Paolo Nimis

Melt migration in the upper mantle along the Romanche Fracture Zone (Equatorial Atlantic)

Textural and petrological data of mantle peridotites sampled in the central and western parts of the Romanche Fracture Zone (Equatorial Atlantic) during the oceanographic expedition PRIMAR-96 (Russian R/V Gelendzhik) are presented. The studied rocks are mantle peridotites carrying patches, pockets and veins/dikes of magmatic origin, interpreted to be the product of various extents of magma impregnation on mantle partial melting residues. Estimated partial melting degrees based on clinopyroxene Ti/Zr ratios are in the ranges 5–13% and 18–20%. In highly impregnated samples, refertilization of residual peridotite minerals precludes a correct evaluation of the degree of melting. Magmatic products occur as pl±cpx±opx±ol±sp aggregates with various textural features. Interstitial pl-rich patches and gabbroic pockets are interpreted to derive from magma migration through the upper mantle by diffusive porous flow in the ductile part of the lithosphere and melt–rock reactions. Metasomatism of the host peridotites is testified by Ti and Cr increase in spinel and Ti, Sr, Zr, Y and LREE increase in clinopyroxene. Veins and dikes reflect channeled magma migration focused by brittle failures at shallower lithospheric levels. Minor or no chemical changes occurred in peridotites impregnated along fractures. The compositions of magmatic minerals in impregnated peridotites are consistent with derivation from variably fractionated melts of probably MORB type. Barometric estimates suggest that the Romanche peridotites were impregnated at minimum depths of ca. 9–12 km. Thermometric estimates for the peridotite hosts are in the range 750–1050 °C. The spread in temperature values is partly ascribed to localized heating by migrating melts of relatively cold peridotites. Our data and the occurrence of both fertile and depleted peridotites in a neighbouring area along the western Romanche FZ are in accord with the hypothesis of small-scale (<100 km) mantle heterogeneity along this fracture zone.

Mantle–crust interactions during Variscan subduction in the Eastern Alps (Nonsberg–Ulten zone): geochronology and new petrological constraints

Abstract We have analyzed the Sm–Nd and Rb–Sr whole-rock and mineral isotope systematics of garnet peridotites and associated eclogites and migmatitic gneisses from the Nonsberg–Ulten zone of the Eastern Alps. The garnet peridotites include coarse-grained varieties, characterized by well-preserved to slightly modified mantle geochemical signatures, and finer-grained varieties enriched in amphibole and LILE. Hydration of some of the most strongly deformed, fine-grained peridotites by crustal fluids caused isotopic disequilibrium between the peridotite minerals, preventing accurate age determinations. The coarse-grained peridotites, the eclogites and the country migmatitic gneisses yield garnet–whole-rock and garnet–clinopyroxene Sm–Nd ages that indicate for all rock types an isotopic homogenization event at ca. 330 Ma. The similar ages suggest that all rock types shared a common history since the incorporation of the peridotites in the crust, and constrain the garnet-facies metamorphism of the peridotites, as well as partial melting of the crust, to an episode of crustal subduction at the end of the Variscan orogenic cycle.

P–T evolution of ‘crustal’ garnet peridotites and included pyroxenites from Nonsberg area (upper Austroalpine), NE Italy: from the wedge to the slab

Abstract Up-to-date thermobarometers have been applied to garnet-bearing peridotites and pyroxenites from Nonsberg area (upper Austroalpine, NE Italy), a remnant of the lower crust of the Central European Variscan belt. The Nonsberg ultramafic rocks record an unusual metamorphic history, from high-temperature, moderate-pressure, spinel-facies conditions to low-temperature, high-pressure, garnet-facies conditions. This peculiar P – T evolution is interpreted as a result of subduction processes, which involved mantle-wedge peridotites overlying a subducting continental slab. The following scenario is proposed based on our P – T estimates and available physical models for subduction zones. Relatively high-temperature (∼1200°C, 1.31–1.58 GPa) mantle-wedge peridotites (spinel lherzolites) were intruded by hot (>1400°C) hydrous melts rising from deeper, inner portions of the wedge. Coarse-grained, garnet-free pyroxenites were segregated from these melts and formed veins in the host peridotites. Convection within the wedge induced by the downgoing slab caused the peridotites to flow towards the slab while cooling at essentially constant pressure. The peridotites were then dragged to greater depths at slightly decreasing, or nearly constant, temperature by the downward mantle flow near the wedge corner and near the wedge-slab interface and eventually emplaced into the underlying slab. Entrainment in the continental crust, either as tectonic slices or as sinking mantle blobs, was probably favored by density contrast between overlying mantle and underlying crust. Mantle flow and entrainment in the cold continental crust caused the peridotites to cool down to ∼850°C before, or while, being subducted together with the slab to depths of about 90 km. During this T -decrease, P -increase path, garnet formed at the expense of spinel and pyroxenes, giving rise to garnet ± spinel-bearing lherzolites and pyroxenites.

Geobarometry from host-inclusion systems: The role of elastic relaxation

Abstract Minerals trapped as inclusions within other host minerals can develop residual stresses on exhumation as a result of the differences between the thermo-elastic properties of the host and inclusion phases. The determination of possible entrapment pressures and temperatures from this residual stress requires the mutual elastic relaxation of the host and inclusion to be determined. Previous estimates of this relaxation have relied on the assumption of linear elasticity theory. We present a new formulation of the problem that avoids this assumption. We show that for soft inclusions such as quartz in relatively stiff host materials such as garnet, the previous analysis yields entrapment pressures in error by the order of 0.1 GPa. The error is larger for hosts that have smaller shear moduli than garnet.

Olivine with diamond-imposed morphology included in diamonds. Syngenesis or protogenesis?

The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of the possible chemical re-equilibration during diamond-forming processes, at least some of the olivines may have existed prior to the diamond (i.e. they are protogenetic). Our results imply that a diamond-imposed morphology alone cannot be considered as unequivocal proof of syngenicity of mineral inclusions in diamonds.

An ion microprobe study of clinopyroxenes in websteritic and megacrystic xenoliths from Hyblean Plateau (SE Sicily, Italy): constraints on HFSE/REE/Sr fractionation at mantle depth

Abstract Eleven clinopyroxenes from websteritic (Cr-diopside- and Al-diopside-bearing) and Al-augite±orthopyroxene±spinel-bearing megacrystic xenoliths occurring in alkaline volcanics of the Hyblean Plateau (SE Sicily) were analysed for rare-earth and other trace elements (Sc, Ti, V, Cr, Sr, Y, Zr) with an ion microprobe. Trace-element analysis shows that Hyblean pyroxenites are crystal/liquid segregates of basaltic magmas characterized by different composition (from tholeiitic to alkaline) and variable LILE enrichment. Megacrystic clinopyroxenes are, most probably, high-pressure liquidus phases of OIB-type alkaline basalts characterized by smooth chondrite-normalized trace-element abundance plots, except for a slight negative Sr anomaly. Clinopyroxenes from websterites crystallized from either alkaline or tholeiitic melts and later experienced metamorphic (orthopyroxene exsolution) and/or metasomatic (interaction with fluids or melts) processes which modified to variable extent their original major-and trace-element signatures. In Hyblean high-pressure alkaline basalts, compositional factors (i.e. major-element activities in the melt and clinopyroxene crystal chemical configuration) caused a significant D Ti increase, mild D REE and D Zr increases, and virtually no D Sr variations with increasing degree of fractionation. In particular, clinopyroxene crystal chemical constraints, namely electrostatic balance and M1-site geometry, appear to play a major role in the creation of HFSE/REE and Sr/REE decoupling in clinopyroxene-bearing mantle rocks. A sharp decrease of D Zr with increasing pressure is predicted from crystal chemical considerations.

Inclusions under remnant pressure in diamond: a multi-technique approach

Measurement of the remnant pressure sustained by a mineral inclusion within a diamond can allow calculation of the source pressure and temperature conditions of the diamond formation. While Raman spectroscopy (point analyses and 2D mapping) has been the most commonly used non-destructive method for measuring this remnant pressure, two new techniques (quantitative birefringence analysis and in situ X-ray diffraction) have recently been developed. In this paper we apply all of these techniques to the study of two diamonds. The first is a diamond twin (macle) from Shandong (China), containing two olivine inclusions. Analysis of the largest inclusion by the birefringence and X-ray techniques returned compatible values for the remnant pressure of 0.1–0.2 GPa. However, while 2D Raman mapping of the diamond host qualitatively confirmed the presence of a radial stress field, the low remnant pressures values were too small to be detected by pressure shifts in the olivine’s Raman spectrum. The second sample studied was a rounded dodecahedral diamond (unknown origin) containing several coesite inclusions. Its morphology prevented quantitative birefringence analysis or Raman mapping being performed but Raman analyses on three inclusions (2.2–2.5 GPa) provided similar results to those obtained by X-ray diffraction from a single inclusion (2.7 GPa); all of these values lie within the range of previous published remnant pressures for coesite in diamond. Calculation of the source pressure and temperature conditions for both diamond samples returned anomalously low values, both below the diamond stability field for a range of mantle temperatures (950–1350 °C). The reasons for this are due to violations of the fundamental assumptions upon which such studies of these elastic effects in diamond are based. Although previous studies showed that olivine inclusions have the potential to record reasonable formation pressures under favourable conditions, the present study re-iterates the serious concerns of using the coesite-in-diamond geobarometer. It is inferred that, as the remnant pressures in the coesite-diamond pair are so high, there is a high probability that the diamond will undergo some plastic deformation, thus reducing the elastic behaviour and leading to severe underestimation of entrapment pressure.

Origin of chromite in mafic–ultramafic-hosted hydrothermal massive sulfides from the Main Uralian Fault, South Urals, Russia

Abstract Mafic–ultramafic-hosted hydrothermal Fe–Cu–(Ni–Co) sulfide ores from the Main Uralian Fault Zone (MUFZ), South Urals (Ivanovka and Ishkinino ore fields), contain a relatively large (up to 3%) proportion of chromite. This association is common for magmatic Fe–Ni–Cu sulfides, but definitely unusual for hydrothermal sulfides. Textural, morphological and compositional data are used here to gain an insight into the origin and significance of this unusual chromite–sulfide association. The studied chromites occur both as broken fragments and as euhedral or subhedral crystals, which are included in the sulfides or scattered in their talc±chlorite±saponite±quartz±carbonate matrix. They are characterized by high Cr/(Cr+Al) ratios (0.58–0.85) and range in composition from magnesiochromite to chromite sensu stricto. Textural, morphological and compositional features, as well as the occurrence of relatively high-silica, low-Ti, low-K melt inclusions in some of the crystals, indicate that the ore-associated chromites (i) are a mixed population of grains derived from mafic–ultramafic mantle and crustal magmatic rocks and mantle peridotite melting residua, (ii) have no genetic relation with the host sulfides and (iii) represent relicts derived from the hydrothermally altered country rocks. The compositions of the chromites and of the melt inclusions denote a clear supra-subduction zone signature. The melts parent to the cumulitic chromites had an arc tholeiitic to, possibly, boninitic affinity. These data suggest that the host mafic–ultramafic complexes formed in an early arc or forearc setting and do not represent obducted portions of MORB oceanic lithosphere. Hence, contrary to previous interpretations, the associated massive sulfides could not originate on a mid-ocean ridge, but rather in an early arc or forearc environment. Given the relatively short life of the western Uralian arc system, the most probable time window for sulfide ore deposition is confined to Early to Middle Devonian time.

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

A single crystal of garnet enclosed in a diamond from the Jericho kimberlite (Slave Craton, Canada) has been investigated using X-ray diffraction and X-ray micro-tomography. The novel experimental approach allowed us to determine the crystal structure of the garnet. The unit-cell edge a and fractional atomic coordinates of oxygen were used to determine the composition via an updated Margules model for garnets. The composition is Pyr0.41(5)Alm0.36(7)Gro0.22(1)Uva0.01(1), which is indistinguishable from the eclogitic garnets found in other Jericho diamonds. We also demonstrated that residual pressures on the inclusion of up to 1 GPa do not affect significantly the determination of the garnet composition by structure refinement.

Dissakisite-(La) from the Ulten zone peridotite (Italian Eastern Alps): A new end-member of the epidote group

Abstract Dissakisite-(La), ideally CaLaAl2MgSi3O12(OH), has been found in the Hochwart peridotite, Ulten zone, Italy. The mineral occurs as centimeter-sized black to very dark brown anhedral nodules and smaller grains. Associated minerals are: olivine, spinel, amphiboles, clino- and orthopyroxenes, and minor clinochlore, uraninite, thorite, thorianite, phlogopite, zircon, apatite, calcite, dolomite, pentlandite, and copper sulfides. The streak is gray-greenish and the luster is vitreous. Mohs hardness is 6.5.7; the mineral is brittle with a conchoidal fracture. The cleavage is imperfect on (001). Dissakisite-(La) is monoclinic, space group P21/m. The unit cell dimensions are a = 8.9616(7), b = 5.7265(5), and c = 10.2353(9) Å, β = 115.193(6)°, V = 475.30(7) Å3, Z = 2. The strongest X-ray powder diffraction lines are: [d(Å) (I)(hkl)] 2.926(100)(113̅), 2.860(53)(020), 2.553(51)(202), 3.526(49)(211̄), 2.699(44)(120). Electron and ion microprobe analysis of the type sample DISS 5 gave the formula (Ca1.195 Mn0.009 Sr0.010 Na0.002 Th0.090 U0.003 La0.315 Ce0.262 Pr 0.019 Nd0.038 Sm0.002 Gd0.001 Er0.001) (Al1.816 Mg0.622 Fe2+0.244 Fe3+0.159 Cr0.148 Ti0.030 Sc0.002 V0.008 Ga0.001 Ni0.010 Zn0.015) (Si2.970 Al0.022 P0.008) O11.991 F0.009 (OH). The La/(La + Ce) ratio is 0.545(16) in the type analysis and 0.543(18) in an average of 70 analyses of the type sample A4310. Ce ≥ La was not observed in any analysis. Dmeas = 3.79(15) g/cm3; Dcalc = 3.84 g/cm3. Radioactivity is appreciable. The optical properties and Raman spectrometry have also been investigated. The mineral formed by hydration and enrichment in LILE and LREE of a peridotite body, in relation to HP-migmatization of the surrounding gneisses during the Variscan orogeny.