Katharina Vogt

Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran

The petrological and geochronological study of the Cenozoic Shaivar Dagh composite intrusion in the Alborz Mountain belt (NW Iran) reveals important clues to decipher complex relations between magmatic and tectonic processes in the central sectors of the Tethyan (Alpine–Himalayan) orogenic belt. This pluton is formed by intrusion at different times of two main magmatic cycles. The older (Cycle 1) is formed by calc-alkaline silicic rocks, which range in composition from diorites to granodiorites and biotite granites, with abundant mafic microgranular enclaves. The younger cycle (Cycle 2) is formed by K-rich monzodiorite and monzonite of marked shoshonitic affinity. The latter form the larger volumes of the exposed plutonic rocks in the studied complex. Zircon geochronology (laser ablation ICP-MS analyses) gives a concordia age of 30.8 ± 2.1 Ma for the calc-alkaline rocks (Cycle 1) and a range from 23.3 ± 0.5 to 25.1 ± 0.9 Ma for the shoshonitic association (Cycle 2). Major and trace element relations strongly support distinct origins for each magmatic cycle. Rocks of Cycle 1 have all the characteristic features of active continental margins. Shoshonitic rocks (Cycle 2) define two continuous fractionation trends: one departing from a K-rich basaltic composition and the other from an intermediate, K-rich composition. A metasomatized-mantle origin for the two shoshonitic series of Cycle 2 is proposed on the basis of comparisons with experimental data. The origin of the calc-alkaline series is more controversial but it can be attributed to processes in the suprasubduction mantle wedge related to the incorporation of subducted melanges in the form of silicic cold plumes. A time sequence can be established for the processes responsible of the generation of the two magmatic cycles: first a calc-alkaline cycle typical of active continental margins, and second a K-rich cycle formed by monzonites and monzodiorites. This sequence precludes the younger potassic magmas as precursors of the older calc-alkaline series. By contrast, the older calc-alkaline magmas may represent the metasomatic agents that modified the mantle wedge during the last stages of subduction and cooked a fertile mantle region for late potassic magmatism after continental collision.

Deep plate serpentinization triggers skinning of subducting slabs

Most of the present-day ocean floor is continuously being consumed in subduction zones, but large fragments of oceanic crust (ophiolites) have also been recognized on land. The process by which oceanic crust is separated from the subducting slab remains enigmatic, and several competing hypothesis have been proposed in the past. Based on numerical experiments, we suggest that serpentinized mantle, formed in the outer rise regions of subduction zones, may provide a mechanically weak horizon within which basal detachment of the oceanic crust is feasible. Deformation of this serpentinized layer may lead to decoupling and separation of oceanic crust from the downgoing slab. Fragments of the former oceanic crust can underplate the accretionary wedge or be exposed on continental crust, whereas the skinned lithospheric part of the slab subducts into the mantle.

Fe-Mg diffusion in spinel: New experimental data and a point defect model

Abstract We have measured Fe-Mg interdiffusion rates (DFe-Mg) in synthetic Mg-Al spinel and a natural (Mg,Fe) aluminous spinel from Sri Lanka (XFe ~ 0.07) at atmospheric pressure over a range of different oxygen fugacities {log10 (fO₂[Pa]) = -14 to -10} and temperatures (750-900 °C). Diffusion couples made of single-crystal spinel and thin films of hercynitic composition (XFe ~ 0.5) were used for the diffusion anneals. The experimentally induced diffusion profiles were analyzed by Rutherford backscattering Spectroscopy to retrieve true depth concentration profiles for Fe. These were fitted numerically by an explicit finite difference scheme that allows compositionally dependent interdiffusion processes to be modeled by relating self- and interdiffusion coefficients. Synthesis of data from the two diffusion couples indicate that: (1) DFe-Mg depends on XMg of spinel, with increasing diffusion rates with increasing XMg. This behavior is opposite of that found in silicates. (2) Self-diffusion coefficients could not be determined from these experiments, but the results indicate that DFe/DMg > 100. (3) DFe-Mg in Mg-spinel is independent of oxygen fugacity, whereas it depends strongly and nonlinearly on oxygen fugacity for the natural spinel. This observation indicates that the mechanisms of diffusion are different in the two kinds of spinel (Fe-bearing vs. Fe-free), which is also seen in the difference in activation energy obtained for these. Moreover, the nonlinear dependence on oxygen fugacity indicates that diffusion occurs by an interstitial mechanism at low-oxygen fugacities and by a vacancy mechanism at highoxygen fugacities in natural, Fe-bearing spinel. (4) Simple Arrhenius relations that describe the data within the range of experimental conditions are: Synthetic magnesium spinel: QFe-Mg = 219 ± 19 kJ/mol, log10DFe-Mg = -7.76 ± 0.90 [m2/s]. Natural Fe-bearing spinel for log10 (fO₂ [Pa]) = -12}: QFe-Mg = 139 ± 18 kJ/mol, log10DFe-Mg = -12.33 ± 0.85 [m2/s]. A model based on point defect considerations that describes the temperature as well as oxygen fugacity dependence of DFe-Mg in Fe-bearing spinel is: D [m2/s] = Dv [m2/s]fO₂ [Pa]mexp{-Qv [J/mol]/RT [K]} + Di [m2/s] fO₂ [Pa]-mexp(-Qi [J/mol]/RT [K]), with Dv = 1.07 × 10-9 ± 1.55 × 10-9 m2/s, Qv = 131 ± 66 kJ/mol, Di = 1.03 × 10-17 ± 7.32 × 10-17 m2/s, Qv = 130 ± 80 kJ/mol and m = 0.34 ± 0.18. Poor coverage of T-fO₂ space by available experimental data results in large uncertainties in the fit parameters. As a result, these expressions are useful for understanding the diffusion behavior in spinels, but not for extrapolation and calculation of diffusion coefficients for cooling rate or other related calculations. Until the parameters can be better constrained through the availability of more data, we recommend that for such calculations, the parameters noted above for Fe-bearing spinels be used for compositions and fO₂ conditions that are close to those of the experiments. (5) DFe-Mg in spinel is faster than DFe-Mg in olivines, pyroxenes, and garnets at most conditions.