Georg F. Zellmer

Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc

The timescales of element transfer at volcanic arcs provide important insights into magma generation, movement and storage beneath destructive margin volcanoes. Here we present major, trace and Sr-, Nd- and U-series isotope data on 6 200 ka old samples from Santorini in the Aegean volcanic arc as a case study of a hazardous destructive margin volcano. Samples range from low-K calc-alkaline basalts to high-K rhyolites, are enriched in light rare earth elements and fluid mobile trace elements, and have negative Ta, Nb and Ti anomalies characteristic of arc magmatism. 87 Sr/ 86 Sr ratios range from 0.7035 to 0.7054 and are negatively correlated with 143 Nd/ 144 Nd ratios, ranging from 0.51285 to 0.51263. Lavas from the second eruptive cycle (V180 ka^V3.6 ka) are in U^Th radioactive equilibrium, and a wholerock, plagioclase, magnetite U^Th mineral isochron from a 67 ˛ 9 ka dacite yields an age of 85 (+22/319) ka (1c). Samples from the Kameni islands (46 AD^1950) show small but significant uranium excesses, with ( 238 U/ 232 Th) = 0.91^ 1.04 and ( 230 Th/ 232 Th) = 0.91^0.97, suggesting that the time since U^Th differentiation is 9 147 (+27/321) ky (1c). A U^Th mineral isochron from the 1940 Kameni dacite yields an age indistinguishable from its eruption age. The Kameni samples show variable Ra depletion, with ( 226 Ra/ 230 Th) = 0.92^0.99. The range in Sr and Nd isotope data reflects progressive crustal assimilation. The combined trace element and isotope data are consistent with a three-component source model involving mantle wedge, sediment melts and slab-derived fluids. While initial 238 U-excesses in the second eruptive cycle have decayed, implying slab-derived fluid addition s 180 ky prior to eruption, for the Kameni lavas fluid addition to the wedge is 9 150 ky. U^Th mineral isochrons indicate that crystal residence times in the subvolcanic plumbing system are short. Fractional crystallisation less than V1 ky prior to eruption of the Kameni dacites is suggested by ( 226 Ra/ 230 Th)6 1. The combined data imply that timescales of magmatic differentiation in crustal reservoirs are short, that transfer time of the slab-fluid signature through the mantle wedge varies through time. fl 2000 Elsevier Science B.V. All rights reserved.

Time scales of magmatic processes

Abstract The development of improved analytical techniques has facilitated the application of short-lived isotopes to the study of magmatic processes, and resulted in a renewed interest in a number of other chronometers. Two approaches have been used to determine the time scales of magmatic processes. Isotopic dating provides absolute ages for the growth of mineral phases. This usually involves analyses of mineral separates such that the textural relations of the individual grains are difficult to establish. An exception is zircon, which can be analysed in situ. The second approach is to use relative chronometry based on major, trace element and isotope profiles in crystals that may have been modified by diffusion. These yield information on how long crystals were at a particular temperature, without indicating when this occurred. The ages are obtained on individual crystals, and so age distributions can be determined on different crystals from the same whole rock. The ages of crystals and the liquid, as represented by the groundmass, in an igneous rock can be different, and in a number of cases it has been shown that even the larger, and therefore typically older crystals formed after the fractional crystallisation responsible for the whole rock composition. One implication is that the processes of magma differentiation responsible for whole rock compositions may not necessarily be inferred from the compositional record of the larger crystals. Different approaches are therefore used to investigate the crystallisation history and the differentiation of magmatic suites. Crystallisation rates are ∼10 −10 –10 −11 cm/s, whereas differentiation to high-silica magmas may take up to 2×10 5 years. The ages of crystals at the time of eruption can range back to 10 5 –10 6 years, the older ages tend to be in the more evolved rock types, and it can take 10 5 years for high-silica magmas to be generated at individual volcanic centres. It appears that the generation of such evolved magmas is thermally controlled, for both fractional crystallisation and the generation of crustal melts, and the rates of fractional crystallisation can, for example, be linked to volcanic power outputs. If crystallisation is in response to magma degassing or decompression, it will be fast and there may be too little time for fractional crystallisation to take place.

Time-scales of magma formation, ascent and storage beneath subduction-zone volcanoes

There is now sufficient information to attempt an integrated model for melt generation, transfer and storage beneath subduction-zone volcanoes. Fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr, to as little as less than 1 kyr, before eruption. This supports models in which fluid addition is closely linked to partial melting, though there may also be evidence for a component of decompression melting. The timing of the onset of fluid addition may be linked to the rate of subduction (i.e. water supply rate) and the angle of subduction, and, consequently, the thermal structure of the mantle wedge. In contrast, contributions from subducted sediments to subduction-zone lava sources appear to occur some 350 kyr–4 Myr before eruption. Evidence for partial melting of the sediment component, combined with the short fluid transfer times, phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate. New 226Ra data permit only a short period of time between fluid addition and eruption. This requires rapid melt segregation, magma ascent by channelled flow and minimal residence time within the lithosphere. Typically, the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs, inferred from some geophysical data to lie within the lithospheric mantle. Mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers, where they can evolve to dacitic compositions via fractionation, typically combined with assimilation, on time-scales of a few thousand years or less.

Some remarks on U–Th mineral ages from igneous rocks with prolonged crystallisation histories

Abstract Mineral isochron dating is a frequently used geochronological tool. One of its assumptions is that the minerals grow over a time period that is small compared to the half-life of the radiogenic isotope system used. In recent years, increasing analytical precision has promoted the use of the short-lived U-series isotope system in order to date young crystallisation events. Three whole-rock zircon U–Th isochrons from the 26.5 ka Oruanui eruption in the Taupo Volcanic Zone, New Zealand, yield pre-eruptive model ages of 5.5±0.8 ka, 9.7±1.7 ka and 12.3±0.8 ka for the sub-63 μm, 63–125 μm and 125–250 μm zircon size fractions, respectively. This suggests that in this case the assumption of instantaneous crystal growth breaks down. Instead, the U–Th data may be explained by continuous zircon growth over a period of ∼90 ka. However, cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector-zoned euhedral rim, and the U–Th data can also be modelled by mixing an older (∼27 ka model age) population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the Oruanui rhyolite. This indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment. It is concluded that conventional U-series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude. In future, microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales.

Three Fe-Ti oxide ore-bearing gabbro-granitoid complexes in the panxi region of the Permian Emeishan large igneous province, SW China

The Permian (∼260 Ma) Emeishan large igneous province of SW China contains three nearly identical gabbro-granitoid complexes that host giant Fe-Ti oxide deposits. The Fe-Ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with A-type granitic plutons. The 264 ± 3 Ma Taihe layered gabbroic intrusion hosts a large magmatic Fe-Ti oxide deposit and is coeval with the Taihe peralkaline, A-type granitic pluton, which is dated at 261 ± 2 Ma. Within the A-type granitic pluton are microgranular enclaves, which have compositions intermediate between the gabbro and host granite. Primitive mantle-normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks. The chondrite-normalized REE patterns show Eu-anomalies changing from positive (Eu/Eu* = 1.5 to 5.9) in the gabbroic intrusion to negative in the enclaves (Eu/Eu* = 0.4 to 0.6) and granites (Eu/Eu* = 0.2 to 0.5). Whole rock εNd(T) values of the gabbroic intrusion (εNd(T) = +2.5 to +3.3) are similar to those of the enclaves (εNd(T) = +1.0 to 2.0) and granite (εNd(T) = +1.5 to +1.9) whereas the zircon εHf(T) values of the gabbro (εHf(T) = +8.1 ± 0.8) are indistinguishable from those of the granites (εHf(T) = +9.2 ± 1.0), suggesting that the parent magmas for all rock types originated from the same mantle source. Geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high-Ti Emeishan flood basalt. The compositional jump from the gabbro to the enclaves is attributed to the crystallization of Fe-Ti oxide minerals. The results of this study and other studies suggest that the magmatic conditions (for example, pressure, composition, fO2), which led to the formation of at least three Fe-Ti oxide bearing gabbro-granitoid complexes, were relatively common during the development of the Emeishan large igneous province.

Lower crustal H2O controls on the formation of adakitic melts

At volcanic arcs, fluids released from the subducting slab lower the solidus of the mantle wedge and cause melting. Furthermore, slab melts may infiltrate the mantle wedge, and have been suggested to generate adakitic (residual garnet) signatures at some arc volcanoes. However, experimental work indicates that the garnet stability field will expand in the lower overriding crust in the presence of somewhat less hydrous melts, suggesting that such signatures may also develop at crustal levels. Here we use geothermometry and plagioclase hygrometry of mafic eruptives from southwest Japan to demonstrate that the adakitic compositions of associated intermediate magmas are of lower crustal origin due to a decrease in the water content of parental melts, and are not generated by partial melting of the eclogitic subducting slab at elevated temperatures. Lower crustal melt evolution at reduced water contents may represent an important process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting. Our results demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.

Dynamics of Crustal Magma Transfer, Storage and Differentiation

Magmas are subject to a series of processes that lead to their differentiation during transfer through, and storage within, the Earth’s crust. The depths and mechanisms of differentiation, the crustal contribution to magma generation through wall-rock assimilation, the rates and timescales of magma generation, transfer and storage, and how these link to the thermal state of the crust are subject to vivid debate and controversy. This volume presents a collection of research articles that provide a balanced overview of the diverse approaches available to elucidate these topics, and includes both theoretical models and case studies. By integrating petrological, geochemical and geophysical approaches, it offers new insights to the subject of magmatic processes operating within the Earth’s crust, and reveals important links between subsurface processes and volcanism.

An introduction to magma dynamics

Abstract A variety of methods have been employed to decipher magmatic systems, including geophysical, petrological, textural and geochemical approaches, and these elucidate a large variety of characteristics of different plumbing systems and magmatic differentiation processes. A common theme to the papers presented in this book is the observation of transport of small volume magma batches with a relatively high frequency, as opposed to less frequent transport of larger magma volumes that would require storage in large crustal reservoirs for long periods of time. The implications of this observation are discussed in the context of a possible tectonic control on crustal magma dynamics.

Petrogenesis of Sr-rich adakitic rocks at volcanic arcs: insights from global variations of eruptive style with plate convergence rates and surface heat flux

Abstract: Using global correlations of effusive eruption style with convergence rates and surface heat flux at volcanic arcs, this study identifies several arcs with excess surface heat flux, related to the presence of unusually hot and H2O-poor magmas. Geophysical evidence suggests that these melts are linked to discontinuities in the subducting slab, which would facilitate upwelling of hot sub-slab asthenospheric mantle. It is demonstrated that excess heat flux arcs are sites where Sr-rich adakitic compositions are generated, and that the arc proportion erupting Sr-rich adakitic magmas is directly proportional to the degree of observed heat flux excess. A model is presented in which the formation of Sr-rich adakitic compositions, which require garnet as a residual phase, is facilitated at excess heat flux arcs as a result of a decrease in H2O content through contributions from sub-slab asthenospheric mantle, leading to an expansion of the garnet stability field. Future geochemical, geophysical and experimental work required to test this model is addressed.

Some first-order observations on magma transfer from mantle wedge to upper crust at volcanic arcs

Abstract The viscosity of lavas erupted at volcanic arcs varies over orders of magnitude. A comparison of the relative abundance of viscous lava dome eruptions indicates that the average viscosity of arc lavas also varies considerably between arcs. It is shown that, for continental or transitional arcs with little within-arc crustal deformation and without underlying slab windows or tears, average lava viscosity is anticorrelated with average surface heat flux. The latter may be influenced by crustal thickness and crustal magma throughput. To constrain the relative contributions of these parameters, variations of average lava viscosity with average crustal thickness and plate convergence rate are assessed. While crustal thickness appears to have little effect on average lava viscosity, a good anticorrelation exists between average lava viscosity and plate convergence rate, with the exception of two arcs that show significant intra-arc crustal deformation. If plate convergence rate is a good proxy of the rate of melt generation within the mantle wedge, these first-order observations indicate that, where the rate of mantle melting is high, crustal magma throughput is rapid and efficient, resulting in low-viscosity melts migrating through a hot overriding crust; in contrast, where the rate of mantle melting is low, crustal magma transfer is slow and inefficient, resulting in high-viscosity melts that may frequently stall within a cool overriding crust prior to eruption. Uranium series geochemical evidence from dome lavas is presented and lends support to this interpretation. Finally, some explanations are offered for the observed average viscosity variations of arcs with underlying slab windows or tears and/or significant intra-arc crustal deformation.