Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

This chapter summarizes the oxygen isotope geochemistry of terrestrial basalts and
their mantle sources, including the conceptual framework for interpreting such data and
the phenomenology of known variations. In particular, the first section outlines the
motivations for and first-order results of oxygen isotope studies of terrestrial and lunar
basalts over the last 30 years; the second section reviews oxygen isotopic fractionations
among phases relevant for studying basalts and mantle rocks; the third summarizes
variations in δ^(18)O of various crustal rocks that may contribute to the petrogenesis of
basalts either as subducted source components or lithospheric contaminants; and the
final and longest section describes observed oxygen isotope variations of major classes
of terrestrial basalts and related mantle nodules with an emphasis on data generated
within the last six years using laser-based fluorination techniques. In the interests of
brevity, I do not describe in detail methods for oxygen isotope analysis or changes in
δ^(18)O of volcanic rocks caused by sub-solidus alteration; however, these issues are
important practical considerations for anyone studying oxygen isotope compositions of
basalts and interested readers are directed to the following references: analytical
methods: Sharp (1990), Mattey and Macpherson (1993), and Valley et al. (1995); basalt
alteration: Muehlenbachs (1986), Alt (1993), and Staudigel et al. (1995).

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Oxygen Isotope Variations of Basaltic Lavas and Upper Mantle Rocks

U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China : Implications for the initial rifting of Rodinia

The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguflu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Υb = 4.2], moderate in the alkali basalts (La7 = 24, La/Υb = 9.2), and very high in the magnesian alkaline magmas (La7 = 43, La/Υb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of ∼17·9-18·6 in the tholeiites and ∼19·0-19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes. © Oxford University Press 2004; all rights reserved.

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Flood and shield basalts from Ethiopia: Magmas from the African superswell

Two distinct phases of continental magmatism are evident in western Arabia The first, from about 30 to 20 Ma, produced tholeiitic-to-transitional lavas emplaced along NW trends The second, from about 12 Ma to Recent, produced transitional-to-strongly-alkalic lavas emplaced along N-S trends The older phase is attributed to passive-mantle upwelling during extension of the Red Sea Basin, whereas the younger phase is attributed to active-mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision The younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the West Arabian Swell after about 14 Ma A variety of evidence suggests that the West Arabian Swell is thermally supported by hot, upwelling asthenosphere In contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the Red Sea Basin, these processes were symmetric across a N-S line marking the central axis of the West Arabian Swell This axis coincides with two fundamental features: the Ha'il-Rutbah Arch in the north, and the Makkah-Madinah-Nafud (MMN) volcanic line in the south The symmetry of magmatism is demonstrated by petrochemical evidence that the MMN harrats were derived by greater degrees of partial melting, at shallower depths, than those harrats lying to the west and east of the MMN line The potential temperature of the asthenosphere is estimated to be about 1436°C beneath the MMN line and about 1354°C beneath the flanking, more undersaturated harrats The source of upwelling is either a mantle plume centrally located beneath the West Arabian Swell or an elongated and extended lobe of hot asthenosphere emanating from the Ethiopian mantle plume Convective flow may have been channelled along a preexisting, regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the AfroArabian Dome This crest of mantle upwelling underlies the MMN line in the north and the Danakil Depression and Ethiopian rift system in the south It also passes through the Red Sea Basin at the midpoint of an unusual, doubly propagating rift system where axial seafloor spreading began 4–5 my ago The NW trend of the Red Sea Basin was well established by crustal attenuation during the older magmatic phase The Pliocene invasion of this basin by a N-S zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

Upwelling asthenosphere beneath western Arabia and its regional implications

We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa (NWA) 856 with Djel Ibone as a synonymous name. It is a fresh, fine-grained basaltic rock consisting mainly of two pyroxenes (total similar to68 vol%: 45% pigeonite, En(61-16)Wo(9-22)Fs(26-68); 23% augite, En(46-26)Wo(34-29)Fs(21-43)) and plagioclase converted to maskelynite (similar to23 vol%, Ab(43-57)Or(1-5)An(54-36)). Accessory minerals include merrillite, Cl-apatite, pyrrhotite, ilmenite, ulvospinel, silica (stishovite and glass), amorphous K-feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (similar to2 vol%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of similar to50 vol% maskelynite. The meteorite is highly fractured at all scales.

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The basaltic shergottite Northwest Africa 856: Petrology and chemistry

http://www.meteorite.fr/en/pdf/NWA817complet.pdf

A new Martian meteorite from Morocco: the nakhlite North West Africa 817

Magma genesis in an ongoing rifting zone: The Tadjoura Gulf (Afar area)

Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions

J. L. Joron

Papers

The basaltic shergottite Northwest Africa 856: Petrology and chemistry

A new Martian meteorite from Morocco: the nakhlite North West Africa 817

Magma genesis in an ongoing rifting zone: The Tadjoura Gulf (Afar area)

Mantle heterogeneity in northeastern Africa: evidence from Nd isotopic compositions and hygromagmaphile element geochemistry of basaltic rocks from the Gulf of Tadjoura and southern Red Sea regions

Geochemistry of basalts from Manda Hararo, Ethiopia: LREE-depleted basalts in Central Afar