Rates of Magma Transfer in the Crust: Insights Into Magma Reservoir Recharge and Pluton Growth Thierry Menand, Catherine Annen, Michel De Saint Blanquat
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Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth Thierry Menand, Catherine Annen, Michel de Saint Blanquat To cite this version: Thierry Menand, Catherine Annen, Michel de Saint Blanquat. Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth. Geology, Geological Society of America, 2015, 43 (3), pp.199-202. 10.1130/G36224.1. hal-03025421 HAL Id: hal-03025421 https://hal.archives-ouvertes.fr/hal-03025421 Submitted on 26 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth Thierry Menand1, Catherine Annen2, and Michel de Saint Blanquat3 1Laboratoire Magmas et Volcans, Université Blaise Pascal–CNRS–IRD, OPGC, 5 rue Kessler, 63038 Clermont-Ferrand, France 2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK 3Geosciences Environnement Toulouse, Université de Toulouse, CNRS, IRD, OMP, 14 avenue Edouard-Belin, 31400 Toulouse, France ABSTRACT Horsman et al., 2009; Leuthold et al., 2012). It is in the upper crust that Plutons have long been viewed as crystallized remnants of large magmas acquire their textural diversity owing to shallow-level crystalliza- magma reservoirs, a concept now challenged by high-precision geo- tion. Incremental growth of plutons is widely accepted, and the issue is chronological data coupled with thermal models. Similarly, the clas- now to identify the incremental processes involved and to quantify their sical view of silicic eruptions fed by long-lived magma reservoirs that rates. The recharge of volcano reservoirs and the growth of plutons seem slowly differentiate between mafic recharges is being questioned by intimately related: in both cases, the key point is the rate of magma trans- petrological and geophysical studies. In both cases, a key and yet fer between a source and a shallower storage level. unresolved issue is the rate of magma transfer in the crust. Here, we use thermal analysis of magma transport to calculate the minimum THERMAL ANALYSIS rate of magma transfer through dikes. We find that unless the crust Magma transport in the crust occurs mainly through dikes (Lister and is exceptionally hot, the recharge of magma reservoirs requires a Kerr, 1991; Clemens and Mawer, 1992; Petford et al., 1993), but there is magma supply rate of at least ~0.01 km3/yr, much higher than the currently no three-dimensional, time-dependent propagation model. We long-term growth rate of plutons, which demonstrates unequivocally therefore focused our analysis on the steady-state propagation of buoyant that igneous bodies must grow incrementally. This analysis argues dikes, and performed a stochastic thermal analysis to quantify the mini- also that magma reservoirs are short lived and erupt rapidly after mum volumetric flow rates that are required for dikes to transport magma being recharged by already-differentiated magma. These findings through Earth’s crust. have significant implications for the monitoring of dormant volcanic Dikes need to propagate fast enough through the crust to avoid systems and our ability to interpret geodetic surface signals related to death by solidification (Spence and Turcotte, 1985; Rubin, 1995). Flow- incipient eruptions. ing magma advects heat along dikes while heat is conducted away by the colder country rocks. If advection is slower than conduction, then INTRODUCTION the magma will solidify and dike propagation will cease. A buoyant dike Petrological and geophysical studies challenge the long-held view intruding rocks of fracture toughness Kc over some height H must thus be that silicic magma reservoirs are long lived, differentiating slowly between fed with a critical minimum magma supply rate, Qc (details are provided mafic recharges. For example, until recently, Santorini volcano, Greece, in the GSA Data Repository1): was understood as a volcano whose shallow dacitic reservoir is regularly 9 1 2 recharged by small mafic spurts (Martin et al., 2008). However, crystal dif- 2 4 33 4 3 9 CTpf()− T∞ ∆ρκgH K fusion chronometry and surface deformation data reveal a different story. Q = c , (1) c 32 LT T g In the case of the ca. 1600 B.C. Minoan caldera-forming eruption, reactiva- ()0f− µ ∆ρ tion was fast (<100 yr) with a transient recharge flux of >0.05 km3/yr, more than 50 times the long-term magma influx that feeds the volcano (Druitt where L is magma latent heat, Cp is its specific heat capacity, T0, Tf, and T∞ et al., 2012). Crucially, reactivation represented a sizable proportion of the are initial magma temperature, freezing temperature, and crustal far-field shallow reservoir, and involved an already-differentiated magma of deep temperature, respectively, m is magma dynamic viscosity, k is its ther- origin (Druitt et al., 2012). Similar conclusions are reached for Santorini’s mal diffusivity, Dr is the density difference between country rocks and recent activity (Parks et al., 2012). The emerging view is thus of a volcano magma, and g is gravitational acceleration. This is the minimum influx of whose shallow reservoir is regularly replenished from depth by high-flux magma a buoyant dike needs to propagate over a height H without freez- batches of already-differentiated magma, the duration of which is much ing. A growing pluton would need to be fed with at least this influx of shorter than that of intervening repose periods. These findings raise key but magma Qc, which would thus represent a short-term magma supply rate, unresolved questions: What determines the recharge rate of shallow reser- otherwise its feeder dike would freeze before it could reach this emplace- voirs? And ultimately, can we identify recharges that will lead to an erup- ment level. The same goes for the recharge of an existing reservoir lying a tion? These are crucial issues in volcanology, and not simply at Santorini. height H above a deeper magma source; Qc would then represent a short- These issues also echo that of the rates at which pluton assembly term magma recharge or replenishment rate. occurs (Petford et al., 2000; Cruden and McCaffrey, 2001; Glazner et al., 2004; de Saint Blanquat et al., 2011). Indeed, the example of Santorini RESULTS fits with models for the generation of evolved magmas in deep crustal hot We used Monte Carlo simulations to calculate the expected range of zones (Annen et al., 2006) and the incremental emplacement and growth critical rates for magmas with viscosities between 10 and 108 Pa⋅s. Mag- of igneous bodies in the upper crust (Menand et al., 2011). According to matic and country-rock parameters were sampled randomly, and three dif- the deep-hot-zone model (Annen et al., 2006), the chemical diversity of ferent ranges of far-field temperatures T∞ were tested: cold, intermediate, arc magmas and granites is generated in the lower crust. After leaving their or hot crusts (Table 1). In each case, T∞ was kept constant throughout the deep hot zones, the vast majority of magmas stall below the surface, many of them as sills and commonly amalgamating together to form larger plu- 1GSA Data Repository item 2015072, supplemental information, is avail- tons as evidenced by geological, geophysical, and geochronological data able online at www .geosociety .org /pubs/ft2015.htm, or on request from editing@ (e.g., Hawkes and Hawkes, 1933; Cruden, 1998, 2006; Benn et al., 1999; geosociety .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO de Saint Blanquat et al., 2001; Al-Kindi et al., 2003; Michel et al., 2008; 80301, USA. GEOLOGY, March 2015; v. 43; no. 3; p. 1–4; Data Repository item 2015072 | doi:10.1130/G36224.1 | Published online XX Month 2014 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 1 TABLE 1. PARAMETER RANGES AND DISTRIBUTIONS USED IN MONTE 106 /yr) H = 5 km CARLO SIMULATIONS 3 105 T∞ = 200 – 400 °C Parameter Distribution Min Max Mean 4 Magma viscosity (Pa·s) Uniform 10 108 – 10 a,b,c 5 5 Latent heat, L (J/kg) Uniform 2.5 × 10 5.5 × 10 – 3 a,d 10 Specific heat, Cp (J/kg K) Uniform 800 1600 – 2 e −6 −6 Thermal diffusivity, κ (m /s) Uniform 0.3 × 10 2.0 × 10 – 102 1/2 f,g,h 6 9 7.5 Fracture toughness, Kc (Pa m ) Log-normal 10 10 10 Density contrast, ∆ρ (kg/m3)i Uniform 100 700 – 101 Magma temperature, T (°C)j Uniform 750 1250 – 0 0 Freezing temperature, 10 T = T – ∆T. ∆T (°C)k,l Uniform 20 200 – f 0 i i 10-1 Far field temperature, T∞ (°C): Cold crust Uniform 50 200 – A 10-2 Intermediate crust Uniform 200 400 – 1 2 3 4 5 6 7 8 Critical magma supply rate (km 10 10 10 10 10 10 10 10 Hot crust Uniform 400 600 – Magma viscosity (Pa⋅s) Note: The (Maximum – Minimum) range of the log(Kc) normal distribution was equated with six standard deviations. References: a—Bohrson and Spera (2001); 1500 H = 5 km b—Kojitani and Akaogi (1994); c—Lange et al. (1994); d—Robertson (1988); B T∞ = 200 – 400 °C e—Whittington et al. (2009); f—Atkinson (1984); g—Delaney and Pollard (1981); µ = 10 4 Pa⋅s h—Heimpel and Olson (1994); i—Spera (2000); j—Mysen (1981); k—Piwinskii and Wyllie (1968); l—Wagner et al.