The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction‐related processes. Less well studied are the coeval calc‐alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction‐related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na‐alkalic magmatism and continental margin calc‐alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB‐like) mantle in the genesis of subduction‐related volcanism.
Abstract The initial interaction between material rising from the African Large Low Shear Velocity Province and the African lithosphere manifests as the Eocene continental large igneous province (LIP), centred on southern Ethiopia and northern Kenya. Here we present a geographically well-distributed geochemical dataset comprising flood basalt lavas of the Eocene continental LIP to refine the regional volcano-stratigraphy into three distinct magmatic units: (1) the highly alkaline small-volume Akobo Basalt (49.4–46.6 Ma), representing the initial phase of flood basalt volcanism derived from the melting of lithospheric–mantle metasomes; (2) the primitive and spatially restricted Amaro Basalt (45.2–39.8 Ma), representing the early main phase of flood basalt volcanism derived from the melting of the upwelling thermochemical anomaly; and (3) the spatially extensive Gamo–Makonnen magmatic unit (38–28 Ma), representing the mature main phase of flood basalt volcanism that has undergone significant processing within the lithosphere and resulted in relatively homogeneous compositions. The focused intrusion of these main phase magmas over 10 myr preconditioned the African lithosphere for the localization of strain during subsequent episodes of lithospheric stretching. The focusing of strain into the region occupied by this continental LIP may have contributed to the initial extension in SW Ethiopia that is associated with the East African Rift.
Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. 30 Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at ∼6 Ma and their geochemistry suggests a garnet lherzolite source region at 90–100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) (206Pb/204Pb = 17·822, 207Pb/204Pb = 15·482, 208Pb/204Pb = 37·767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with 206Pb/204Pb = 16·991 and 87Sr/86Sr = 0·704704. The youngest lavas, dated as younger than ∼8 ka, have the highest K2O contents (up to 5·2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0·7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.
Abstract Gallium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glassy matrices from a metaluminous trachyte and five peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley. Abundances in the glasses range from 28.9 to 33.3 ppm, comparable with peralkaline rhyolites elsewhere. Phenocryst Ga abundances (in ppm) are: sanidine 31.5–45.3; fayalite 0.02–0.22; hedenbergite 3.3–6.3; amphibole 12; biotite 72; ilmenite 0.56–0.72; titanomagnetite 32; chevkinite-(Ce) 364. The mafic phases and chevkinite-(Ce) are enriched in Ga relative to Al, whereas Ga/Al ratios in sanidine are smaller than in coexisting glass. Apparent partition coefficients range from <0.01 in fayalite to 12 in chevkinite-(Ce). Coefficients for hedenbergite, ilmenite and titanomagnetite decrease as melts become peralkaline. The sharp increase in Ga/Al in the more fractionated members of alkaline magmatic suites probably results from alkali feldspar-dominated fractionation. Case studies are presented to show that the Ga/Al ratio may be a sensitive indicator of such petrogenetic processes as magma mixing, interaction of melts with F-rich volatile phases, mineral accumulation and volatile-induced crustal anatexis.
We have developed a technique to determine the variability of trace elements (including Li, B, Na, Mg, Mn, Cu, Zn, Sr and Ba) within foraminifera tests using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). This technique has a high spatial resolution (width 40–80 μm, depth >0.5 μm), is reproducible (<8% external reproducibility) and has low detection limits (generally <0.05 μg g −1 ). We demonstrate that normalization of data to a calcite standard usually gives results that are more consistent with solution ICP‐MS data than normalization to NIST 612. Rastering into the wall of the final chamber of Globigerinoides sacculifer shows that the outermost ∼1.5 μm of the test is enriched in trace elements, including Mn, relative to the interior, indicating the presence of a contaminant surface phase. Pustule calcite close to the aperture of Globorotalia tumida has different Sr/Ca and Ba/Ca compared to other parts of the test. Sr/Ca and Ba/Ca partition coefficients suggest that the pustule calcite is diagenetic and/or formed in a more open system. Multiple analyses of Orbulina universa tests recovered from a latitudinal transect in the North Atlantic give a relationship between Mg/Ca and sea surface temperature similar to that reported elsewhere in the literature. These initial data demonstrate the validity of this technique, and show that it is potentially an extremely powerful microanalytical tool for palaeoceanographic and palaeontological studies.
Research Article| October 01, 1998 Earliest magmatism in Ethiopia: Evidence for two mantle plumes in one flood basalt province Rhiannon George; Rhiannon George 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar Nick Rogers; Nick Rogers 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar Simon Kelley Simon Kelley 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information Rhiannon George 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Nick Rogers 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Simon Kelley 1Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (10): 923–926. https://doi.org/10.1130/0091-7613(1998)026<0923:EMIEEF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rhiannon George, Nick Rogers, Simon Kelley; Earliest magmatism in Ethiopia: Evidence for two mantle plumes in one flood basalt province. Geology 1998;; 26 (10): 923–926. doi: https://doi.org/10.1130/0091-7613(1998)026<0923:EMIEEF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Tertiary magmatism in Ethiopia has been linked to the thermal influence of the Afar mantle plume. However, new laser 40Ar/39Ar ages for the volcanic succession in southern Ethiopia confirm the presence of two distinct magmatic phases at 45–35 Ma and 19–12 Ma. The earliest phase predates both extension and magmatism in northern Ethiopia by 15 m.y. and cannot be related to any simple model of melting in response to extension over a single mantle plume. We propose a model in which the Ethiopian province was initially related to the thermal influence of the Kenyan, and subsequently, the Afar mantle plume during northward movement of the African plate in the Tertiary. Support for this model comes from paleogeographic evidence that places southern Ethiopia ∼1000 km farther south than its current position during the early melting event at 45 Ma. Moreover, the rate of migration of the onset of magmatism from southern Ethiopia to Tanzania is similar to the rate of migration of the African plate over the same period. Comparable eruption rates in southern Ethiopia and Kenya further strengthen this link. In the light of this evidence, eruption rates ascribed to melting of the Afar mantle plume may be overestimated, which calls into question the potential for the Afar mantle plume to have had a significant effect on the biosphere. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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