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

• Published in: Nature (London) 2018-12, Vol.564 (7736), p.405-409
• Main Authors: Jackson, M. D., Blundy, J., Sparks, R. S. J.
• Format: Article
• Language: English
• Subjects: 704/2151/209; 704/2151/2809; 704/2151/431; 704/2151/598; Cold flow; Cold storage; Composition; Crust (Geology); Crystallization; Crystals; Differentiation; Earth; Earth crust; Flow; Fractional crystallization; Geochemical cycles; Geological research; Geology; Humanities and Social Sciences; Lava; Letter; Magma; Magmatic differentiation; multidisciplinary; Natural history; Organic chemistry; Percolation; Permeability; Reservoirs; Science; Science (multidisciplinary); Solidus; Trace elements; Volcanology
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-018-0746-2

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 – 9 . A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space 1 , 10 . However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts 1 , 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 crystals 10 . Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface 11 – 13 . These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data 6 – 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 reservoirs 14 , 15 . Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface 16 . 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 chambers 14 . Magma storage and differentiation in the Earth’s crust mainly occurs by reactive melt flow in long-lived mush reservoirs, rather than by fractional crystallization in magma chambers, as previously thought.