Estimating the distribution of strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in the Precambrian of Finland

A method to estimate the 87Sr/86Sr ratio of a rock based on its age and Rb/Sr ratio is presented. This method, together with data from the Rock Geochemical Database of Finland (n=6544) is used to estimate the 87Sr/86Sr ratios in the Precambrian of Finland and in its different major units. A generalization to cover the whole area of Finland is achieved by smoothing of estimation points. The estimation method is evaluated by comparing its results to published Rb-Sr isotope analyses (n=138) obtained on the Finnish Precambrian. The results show correspondence to different geological units of Finland, but no systematic difference between Archaean and younger areas is evident. Evaluation of the method shows that most of the estimates are reliable and accurate to be used as background material for provenance studies in archaeology, paleontology and sedimentology. However, some granitic rocks may have large (>1.0 %) relative errors. Strontium concentration weighted average of the estimates differs only by 0.001 from the average 87Sr/86Sr ratio (0.730) of the rivers on the Fennoscandian shield

Interaction between mantle-derived magma and lower arc crust: quantitative reactive melt flow modelling using STyx

International audienc

Lithosphere destabilization by melt weakening and crust-mantle interactions-implications for generation of granite-migmatite belts

Orogenic crustal anatexis is a still poorly understood process due to the complexity of the thermal and geodynamical interaction between mantle and crustal processes during and after continental collision. Here, we present a novel conceptual model for the formation of granite migmatite belts: we propose that convective thinning of the lithosphere results in minor amounts of partial melts within the lowermost crust that trigger further instabilities. This will lead to a positive feedback effects between melt weakening, mantle upwelling and wholesale mantle lithosphere removal, causing a strong pulse of mantle and crustal melting. We test this model numerically, and results show that his process, taking between 20 and 50 Myrs in total, can explain the temporal evolution of melting in granite‐migmatite zones and associated mantle derived mafic rocks, and provides a heat source for crustal melting without the need for other processes, such as slab break‐off or increased radiogenic heating. Furthermore, the generation of a refractory residue after mantle and crustal melting is also shown to control the progress of the lithospheric mantle removal, providing another feedback mechanism between melting and lithospheric re‐equilibration

A numerical approach to melting in warm subduction zones

The complex feedback between dehydration and melting in hot subduction zones is quantitatively addressed in this study. We present an integrated numerical tool that combines a high-resolution thermo-mechanical subduction model with a thermodynamic database that allows modeling metamorphic devolatilization, and subsequent re-hydration and melting reactions. We apply this tool to quantify how the hydration state of a lithologically layered subducting slab varies during interaction with the hot mantle wedge and how this affects any melting taking place in the subducting crust or the overlying mantle wedge. Total crustal dehydration is achieved before any crustal melting can occur, even in very young subducting slabs. Significant oceanic crust melting is only achieved if the metamorphic fluids from the dehydrating underlying subducting slab mantle are fluxed through the dry eclogites. But our models further demonstrate that even if the oceanic crust can melt in these specific conditions, the preceding crustal dehydration will simultaneously result in extensive mantle wedge melting at lower pressures than for colder slabs. The significant mantle wedge melting implies that also for hot subduction zones, most of the melt feeding the overriding plate is of mantle origin

Rcrust: a tool for calculating path-dependent open system processes and application to melt loss

International audienc

Sub-lithospheric small-scale convection—a mechanism for collision zone magmatism

We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau