Synthesis piston cylinder experiments were carried out in the range 2.0-4.5 GPa and 680-1,050 °C to investigate phase relations in subducted continental crust. A model composition (KCMASH) has been used because all major ultrahigh-pressure (UHP) minerals of the whole range of rock types typical for continental crust can be reproduced within this system. The combination of experimental results with phase petrologic constraints permits construction of a UHP petrogenetic grid. The phase relations demonstrate that the most important UHP paragenesis consists of coesite, kyanite, phengite, clinopyroxene, and garnet in subducted continental crust. Below 700 °C talc is stable instead of garnet. As most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize UHP metamorphism in subducted crust. A general feature, however, is the absence of feldspars at H 2 O-saturated conditions. Plagioclase is never stable at UHP conditions, but K-feldspar can occur in H 2 O-undersaturated rocks. Mineral compositions in the experiments are fully buffered by coexisting phases. The Si content of phengite and biotite increase with increasing pressure. At 4.0 GPa, 780 °C, biotite contains 3.28 Si per formula unit, which is most probably caused by solid solution of biotite with talc. Above 800 °C, the CaAl 2 SiO 6 component in clinopyroxene buffered with kyanite, coesite and a Mg-phase increases with increasing temperature, providing a tool to distinguish between 'cold' and 'hot' eclogites. Up to 10% Ca-esk-olaite (Ca 0.5 [] 0.5 AlSi 2 O 6 ) in clinopyroxene has been found at the highest temperature and pressure investigated (>900 °C, 4.5 GPa). Garnet buffered with coesite, kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature. Although the investigated system represents a simplification with respect to natural rocks, it helps to constrain general features of subducted continental crust. The observed phase relations and phase compositions demonstrate that at pressures >3.0 GPa and temperatures >800 °C continental crust can retain significant amounts of H 2 O (>1 wt%), whereas K-free mafic or ultramafic rocks are dry at these conditions. UHP parageneses are only preserved if the whole exhumation path is situated within the stability field of phengite, i.e. if there is cooling during exhumation or if the whole exhumation occurred at T <700 °C. In contrast, break down of phengite and concomitant partial melting in terranes that show isothermal decompression may lead to a complete recrystallization of the subducted crust during exhumation. The density of UHP rocks can be estimated on the basis of the established phase relations. Pelitic rocks are likely to have a density close to mantle rocks (3.3 g/cm 3 ) because of significant amounts of dense garnet and kyanite whereas granitic rocks are less dense (3.0 g/cm 3 ). Hence, subducted average continental crust is most probably buoyant with respect to mantle rocks and tends t...
The palladium-catalyzed Sonogashira reaction can be used to build optically active, oligomeric 1,2,3-substituted ferrocenes up to the tetramer, as well as polymers, by sequential coupling of optically active (ee > 98 %), planar chiral iodoferroceneacetylenes and ferroceneacetylenes. (SFC)-1-Iodoferrocene-2-carbaldehyde (1) was reduced to the alcohol and methylated to give the corresponding methyl ether, which was Sonogashira-coupled with HC(triple bond)CSiEt3, resulting in (RFc)-1-(C(triple bond)CSiEt3)-2-methoxymethylferrocene (4) (79%, three steps). Orthometalation with tBuLi followed by quenching with 1,2-diodoethane gave (RFc)-1-(C(triple bond)CSiEt3)-2-methoxymethyl-3-iodoferrocene (5). Deprotection of the acetylene with nBu4NF resulted in (RFc)-1-ethynyl-2-methoxymethyl-3-iodoferrocene (6), which was Sonogashira-coupled with itself to produce an optically active polymer. Deprotection of 4 with nBu4NF and Sonogashira coupling of the product with 5 resulted in the dinuclear ferrocene 9. Deprotection of 9 and coupling with 5, followed by deprotection of the resulting acetylene 11, gave the trinuclear ferrocene 12. Another such sequence involving 11 and 5 produced a tetranuclear ferrocene 13. To study the electronic communication in such oligomers in more detail, two symmetrical, closely interrelated, trinuclear ferrocenes 18 and 19 were synthesized. The redox potentials of all the ferrocenes and the ferroceneacetylene polymer were determined by cyclic and square-wave voltammetry. All the metallocenes were investigated by UV/Vis spectroscopy. A linear relationship was found between lambdamax and l/n (n=number of ferrocene units in the oligomer). The polymer displayed two redox waves in the cyclic voltammogram, at 0.65 and 0.795 V. The corresponding mixed-valence oligoferrocene cations were synthesized from four ferroceneacetylenes, and their metal-metal charge transfer bands were examined by UV/Vis-NIR. The resonance exchange integrals Had, calculated on the basis of spectral information from the metal - metal charge transfer (MMCT) bands, were between 290 and 552 cm-1.
The reactions of 1,3-bis(bromomethyl)-2-fluorobenzene with the bis(trifluoroacetamides) of 1,3-bis(aminomethyl)-2-fluorobenzene and 1,3-bis(aminomethyl)benzene yield the respective 1+1-condensed [3.3]-m-cyclophanes, respectively termed F(2)-phane (yield 39%) and HF-phane (yield 48%) without trifluoroacetamide groups. The reactions of F(2)-phane with 1,8-diiodo-3,6-dioxaoctane and 1,11-diiodo-3,6,9-trioxaundecane result in the respective 1+1-addition products 1,10-diaza-25,26-difluoro-4,7-dioxatetracyclo[8.7.7.1(12,16).1(19,23)]hexaeicosa-12,14,16(25),19,21,23(26)-hexene (= F(2)-[2.1.1]-cryptand) (yield 5%) and 1,13-diaza-28,29-difluoro-4,7,10-trioxatetracyclo[11.7.7.1(15,19).1(22,26)]nonaeicosa-15,17,19(28),22,24,26(29)-hexene (= F(2)-[3.1.1]-cryptand) (yield 39%). Analogous reactions of the HF-phane give the related macrocycles HF-[2.1.1]-cryptand (yield 68%) and HF-[3.1.1]-cryptand (yield 76%). The coordination of alkali and alkaline earth metal ions by these fluorophane cryptands results in significant shifts of the (19)F NMR resonances: F(2)-[2.1.1]-cryptand, delta -100.70 ppm; its Li(+) complex, delta -129.23 ppm. The (1)J(CF) coupling constant for such complexes is correlated with the degree of interaction between CF units and metal ions, and the most pronounced decrease (262 Hz to 232 Hz) is found for the lithium complex of the F(2)-[2.1.1]-cryptand. Competition experiments show the difluoro F(2)-[3.1.1]-cryptand to form significantly stronger complexes with Na(+) than the monofluoro HF-[3.1.1]-cryptand. In the crystal structure of F(2)-[2.1.1]-cryptand.NaCF(3)SO(3), the sodium ion displays an unusual F(2)O(4)N coordination sphere with extremely short sodium-fluorine distances: CF.Na(+) = 229.8(3), 235.7(4) pm; O-Na(+) = 228.5(4), 242.0(4), 243.8(4), 247.6(4) pm; N-Na(+) = 285.1(7) pm. In the closely related crystal structure of HF-[3.1.1]-cryptand.NaClO(4), the metal has an FO(5)N coordination sphere: CF.Na(+) = 236.0(4) pm; O-Na(+) = 234.8(6), 239.3(6), 240.3(12), 240.6(6), 285.7(17) pm; N-Na(+) = 272.3(6) pm. In the crystal structure of F(2)-[3.1.1]-cryptand.HCF(3)SO(3), the proton is in a pseudotetrahedral environment: N-H = 84 pm; O.HN = (216 pm; CF.HN = 224, 236 pm. This, however, is not considered indicative of significant CF.HN hydrogen bonding.
Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. We present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite-hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at ca. 670 °C / 1.6 GPa, displaying successive crystallization stages. Each stage is characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt.%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite-hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt.%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al, and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr-V-Al-Fe3+ mass balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass balance analysis also reveals that in order to account for the mass conservation of fluid-immobile elements such as Cr-V-Al-Fe3+, new magnetite formation is required across the antigorite-breakdown reaction. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89-91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90-91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz-fayalite-magnetite (QFM) buffer. This is consistent with the observation that the Fe-Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why arc magmas are comparatively oxidized.
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