Making magma crystal clear

Precious metal deposits, volcanic super-eruptions and even mass extinction events – detailed examination of magma can tell us so much about the Earth’s dramatic past. And how can such detail be determined? Professor Urs Schaltegger thinks the answer is gleaming away beneath our feet….     Rocks are molten at high temperatures, and large reservoirs containing liquid magma are formed inside the Earth, deep in the crust. Depending on various factors, magma can crystallise at depth, forming intrusions (which are also referred to as "plutons"), or it can migrate to the surface, where it erupts, forming a volcano. Furthermore, it can also act as a source for large ore deposits, including gold and copper. The volume of such a magmatic reservoir, as well as its flux (the rate of migration of the magma volume through the crust) are the two dominant parameters which determine whether a system has the potential to generate a violent volcanic eruption, form a giant ore deposit, or whether the magma will completely solidify during its ascent through 25 kilometres of colder crust, and thus not reach the surface. The volumes of magma systems hidden at depth, as well as their integrated magma fluxes in the past cannot be determined directly. However, my team of researchers at the University of Geneva have developed a novel approach to precisely estimate what the volumes of these fossilised magma systems may be¹. Many volcanic and plutonic rocks contain a mineral called zircon, which is a small accessory mineral phase (a few 100 micrometres long), and hosts small amounts of uranium in its crystal lattice. Some uranium isotopes (²³⁸U or ²³⁵U) are unstable, and transform into stable lead isotopes (²⁰⁶Pb or ²⁰⁷Pb) at known rates, forming the physical basis for a radiometric clock, a "geochronometer". Measurements of the abundance of these isotopes are made in a dust-free clean laboratory using micro-analytical techniques, enabling precise isotopic analysis of picograms (10^-12 grams) of lead and nanograms (10^-9 g) of uranium, using thermal ionisation mass spectrometry. U-Pb geochronology is the most precise dating tool that can be applied to any mineral that contains a sufficient amount of uranium. Among these minerals are phosphates, silicates and oxides, but the silicate zircon (ZrSiO₄) is the most suitable candidate because of its high chemical and mechanical resistivity, and thus resistance to change during geological transformations. Ages of distinct regions of a single crystal may be obtained using highly spatially resolved sampling techniques, using focussed ion beams or a laser. These ages can record protracted and sometimes complex crystallisation histories, which collectively formed the single mineral grain over millions of years. The highest-precision age information is obtained by a technique called "isotope dilution", which uses synthetic mono-isotopic tracer solutions to obtain measurements of isotope concentrations with the best precision. The procedure requires that complete, single grains of zircon are dissolved, and the uranium and lead isotopes are isolated and extracted for analysis. The isotope dilution technique is the only known method that is sufficiently precise to resolve age differences of a few 10,000 years within a magma that crystallised approximately 40 million years ago. [caption id="attachment_39230" align="alignright" width="200"] Scanning electron microscope image of a zircon. The length of the crystal is about the same as the diameter of a human hair (100 micrometers).Credit: U. Schaltegger, J.F. Wotzlaw, R. Martini, A. Martignier[/caption] The assemblage of zircon crystals that can be extracted from a plutonic rock do not form instantly, at the same moment. Rather, zircon crystallises over a prolonged period of time characterised by cooling of the magma through temperatures 750 and 700°C, if it is saturated in silica and zirconium. Consequently, the analysis of many single zircon crystals from the same rock sample may yield a different age for each crystal if your analyses are sufficiently precise. However, the number of zircons crystallising in a given volume of magma is not constant. Fewer zircons crystallise per unit time when the temperatures are elevated (due to, for example, the influx of new, hot magma). A larger quantity of crystals will form during the subsequent temperature decrease. Therefore, a frequency distribution of the number of zircons, with respect to time, is considered as an inverse "fever curve" of the magma system. Numerical modelling of zircon crystallisation in a given magma was carried out by researchers at the University of Geneva, Switzerland, after taking varying parameters, such as depth and thermal gradient, into account. The models demonstrate that the shape of the abundance distribution curves depends on the total volume of magma, and on the magma flux. If the size and flux are small, the magma cools and crystallises readily at 20-10 km depth, forming plutons in the middle part of the crust. Examples of this scenario are the Adamello Massif in the northern Italian Alps, and the granites of the Torres del Paine in Patagonia (Chile). If the volume and the flux are sufficiently large, a considerable portion of the crust gets heated up. Hot incoming magma from the mantle is added to the magma system and maintains high temperatures over several ten to hundred thousands of years, eventually leading to the presence of a large body of buoyant magma in the crust. Such conditions are favourable for giant volcanic eruptions ("super-eruptions"), such as the 5000 cubic kilometre large ash fall deposit of the Fish Canyon Tuff (Colorado), which is 28.2 million years old, and the 770,000 year old Bishop Tuff in California (United States). The numerical model also shows that the formation of giant copper or gold deposits, such as the  world’s largest copper mine in the porphyry of Bingham Canyon close to Salt Lake City (Utah, USA), requires certain, well defined conditions, which include large magma volumes but considerably lower magma fluxes compared to the previous examples. This newly developed technique may lead to a more quantitative and probabilistic understanding of magmatic processes in the Earth's crust. The results of uranium-lead age determinations of zircon from a sample of an active volcano may help to establish estimates of the hazardous potential of this magma system. Is the magma system large enough, and is the reconstructed magma flux sufficiently high to render this volcano potentially dangerous? However, more concrete predictions, especially of the timing and frequency of volcanic activity, cannot be made with this method. The new approach may also have the potential for more quantitative exploration for metal commodities. The model may be capable of predicting whether a magmatic system fulfils the size and flux requirements to have the potential to form a giant ore deposit, justifying further exploration. Volcanic super-eruptions were recurrent events in the geological past, which not only re-shaped the surface of the Earth, but also had a dramatic impact on the environment and life. Super-eruptions include volcanic events such as the 1883 eruption of Krakatoa, or the explosive eruption of Toba 74,000 years B.C., both of which were situated in Indonesia. However, geological studies have shown that the boundaries between geological periods (e.g. the boundary between the Triassic and Jurassic, or Cretaceous and Paleogene) are marked by dramatic changes in biodiversity, which are often referred to as "mass extinctions". In most cases, scientific work has demonstrated that these biotic events were caused by giant catastrophic volcanic eruptions of so-called "Large Igneous Provinces", which were several orders of magnitude larger than the previously mentioned examples. High-precision uranium-lead dating of zircon in volcanic ash beds from fossil-bearing, marine sediments can be used to make precise temporal correlations between biotic events and volcanism. The massive injection of volcanic gases (SO₂, CO₂) in the atmosphere, during 500,000 years of basalt eruption in the Siberian Traps, 252 million years ago, deteriorated the global climate and led to the extinction of up to 96% of all marine species. Similarly, volcanic activity of the Deccan Traps in India led to the extinction of 73% of all species on the Earth, including the dinosaurs. This last event may have occurred in combination with the impact of a meteorite within the same period of time. Tiny zircon crystals thus provide us with an opportunity to reconstruct dramatic climatic changes in the past, and relate them to volcanic super-eruptions. Author Professor U. Schaltegger, Président de la Section des Sciences de la Terre et de l’Environnement, Commission de Recherche, Université de Genève References

1. Caricchi, Simpson and Schaltegger, Nature 511, 457-461, 2014