Alpe Arami Research Articles

Evidence of majoritic garnet from the mantle transition zone in the Alpe Arami garnet peridotite

ABSTRACT Ultra-high-pressure (UHP) peridotites found along collisional zones record rare information from deep within the Earth. However, the estimation of depth of origin for these UHP rocks has been controversial. A major controversy remains related to the conjectural proposition of mantle transition zone (410–660 km) origin of the Alpe Arami (AA) garnet peridotite massif in the Swiss Alps. In this contribution, we show micro-textural evidence of precursor majoritic garnet by documenting exsolved rutile, high-Al orthopyroxene, jadeite-rich clinopyroxene and olivine within the AA garnets in this peridotite. We also document an unforeseen texture of olivine with ‘necklace’ like enstatite corona in the kelyphite formed after decomposition of relict garnet. These olivines bear FeTiO3 and Cr-spinel exsolution needles indicating retrogression from high-pressure Mg2SiO4. Thus, the occurrence of retrogressed high-pressure Mg2SiO4 with enstatite corona in kelyphite suggests majorite breakdown to precipitate high-pressure Mg2SiO4 near mantle transition zone (MTZ) depth. The SiO2 released during decompression of majoritic garnets reacts with the high-pressure Mg2SiO4 to produce the enstatite corona. Our documented micro-textures show high-pressure Mg2SiO4 are breakdown product of precursor majoritic garnet, indicating that these micro-textures of the AA peridotite massif are sourced from the mantle transition zone (MTZ).

Lu–Hf garnet systematics of a polymetamorphic basement unit: new evidence for coherent exhumation of the Adula Nappe (Central Alps) from eclogite-facies conditions

The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks.

The Alpe Arami story: Triumph of data over prejudice

The Alpe Arami garnet peridotite of the Southern Swiss Alps is associated with eclogites and included within quartzofeldspathic gneisses. Controversy has swirled around the depth of origin of this massif since the 1970s when application of the newly-developed technique of thermobarometry suggested a depth of last equilibration of greater than 120 km. Such controversy accelerated in 1996 when we reported microstructural evidence of extensive precipitation of ilmenite and spinel from oli- vine and proposed a much greater depth of origin. Subsequent experiments showed that it was possible to dissolve the observed amount of TiO2 in olivine, but only at depths in excess of 300 km, agreeing with the earlier proposal. In 1999 we added new, independent, evidence concerning exsolution of high-pressure clinoenstatite from diopside that in-and-of-itself required a depth of origin in excess of 250 km. Subsequently, we also added evidence from the surrounding eclogites of very high pressures and experimental evidence that the pyroxenes included in the amoeboid garnets of this rock had ex- solved from a majoritic parent at perhaps even greater pressures. In refutation of the first two of these observations, suggestions were made that (i) we had made a serious error in our estimate of how much ilmenite was present in olivine (and therefore how much TiO2 had been dissolved in olivine); (ii) the il- menite had not been exsolved from olivine but former titanian clinohumite had been present and broke down to yield the ilmenite; (iii) the pyroxene exsolved from diopside had been high-temperature cli- noenstatite. In all three of these cases, the alternatives offered were claimed to be accommodated at low pressures. Here we review the essence of this controversy and show that the only scenario that can ex- plain all of the data is the one that we originally proposed; indeed, the more recent data have strongly supported that interpretation and pushed the minimum origin of the massif to depths approaching 400 km.

Tertiary subduction, collision and exhumation recorded in the Adula nappe, central Alps

Abstract The Adula nappe in the Central Alps represents a lithospheric mélange assembled in a south-dipping subduction zone during the Tertiary orogenic cycle. It consists of several heterogeneous lobes which are stacked in a forward-dipping duplex geometry. Eclogites, garnet peridotites and garnet-white-mica schists record southward-increasing peak pressure conditions which culminate at 12–17 kbar/500–600 °C in the north and 30 kbar/800–850 °C in the south. Some studies infer even higher peak pressures for the garnet peridotite body of Alpe Arami. The present-day metamorphic field gradient for peak pressures exceeds the lithostatic pressure gradient. So far, only eclogites and garnet peridotites from the Cima Lunga complex in the south and the adjacent Southern Steep Belt have yielded Tertiary metamorphic ages for the peak-pressure stage. Some recent studies propose that the Adula nappe got assembled after the formation of high-pressure assemblages in eclogites and garnet peridotites and reject regional high-pressure conditions in Tertiary times. This scenario, however, is in conflict with the observed continuity of metamorphic field gradients and post-peak-pressure structures. Amphibolite facies conditions post-date formation of the Central Alpine nappe stack. In this paper, the associated field gradient is explained through southward-increasing temperatures during near-isothermal decompression. The main mylonitic foliation in the Adula nappe post-dates peak-pressure conditions. It is associated with top-to-the-north shearing and southward-increasing amounts of decompression from eclogite facies to amphibolite facies conditions. Also, the present-day supra-lithostatic field gradient for peak pressures probably results from this deformation phase and is here related to substantial vertical flattening during northward shearing. All subsequent structures affect established nappe boundaries. Pervasive Oligocene deformation events in the Adula nappe are coeval with intense shearing along the so-called Insubric mylonites and occur during ongoing isothermal decompression to around 5 kbar. They are associated with orogen-oblique to orogen-parallel stretching of unspecified amount which may considerably contribute to the exhumation of the Lepontine dome already before the onset of the well-known Miocene extension.

Clinoenstatite exsolution in diopsidic augite of Dabieshan: Garnet peridotite from depth of 300 km

Clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the Bixiling massif of the ultrahigh-pressure metamorphic (UHPM) terrane in the Dabie Mountains, China. Using transmission electron microscopy, we show here that although the lamellae are low clinoenstatite (Lclen) with P2 1 /c space-group symmetry, they contain nanometer-scale antiphase domains indicating that the originally precipitating phase had C2/c space-group symmetry. The lamellae are oriented parallel to [010] and ~18° to [001] of the host.approximately parallel to (401). Both the very cold exhumation path of the Dabie UHPM terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high-temperature clinoenstatite (HTclen). We conclude that the lamellae precipitated as high-pressure clinoenstatite (HPclen) and subsequently inverted to Lclen. Analysis of the incomplete high-pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of ~9 GPa for Dabie and about 12 GPa for similar exsolution previously observed in the Alpe Arami peridotite, Switzerland. The Bixiling complex is a crustal cumulate, hence this result extends the minimum depth of subduction of continental rocks in the Dabie/SuLu orogen to ~300 km (P > 9 GPa).

Exsolution of ilmenite and Cr-Ti magnetite from olivine of garnet-wehrlite

Exsolution of rod-like ilmenite (Ilm) and Cr-Ti magnetite (Mt) have been found in olivine of garnet-wehrlite from the core of Chinese Continental Sciences Drilling (CCSD). Their composition, morphology, crystal structure and their topotaxies with host olivine have been studied in detail by the transmission electron microscopy (TEM) and electron probe microanalyis (EPMA) technique. It shows that rod-like Ilm exsolution reported in this paper has similar characteristic with that of Alpe Arami olivine, while Cr-Ti magnetite exsolution mentioned in this paper has large discrepancy with chromite exsolution in Alpe Arami olivine. These observations suggest that both of the exsolutions found in this paper should be solid solution phases in â-olivine at their first period, then experiencing decomposition of solid solution and therefore forming Ilm and Cr-Ti magnetite exsolution with the pressure decreasing. So, this garnet-wehrlite perhaps had been ever located in mantle transition zone with a minimum depth of 300 km.

Ilmenite exsolution in olivine from the serpentinite body at Zöblitz, Saxonian Erzgebirge – microstructural evidence using EBSD

AbstractIn the large serpentinite body at Zöblitz, which is part of the Gneiss-Eclogite Unit of the Saxonian Erzgebirge, ilmenite rods have been detected in olivine. Although these rods are ≤1 μm wide, they can be unequivocally identified as ilmenite using electron backscatter diffraction. We also applied this method to prove that ilmenite is topotactically intergrown with the olivine host ([100]ol ‖ [001]ilm). This relation was found previously, e.g. for ilmenite exsolved from olivine of garnet peridotite from Alpe Arami, Swiss Alps. The degree of this exsolution phenomenon in the Alpe Arami rocks was taken as an indication of the original formation of a Ti-bearing mineral at depths of ∽300 km. As in the olivines of the serpentinite from Zöblitz, the density of the ilmenite rods is significantly less than in the rock from Alpe Arami. We think that our observation is compatible with previous P-T estimates of close to 4 GPa and 1000–1100°C for the ultramafic bodies in the central Saxonian Erzgebirge.

Li–Be–B systematics in the ultrahigh-pressure garnet peridotite from Alpe Arami (Central Swiss Alps): implications for slab-to-mantle wedge transfer

The abundances of lithium, beryllium and boron in mineral grains from the ultrahigh-pressure garnet peridotite of Alpe Arami were studied by secondary ion mass spectrometry. In order to eliminate the ubiquitous B contamination problem, polished thin sections were cleaned twice in an ultrasonic cleaner using ultrapure water from a Milli-Q water purification system (Millipore ®). Furthermore, a small aperture was used during instrumental analysis so that only the secondary ion beam from the center of the sputtered crater was sampled. Boron abundances measured with the new techniques are generally very low and increase from garnet (10–28 ng/g) via orthopyroxene (25–82 ng/g) and clinopyroxene (26–301 ng/g) to olivine (57–260 ng/g). From these values and the modal composition of the peridotite, a bulk-rock boron content of ∼110 ng/g can be estimated. Beryllium abundances are also very low and increase from olivine (∼0.1–1.0 ng, in rare cases up to 3.4 ng/g) via garnet (<2 ng/g, in rare cases up to 14 ng/g) and orthopyroxene (7–18 ng/g) to clinopyroxene (38–148 ng/g). Bulk-rock Be abundances are estimated to ∼11 ng/g. Our new data, together with additional Li and Be data from an earlier study [J. Paquin, R. Altherr, Contrib. Mineral. Petrol. 143 (2002) 623–640] strongly suggest that the Alpe Arami peridotite was subjected to a metasomatic overprint by aqueous solutions after the peak of Alpine metamorphism (∼5.9 GPa/1180°C) but before the formation of secondary amphibole (at <3 GPa), most likely during exhumation in a supra-subduction zone setting. While this metasomatism led to a substantial addition of Li to the peridotite, the addition of B was very limited and no detectable enrichment in Be was found. Assuming chemical equilibrium between the rims of clinopyroxene grains and the metasomatizing fluid and using experimentally determined cpx/fluid partition coefficients, minimum and maximum abundances of Li, Be and B in the slab fluid were estimated. We obtained concentrations of 12–175 μg/g Li, 8–206 ng/g Be and 1–12 μg/g B. The results of this study indicate that Li and B were effectively decoupled during percolation of slab-released aqueous solutions through the mantle wedge. While substantial amounts of Li were incorporated into the peridotite minerals, most of the B remained within the percolating fluid, due to its lower bulk-rock/fluid partition coefficient as compared to Li. Most probably, the metasomatizing fluids were derived from subducted altered oceanic crust or serpentinized peridotite.

Fast exhumation of the ultrahigh‐pressure Alpe Arami garnet peridotite (Central Alps, Switzerland): constraints from geospeedometry and thermal modelling

AbstractPrevious studies suggest that the metamorphic evolution of the ultrahigh‐pressure garnet peridotite from Alpe Arami was characterized by rapid subduction to a depth of c. 180 km with partial chemical equilibration at c. 5.9 Gpa/1180 °C and an initial stage of near‐isothermal decompression followed by enhanced cooling. In this study, average cooling rates were constrained by diffusion modelling on retrograde Fe–Mg zonation profiles across garnet porphyroclasts. Considering the effects of temperature, pressure and garnet bulk composition on the Fe–Mg interdiffusion coefficient, cooling rates of 380–1600 °C Myr−1 for the interval from 1180 to 800 °C were obtained. Similar or even higher average cooling rates resulted from thermal modelling, whereby the characteristics of the calculated temperature‐time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process. The very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c. 15 mm yr−1 or more. These results agree with the range of exhumation rates (16–50 mm yr−1) deduced from geochronological results. It is suggested that the Alpe Arami peridotite passively returned towards the surface as part of a buoyant sliver, caused as a consequence of slab breakoff.

Quantitative 3D measurement of ilmenite abundance in Alpe Arami olivine by confocal microscopy: Confirmation of high-pressure origin

A critical aspect of the debate about the origin and conditions of metamorphism of the Alpe Arami (AA) peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much TiO 2 might have been dissolved at high pressure and temperature. We have now determined quantitatively the 3-dimensional distribution of ilmenite in AA olivine by confocal laser scanning microscopy (CLSM). The CLSM measurements show an average concentration of 0.31 vol% of ilmenite in olivine, with individual grains containing up to 1.2 vol%. This translates into average and maximum concentrations of 0.23 and 0.9 wt% TiO 2 in olivine, respectively, and confirms the original estimation of maximum concentration of ~1 vol% TiO 2 . The vast majority of ilmenite in AA olivine is distributed randomly (although topotactically oriented) and, in all cases, is accompanied by chromite in a ratio of ~4:1. These observations are consistent with an origin of the ilmenite (and chromite) by exsolution from an olivine solid solution at P = 9–12 GPa and temperatures above the stability field of titanian clinohumite, but are not consistent with suggested breakdown of titanian clinohumite. Combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of H 2 O, and that the high solubility of TiO 2 is probably explained by pressure-induced accommodation of Ti in the tetrahedral site of silicates.

Olivine [100] normal to foliation: lattice preferred orientation in prograde garnet peridotite formed at high H2O activity, Cima di Gagnone (Central Alps)

Automated electron backscattered diffraction (EBSD) was applied using a scanning electron microscope to obtain lattice preferred orientation (LPO) data for olivine in garnet peridotites of the Central Alps. As a reference frame, the LPOs of enstatite were also investigated. In the garnet peridotite at Cima di Gagnone (CDG), a weak foliation carrying a distinct lineation is present. The lineation is characterized by elongated enstatite, olivine and poikiloblastic garnet. Olivine shows a very unusual LPO with [100] normal to foliation and [001] parallel to lineation. Achsenverteilungsanalyse (AVA) maps demonstrate that [001] of olivine grains corresponds quite well to their maximum length axes which are preferentially parallel to the lineation. Numerous planar hydrous defects within (001) planes of olivine are marked by palisades of ilmenite rods and show a preferred orientation normal to lineation. Calculated P-wave velocities for CDG are fastest (8.32 km s−1) normal to foliation with a relatively low anisotropy (2.9%). Compared to mantle peridotites with the usual (010)[100] LPO where the fastest Vp direction is towards the lineation, the relationship between flow geometry and seismic anisotropy is significantly different at CDG. Several mechanisms for the formation of the LPO type at CDG are considered, with glide possible on (100)[001] of olivine. On the basis of field data as well as petrographic and petrologic evidence, it has been demonstrated that the CDG garnet peridotite formed by prograde metamorphism from a hydrous protolith at pressures and temperatures of about 3.0 GPa and 750 °C, respectively. The CDG LPO is interpreted to have formed during hydrous subduction zone metamorphism. The same interpretation may hold for the previously investigated olivine LPO at Alpe Arami, which is similar to that at the nearby CDG. The observed anomalous LPO is no proof for ultradeep (>3.0 GPa) conditions.

Subduction-related lithium metasomatism during exhumation of the Alpe Arami ultrahigh-pressure garnet peridotite (Central Alps, Switzerland)

The inter- and intragrain distribution of Li and Be in the subduction-related ultrahigh-pressure (UHP) garnet peridotite from Alpe Arami, Central Swiss Alps, was studied using secondary ion mass spectrometry. The data indicate substantial Li infiltration during exhumation of this ultramafic body. Orthopyroxene porphyroclasts and neoblasts are characterised by low Li contents (0.11–0.36 µg/g) typical of depleted peridotites, whereas Li zonation profiles across porphyroclasts of garnet and clinopyroxene document a metasomatic addition of Li. Small clinopyroxene grains in the matrix contain extremely high and variable abundances of Li (4–16 µg/g). In marked contrast to the behaviour of Li, the abundances of Be (77–134 ng/g) are similar in all textural types of clinopyroxene. Olivine porphyroclasts and neoblasts are characterised by somewhat elevated Li contents (0.95–1.79 µg/g), typical of fertile lherzolites. All textural types of clinopyroxene in the Alpe Arami peridotite are enriched in Li, providing evidence for infiltration of Li-rich and Be-poor aqueous solutions after the peak of UHP metamorphism. The lack of Li enrichment in orthopyroxene is consistent with orthopyroxene dissolution and formation of secondary olivine and clinopyroxene during metasomatism. Cr-diopside pyroxenite veins and boudins within the peridotite show low abundances of Li, with 0.7–2.5 µg/g in clinopyroxene and 1.1–1.5 µg/g in olivine. These pyroxenites likely represent precipitates from aqueous solutions which infiltrated the host peridotite after Li enrichment of the peridotite. A slab-derived nature of the metasomatic agent is suggested by the general lack of Ti enrichment in the Alpe Arami rocks.

Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

AbstractObservations of oriented SiO2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher‐pressure metamorphism than previously realized. We employed various calibrations of the Fe2+ − Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl2SiO6), of garnet‐pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.

Comment on ‘New Constraints on the P–T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)’ by Paquin &amp; Altherr (2001)

The ‘Alpe Arami controversy’ seems destined not to to is representative of the P–T recorded by the Alpe Arami end as more and more new analytical data become peridotite during Alpine subduction metamorphism dated at available. Recently, Paquin & Altherr (2001; hereafter 43 Ma (Gebauer et al., 1992; Becker, 1993; Gebauer, PA (3) a short-lived thermal event at T [ 1100°C, element zoning profiles in garnets and in various genprobably related to magmatic intrusion at 35 Ma (see erations of pyroxenes from the Alpe Arami peridotite Gebauer, 1996), caused complete re-equilibration of garbody. The results led the authors to conclude that the net Fe–Mg and Ni, but left pyroxene Ca, Cr, Al core peridotite experienced peak metamorphic conditions of contents virtually unaltered; (4) no unequivocal baro1180°C and 5·9 GPa. These values are in fairly good metric estimate can be given for this high-T stage on the basis of available data, although pressures not very agreement with the high P–T estimates of Brenker & dissimilar from those achieved at the peak of subduction Brey (1997), but are at variance with the more recent metamorphism are the most likely. estimates of Nimis & Trommsdorff (2001; hereafter NT Harley, 1984; perature estimates are not in conflict with each other and Krogh Ravna, 2000), Co and Ni partitioning between reflect two distinct stages of equilibration characterized by clinopyroxene (Cpx) and orthopyroxene (Opx) (Seitz et different thermal conditions; (2) the low-T stage coral., 1999), and Ni partitioning between Ol and Grt (Canil, 1999). When coupled with the Opx–Grt barometer of responds to N&T’s estimates of 840°C and 3·2 GPa and

New Constraints on the P–T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)

The metamorphic evolution of the garnet peridotite body of Alpe Arami, Central Alps, is a matter of current controversy. In this paper, the inter- and intragrain distribution of major and trace elements obtained by electron and ion probe microanalyses is used to better constrain the P–T evolution of this peridotite. Using the compositions of homogeneous porphyroclast cores, peak metamorphic conditions of 1180 ± 40°C and 5·9 ± 0·3 GPa are estimated, based on consistent results from the application of several independent thermometers (Fe–Mg exchange between garnet, pyroxenes and olivine, Ni exchange between garnet and olivine, Co and Ni exchange between orthopyroxene and clinopyroxene), the Al-in-orthopyroxene barometer and the Ca–Cr systematics of garnet. Orthopyroxene and clinopyroxene porphyroclasts are, however, not in equilibrium with respect to some elements with low diffusivities, such as Ca, Ti, Cr, V and Sc. This disequilibrium appears to be the main cause for the lower P–T values suggested by some of the previous workers. On the other hand, there is no evidence for an ultradeep (>200 km) origin of the Alpe Arami body as postulated recently. Chemical zonation profiles across mineral grains suggest that during retrograde evolution a near-isothermal decompression was followed by accelerated cooling.

Genesis of ilmenite rods and palisades along humite-type defects in olivine from Alpe Arami

Rod-shaped ilmenite inclusions in olivine have been observed in numerous peridotite lenses of the Adula–Cima Lunga unit, Central Alps. The rods are elongated parallel to [010]ol and identical in size, shape and concentration to those reported from garnet peridotite at Alpe Arami (AA). TEM investigation of the rods at AA, Cima di Gagnone (CDG) and Monte Duria (MD) demonstrates that the titanate inclusions correspond to a Mg-bearing ilmenite with exclusively a trigonal crystal structure. No other FeTiO3 polymorphs have been identified. The shape of the ilmenite inclusions concurs with minimum topotactic misfits between the lattices of olivine and Mg-bearing ilmenite. Optical microscopy reveals that the ilmenite rods are not randomly distributed within olivine but have, at all localities, a preferred alignment parallel to (001)ol, forming palisades. TEM images of the ilmenite palisades at AA and CDG show that they are controlled by the presence of (001) planar defects in olivine along which the rods nucleated. The defects have been characterised by high-resolution TEM and identified as 4.4 A-wide humite-type layers, i.e. (Mg2SiO4)·Mg(OH,F)2, intergrown within olivine. In such layers substitution of up to 50 mol% MgH2 by Ti4+ is possible. Assuming Ti-saturated humite slabs, the maximum density of (001) faults observed at AA, approximately 1900 mm–1, can accommodate about 170 ppm TiO2 in olivine. This represents about half the values obtained by UV-laser ablation ICP-MS and EMPA analysis. Evidence for annealing of the defects, however, suggests that only parts of the defects are still preserved. The formation of ilmenite rods is considered to be a consequence of the breakdown of isolated humite layers in olivine. The apparent TiO2 content in olivine at AA, CDG and MD is regarded as a function of the Ti-humite defect density.

Revised Thermobarometry of Alpe Arami and other Garnet Peridotites from the Central Alps

sensitivity of garnet–olivine chemical exchanges to temperature variThe four, currently best constrained, independent thermobarometers ations, and unreliability of garnet–orthopyroxene thermometry at lowfor garnet peridotites, namely Taylor’s (Neues Jahrbuch fur temperature conditions may account for the observed inconsistencies. A Mineralogie, Abhandlungen 172, 381–408, 1998) pyroxene discrepancy between thermobarometric data for the garnet lherzolites solvus and Krogh’s (Contributions to Mineralogy and Petand the associated eclogites is non-existent in our example, and in rology 99, 44–48, 1988) clinopyroxene–garnet Fe–Mg exchange other localities it may also be a thermobarometric artefact. thermometers, and Taylor’s (1998) Al-in-orthopyroxene and Nimis & Taylor’s (Contributions to Mineralogy and Petrology, 139, 541–544, 2000) Cr-in-clinopyroxene barometers, have been applied to garnet lherzolites from the Central Alps. Analyses from

Mineralogical and experimental evidence for very deep exhumation from subduction zones

In 1996, we proposed that the Alpe Arami lherzolite, Switzerland, contains evidence within it implying origin at a depth of greater than 300 km. Suggestion of such extraordinary depth of exhumation of these rocks has been controversial. Principal amongst the original evidence was a very high concentration of oxide precipitates in olivine. From the abundance, morphology, crystallography, and topotaxy of these oxides, it was argued that the inferred very high solubility of highly-charged cations (principally Ti and Cr) represented a previously unrecognized mantle environment and that pressures in excess of 10 GPa were the most likely explanation of the observations. We have now successfully completed high-pressure experiments to test whether there are any conditions under which olivine can dissolve the high concentrations of TiO 2 we originally inferred (>0.6 wt%). We answer the question in the affirmative for P≥10 GPa, consistent with our hypothesis. In addition to these experimental results, we also have discovered in the same rocks exsolution lamellae of clinoenstatite in diopside. These lamellae contain antiphase domains which indicate that the originally-precipitating phase was a C2/c pyroxene; additional geologic and crystallographic observations strongly suggest that the precipitating phase was high-pressure clinoenstatite, thereby providing independent evidence of a minimum depth of origin of this massif of 250 km. To those observations, we add here exsolution of SiO 2 from omphacite of Alpe Arami eclogite. Discoveries similarly implying very great depth of exhumation of mantle rocks and/or subduction to and return from such depths are now known from other continental collision terranes. In particular, recent discovery of exsolution of pyroxenes from garnets in the peridotites of the Western Gneiss terrane of Norway provides unambiguous evidence of a very deep origin for these peridotites (>185–200 km), and discovery of microdiamonds in metasediments of the Erzgebirge of Saxony adds to the growing list of continental terranes exhumed from >100 km. Realization that such deep subduction and exhumation have occurred multiple times spanning the entire Phanerozoic strongly suggest that this phenomenon is a normal process of continental collision rather than a bizarre curiosity. Whether such exhumation is a single- or multiple-step process is an important question for future research.

Geodynamic cycles of subcontinental lithosphere in the Central Alps and the Arami enigma

Major masses of metamorphosed ultramafic rocks in the Central Alps, Western Alps and Liguria are former subcontinental lithospheric mantle. It occurs in two, tectonically distinct ophiolitic zones which paleogeographically correspond to the Jurassic, South Penninic, Piemontese ocean belt and to the more external, Cretaceous, North Penninic, Valais ocean belt, respectively. In both belts, the geodynamic cycle of pieces of subcontinental lithospheric mantle started with its exhumation during continental rifting and resulted in the exposure of denuded mantle in the Tethyan ocean basins. With the onset of oceanic closure, these mantle rocks were subducted and, after continental collision, integrated in the Alpine mountain chain and again exhumed. Combined evidence from two key areas of the Central Alps is presented. At Val Malenco, a stable continental crust-mantle transition (1.0–0.9 GPa; 800–600°C) welded by a 270 Ma gabbro intrusion existed into the Triassic. During Jurassic rifting, Malenco mantle with attached pieces of lower crust was rapidly exhumed and exposed in the Piemontese ocean basin near its Adriatic margin. There, an ‘Alpine type ophiolite suite’ developed, containing serpentinized ex-subcontinental mantle, rodingitized Permian gabbro, ophicarbonate rocks and MOR basalts. During Cretaceous convergence, the rifted margin of Val Malenco was only moderately subducted (∼450°C, 0.6 GPa) as it remained in a supra-subduction wedge. In the Western Alps and Liguria, rifted margins of the Piemontese belt were subducted to eclogite conditions (∼3 GPa, 650°C), but never beyond the stability of Antigorite. In the North-Penninic Adula-Cima Lunga nappe of the Central Alps, however, a suite analogous to Val Malenco was subducted to much higher metamorphic conditions (∼3 GPa, 830°C) during Eocene collision. Successive regional metamorphic prograde stages are proven by overgrowth textures in mafic rocks up to kyanite eclogite and in ultramafic rocks up to garnet peridotite. The inferred ultradeep garnet lherzolite of Alpe Arami forms part of the prograde Alpine suite of Adula-Cima Lunga. Ilmenite rods in olivine, as at Arami, invoked as ‘proof’ for an ultradeep origin, have been detected in over 10 localities. They have been shown to form from planar, Ti-bearing, (OH)-defects at conditions below 3 GPa. No direct evidence for an ultradeep origin for Alpe Arami has been presented so far.

The solubility of TiO 2 in olivine: implications for the mantle wedge environment 1

One characteristic of many subduction zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite (TiCH) and/or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Lepontine Alps, while lacking TiCH, displays instead large numbers of FeTiO 3 rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO 2 in olivine was greater than 0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO 2 in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350-1700 K. Our results on anhydrous compositions show that, whereas solubility of TiO 2 was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.5 wt.% at 8 and 10 GPa, and to 1 wt.% at 12 GPa, all at 1600 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from depths > 300 km (corresponding to pressures > 10 GPa) but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones. © 2000 Elsevier Science B.V. All rights reserved.