Magma Emplacement Rate Research Articles

Does a Shallow Magma Reservoir Exist in the Wudalianchi Volcanic Field? Constraints From Magnetotelluric Imaging

AbstractWe obtain new images of the 3D resistivity structure of the Wudalianchi volcanic field (WVF). No low‐resistivity structure is found beneath Laoheishan and Huoshaoshan volcanoes in the WVF, which challenges the hypothesis of one or more magma reservoirs in the upper crust of this region. We observe a low‐resistivity body beneath Weishan volcano; however, estimated magma emplacement rates conflict with the observed volume of that body. Due to the relatively low geothermal gradient and surface heat flow in the area, magma would cool rapidly and could not be sustained for a long time unless new magma was regularly introduced into the system, but our observations do not support this scenario. We suggest that the magma in the WVF mainly originates from the mantle or the base of the crust with little crustal residence, and that the low‐resistivity body in the upper crust is likely due to saline aqueous free fluids.

Copper-mineralised porphyries sample the evolution of a large-volume silicic magma reservoir from rapid assembly to solidification

Volatile-rich intermediate to silicic magmatic systems can feed devastating volcanic eruptions but also generate valuable magmatic-hydrothermal ore deposits that supply most of the world's copper. Understanding the geometry, dynamics and timescales of these magmatic systems is critical in developing models for predicting the occurrence of ore deposits and future large volcanic eruptions. Here, we use zircon petrochronology from an equigranular monzonite and successively emplaced porphyry dykes to reconstruct the time – temperature – composition evolution of the magma that sourced the giant Cu-Mo-Au deposit at Bingham Canyon (USA). Combining high-precision CA-ID-TIMS geochronology with in-situ trace element analyses by LA-ICP-MS shows the intra-grain, inter-sample and temporal geochemical changes recording the evolution of the magmatic system over 817 ± 62 kyr. Systematic variation of zircon chemistry with time indicates crystallisation from a coherent magma reservoir. After reservoir assembly its thermal and chemical state was controlled by protracted monotonous crystallisation over ∼650 kyr with rapid cooling over the first 200 kyr followed by a longer period approaching the granite solidus. Porphyry Cu-Au ore formation occurred after the early drop in magma temperature that resulted in large-scale fluid saturation and expulsion into the sub-volcanic environment but main Mo-mineralisation occurred after protracted low-temperature magma storage and the emplacement of the last porphyry. Zircons do not quantify the depth of this reservoir but integrating independent geophysical evidence with 2-D thermal modelling indicates that the time – temperature evolution recorded by the zircons is consistent with rapid incremental assembly of this large pluton (magma emplacement rate ≳ 0.0065 km3/yr) by initially zircon-undersaturated monzonitic magma into pre-heated upper crust. Our results indicate that massive fluid expulsion from rapidly-formed, large magma reservoirs containing mobile but mushy magma (>40 wt.% melt) can occur in the upper crust, favouring the formation of giant porphyry copper deposits.

Modeling water exsolution from a growing and solidifying felsic magma body

We modeled water exsolution from a growing and crystalizing felsic magma body using a conductive thermal model with simplified physical mechanisms of volatile transport. Magma solidification leads to the release of the water initially dissolved in the melt. Solidification behaviour depends, in turn, on dissolved water content. The water is channeled where the magma is solidified enough to form a crystal network and rapidly ascends until it encounters solid rock or liquid-rich magma. The rate of water exsolution depends on the basal magma emplacement rate, the cooling rate, and the magma initial water content. Water-rich layers form and are eventually trapped in the solid rock as cooling and crystallization proceeds. We ran our model with a quasi-eutectic granite composition and a non-eutectic monzogranite composition. The non-eutectic magma crystallizes on a wider range of temperature than the eutectic magma, produces more mush, and is more prone to the formation of water-rich lenses. Exsolved water accumulates on the sides of the chamber for both tested compositions and also above the top of the chamber for the non-eutectic composition. The actual presence of these water-rich layers in nature and their lifetime depend on whether the water is further released by fracturing. Our results account for the observed decoupling of volatiles from magma and for the episodic nature of volcano deformation. It has implications for the interpretation of magma chambers tomography as water-rich lenses would be difficult to distinguish from melt-rich lenses.

The contribution to exogenic CO2 by contact metamorphism at continental arcs: A coupled model of fluid flux and metamorphic decarbonation

Recent work has suggested a possible temporal coincidence between greenhouse intervals and enhanced arc volcanism, motivating the hypothesis that magmatic and metamorphic CO₂ emissions from volcanic arcs, particularly those intersecting crustal carbonates, may play a strong role in modulating the long-term carbon budget of the exogenic system. When hot fluid exsolving from arc magmas interacts with carbonate sequences on active margins, contact metamorphism releases CO₂ to metasomatic fluids that transport carbon to shallow reservoirs. To estimate the magnitude of CO₂ release, here we model how the infiltration of silica-saturated magmatic water into a porous medium facilitates the decarbonation reaction in contact aureoles. Analytical scalings and numerical simulations show that the propagation rate of the reaction front scales with the ratio of the infiltration flux to the mass of the rate-limiting reactant, and accordingly the CO₂ flux increases linearly with the infiltration flux. This simple relationship allows for scaling to predict regional and global scale CO₂ release at continental arcs if magma emplacement rate is known. Using the global rate of continental arc magma emplacement, we estimate that the present-day contact-metamorphic CO₂ release range from ∼0.06 to 0.9 Tmol/yr, half-to-one orders of magnitude smaller than the field-based estimates of carbon output in modern arcs (1.5–3.5 Tmol/yr). Yet, the extrapolated CO₂ release from Cretaceous continental arcs via simple infiltration-induced decarbonation is comparable to the release from mid-ocean ridges. CO₂ released from continental arcs amplifies the background flux of CO₂ from direct degassing of the magma, and therefore may have been key in causing the climatic greenhouse interval in the Cretaceous when there was heightened arc activity. Thus, our result supports the hypothesis that global arc flare-ups at continental margins effectively increase CO₂ outgassing coinciding with green-house intervals in the geological past. The contribution by arcs to the tectonic CO₂ input could be significant, which needs field-based studies to revise long-term climate models.

Seafloor expression of oceanic detachment faulting reflects gradients in mid-ocean ridge magma supply

Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13°20′N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

AbstractDespite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.

Magma Emplacement Rates and Porphyry Copper Deposits: Thermal Modeling of the Yerington Batholith, Nevada

Abstract Many porphyry copper deposits are associated with granitoid plutons. Porphyry copper deposit genesis is commonly attributed to degassing of pluton-forming intermediate to silicic magma chambers during slow cooling and crystallization. We use numerical simulations of thermal evolution during pluton growth to investigate the links between pluton construction, magma accumulation and solidification, volatile release, and porphyry copper deposit formation. The Jurassic Yerington batholith, Nevada, serves as a case study because of its exceptional exposure, revealing the geometry of three main intrusions. The last intrusion, the Luhr Hill granite, is associated with economic porphyry copper deposits localized over cupolas where dikes and fluid flow were focused. Our simulations for the conceptual model linking porphyry copper deposits with the presence of large, highly molten magma chambers show that the Luhr Hill granite must have been emplaced at a vertical thickening rate of several cm/yr or more. This magma emplacement rate is much higher than the time-averaged formation rates of other batholiths reported in the literature. Such low rates, although common, do not lead to magma accumulation and might be one of the reasons why many granitoid plutons are barren. Based on our results, we formulate the new testable hypothesis of a link between porphyry copper deposit formation and the emplacement time scale of the associated magma intrusion.

Compositional zonation of the shallow La Gloria pluton (Central Chile) by late-stage extraction/redistribution of residual melts by channelization: Numerical modeling

The origin of highly evolved magmas (e.g. rhyolites) has been a long-standing controversy in earth sciences. They are commonly thought to be generated in the upper crust by melt extraction from mush zones, but due to the rapid cooling of magma reservoirs in such shallow and typically cold environments, high magma emplacement rates of intermediate magmas are thought to be necessary to maintain large silicic mushes above the solidus long enough for the high-SiO2 melts extraction to occur. Late-stage redistribution of interstitial melts (i.e. heat and mass) by channels/dikes within those mushes has been invoked as a mechanism to preserve silicic mushes above their solidi for longer periods (i.e. delaying their final crystallization), but the nature of this process and its implications on plutons zonation are still poorly understood.Here, using time-dependent numerical modeling, we study the feasibility of late-stage interstitial melt extraction/redistribution by channels/dikes from a crystalline mush. Our model accounts for magma fluid dynamics, extraction of residual melts and thermal evolution of the crystallizing magma system and its hosting rocks, considering the thermal effect of the redistributing material. The model was applied to explain the anatomy of the well-documented La Gloria pluton (LGP, Central Chile), which exhibits increasing contents of SiO2 and abundant leucocratic dikes toward the margins, interpreted as trapped residual melts generated elsewhere in the magma chamber.Our results suggest that favorable conditions for extracting late-stage residual melts are reached at temperatures of ~750°C (60vol% crystallinity), at least for compositions similar to LGP dikes. Simulations correspond to 30kyr of reservoir cooling, when the concentric compositional zonation of LGP is reproduced after a short period of extraction (<15kyr) and outward redistribution of silicic melt, with an extracted mass fraction of up to 0.17. An inward growing crystal-dominated domain is the main source of interstitial melts. A total extracted melt volume of ~7.5km3 and a mean extraction rate of ~0.5km3/kyr were calculated for LGP conditions. For small magma reservoirs, the extracted rhyolitic melt could reach the upper levels of the magma chamber and the overlying host rock, but it seems unlikely that it can reach the surface to feed large rhyolitic eruptions, at least once recharges have stopped.

Crustal Magmatic Systems from the Perspective of Heat Transfer

Crustal magmatic systems are giant heat engines, fed from below by pulses of hot magma, and depleted by loss of heat to their surroundings via conduction or convection. Heat loss drives crystallization and degassing, which change the physical state of the system from relatively low-viscosity, eruptible melt, to high-viscosity, immobile, partially molten rock. We explore the temporal evolution of incrementally grown magmatic systems using numerical models of heat transfer. We show that their physical characteristics depend on magma emplacement rates and that the majority of a magma system9s lifetime is spent in a highly crystalline state. We speculate about what we can, and cannot, learn about magmatic systems from their volcanic output.

On the longevity of large upper crustal silicic magma reservoirs

Research Article| July 01, 2013 On the longevity of large upper crustal silicic magma reservoirs Sarah E. Gelman; Sarah E. Gelman 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Francisco J. Gutiérrez; Francisco J. Gutiérrez 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA2Advanced Mining and Technology Center, and Departamento de Geología, Universidad de Chile, Santiago, Chile Search for other works by this author on: GSW Google Scholar Olivier Bachmann Olivier Bachmann 1Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA3Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (7): 759–762. https://doi.org/10.1130/G34241.1 Article history received: 15 Nov 2012 rev-recd: 08 Feb 2013 accepted: 12 Feb 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sarah E. Gelman, Francisco J. Gutiérrez, Olivier Bachmann; On the longevity of large upper crustal silicic magma reservoirs. Geology 2013;; 41 (7): 759–762. doi: https://doi.org/10.1130/G34241.1 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 Understanding the processes involved in the formation and maturation of upper crustal magma reservoirs, ultimately sourcing the largest volcanic eruptions on Earth, is one of the most fundamental aspects of volcanology. While such reservoirs are known to assemble incrementally over extended periods of time, debate persists regarding the time scales of melt preservation in the cold upper crust. If rapid cooling individually freezes incoming replenishing intrusions, accumulations of eruptible magma are impossible, precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates. Recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival, and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates, and subsist only ephemerally, making their long-term evolution less predictable. Motivated by geological observations suggesting the contrary, we have improved upon these models by incorporating two fundamental features of natural systems not previously considered: (1) a nonlinear crystallization-temperature relationship adapted for upper crustal silicic magmas and (2) a temperature-dependent thermal conductivity. We demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces. While the crystallization-temperature path plays the most significant role in maintaining a large pool of eruptible magma, the incorporation of temperature-dependent thermal properties significantly extends the lifetime of such reservoirs. Furthermore, while deeper emplacement levels (e.g., 10 km depth) can both extend magma survival and increase melt availability, we show that a 5 km depth can also provide an adequate magma storage environment. These results provide strong support for long-lived upper crustal mushes as a staging ground for accumulation of highly eruptible, crystal-poor silicic magmas, and further assert the evolutionary link between volcanic and plutonic systems. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence for a two‐phase Palmer Land event from crosscutting structural relationships and emplacement timing of the Lassiter Coast Intrusive Suite, Antarctic Peninsula: Implications for mid‐Cretaceous Southern Ocean plate configuration

New analysis of the relationships between geological structural data and radiometric ages for the Lassiter Coast Intrusive Suite indicate that the collisional mid‐Cretaceous Palmer Land Event orogeny in the Antarctic Peninsula has had two kinematic phases, forming an intersection orocline, one of which can be related to Cretaceous Southern Ocean plate motions. Both are compressional phases along the Eastern Palmer Land Shear Zone: Phase 1 occurred at ∼107 Ma with a principal paleostrain axis of 341°, and is best expressed in southern Palmer Land although evident elsewhere on the Antarctic Peninsula; Phase 2 occurred at ∼103 Ma with a principal paleostrain axis of 259.5°, but is confined to between 68°S and 74°S. A peak in Lassiter Coast Intrusive Suite magma emplacement rate was coeval with Phase 1, whereas Phase 2 may have coincided with a lull. During Phase 1, the allochthonous Central and Western Domain terranes may have been transported to the Gondwana margin, represented by the para‐autochthonous Eastern Domain, on board the Phoenix plate or on board the South American plate. The variable provenance indicators from the Central and Western terranes can be cited to support either, or a combination, of these scenarios. The convergence direction evident from Phase 2 structures cannot easily be reproduced in regional plate kinematic models. This, and the localized evidence for superposition of Phase 2 structures on Phase 1 structures suggests that Phase 2 is unlikely to have occurred solely in response to changes in the kinematics of the large plates considered.

Implications of incremental emplacement of magma bodies for magma differentiation, thermal aureole dimensions and plutonism–volcanism relationships

Field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets (sills). Emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation, country rock metamorphism and assimilation, and for the formation of large magma chambers in the upper crust. When a magma body begins to grow by slow accretion of sills, each successive intrusion solidifies before the injection of the next one. When the system is thermally mature, sill temperatures equilibrate above the solidus, melts accumulate and older sills can re-melt. The time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body. The transition from a mafic crystal-poor magma to a partially molten rock that retains a highly differentiated melt is fast, whereas the resulting evolved residual melt can reside in the crust for protracted periods. As long as temperatures in the system are relatively low, highly differentiated melts are generated, which may explain the bi-modal character and the absence of intermediate compositions in some magmatic provinces. The level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth. Metamorphism, partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock–magma body boundary. If the magma emplacement rate is low, the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body. Understanding emplacement of magma bodies is fundamental for our understanding of the plutonism–volcanism relationship. Magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions. The time-averaged emplacement rates of plutons are lower than this critical emplacement rate. Eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust. This suggests that the emplacement of magma bodies is a multi-timescale process with the development of large magma chambers corresponding to the highest magma fluxes. Because they control magmatic processes and the impact of magma intrusion on the country rock, future studies should focus on magma emplacement rates and on magma emplacement geometries. These studies should integrate field observation on plutons and geophysical data on active magmatic systems, coupled with laboratory experiments and numerical simulations.

Significance of widespread low effusion rate eruptions over the past two million years for delivery of magma to the overlapping spreading centers at 9°N East Pacific Rise

Ever since the discovery that the mid-ocean ridge axis deepens at most axial discontinuities, there has been vigorous debate about whether ridge axis depth and shape reflect magma supply. Here we report evidence, from one of the most thoroughly studied ridge discontinuities on a fast-spreading mid-ocean ridge, that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history. This pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off-axis discordant zone of the East Pacific Rise (EPR) 9°N Overlapping Spreading Centers (OSC) than on typical EPR seafloor. Detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current OSC basin and ridge tips, rather than off-axis. Pillow lava, erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows, primarily builds mounds on the seafloor. Crustal thickness, a proxy for long-term magma supply, does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions. To resolve the problem of achieving the same magma emplacement rate with lower effusion rates, without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations, we propose that eruptions occur across the entire OSC basin. The current configuration of mid-crustal melt sills within the OSC as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the OSC basin via vertically propagating dikes. Instead, we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid-crustal melt sills outside of the overlap zone.

Revised definition of Large Igneous Provinces (LIPs)

Much has been learned about Large Igneous Provinces (LIPs) and their database greatly expanded since their first formal categorization in the early 1990s. This progress provides an opportunity to review the key characteristics that distinguish LIP events from other melting events of the upper mantle, and to reassess and revise how we define LIPs. A precise definition is important to correctly recognize those LIP events with regional to global effects, and to aid in refining petrogenetic models of the origin of LIPs. We revise the definition of LIPs as follows: “Large Igneous Provinces are magmatic provinces with areal extents > 0.1 Mkm 2, igneous volumes > 0.1 Mkm 3 and maximum lifespans of ∼ 50 Myr that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (∼ 1–5 Myr), during which a large proportion (> 75%) of the total igneous volume has been emplaced.” They are dominantly mafic, but also can have significant ultramafic and silicic components, and some are dominated by silicic magmatism. In this revision, seamounts, seamount groups, submarine ridges and anomalous seafloor crust are no longer considered as LIPs. Although many of these are spatially-related features post-dating a LIP event, they are constructed by long-lived melting anomalies in the mantle at lower emplacement rates, and contrast with the more transient, high magma emplacement rate characteristics of the LIP event. Many LIPs emplaced in both continental and oceanic realms, are split and rifted apart by new ridge spreading centres, which reinforce the link with mid-ocean ridges as a post-LIP event. Three new types of igneous provinces are now included in the LIP inventory, to accommodate the recognition of a greater diversity of igneous compositions, and preserved expressions of LIP events since the Archean: 1) giant diabase/dolerite continental dyke swarm, sill and mafic–ultramafic intrusion-dominated provinces; 2) Silicic LIPs; and 3) tholeiite–komatiite associations, which may be Archean examples of LIPs. A revised global distribution of LIPs for the Phanerozoic is presented. Establishing the full extent of LIPs requires well-constrained plate reconstructions, and at present, plate reconstructions for the Precambrian are poorly known. However, the possibility of reconstructing the LIP record back to and into the Archean and using this expanded LIP record to better constrain the origins and effects of LIPs is an exciting frontier, and our revised definition is a contribution to that effort.

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya) has been modelled as the accretion of successive sills. The leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection, and by a thin (<100 m) upper thermal aureole. These characteristics were used to constrain the maximum magma emplacement rate. Models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period. Additionally, the hypothesis of a tectonic top contact, with unroofing limiting heat transfer during magma emplacement, was evaluated. In this latter case, the upper limit for the emplacement rate was estimated at 3·4 mm/year (or 1·5 Myr for 5 km of granite). Geological and thermobarometric data, however, argue against a major role of fault activity in magma cooling during the leucogranite emplacement. The best model in agreement with available geochronological data suggests an emplacement rate of 1 mm/year for a relatively shallow level of emplacement (granite top at 10 km), uninterrupted by a long repose period. The thermal aureole temperature and thickness, and the isotopic heterogeneities within the leucogranite, can be explained by the accretion of 20–60 m thick sills intruded every 20 000–60 000 years over a period of 5 Myr. Under such conditions, the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites.

CO 2 fluxes from mid-ocean ridges, arcs and plumes

Estimates of CO 2 emissions at spreading centres, convergent margins, and plumes have been reviewed and upgraded using observed CO 2/ 3He ratios in magmatic volatiles, 3He content estimates in the magmatic sources, and magma emplacement rates in the different tectonic settings. The effect of volatile fractionation during magma degassing, investigated using new rare gas and CO 2 abundances determined simultaneously for a suite of Mid-Ocean Ridge (MOR) basalt glasses, is not the major factor controlling the spread of data, which mainly result from volatile heterogeneity in the mantle source. The computed C flux at ridges (2.2±0.9)×10 12 mol/a, is essentially similar to previous estimates based on a more restricted data base. Variation of the C flux in the past can be simply scaled to that of spreading rate since the computed C depends mainly on the volatile content of the mantle source, which can be considered constant during the last 10 8 a. The flux of CO 2 from arcs may be approximated using the CO 2/ 3He ratios of volcanic gases at arcs and the magma emplacement rate, assuming that the 3He content of the mantle end-member is that of the MORB source. The resulting flux is ∼2.5×10 12 mol/a, with approx. 80% of carbon being derived from the subducting plate. The flux of CO 2 from plumes, based on time-averaged magma production rates and on estimated contributions of geochemical sources to plume magmatism, is ≤3×10 12 mol/a. Significant enhancements of the CO 2 flux from plumes might have occurred in the past during giant magma emplacements, depending on the duration of these events, although the time-integrated effect does not appear important. The global magmatic flux of CO 2 into the atmosphere and the hydrosphere is found to be 6×10 12 mol/a, with a range of (4–10)×10 12 mol/a. Improvement on the precision of this estimate is linked to a better understanding of the volatile inventory at arcs on one hand, and on the dynamics of plumes and their mantle source contribution on the other hand.

The correlation between geomagnetic field reversals, Hawaiian volcanism, and the motion of the Pacific plate

The correlation between geomagnetic field reversals and volcanism is investigated, according to the speculated consequence on volcanoes of the transient electric currents in the geodynamo, through Joule's heating, before and after every reversal event. We evaluate the temporal variation during the last ~ 70 Ma both of the magma emplacement rate Q(t) from the Hawaii hot spot, and of the speed v(t) of the Pacific plate, by means of the observed volumes of islands and seamounts along the Hawaii/Emperor Seamounts chain, and their respective radiometric datings. Results confirm expectations. A justification of the volcanic crises that lead to the generation of the large igneous provinces during the last ~ 250 Ma also emerged. We describe in detail the complex pattern of the timings of the different effects. Joule's power is generally responsible for ~ 75-80% of magmatism, and friction power only for ~ 20-25%; but, on some occasions almost ~ 100% is fuelled by friction alone. The visco-elastic coupling between lithosphere and asthenosphere results ~ 96% viscous, and ~ 4% elastic.