Tyrone O. Rooney

Elevated mantle temperature beneath East Africa

The causes of magmatism at magmatic rifted margins and large igneous provinces (LIPs) are uncertain because the condition of the mantle that underlay them during formation can no longer be directly observed. Therefore, whether the mantle was characterized by elevated potential temperatures ( T P ), small-scale convection, or anomalously fertile composition is debated. East Africa is an ideal area in which to address this problem because it contains both the young African-Arabian LIP and the tectonically and magmatically active East African Rift system. Here we present mantle T P estimates for 53 primitive magmas from throughout the region to reveal that thermal anomalies currently peak in Djibouti (140 °C above ambient upper mantle). Slightly warmer conditions accompanied the Oligocene African-Arabian LIP, when the T P anomaly was 170 °C. These values are toward the low end of the global temperature range of LIPs, despite the markedly slow seismic velocity mantle that underlies the region. Mantle seismic velocity anomalies in East Africa cannot, therefore, as is often assumed, be attributed simply to elevated mantle temperatures. We conclude that CO 2 -assisted melt production in the African superplume contributes to the markedly slow seismic velocities below East Africa.

Heads and tails: 30 million years of the Afar plume

Abstract Primitive recent mafic lavas from the Main Ethiopian Rift provide insight into the structure, composition and long-term history of the Afar plume. Modern rift basalts are mildly alkalic in composition, and were derived by moderate degrees of melting of fertile peridotite at depths corresponding to the base of the modern lithosphere (c.100 km). They are typically more silica-undersaturated than Oligocene lavas from the Ethiopia-Yemen continental flood basalt province, indicating derivation by generally smaller degrees of melting than were prevalent during the onset of plume head activity in this region. Major and trace element differences between the Oligocene and modern suites can be interpreted in terms of melting processes, including melt-induced binary mixing of melts from the Afar plume and those from three mantle end-member compositions (the convecting upper mantle and two enriched mantle sources). The Afar plume composition itself has remained essentially constant over the past 30 million years, indicating that the plume is a long-lived feature of the mantle. The geochemical and isotopic compositions of mafic lavas derived from the Afar plume support a modified single plume model in which multiple plume stems rise from a common large plume originating at great depth in the mantle (i.e. the South African superplume).

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate and timing of extension; alternatively, pre-existing plate architecture and/or the evolving kinematics of extension during breakup may exert first order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along-strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows, each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en-echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

The role of continental lithosphere metasomes in the production of HIMU-like magmatism on the northeast African and Arabian plates

Intraplate alkaline lavas typically exhibit isotopic characteristics that require a source with long-term isolation from the convecting asthenosphere, such as in the sub-continental lithosphere mantle or a mantle boundary layer. Melting of metasomatically enriched domains, or metasomes, within the lithospheric mantle provides a viable mechanism for generating the geochemical characteristics of intraplate alkaline basalts. The origins and distribution of these metasomes have been attributed to recent enrichment of the lithosphere by a mantle plume or ancient events that occurred during the early evolution of the sub-continental lithosphere mantle. Here, we present a geochemical study of Ethiopian Miocene intraplate alkaline lavas: melts of a lithospheric mantle that was enriched metasomatically during lithospheric stabilization and by recent plume-lithosphere interaction. We find that these lavas have geochemical characteristics consistent with melting of an amphibole-bearing lithospheric-mantle metasome. New Pb and Hf isotope data for these lavas require a HIMU-like source component, similar to other alkaline lavas erupted through the Horn of Africa, Sudan, and Egypt, and adjacent Arabian plate lithospheres. The isotopic characteristics of this component are distinct from the Afar plume mantle source and instead are consistent with the long-term evolution of a lithospheric metasome created during a Neoproterozoic subduction event associated with the Pan-African orogeny. The widespread distribution of easily fusible lithospheric metasomes within the continental lithosphere mantle may facilitate magma generation without the need for substantial lithospheric thinning or elevated mantle potential temperatures. Mantle heterogeneity of this nature has implications for the source origin of HIMU magmas associated with continental lithosphere.

The protracted development of focused magmatic intrusion during continental rifting

The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

Enhanced East Pacific Rise hydrothermal activity during the last two glacial terminations

Searching sediment for climate signals Sediments on the ocean floor may provide clues about the interplay between ice ages and mid-ocean ridge magma production. Lund et al. present well-dated and detailed sediment records from hydrothermal activity along the East Pacific Rise. The sediments show changes in metal fluxes that are tied to the past two glaciations. Ice age changes in sea level alter magma production, which is manifested by changes in hydrothermal systems. The apparent increase in hydrothermal activity at the East Pacific Rise around the past two glacial terminations suggests some role in moderating the size of ice sheets. Science, this issue p. 478 Sediments from hydrothermal vents may provide insight into interplay between glaciation and hydrothermal activity. Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.

Crystal fractionation processes at Baru volcano from the deep to shallow crust

Linking shallow and deep crustal processes at volcanic arcs is an important component in evaluating the growth and evolution of the continental crust. Commonly, deep crustal processes and the nature of subarc lithosphere are studied long after the volcanism has ceased in obducted arc terranes. In active arcs, studies of deep crustal processes focus on cumulates derived from middle-lower crustal levels. Although uncommon in the erupted magmas, these cumulates are required by crustal differentiation models of arc magmatism. Quaternary magmas at Baru volcano in Panama contain ubiquitous amphibole-bearing cumulates that provide an opportunity to probe the magma plumbing system of an active arc volcano. We have determined that these cumulates are related to their host magmas by crystal fractionation processes. Pressure and temperature estimates for amphiboles within these cumulates and the host rock are consistent with sampling of mush/magma zones from throughout the arc crust. These mush zones would be localized in deep hot crustal zones where magmatic differentiation of water-saturated arc magmas takes place by crystallization of amphibole-rich cumulates. The identification of middle-lower crustal cumulates is not exclusive to Baru volcano; similar cumulates are common throughout the Panamanian arc and are consistent with a widespread amphibole-rich layer present within the arc crust of Panama. Our results highlight the importance of amphibole fractionation in the differentiation sequence of island arcs effectively driving the residual magma to the average andesitic composition of the continental crust.

Conditions of melt generation beneath the Taupo Volcanic Zone: The influence of heterogeneous mantle inputs on large-volume silicic systems

Many arc silicic igneous provinces exhibit compositional variability defined by oscillation between dry and wet rhyolites. The origins of this variability are often uncertain due to the poor constraints on the compositions of the mantle-derived inputs to the lower crustal hybridization zones. The Taupo Volcanic Zone (TVZ) in New Zealand, the most productive of modern silicic igneous provinces, exhibits variability of rhyolite compositions, but small-volume coeval basaltic eruptions also occur, making it an ideal location to study the mantle contributions to these distinctive types of rhyolite. Here we present major and trace element data from 12 magmatic centers that reveal that mafic units from the northern portion of the central TVZ are enriched in large-ion lithophile elements (LILE) in comparison to the southern TVZ. The results of models quantifying slab-derived contributions to the mantle suggest that the observed heterogeneity in LILE concentrations, and volatile fugacities, can be explained by variable amounts of subduction-derived fluid within the melting region of the basalts. These new data and modeling results provide the first direct evidence that the spatial diversity in the flux of mantle-derived basalts and associated volatile elements into the lower crustal differentiation system of the TVZ are coincident with wet to dry rhyolite compositional variability.

Magmatic consequences of the transition from orthogonal to oblique subduction in Panama

The closure of the Central American Seaway is linked with tectonic and magmatic processes that have controlled the evolution of the Isthmus of Panama. We focus on the terminal stages of arc activity in the Central Panama region, and present new geochemical data from ∼9 Ma explosive silicic volcanism preserved in three syngenetic tuff beds from the Gatun Formation. The magmatic evolution of the Gatun Formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral, with amphibole appearing only relatively late in the fractionation sequence. Our data shows Gatun lavas exhibit a strong subduction signature, consistent with plate reconstruction models showing arc-normal subduction from Costa Rica to Panama pre-8.5 Ma. However, large ion lithophile elements are depleted in the Gatun Formation in comparison to other regional suites, indicative of a lower flux of subduction fluid to the Gatun Formation mantle source, which is explained by a shift towards magma generation by decompression following the collision of the arc with South America. Oblique subduction commencing ∼8.5 Ma resulted in the shutdown of normal arc activity throughout Panama. We interpret subsequent regional Quaternary adakitic volcanism as a response to this oblique subduction. The now more refractory mantle wedge required greater fluid flux in order to melt. The resultant volatile-rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites. Deep fractionation was potentially enhanced by changing stress regimes on the upper-plate caused by oblique subduction. This article is protected by copyright. All rights reserved.