Chemical differentiation, cold storage and remobilization of magma in the Earth’s crust

Abstract

The formation, storage and chemical differentiation of magma in the Earth’s crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high-melt-fraction ‘magma chamber’ paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low-melt-fraction ‘mush reservoirs’1,2,3,4,5,6,7,8,9. A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1,10. However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts1,11. Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, caused by buoyant melt percolating upwards through, and reacting with, the crystals10. Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface11,12,13. These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data6,7,8,9. Changes in local bulk composition caused by reactive melt flow, rather than large increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool- or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid- to lower-crustal reservoirs14,15. Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface16. We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallization in magma chambers14.

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Fig. 1: Temperature, melt fraction, bulk composition and incompatible trace-element concentration as a function of depth during the growing and active phases of the reservoir.
Fig. 2: Cold storage and rapid remobilization of magma.
Fig. 3: Geochemical consequences of reactive melt flow in crustal magma reservoirs.
Fig. 4: Reactive flow of buoyant melt at low melt fraction.

Data availability

No original data are reported that were not created using the software code (MUSHREACT). Data can be recreated using the code.

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Acknowledgements

M.D.J. and J.B. acknowledge funding from NERC Grant NE/P017452/1 “From arc magmas to ores (FAMOS): A mineral systems approach”. This paper is FAMOS contribution F05. M.D.J. also acknowledges sabbatical support from the Department of Earth Science and Engineering, Imperial College London, during which part of the research reported here was undertaken. R.S.J.S. acknowledges support from a Leverhulme Trust Emeritus Fellowship.

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Nature thanks O. Bachmann, C. Till and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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M.D.J. wrote the code and produced the numerical results. J.B. prepared the phase equilibria model and calibrated this to experimental data. R.S.J.S. provided information on context and background for the study. All authors jointly designed the numerical experiments presented and drafted the manuscript text. M.D.J. prepared the figures.

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Correspondence to M. D. Jackson.

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Extended data figures and tables

Extended Data Fig. 1 Phase behaviour and compositions of the modelled system.

a, Static melt fraction versus temperature for the modelled basalt and crust, extracted from the binary phase diagram for the chosen initial bulk compositions. Also shown are experimental equilibrium melting/crystallization data for metagreywackes and basalt over the pressure range12,73,74 400–900 MPa. Triangles denote data from ref. 73; circles denote data from ref. 74; squares denote data from ref. 12. Static melt fraction denotes the melt fraction obtained if there is no relative motion of melt and crystals, so the bulk composition remains constant. b, SiO2 content versus temperature modelled here. Also shown are experimental data corresponding to those shown in a. c, Melt fraction versus temperature obtained from the numerical model (data extracted from Supplementary Video 1 at three snapshots in time (0.97 Myr, 1.39 Myr and 1.66 Myr after the onset of sill intrusions) corresponding to Fig. 1 and Extended Data Fig. 3b. Reactive flow in the mush decouples temperature and melt fraction: high melt fraction can be found at low temperatures where reactive melt flow has caused the bulk composition of the mush to evolve and vice versa.

Extended Data Fig. 2 Maximum melt fraction as a function of time.

a, Following a single sill intrusion during the incubation period. b, Over the life of the reservoir. In a, the sill cools rapidly, with the melt fraction falling below 0.7 (that is, the crystallinity exceeding 30%) within 63 years after intrusion, and the sill solidifying within 225 years. The sharp decrease in melt fraction before full solidification is physical and represents the arrival of the solidification front during crystallization at the eutectic. In b, during the ‘incubation phase’, maximum melt fraction spikes after each sill intrusion, but decreases rapidly and falls to zero between sill intrusions. The incubation phase ends when the melt fraction remains greater than zero between sill intrusions. During the ‘growing phase’, the maximum melt fraction at the top of the mush reservoir increases in response to the reactive flow of buoyant melt. Spikes in melt fraction correspond to ongoing sill intrusions deeper in the reservoir. Melt fraction at the top of the mush increases until, during the ‘active phase’, evolved, low-crystallinity (<30%) magma is present, which is likely to leave rapidly and ascend to shallower crustal levels. New sill intrusions cease and, some time later, the melt fraction at the top of the mush also starts to decrease. Overall, the reservoir is cooling. This is the ‘waning phase’, at the end of which the reservoir has completely solidified. Data in both plots were extracted from Supplementary Video 1.

Extended Data Fig. 3 Snapshots showing temperature, melt fraction, bulk composition and incompatible trace-element concentration as a function of depth through a crustal section at 18 km during the incubation and waning phases of the reservoir.

a, 0.82 Myr after the onset of sill intrusions. b, 1.66 Myr after the onset of sill intrusions. Snapshots are taken from Supplementary Video 1. At early times, during the incubation phase (a), individual sills cool rapidly. During the growing phase (not shown here; see Fig. 1a), a persistent magma reservoir forms but the melt fraction is low and relatively uniform. However, buoyant melt migrates upwards and begins to accumulate at the top of the reservoir. During the active phase (not shown here; see Fig. 1b), a high-melt-fraction layer forms. At late times, during the waning phase (b), sill intrusions cease and the mush cools and solidifies. The shaded areas in all plots denote the vertical extent of basalt intrusion at that time. Equivalent results for intrusions at 10 km depth are shown in Extended Data Fig. 4.

Extended Data Fig. 4 Snapshots showing temperature, melt fraction, bulk composition and incompatible trace-element concentration as a function of depth through a crustal section at 10 km depth during the incubation and waning phases.

a, 0.82 Myr after the onset of sill intrusions. b, 1.66 Myr after the onset of sill intrusions. Snapshots are taken from Supplementary Video 2. The results are qualitatively very similar to those obtained at 18 km depth (Extended Data Fig. 3). During the incubation phase (a), individual sills cool rapidly. During the waning phase (b), sill intrusions cease and the mush cools and solidifies. The shaded areas in all plots denote the vertical extent of basalt intrusion at that time.

Extended Data Fig. 5 Snapshots showing temperature, melt fraction, bulk composition and incompatible trace element concentration as a function of depth through a crustal section at 10 km depth during the growing and active phases.

a, 0.99 Myr after the onset of sill intrusions. b, 1.39 Myr after the onset of sill intrusions. Snapshots are taken from Supplementary Video 2. The results are qualitatively very similar to those obtained at 18 km depth (Fig. 1). During the growing phase (a), a persistent mush reservoir forms but the melt fraction is low. Buoyant melt migrates upwards and begins to accumulate at the top of the reservoir. During the active phase (b), the accumulating melt forms a high-melt-fraction layer containing mobile magma. The composition of the melt in the layer is evolved and enriched in incompatible trace elements. Elsewhere in the mush, the melt fraction remains low. The shaded areas in all plots denote the vertical extent of basalt intrusions at that time.

Extended Data Fig. 6 Cold storage and rapid remobilization of magma in a reservoir at 10 km depth.

Results are qualitatively very similar to those obtained at 18 km depth (Fig. 2). a, Melt fraction as a function of depth at the first snapshot after remobilization at 10 km (1.441 Myr after the onset of sill intrusions). The shaded area denotes intruded basalt. The reactive flow of buoyant melt produces a high-melt-fraction layer that migrates upwards. b, Temperature and melt fraction as a function of time at a depth of 10 km. Similar results are obtained over the depth range 10–10.5 km. Early sill intrusions rapidly cool and crystallize. The crystals are kept in ‘cold storage’ at sub-solidus temperature, but the temperature gradually increases in response to sill intrusions deeper in the reservoir. Soon (<0.3 kyr) after the temperature exceeds the solidus, the high-melt-fraction layer arrives at this depth and the reservoir is remobilized: the melt fraction increases rapidly to form a low-crystallinity magma. The melt fraction increases much more rapidly and to a higher value than would be possible by melting alone. c, Temperature and melt fraction as a function of time at a depth of 12 km. Similar results are obtained over the depth range 10.5–15 km. Melt fraction remains low because reactive flow has left a refractory residue at this depth. There is no remobilization, despite the increase in temperature. Data were extracted from Supplementary Video 2.

Extended Data Fig. 7 Geochemical consequences of reactive melt flow in crustal magma reservoirs at 10 km depth.

a, Intrusion of mafic sills; b, intrusion of intermediate sills. Both plots show SiO2 content of low-crystallinity (crystal fraction <30%) magmas. Solid curves show bulk magma composition (melt plus crystals); dashed curves show melt composition alone. The peak at low SiO2 corresponds to magma within the intruding sills; the peak at high SiO2 corresponds to magma within high-melt-fraction layers near the top of the reservoir. In a, measured data from the Snake River Plain (SRP) are shown for comparison29; the bimodality is clear although the basalt has a lower SiO2 content than modelled here. Bimodal compositions correspond to (1) the magma intruded into the reservoir, and (2) the most evolved composition obtained by differentiation.

Extended Data Fig. 8 Cool storage and rapid remobilization of magma in a reservoir created by intrusion of intermediate magma at 10 km depth.

Results are qualitatively similar to those obtained by intruding basalt magma. a, Melt fraction as a function of depth at the first snapshot after remobilization at a depth of 11.4 km (1.28 Myr after the onset of sill intrusions). Reactive flow of evolved, buoyant melt produces a high-melt-fraction layer that migrates upwards. b, Temperature and melt fraction as a function of time at a depth of 11.4 km. Early sill intrusions rapidly cool and crystallize. The crystals are kept in ‘cool storage’ at near-solidus temperature. At 1.28 Myr, the high-melt-fraction layer arrives at this depth and the reservoir is remobilized: the melt fraction increases rapidly to form a low-crystallinity magma. The melt fraction increases much more rapidly and to a higher value than would be possible by melting alone. Melt fraction deeper in the reservoir remains low because reactive flow has left a refractory residue at this depth.

Extended Data Fig. 9 Consequences of emplacement during under- and over-accretion.

a, Melt fraction as a function of depth during under-accretion, at the first snapshot after remobilization at a depth of 22 km (1.02 Myr after the onset of sill intrusions). Reactive flow of evolved, buoyant melt produces a high-melt-fraction layer that migrates upwards. b, Temperature and melt fraction as a function of time at a depth of 22 km during under-accretion. Similar results are obtained over the depth range 22–22.5 km. Under-accretion causes the sill intrusion depth to increase progressively from 18 km; in this case, an intrusion at 22 km occurs at 0.75 Myr ago that rapidly cools and crystallizes. The crystals are kept in ‘cool storage’ at a close-to-solidus temperature. At 1.02 Myr the high-melt-fraction layer arrives at this depth and the reservoir is remobilized. c, Melt fraction as a function of depth during over-accretion, at a snapshot in time (1.53 Myr after the onset of sill intrusions). In this case, the high-melt-fraction layer has migrated into the overlying country rock. d, Temperature and melt fraction as a function of time at a depth of 17.5 km during over-accretion, close to the top of the active magma reservoir. Similar results are obtained over the depth range 17.5–18 km. Crystals in the magma are sourced from the country rock and may be genetically unrelated to the melt. There is no cold storage of crystals brought into the reservoir by basaltic sill intrusions, as intrusion occurs deeper in the reservoir. In a and c, the shaded area denotes intruded basalt.

Extended Data Fig. 10 Sensitivity analysis.

a, A frequency plot showing values of the dimensionless scaling factor κ calculated using equation (12). Values of the input values were varied uniformly over the range given in Extended Data Table 1 in a simple Monte Carlo analysis89. b, Incubation and activation time; c, Cold storage time and eruptible magma composition. Error bars and shaded regions in b and c denote the effect of varying the dimensionless scaling factor κ over the range 0.028 < κ < 2,160. Error bars on the incubation time are within the symbol size. Dashed lines denote the fit to the incubation time of the form q−2, where q is the intrusion rate. Colours in b and c denote different initial emplacement depths of 10 km, 18 km and 30 km. Models were run for a maximum 20 km of intruded basalt.

Extended Data Table 1 Parameters used in the numerical experiments

Supplementary information

41586_2018_746_MOESM1_ESM.mp4

The incubation phase of the reservoir occupies the first 0.94 Myr: the melt fraction falls to zero after each sill intrusion (Extended Data Fig. 3). The growing phase lasts from 0.94 Myr ago to 1.08 Myr ago: the melt fraction remains greater than zero between sill intrusions and melt migrates upwards. The active phase lasts from 1.08 Myr ago to 1.66 Myr ago: a layer of evolved, eruptible magma has accumulated. The waning phase completes the reservoir life, ending at 1.7 Myr ago.

41586_2018_746_MOESM2_ESM.mp4

The incubation phase of the reservoir occupies the first 0.86 Myr: the melt fraction falls to zero after each sill intrusion. The growing phase lasts from 0.94 Myr ago to 1 Myr ago: the melt fraction remains greater than zero between sill intrusions and melt migrates upwards. The active phase lasts from 1 Myr ago to 1.66 Myr ago: a layer of evolved, eruptible magma has accumulated. The waning phase completes the reservoir life, ending at 1.8 Myr ago.

Video 1: Time evolution of temperature, melt fraction, bulk composition and incompatible trace element concentration as a function of depth for a magma reservoir at 18 km depth.

The incubation phase of the reservoir occupies the first 0.94 Myr: the melt fraction falls to zero after each sill intrusion (Extended Data Fig. 3). The growing phase lasts from 0.94 Myr ago to 1.08 Myr ago: the melt fraction remains greater than zero between sill intrusions and melt migrates upwards. The active phase lasts from 1.08 Myr ago to 1.66 Myr ago: a layer of evolved, eruptible magma has accumulated. The waning phase completes the reservoir life, ending at 1.7 Myr ago.

Video 2: Time evolution of temperature, melt fraction, bulk composition and incompatible trace element concentration as a function of depth for a magma reservoir at 10 km depth.

The incubation phase of the reservoir occupies the first 0.86 Myr: the melt fraction falls to zero after each sill intrusion. The growing phase lasts from 0.94 Myr ago to 1 Myr ago: the melt fraction remains greater than zero between sill intrusions and melt migrates upwards. The active phase lasts from 1 Myr ago to 1.66 Myr ago: a layer of evolved, eruptible magma has accumulated. The waning phase completes the reservoir life, ending at 1.8 Myr ago.

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Jackson, M.D., Blundy, J. & Sparks, R.S.J. Chemical differentiation, cold storage and remobilization of magma in the Earth’s crust. Nature 564, 405–409 (2018). https://doi.org/10.1038/s41586-018-0746-2

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Keywords

  • Local Bulk Composition
  • Melt Fraction
  • Buoyant Melt
  • Sill Intrusions
  • Intrusion Depth

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