ALBERT

Notes: Zusammenfassung Basanitische Ganggesteine (Lokalbezeichnung Ehrwaldite) treten in einer schmalen, über fünfzig Kilometer langen, Ost-West verlaufenden Zone der Lechtal-Decke in den Nördlichen Kalkalpen auf. Die Ganggesteine durchschlagen mesozoische Sedimente bis zur Unterkreide, welche in unmittelbarer Umgebung der Gänge kontakt-metamorphisiert wurden. Die basanitischen Gesteine enthalten neben zum Teil noch frischem Olivin (fo 91–87) wechselnde Anteile von Titanaugit, Kaersutit, Titanbiotit und Zeolithen. Nebengemengteile sind Titanomagnetit, Picotit, Apatit, Ilmenit, Karbonat und Sulfide. Mantelxenolithe und Mantelxenokristalle können bis zu fünf Prozent des Gesteins ausmachen. Die Gesamtgesteinschemie der Ehrwaldite belegt deren Charakter als primitive Mantelschmelzen und ist typisch für Nephehnbasanite (ne = 11–17 Gew.%, mg-Zahl = 74–78). Alle Spurenelementgehalte der Ehrwaldite stimmen ausgezeichnet mit denjenigen von Basaniten und Olivinnepheliniten tertiärer und quartärer Vulkanprovinzen in Europa überein. Dasselbe gilt für die Isotopencharakteristik der Gänge (mittleres εNd von + 4.7, mittleres εSr von −8.2). Die Daten belegen ähnliche Mantelausgangsgesteine und Schmelzcharakteristika wie für andere europäische Alkaligesteine und damit Ähnlichkeiten des europäischen subkontinentalen Mantels an verschiedenen Orten für den Zeitraum der letzten 100 Millionen Jahre. Die Isotopendaten stehen im eindeutigen Gegensatz zur Isotopencharakteristik des penninischen subozeanischen Mantels. Geochemische Daten und laufende Piston Cylinder Experimente ergeben eine Ursprungstiefe der Ehrwaldite als Mantelteilschmelzen von 〉 80 km bei Temperaturen von mindestens 1250 °C. K-Ar Gesamtgesteinsalter der Ehrwaldite an petrographisch verschiedenen Proben aus verschiedenen Aufschlüssen ergeben sehr konsistente Alter von ca. 100 Millionen Jahren (oberes Alb). Nach ihrer Platznahme erlitten die Ehrwaldite Bedingungen der »Diagenesezone« mit Temperaturen um 120 °C, wobei die primär gebildeten Zeohthe überlebten. Für die Entwicklung des Ostalpins westlich von Innsbruck bedeutet dies, daß zur Zeit des Magmenaufstiegs keine Subduktionszone unter den Nördlichen Kalkalpen bestand. Dieser Magmenaufstieg erfolgte in einem Gebiet mit Extension, als die Nördlichen Kalkalpen noch keinen Deckenbau aufwiesen und auf einem kontinentalen Sockel lagen, weit entfernt von einer penninischen Subduktionszone. Der Aufstieg der basanitischen Ehrwalditschmelzen könnte in einem Horst-Graben System erfolgt sein, welches möglicherweise Beziehungen zu transpressiver Tektonik aufwies.

Notes: Abstract Melting experiments were performed on a natural mica-amphibole-rutile-ilmenite-clinopyroxene (MARID) sample from the Kaapvaal mantle lithosphere (AJE137) at 20 to 35 kbar and 800 to 1450°C. A solidus was determined at 1260°C and 30 kbar above which phlogopite, clinopyroxene and olivine were stable with an alkali-rich silicate melt. Olivine is the only crystallizing phase just below the liquidus of the AJE137 bulk composition and K-richterite was only stable in the subsolidus region (≤ 1100°C at 30 kbar). These results are consistent with previous studies in more simple systems. In experiments with 10 wt% added water the solidus was depressed by ca. 300°C and K-richterite was stabilized above this solidus. MARIDs represent a potential lowtemperature component in the lithospheric mantle beneath the Kaapvaal Craton of southern Africa. The addition of 〉 10 wt% water (with less than a 120°C rise of temperature above the geotherm) to this mantle region would create conditions for the melting of this component. This may then be incorporated in any continental flood basalt parent magma that traverse this lithospheric mantle. The derivation of MARIDs from a silicate melt of their bulk composition, even if water saturated, is considered unlikely as such small degree melts could not sustain the elevated liquidus temperatures required (〉 1200°C at 30 kbar) in a cold (〈 800°C at 30 kbar) mantle lithosphere. MARID xenoliths may be produced by the interaction of an alkali-rich fluid with a peridotite or as the residue to a group II kimberlitic parent magma that has undergone fractionation of olivine and the exsolution of a carbonatite component.

Notes: Abstract Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na 〉 0.5 apfu; Mg/(Mg + Fe2+) 〉 0.7] coexisting with basic and ultrabasic melts was utilized for calibration of geobarometers based on unit-cell volume (Vcell) vs M1-site volume (VM1). The clinopyroxene database includes over one hundred experiments from literature and sixteen previously unpublished experiments on basanite and picrobasalt starting materials. The coexisting melts span a wide range of petrologically relevant anhydrous and hydrous compositions (from quartz-normative basalt to nephelinite, excluding high-Al basalts and melts coexisting with garnet or melilite) at pressure conditions pertinent to the earth's crust and uppermost mantle (P= 0–24 kbar) in a variety of fO 2 conditions (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Lc ± Ne ± Spl ± Amp ± Ilm). As previously found for near-liquidus products of basaltic melts, the experimental clinopyroxenes follow two distinct trends: (i) at a given P, Vcell is linearly and negatively correlated with VM1. This corresponds with the extent of Tschermak-type substitutions, which depends strongly on aSiO2 and a CaO; (ii) for a fixed melt composition, Vcell and VM1 decrease linearly as P increases, due to a combination of M1, M2 and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of Vcell and VM1. A simplified solution for anhydrous magmas reproduced the experimental pressures with an uncertainty of 1.75 kbar (=1 ; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capable of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1 ; max. dev. = 5.4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volumes for thermal expansivity and compressibility. The corrected formulation is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase of calculated P. The applicability of the T-dependent formulation was tested on hydrous ultramafic to gabbroic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography and experimentally determined phase relations. The pressure estimates obtained by clinopyroxene structural geobarometry closely matched those predicted by phase equilibria of a picrobasaltic melt parental to the investigated magmatic rocks. To facilitate application of the present geobarometers, both anhydrous and corrected solutions were implemented as MS-DOS® and UNIX® software programs (CpxBar) designed to permit retrieval of the pressure of crystallization directly from a chemical analysis or from uncorrected unit-cell and M1-site volume X-ray data.

Notes: Abstract  Single crystals of the garnet Mn2+ 3Mn3+ 2[SiO4]3 and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn3+ 2[GeO4]3 (space group I41/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn3+-O=1.995 (4) Å (octahedron), Mn2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn2+ 3Mn3+ 2[SiO4]3 were assigned based on a calderite Mn2+ 3Fe3+ 2[SiO4]3 model extrapolated from andradite and grossular literature data.