The Formation of Amphibole-Plagioclase White Pods in Eclogitic Rocks from NE Sardinia, Italy

Gabriele Cruciani (francmar@unica.it)¹, Gianfranco Carcangiu², Anna Maria Caredda (acaredda@unica.it)¹, Marco Franco Elter³ & Marcello Franceschelli (francmar@unica.it)¹

¹ Dip. Scienze della Terra, via Trentino, 51, 09127 Cagliari, Italy

² Centro Studi Geominerari e Mineralurgici C.N.R., P.zza d'Armi, 16, 09127 Cagliari, Italy

³ Dip. Studio del Territorio e sue Risorse, Corso Europa, 26, 16132 Genova, Italy

Lenses of metabasites with relics of eclogite facies assemblages, hosted in the Hercynian migmatite of NE Sardinia, are transected by a series of small-scale shear zones. These rocks exhibit a compositional banding made up of garnet-pyroxene rich and amphibole-plagioclase rich layers. The garnet-pyroxene rich layers are made up of poikiloblastic garnet with inclusion relics of omphacitic pyroxene, plagioclase-clinopyroxene symplectite after omphacite, orthopyroxene, amphibole, and quartz. Garnet surrounded by plagioclase-clinopyroxene symplectite shows a corona made up of Ca-plagioclase and amphibole. The modal proportions of garnet, pyroxene, and plagioclase reaches 70-80%. The amphibole-plagioclase rich layers consist of plagioclase, amphibole, and relics of clinopyroxene and plagioclase symplectite, orthopyroxene, and colourless amphibole. The modal proportion of amphibole plus plagioclase reaches 80-90%. The amphibole-plagioclase rich layers contain strongly oriented white pods, 1-5 mm in size, made up of amphibole, plagioclase, ilmenite, quartz, and relics of garnet and pyroxene. White pods strike N 80°, dip N 20°, and crosscut the eclogite banding at high angles. A series of textural and mineralogical changes, which can be observed as the strain increases into the shear zones, showed that the amphibole-plagioclase rich layers are formed at the expense of garnet-pyroxene rich layers. During the amphibolite stage (T = 550-635°C and P = 4-7 kbar) the widespread destabilization of garnet and calcic pyroxene via the simplified reaction: (1) garnet + clinopyroxene + Na-rich plagioclase + rutile + H₂O = amphibole + Ca-rich plagioclase ± ilmenite produced the most significant textural changes in the two layers. In the garnet-pyroxene layers the reaction (1) gave rise to a well-developed corona texture of plagioclase and amphibole between garnet and calcic pyroxene, whereas in the amphibole-plagioclase layers reaction (1) led to the disappearance of garnet crystals and to the formation in the garnet sites of white pods made up of plagioclase and amphibole.

Trace Element Partitioning for Rutile in Melted MORB

Arnaud Dardon (dardon@opgc.univ-bpclermont.fr)¹, Max W. Schmidt², Riccardo Vannucci³, Piero Botazzi⁴ & Gilles Chazot¹

¹ Universite Blaise Pascal - UMR 6524, 5, rue Kessler, 63038 Clermont-Ferrand, France

² CNRS - UMR 6524, 5, rue Kessler, 63038 Clermont-Ferrand, France

³ Dip. Scienze della Terra, Universita di Pavia, 27100 Pavia, Italia

⁴ CNR-CS per la Cristallochimica e la Cristallografia, 27100 Pavia, Italia

Partitioning of High Field Strength Elements (HFSE), Rare Earth Elements (REE) and other elements (Th, U) between rutile and silicate melt has been investigated by experiments on synthetic MORB compositions doped with a series of trace elements to a concentration level of 500 ppm. Experiments were conducted in a piston cylinder apparatus (1/2 inch), under different oxygen fugacities (>CCO; Mn₃O₄-MnO), pressure (> 20 kbar) and temperature (950 - 1200°C). Trace element contents of melt, rutile and other minerals were analysed by laser ablation ICP-MS.

All melts are in equilibrium with an eclogitic assemblage (Cpx, Gar, Ru, ± Pl) and have adakitic compositions at T < 1100°C. Rutile has a simple structure where Ti is in octahedric coordination. Compatibility of elements is a function of their ionic radii in six-fold coordination and charge, so that Nb, Ta, Hf and Zr are compatible in rutile because both their ionic radii and charge are close to that of Ti. All REE are incompatible.

Both oxygen fugacity and melt composition control the variation of partition coefficients. Oxygen fugacity is particularly critical for U which is not compatible under low fugacity, while it behaves as a compatible element under high fugacity. Under the latter conditions, rutile may operate significant Th/U fractionation in equilibrium melts, thus providing a plausible explanation for Th/U variation in natural adakites.

Melt composition exerts the major control on HFSE partitioning. Ti, Nb and Ta contents largely vary as a function of NBO/T values. Although always greater than unity, their partition coefficients vary by a factor 2.5 to 7 between 950 and 1160°C, and are thus a function of melting rate, a parameter that must be accounted for in modelling adakite genesis.

The Role of Local Octahedral Composition on the OH-F Exchange in Monoclinic Amphiboles

Giancarlo Della Ventura (dellaven@uniroma3.it)¹, Jean-Louis Robert², François Delbove², Josè Sergent² & Frank C. Hawthorne³

¹ Dipartimento di Scienze della Terra, Università degli studi della Calabria, 87036 Arcavacata di Rende (CS), Italy

² ISTO, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France

³ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

Amphiboles containing significant amounts of fluorine are common in nature, however systematic data on OH/F long range (LRO) and short-range (SRO) ordering patterns in the amphibole structure are still rather scarce (Della Ventura et al., 1998; Robert et al., 1999a and b). In this work we have replaced OH with F in an amphibole structure which has a composition intermediate between richterite and pargasite. Syntheses have been done at 3 kbar PH₂O and 900°C on the join Na(Na_0.6Ca_1.4)(Mg_4.6Al_0.4)(Si_7.2Al_0.8)O₂₂(OH,F)₂ corresponding to a molar composition 60% richterite and 40% pargasite. Structural variations in the amphibole as a function of F have been characterized by a combination of EMPA, powder X-ray diffraction, and infrared spectroscopy. EMPA analyses show that for increasing F in the system (i) the ^[6]Al content decreases significantly whereas the Al content at the T sites is constant; (ii) the fluorine content is systematically lower than the nominal one and for the most F-rich nominal composition there is still a significant amount of OH in the amphibole. The evolution of cell parameters and IR spectra across the join indicate that the solubility of F in the amphibole is restricted to about 1.0 apfu. The OH-stretching spectra are consistent with a two-mode behaviour typical of A-site filled amphiboles and show that those OH-groups locally bonded to ^[M1]Mg^[M1]Mg^[M3]Mg configurations are preferentially replaced by F whereas those OH-groups locally bonded to ^[M1]Mg^[M1]Mg^[M3]Al configurations are not replaced by F. The obtained results can be used as a general model for the OH/F exchange in the amphibole structure.

Della Ventura G, Robert JL & Hawthorne FC, Can. Mineral, 36, 181-186, (1998).

Robert JL, Della Ventura G & Hawthorne FC, Am. Mineral, 84, 86-91, (1999a).

Robert JL, Della Ventura G, Welch MD & Hawthorne FC, Am. Mineral, in press, (1999b).