Fluid-assisted zircon and monazite growth within a shear zone: A case study from Finnmark, Arctic Norway

The U-Pb ages, REE content, and oxygen isotopic composition of zircon rims developed within a major shear zone in the Kalak Nappe Complex (KNC), Arctic Norway have been determined along with the age of monazite crystals. Different generations of granitic veins have been distinguished based on both field criteria and monazite ages of 446 ± 3 and 424 ± 3 Ma. Within each of these veins, inherited zircon cores are mantled by homogeneous low CL-response zircon rims which yield a range of concordant U-Pb dates of ca. 470-360 Ma. Significant numbers of zircon rims coincide with the timing of monazite crystallization. The zircon rims have moderate light REE enrichment compared to cores, distinctive (Sm/La)n values of less than 12, and La between 0.3 and 10 ppm. This indicates free elemental exchange between newly formed zircon rims and the surrounding matrix. The rims have calculated accumulated alpha-radiation dosages corresponding with a crystalline structure and d18O values of 1‰. This implies rim crystallization directly from a zirconium-saturated hydrothermal fluid which was modified by some silicate melt. Growth of the zircon rims was prolonged and locally variable due to preferential fluid flow. A third type of zircon can be recognized, forming both rims and cores, with high alpha-radiation doses, and significant enrichment in La, Pr, and Eu. These are interpreted as low-temperature hydrothermally altered metamict zircons. The high volatile input and partial melting in the shear zone favoured prolonged zircon rim growth due to its ability to easily nucleate on inherited seeds. On the other hand, monazite, susceptible to dissolution and re-growth, crystallized in brief episodes, as has been predicted from theoretical phase diagrams. From a regional perspective, these results elucidate cryptic Ar-Ar cooling ages, providing the first record of a Late Ordovician heating and cooling phase within the KNC prior to the climactic Scandian collision. © Springer-Verlag 2009

A compositional tipping point governing the mobilization and eruption style of rhyolitic magma

International audienceThe most viscous volcanic melts and the largest explosive eruptions on our planet consist of calcalkaline rhyolites. These eruptions have the potential to influence global climate. The eruptive products are commonly very crystal-poor and highly degassed, yet the magma is mostly stored as crystal mushes containing small amounts of interstitial melt with elevated water content. It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions can switch repeatedly between effusive and explosive eruption styles and this transition is difficult to attribute to the rheological effects of water content or crystallinity. Here we measure the viscosity of a series of melts spanning the compositional range of the Yellowstone volcanic system and find that in a narrow compositional zone, melt viscosity increases by up to two orders of magnitude. These viscosity variations are not predicted by current viscosity models and result from melt structure reorganization, as confirmed by Raman spectroscopy. We identify a critical compositional tipping point, independently documented in the global geochemical record of rhyolites, at which rhyolitic melts fluidize or stiffen and that clearly separates effusive from explosive deposits worldwide. This correlation between melt structure, viscosity and eruptive behaviour holds despite the variable water content and other parameters, such as temperature, that are inherent in natural eruptions. Thermodynamic modelling demonstrates how the observed subtle compositional changes that result in fluidization or stiffening of the melt can be induced by crystal growth from the melt or variation in oxygen fugacity. However, the rheological effects of water and crystal content alone cannot explain the correlation between composition and eruptive style. We conclude that the composition of calcalkaline rhyolites is decisive in determining the mobilization and eruption dynamics of Earth’s largest volcanic systems, resulting in a better understanding of how the melt structure controls volcanic processes