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

Isotopic fingerprinting has long been used to trace magmatic processes and the components that contribute to magmas. Recent technological improvements have provided an opportunity to analyze isotopic compositions on the scale of individual crystals, and consequently to integrate isotopic and geochemical tracing with textural and petrographic observations. It has now become clear that mineral phases are commonly not in isotopic equilibrium with their host glass/groundmass. Isotopic ratios recorded from core to rim of a mineral grain reflect the progressive changes in the magma composition from which the mineral crystallized. The sense of these changes and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes such as magma mixing, contamination and recharge.

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Microsampling and Isotopic Analysis of Igneous Rocks: Implications for the Study of Magmatic Systems

Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of...

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Geological archive of the onset of plate tectonics

Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems (Sorby 1858; Roedder 1979, 1984). After trapping, melt inclusions are potentially isolated from external melt and thus provide a way to investigate melts trapped during magmatic evolution—driven by processes such as crystal-liquid separation, vapor saturation and degassing, magma mixing and assimilation—which can dramatically alter the compositions of the eventual erupted (or intruded) magmatic end products. Melt inclusions are a powerful tool for the study of basaltic magma systems and their mantle source regions, and are widely used to study the origin and evolution of mantle-derived magmas. Melt inclusions have specific uses in the study of volatile elements (see chapters by Metrich and Wallace 2008, Moore 2008, and Blundy and Cashman 2008), but also provide unique information about the range of melt compositions present within basaltic magmatic systems, and how these reflect mantle sources and the processes that occur during melt generation, evolution, transport and eruption. This review outlines techniques used to obtain chemical and other information from melt inclusions, discusses the processes which lead to melt inclusion trapping in phenocryst minerals, examines the possible means by which melt inclusion compositions might be fractionated during trapping or during subsequent re-equilibration with the host mineral or external melt, and discusses some implications of melt inclusion compositions for the nature of basaltic melt generation and transport systems.

This review is largely restricted in scope to studies of volcanic rocks of basaltic and related composition. This refers to rocks erupted as lavas or tephra with broadly basaltic compositions: SiO2 ~45–52 wt%, relatively high MgO and FeO, and typically containing one or more of the following minerals …

Melt Inclusions in Basaltic and Related Volcanic Rocks

https://cyberleninka.org/article/n/862582.pdf

Calibration and correction of LA-ICP-MS and LA-MC-ICP-MS analyses for element contents and isotopic ratios

Lithium, boron, and lead isotope systematics of glass inclusions in olivines from Hawaiian lavas: evidence for recycled components in the Hawaiian plume

The evolution of the late Archean Belingwe greenstone belt, Zimbabwe, is discussed in relation to the geochemistry of the ultramafic to mafic volcanic rocks. Four volcanic types (komatiite, komatiitic basalt, D-basalt and E-basalt) are distinguished in the 2� 7Ga Ngezi volcanic sequence using a combination of petrography and geochemistry. The komatiites and D-basalts are rocks in which isotopic systems and trace elements are depleted. Chemical variations in komatiites and D-basalts can be explained by fractional crystallization from the parental komatiite. In contrast, komatiitic basalts and E-basalts are siliceous and display enriched isotopic and trace element compositions. Their chemical trends are best explained by assimilation with fractional crystallization (AFC) from the primary komatiite. AFC calculations indicate that the komatiitic basalts and E-basalts are derived from komatiites contaminated with � 20% and � 30% crustal material, respectively. The volcanic stratigraphy of the Ngezi sequence, which is based on field relationships and the trace element compositions of relict clinopyroxenes, shows that the least contaminated komatiite lies between highly contaminated komatiitic basalt flows, and has limited exposure near the base of the succession. Above these flows, D- and E-basalts alternate. The komatiite appears to have erupted on the surface only in the early stages, when plume activity was high. As activity decreased with time, komatiite magmas may have stagnated to form magma chambers within the continental crust. Subsequent komatiitic magmas underwent fractional crystallization and were contaminated with crust to form D-basalts or E-basalts.

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The geochemistry of ultramafic to mafic volcanics from the belingwe greenstone belt, Zimbabwe: Magmatism in an archean continental large igneous province

Controversy exists as to whether the 2.7 Ga Belingwe greenstone belt, Zimbabwe, is autochthonous or allochthonous. In this study we report direct evidence for an autochthonous continental setting for the Belingwe greenstone belt. Garnet and clinopyroxene xenocrysts were discovered in the fresh ultramafic komatiites. Major and trace element compositions of these xenocrysts suggest that they originated from mafic lower crust, presumably garnet granulite in composition, at a low temperature of ∼600 or ∼700 °C. Furthermore, in a komatiitic basalt sample, we have discovered orthopyroxene and clinopyroxene xenocrysts that may have originated from lithospheric mantle beneath the continental margin. Discoveries of these xenocrysts indicate that the komatiite magma was transported to the surface and quenched rapidly enough to prevent the complete melting of continental fragments.

Kenji Shimizu

Papers

Lithium, boron, and lead isotope systematics of glass inclusions in olivines from Hawaiian lavas: evidence for recycled components in the Hawaiian plume

The geochemistry of ultramafic to mafic volcanics from the belingwe greenstone belt, Zimbabwe: Magmatism in an archean continental large igneous province

Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe