low Zr and high Mg. Hole 335 penetrated tholeiite s rich in heavy rare earths and Y, and with many other distinct geochemical characteristics. The tholeiites of 335 were exceptionally uniform over a depth of 102 m, and the top and bottom halves of this sequence are geochemically indistinguishable. Oxygen, strontium, and lead isotope evidence combines with the geochemical data to indicate little post-extrusion metasomatism.A marked positive correlation exists between all of the elements Ti, Mn, Nb, Zr, Y, Rb, Zn, K, U, and Ba, and in turn these all correlate positively with Fe and Fe/Fe + Mg). Sc also shows a significant linear correlation with Fe. In all subsets among the elements listed so far, the 'low-Zr' (≤ 50 ppm) and the 'normal' basalts form two distinct groups. Other element pairs or groups clearly show the existence of the three sets, normal, low-Zr/high-Ca. and low-Zr/high-Mg in the form of tri-linear or Y-shaped plots. The interrelationships of Mg, Fe, Ca, Sr. Cr, V, Ni, Co, Y, and the REE fall into this category. The trace elements in this set are those that proxy for major elements in the principal silicates or spinels, whereas most of the previous set (Ti, etc.) may be dominantly in minor phases. The trace-element abundances in the low-Zr units, the negative correlation of Cr with V, a positive correlation of Cr with Ca and Cr with Mg, positive correlation of most Y and REE with Ca, and the details of REE distribution (La-rich) are used to argue that partial melting of garnel-free lherzolite is the dominant cause of the composition of the tholeiites in 332A to 334. Basalts from 335 are of different origin and were produced either from La-impoverished. heavy REE-rich mantle, or from a multi-stage partial melting process involving a garnet-lherzolite source. The high-Ca and high-Mg basalts both of which are Cr-rich and picotite-bearing and are respectively bytownite- and olivine-phyric, are considered to be the products of high percentage partial melting. It is argued that the normal tholeiites were produced at a liquid–olivine–labradorite–pytoxene perilectic at early stages of partial melting and that the low-Zr varieties by higher percentage melting at and beyond a liquid–olivine–bytownite–Cr–spinel peritectic, almost to an ultrabasic level of melting.The lead isotope and geochemical data for DSDP 37 and various Atlantic basalt suites indicate a heterogeneous mantle. We believe that this heterogeneity is due to a large-scale gneissic mantle structure, consisting of layers or slabs created and fractionated much earlier in geological history. On arriving at accreting plate margins these slabs give the margins distinctive geochemical and isotopic characteristics over distances on the 500–1000 km scale, within which various degrees of partial melting of material derived from various types of 'barren mantle' (plus former crust?) operate to produce individual basalt horizons.