Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

Basalts from many Southern Hemisphere regions have anomalous Sr and Pb isotopic characteristics. This article shows that the isotopic mantle anomaly is globe-encircling in extent, centred on latitude 30° S. Arguments suggesting that this mantle anomaly has been in existence for billions of years place severe constraints on mantle convection models.

A large-scale isotope anomaly in the Southern Hemisphere mantle

The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr# 0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

The submarine hydrothermal activity on and near the Galapagos Rift has been explored with the aid of the deep submersible Alvin Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Galapagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley The vent areas are populated by animal communities They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis

https://science.sciencemag.org/content/sci/203/4385/1073.full.pdf

Submarine Thermal Springs on the Galápagos Rift

The petrology of the mid-Atlantic ridge between 22° and 23°N latitude may be typical of those portions of the ridge characterized by a linear topography parallel to the axis, a well-developed median valley, and an absence of volcanic cones. Submarine basalt lavas dredged at fifteen stations on the crest of the ridge are of three eruptive facies, all derived from essentially identical magmas; (1) pillow lavas, (2) sideromelane-rich tuffs, and (3) massive, mainly holocrystalline basalts. This association is well known from continental exposures of ancient submarine lavas. Chemically, the lavas are oceanic tholeiites and thus support the view that these low-potassium olivine basalts are by far the dominant eruptive on the deep-sea floor. In the 22° area, they are probably the product of voluminous fissure eruptions. The oceanic tholeiite is evidently the counterpart of the continental flood basalts, but it differs compositionally from these, especially in a lower potassium content. As a further characterization of the basalts, seven new analyses of major, minor, and trace elements are presented. Post-cooling hydrothermal metamorphism under some overburden has transformed some of these basalts into greenschists and lower-grade metamorphic rocks. New data indicate that faulting and shearing along the median valley combined with the introduction of hot, probably saline solutions were major agents in the metamorphism.

Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude

The abundant glassy rinds and chilled contacts of submarine extrusions and intrusions provide the most reliable indicators of primary magmatic compositions of deep-sea igneous rocks. They are homogeneous at the precision of electron microprobe analyses, so that even small sub-samples are reliable indicators of the composition of larger eruptive units. Analyses of such sea-floor glasses by electron microprobe techniques for Si, Al, Fe, Mg, Ca, Na, K, Ti, P reveal an extremely diverse compositional spread, ranging form highly differentiated basalt to varieties extremely depleated in large ion lithophile elements. A preliminary look at glasses from active spreading centers suggests that the FETI basalt group (high FeO and TiO2, low MgO) is more common at rapidly spreading ridges (East Pacific rise system) than at slower spreading ridges (mid-Atlantic ridge system). The data presented here appear for the most part inadequate for meaningful assessment of chemical gradients along spreading centers. There is strong evidence, though, that some segments of spreadings centers have erupted similar lava compositions over tens of millions of years. These similar chemical groups are symmetrically distributed on each side of spreading segments as is expected from sea-floor spreading models. There is very little overlap in the compositions of the basaltic chemical groups at the level precision of the electron probe analyses. However, there are a few Atlantic chemical groups that occur both in the North and South Atlantic. Even these intra-ocean matches of chemical groups from one locality to another are, however remarkably few. This survey shows the great predominance of various basalt types at spreading centers, but other rock types, including soda rhyolite from the Galapagos rise, occur rarely.

Chemical Diversity of Abyssal Volcanic Glass Erupted Along Pacific, Atlantic, and Indian Ocean Sea-Floor Spreading Centers

Fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed. Glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals. The basal 400–500 m of the sequence consists of an andesite-dacite-rhyolite assemblage containing abundant hyaloclastites. The remainder of the section comprises a basalt–basaltic andesite assemblage with high MgO and low TiO 2 and total iron. The lower sequence is interpreted as an evolved arc-tholeiite suite; the upper has some similarities to boninitic lavas. The close association in time and space of these two suites is similar to that observed in the Mariana and Bonin arcs and suggests that all of the Troodos lavas were erupted in a subduction-zone environment, most probably in an incipient arc or fore-arc.

Volcanic glass compositions of the Troodos ophiolite, Cyprus

The Verna Fracture Zone in the North Atlantic (9 to 11° N), which has been identified as a transform fault zone, contains exposures of serpentinized peridotites, while its adjacent ridge segments are floored mainly by typical abyssal ocean ridge basalts. This petrologic contrast correlates with the greater frequency of volcanic eruptions along the actively spreading ridge segments compared to the transform fault zone. Where rifting components occur across transform faults, exposures of the deeper zone of oceanic crust may result. The bathymetry of the Verna Fracture Zone suggests that some uplift parallel to the fracture zone as well as rifting led to exposures of deeper rocks. The basalts from the adjacent ridge axes contain ‘xenocrysts’ of plagioclase and olivine and more rarely of chromite. These appear to have a cognate origin, perhaps related to cooling and convection in near surface magma chambers. The basalts from the ridge axes, offset and on opposite sides of the transform fault, have similar features and compositions. The plagioclase peridotites have mineralogical features which indicate equilibration in the plagioclase pyrolite facies, suggesting maximum equilibration depths of around 30 km for a temperature of around 1200 °C. The chemical characteristics of the Vema F.Z. peridotites suggest that they may be undifferentiated mantle, emplaced as a subsolidus hot plastic intrusion or as a crystal mush. The abundance of peridotites and serpentinized peridotites is believed to reflect their abundance in seismic layer three of the oceanic crust.

William G. Melson

Papers

Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude

Chemical Diversity of Abyssal Volcanic Glass Erupted Along Pacific, Atlantic, and Indian Ocean Sea-Floor Spreading Centers

Volcanic glass compositions of the Troodos ophiolite, Cyprus

Petrology of a transform fault zone and adjacent ridge segments

Metamorphism in the Mid-Atlantic Ridge, 22°N latitude