Ocean Floor Basalts Research Articles

Major element variations in ocean-floor basalts

Although ocean-floor basalts form a very convergent group from the point of view of their major element compositions, the variation that they do show can be interpreted in geological terms to a certain extent, though analytical variation must contribute substantially to that observed. Multivariate statistical methods demonstrate a high-level fractional crystallization trend in 94 selected ocean-floor basalts caused by precipitation of calcic plagioclase, forsteritic olivine and augite. After this effect is subtracted, a moderate amount of variation remains which can be interpreted as caused by crystal-liquid processes involving amphibole, though this is less certain. A new average of ocean-floor basalt compositions is presented, and it is suggested that the term ‘oceanic tholeiite’ should not be retained.

K, Rb, Cs, Sr and Ba contents and Sr isotope ratios of ocean floor basalts

Reliable analyses of K, Rb and Cs in dredge basalts require samples which show no petrographic evidence of alteration minerals and which have H 2 O + contents less than 0.7 %. Very fresh glass samples usually have H 2 O + levels of 0.1 to 0.2 %, and this probably represents the primary level of H 2 O in most submarine basalt magmas. A dredge haul containing both basalt and andesite was studied for major element and trace element variations. The major elements were consistent with a differentiation model involving crystallization of plagioclase, olivine, clinopyroxene and titanomagnetite. This differentiation had little effect on Sr concentration and on K/Rb and K/Cs ratios; these parameters are thus especially useful in studying mantle chemistry and partial melting processes. Twenty-eight unaltered dredge basalts were analysed, with K/Rb ratios varying from 360 to 1350. K contents of most samples, after correction for high level (shallow) differentiation processes, fall in the range 500 to 1200 parts/10 6 . A comparison of unaltered basalts from ‘fast-spreading ridges’ and ‘slowspreading ridges’ shows that K, Rb, Cs and Sr contents and K/Rb, K/Cs and Rb/Sr ratios are identical for both environments, while Ba contents and 87 Sr/ 86 Sr ratios may be significantly different. Thus mantle chemistry appears to be largely decoupled from the dynamic processes of plate movement. It is shown that submarine ridge basalts have lower 87 Sr/ 86 Sr ratios than the basalts of the oceanic islands, suggesting an early depletion of parts of the mantle in the dispersed elements. Average values for 15 samples from 11 different ridge localities: K = 1160 parts/10 6 ; Rb = 1.11 parts/ 10 6 ; Cs = 0.016 parts/10 6 ; Sr = 135 parts/10 6 ; Ba = 10 parts/10 6 ; K/Rb = 1060; K/Cs = 70000; Rb/Sr = 0.0082; 87 Sr/ 86 Sr = 0.70265.

III. Weathered And Metamorphosed Basalts. Altered basalts from Swallow Bank, an abyssal hill in the NE Atlantic, and from a nearby seamount

Three hundred and fifty-seven specimens dredged from Swallow Bank in 1958 are vesicular hypocrystalline porphyritic basalts. Chemical, thin section and X-ray studies were made. The least altered specimens gave an analysis close to the tholeiitic ocean floor basalt type. The glassy selvage of pillows altered sequentially to palagonite, fibro-palagonite and montmorillonite. Within the flows the mesostasis and pyroxene phenocrysts have been replaced by chlorophaeite, fibro-chlorophaeite and obscure chlorites, the calcic cores of plagioclase phenocrysts by a mesh of orthoclase plates and their sodic rims by (?) montmorillonite. Similar, but more extensive, mineralogical changes have affected the lavas from the seamount. Chemically, at Swallow Bank, the alteration of the flow interior involved increases in K 2 O (from 1.0 to 3.5g/100g), in H 2 O + (from 1.2 to 5.2 %) and in oxidation ratio, Fe 2 O 3 :Fe20 3 + FeO (from 71 to 98%) with concomitant loss in CaO (from 11.1 to 2.3 %) and MgO (from 4.7 to 1.8%). In these rocks radioactivity increased, seismic velocity decreased and intensity of magnetization remained substantially unchanged. Similar altered rocks could be widespread in layer 2 beneath the ocean.

Rb, Sr, Y, Zr and Nb in some ocean floor basaltic rocks

X-ray fluorescence measurements have been made of Rb, Sr, Y, Zr and Nb in 35 samples of ocean floor basaltic rocks from four different areas of the oceans. Samples include fresh, weathered and metamorphosed basalts and dolerites. Comparison is made with K and Ti determinations on the same samples. Rb, K and, to a lesser extent, Sr are affected by processes of ocean floor weathering and metamorphism, but abundance of Y, Zr, Nb and Ti seem to be unaffected even by severe secondary processes. High correlations are found between Y and Ti, and Zr and Ti for both fresh basalts and the group of samples as a whole. These correlations define a narrow composition space in terms of these elements for ocean floor basaltic rocks even when they are severely altered. It is possible that this property might be used to identify altered ocean floor basalts and to distinguish them from volcanics from other sources.

Ancient and modern volcanic rocks: A trace element model

In terms of the relative abundance of K, Rb, Cs, Sr and Ba, modern oceanic and island-arc basalts may be viewed as a gradational family ranging from alkali basalt (enriched in these elements) to low-K-tholeiite, such as ocean-floor basalt (depleted in these elements). In island arcs, the contents of these trace-elements in basalts are proportional to the depth to the seismic zone. Our model for this relationship suggests derivation of alkali basalts from non-depleted mantle by low degrees of partial melting at relatively great depths; the low-K tholeiites are satisfactorily explained as products of high degrees of partial melting of residual or “depleted” mantle at relatively shallow depths. Some of these tholeiites are shown to be contaminated with oceanic sediment, and the usefulness of Cs and Ba concentrations to detect such contamination is established. New trace-element analyses reveal that the basalts of the Archaean (2.7 by) greenstone belts of Canada average 2100 ppm K, 6 ppm Rb, 0.4 ppm Cs, 175 ppm Sr and 70 ppm Ba. These trace-element abundances show that the Archaean basalts are most closely related to the modern low-K tholeiites of island arcs. This analogy leads us to propose that Archaean basalts are derived by high degrees of melting at shallow depths. We find no evidence for the presence of residual or “depleted” mantle during the Archaean. Our model implies a thin lithosphere during the Archaean and we suggest that Archaean volcanics evolved in an environment dominated by multiple thin oceanic plates with arc-trench boundaries and shallow subduction zones.