Olivine geospeedometry from non-convergent M-site ordering

The mineral olivine - (Fe,Mg,Mn)2SiO4 - is the dominant phase in the Earth's upper mantle, and is also present in a wide range of igneous rocks. Metal cations in olivine crystals are partitioned between two structurally distinct octahedral sites, a property which could in principle be used to obtain important information regarding the thermal history of the host rock. But attempts to establish the temperature and pressure dependence of cation ordering, mainly from the room-temperature structures of samples that have been annealed and quenched1-3, have yielded contradictory information. In fact, recent studies have shown that considerable re-ordering occurs during the quenching process4,5, and thus cation ordering is unlikely to be representative of high- temperature equilibration. Here we present a new model of the thermodynamics and kinetics of metal partitioning in olivine, derived from in situ neutron-diffraction measurements of cation ordering in the synthetic olivine (Fe0.5Mn0.5)2SiO4. Our results suggest that the room-temperature structure of a quenched olivine reflects the rate at which the mineral cooled. The extension of this approach to common rock-forming olivines should provide a valuable 'geospeedometer' for determining the cooling rates of rocks that have cooled relatively rapidly

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

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 chambers[14]