Mélanie Barboni

Volcanic–plutonic parity and the differentiation of the continental crust

The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks—volcanic and plutonic rocks—and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.

Warm storage for arc magmas

Significance The increasingly popular notion that steady-state magma chambers are highly crystallized, and thus only capable of erupting during brief (<1 ka) reheatings, implies that melt detection beneath volcanoes warns of imminent eruption. By integrating the microgeochronology and geochemistry of zircons from lavas with those from components crystallized within the magma chamber and incorporated during eruption, we show that the Soufrière (Saint Lucia) volcanic reservoir was instead eruptible over long (>100 ka) timescales. Together with data from other volcanic complexes, we show that arc magmas may generally be stored warm (are able to erupt for >100 ka). Thus geophysical detection of melt beneath volcanoes represents the normal state of magma storage and holds little potential as an indicator of volcanic hazard. Felsic magmatic systems represent the vast majority of volcanic activity that poses a threat to human life. The tempo and magnitude of these eruptions depends on the physical conditions under which magmas are retained within the crust. Recently the case has been made that volcanic reservoirs are rarely molten and only capable of eruption for durations as brief as 1,000 years following magma recharge. If the “cold storage” model is generally applicable, then geophysical detection of melt beneath volcanoes is likely a sign of imminent eruption. However, some arc volcanic centers have been active for tens of thousands of years and show evidence for the continual presence of melt. To address this seeming paradox, zircon geochronology and geochemistry from both the frozen lava and the cogenetic enclaves they host from the Soufrière Volcanic Center (SVC), a long-lived volcanic complex in the Lesser Antilles arc, were integrated to track the preeruptive thermal and chemical history of the magma reservoir. Our results show that the SVC reservoir was likely eruptible for periods of several tens of thousands of years or more with punctuated eruptions during these periods. These conclusions are consistent with results from other arc volcanic reservoirs and suggest that arc magmas are generally stored warm. Thus, the presence of intracrustal melt alone is insufficient as an indicator of imminent eruption, but instead represents the normal state of magma storage underneath dormant volcanoes.

Early formation of the Moon 4.51 billion years ago

Data on lunar zircons require the formation of the Moon by 4.51 Gy, therefore within the first 60 My of the solar system. Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system.

Zircon age-temperature-compositional spectra in plutonic rocks

Geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (Tsat) and rock solidification (Tsolid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at Tsat decreasing continuously to volumetrically minor young, cold crystallization at Tsolid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional change over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. These findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record. INTRODUCTION Zircon is an inimitable chronicler of the temporal, thermal, and compositional evolution of many crustal magmatic systems. U-Pb zircon geochronology by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS; Mattinson, 2005) is the most robust method available for constraining the tempo of pluton assembly and the longevity of magma reservoirs in deep time, having been applied to discern the incremental nature of composite intrusive suites (Coleman et al., 2004). As a result of steadily improved analytical precision, CA-ID-TIMS can resolve heterogeneous U-Pb zircon dates, or zircon spectra, in many igneous rocks at the hand-sample scale (Miller et al., 2007; Schoene et al., 2012; Broderick et al., 2015). Such studies demonstrate that zircon age distributions can reflect protracted zircon crystallization time scales, supplanting convention that zircon retains a singular emplacement age of its host rock (e.g., see discussion in Samperton et al., 2015). Therefore, zircon age information combined with complementary petrologic data from zircon and other phases can be used to constrain the time scales of magma emplacement, calculate supersolidus cooling rates, and track magma compositional evolution. An important step in using zircon to further quantify magmatic processes is to compare zircon geochronology of natural systems with theoretical and experimental models for zircon saturation and growth (Watson, 1996; Harrison et al., 2007; Ferry and Watson, 2007; Boehnke et al., 2013; Bindeman and Melnik, 2016). Ab initio diffusive saturation calculations predict noninstantaneous, nonlinear volumetric zircon growth during monotonic magma cooling, with an asymmetric distribution of zircon mass crystallized as a function of time (t) or temperature (T) (Watson, 1996). This model predicts high initial crystallization rates at the system’s zircon saturation temperature (Tsat) and continuous, near-exponentially decreasing growth to the solidus temperature (Tsolid). While the treatment of Watson (1996) has been employed to describe zircon dissolution during thermal rejuvenation (Frazer et al., 2014), no attempt has been made to quantitatively compare theoretical expectations with measured zircon age spectra. If a zircon population is the product of crystallization in a cooling, closed system, then the following criteria may be observed: (1) resolvable dispersion in U-Pb zircon dates, reflecting protracted crystallization time scales; (2) trends in zircon composition through time, reflecting evolving melt composition during fractional crystallization; (3) a systematic decrease in Ti-in-zircon crystallization temperature with time, reflecting magma undercooling (Ferry and Watson, 2007); and (4) asymmetric distributions of zircon mass crystallized as functions of time and temperature (Watson, 1996). While such features have been previously documented individually (e.g., Ickert et al., 2011; Tierney et al., 2016), no prior study has demonstrated these criteria simultaneously. As such, the geochronological, experimental, and modeling perspectives of zircon crystallization have yet to be concisely unified, and our ability to link U-Pb geochronology, petrology, and numerical modeling in magmatic systems thus remains limited. Here we address this shortcoming through geochronological, geochemical, and thermometric characterization of zircon from mid-crustal granitoids. PREVIOUS GEOCHRONOLOGY AND HYPOTHESIS We use the Bergell Intrusion, Central Alps, Europe (Fig. 1), as an excellent locale to test the four criteria presented here. Thermal insulation of *Current address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA; E-mail: samperton1@llnl.gov. †Current address: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA. GEOLOGY, November 2017; v. 45; no. 11; p. 983–986 | Data Repository item 2017333 | doi:10.1130/G38645.1 | Published online 23 August 2017