The Alpe Arami story: Triumph of data over prejudice

Abstract

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 olivine 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 exsolved 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 ilmenite 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 clinoenstatite. 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 explain 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.

Keywords

Alpe Arami / TiO₂ in olivine / high-pressure clinoenstatite / deep exhumation

References

[1] Akaogi M., Ito E., Navrotsky A.. Olivine Modified Spinel-Spinel Transition in the System Mg₂SiO₄-Fe₂SiO₄: Calorimetric Measurements, Thermochemical Calculation, and Geophysical Application. J. Geophys. Res., 1989, 94: 15671-15685.

[2] Angel R. J., Chopelas A., Ross N. L.. Stability of High-Density Clinoenstatite at Upper-Mantle Pressures. Nature, 1992, 358(6384): 322-324.

[3] Arlt T., Kunz M., Stolz J., . P-T-X Data on P2₁/c-Clinopyroxenes and Their Displacive Phase Transitions. Contrib. Mineral. Petrol., 2000, 138(1): 35-45.

[4] Bakun-Czubarow N.. Quartz Pseudomorphs after Coesite and Quartz Exsolutions in Eclogitic Omphacites of the Zlote Mountains in the Sudetes (SW Poland). Archiwum Mineralogiczne, 1992, 48(1–2): 3-25.

[5] Becker H.. Garnet Peridotite and Eclogite Sm-Nd Mineral Ages from the Lepontine Dome (Swiss Alps): New Evidence for Eocene High-Pressure Metamorphism in the Central Alps. Geology, 1993, 21: 599-602.

[6] Boyd F. R.. A Pyroxene Geotherm. Geochimica et Cosmochimica Acta, 1973, 37(12): 2533-2546.

[7] Bozhilov K. N., Green H. W., Dobrzhinetskaya L. F.. Quantitative 3D Measurement of Ilmenite Abundance in Alpe Arami Olivine by Confocal Microscopy: Confirmation of High-Pressure Origin. Am. Mineral., 2003, 88: 596-603.

[8] Bozhilov K. N., Green H. W., Dobrzhinetskaya L.. Clinoenstatite in Alpe Arami Peridotite: Additional Evidence for very High Pressure. Science, 1999, 284(5411): 129-132.

[9] Brenker F. E., Brey G. P.. Reconstruction of the Exhumation Path of the Alpe Arami Garnet-Peridotite Body from Depths Exceeding 160 km. J. Metamorphic Geology, 1997, 15(5): 581-592.

[10] Buiskool-Toxopeus J. M. A.. Petrofabrics, Microtextures and Dislocation Substructures of Olivine in a Peridotite Mylonite (Alpe Arami, Switzerland). Leidse Geol. Med., 1976, 51(1): 1-35.

[11] Dobrzhinetskaya L. F., Bozhilov K. N., Green H. W.. The Solubility of TiO₂ in Olivine: Implications for the Mantle Wedge Environment. Chem. Geol., 2000, 160(4): 357-370.

[12] Dobrzhinetskaya L. F., Green H. W., Renfro A. P., . Precipitation of Pyroxenes and Mg₂SiO₄ from Majoritic Garnet: Simulation of Peridotite Exhumation from Great Depth. Terra Nova, 2004, 16(6): 325-330.

[13] Dobrzhinetskaya L. F., Green H. W., Renfro A. P., . Chen J. H., Wang Y. B., Duffy T. S., . Decompression of Majoritic Garnet: An Experimental Investigation of Mantle Peridotite Exhumation. Advances in High-Pressure Technology for Geophysical Applications, 2005, Amsterdam: Elsevier 265 287

[14] Dobrzhinetskaya L. F., Green H. W., Wang S.. Alpe Arami: A Peridotite Massif from Depths of more than 300 km. Science, 1996, 271(5257): 1841-1845.

[15] Dobrzhinetskaya L. F., Schweinehage R., Massonne H. J., . Silica Precipitates in Omphacite from Eclogite at Alpe Arami, Switzerland: Evidence of Deep Subduction. J. Metamorphic Geology, 2002, 20(5): 481-492.

[16] Ernst W. G.. Mineralogic Study of Eclogitic Rocks from Alpe Arami, Lepontine Alps, Southern Switzerland. J. Petrology, 1977, 18(3): 371-398.

[17] Ernst W. G.. Petrochemical Study of Lherzolitic Rocks from the Western Alps. J. Petrology, 1978, 19(3): 341-392.

[18] Ernst W. G.. Petrogenesis of Eclogites and Peridotites from the Western and Ligurian Alps. Am. Mineral., 1981, 66: 443-472.

[19] Gayk T., Kleinschrodt R., Langosch A., . Quartz Exsolution in Clinopyroxene of High-Pressure Granulite from the Münchberg Massif. European Journal of Mineralogy, 1995, 7: 1217-1220.

[20] Gebauer, D., 1996. A P-T-t Path for an (Ultra-) High-Pressure Ultramafic/Mafic Rock-Association and Its Felsic Country-Rocks Based on SHRIMP-Dating of Magmatic and Metamorphic Zircon Domains, Example: Alpe Arami (Central Swiss Alps). In: Basu, A., Hart, S. R., eds., Earth Processes: Reading the Isotopic Code. Geophysical Monograph, 95: 307–329

[21] Green H. W., Dobrzhinetskaya L. F., Bozhilov K. N.. Determining the Origin of Ultra-high Pressure Lherzolites (Response). Science, 1997, 278: 704-707.

[22] Green H. W., Dobrzhinetskaya L. F., Riggs E. M., . Alpe Arami: A Peridotite Massif from the Mantle Transition Zone?. Tectonophysics, 1997, 279(1–4): 1-21.

[23] Green H. W., Dobrzhinetskaya L., Bozhilov K. N.. Mineralogical and Experimental Evidence for very Deep Exhumation from Subduction Zones. J. Geodynamics, 2000, 30(1–2): 61-76.

[24] Hacker B. R., Sharp T., Zhang R. Y., . Determining the Origin of Ultrahigh-Pressure Lherzolites (Discussion). Science, 1997, 278(5338): 701-707.

[25] Haggerty S. E., Sautter V.. Ultradeep (Greater than 300 Kilometers), Ultramafic Upper Mantle Xenoliths. Science, 1990, 248(4958): 993-996.

[26] Hermann J., O’Neill H. S. C., Berry A. J.. Titanium Solubility in Olivine in the System TiO₂-MgO-SiO₂: No Evidence for an Ultra-deep Origin of Ti-Bearing Olivine. Contrib. Mineral. Petrol., 2005, 148(6): 746-760.

[27] Irifune T., Sekine T., Ringwood A. E., . The Eclogite-Garnetite Transformation at High Pressure and Some Geophysical Implications. Earth and Planetary Science Letters, 1986, 77(2): 245-256.

[28] Katayama I., Parkinson C. D., Okamoto K., . Supersilicic Clinopyroxene and Silica Exsolution in UHPM Eclogite and Pelitic Gneiss from the Kokchetav Massif, Kazakhstan. Am. Mineral., 2000, 85: 1368-1374.

[29] Liu X. W., Jin Z. M., Green H. W.. Clinoenstatite Exsolution in Diopsidic Augite of Dabieshan: Garnet Peridotite from Depth of 300 km. Am. Mineral., 2007, 92: 546-552.

[30] Medaris L. G., Carswell D. A.. Carswell D. A.. The Petrogenesis of Mg-Cr Garnet Peridotites in European Metamorphic Belts. Eclogite Facies Rocks, 1990, London: Blackie-Glasgow 260 290

[31] Möckel J. R.. Structural Petrology of the Garnet Peridotite of Alpe Arami (Ticino, Switzerland). Leidse Geol. Med., 1969, 42: 61-130.

[32] Nee, P. Y., Zhao, S., Green, H. W., et al., 2009. Experimental Studies of the CaEsk Component in Pyroxene at High PT in Multianvil Apparatus. American Geophysical Union, Fall Meet, Abstract, MR13A-1657

[33] Nestola F., Tribaudino M., Ballaran T. B.. High Pressure Behavior, Transformation and Crystal Structure of Synthetic Iron-Free Pigeonite. Am. Mineral., 2004, 89: 189-196.

[34] Nimis P., Trommsdorff V.. Revised Thermobarometry of Alpe Arami and Other Garnet Peridotites from the Central Alps. J. Petrology, 2001, 42(1): 103-115.

[35] Nimis P., Trommsdorff V.. Comment on ‘New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridoite Body (Central Alps, Switzerland)’ by Paquin & Altherr (2001). J. Petrology, 2001, 42(9): 1773-1779.

[36] Olker B., Altherr R., Paquin J.. Fast Exhumation of the Ultrahigh-Pressure Alpe Arami Garnet Peridotite (Central Alps, Switzerland): Constraints from Geospeedometry and Thermal Modelling. J. Metamorphic Geology, 2003, 21(4): 395-402.

[37] Paquin J., Altherr R.. New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland). J. Petrology, 2001, 42(6): 1119-1140.

[38] Paquin J., Altherr R.. ’New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridoite Body (Central Alps, Switzerland)’: Reply to Comment by Nimis and Trommsdorff (2001). J. Petrology, 2001, 42(9): 1781-1787.

[39] Paquin J., Altherr R.. Subduction-Related Lithium Metasomatism during Exhumation of the Alpe Arami Ultrahigh-Pressure Garnet Peridotite (Central Alps, Switzerland). Contrib. Mineral. Petrol., 2002, 143(5): 623-640.

[40] Paquin J., Altherr R., Ludwig T.. Li-Be-B Systematics in the Ultrahigh-Pressure Garnet Peridotite from Alpe Arami (Central Swiss Alps): Implications for Slab-to-Mantle Wedge Transfer. Earth and Planetary Science Letters, 2004, 218(3–4): 507-519.

[41] Risold A. C., Trommsdorff V., Grobéty B.. Genesis of Ilmenite Rods and Palisades along Humite-Type Defects in Olivine from Alpe Arami. Contrib. Mineral. Petrol., 2001, 140(5): 619-628.

[42] Robinson P., Ross M., Nord G. L., . Exsolution Lamellae in Augite and Pigeonite: Fossil Indicators of Lattice Parameters at High Temperature and Pressure. Am. Mineral., 1977, 62: 857-873.

[43] Shinmei T., Tomioka N., Fujino K., . In Situ X-Ray Diffraction Study of Enstatite up to 12 GPa and 1 473 K and Equations of State. Am. Mineral., 1999, 84: 1588-1594.

[44] Smith D. C.. Smith D. C.. A Review of the Peculiar Mineralogy of the ‘Norwegian Coesite-Eclogite Province’, with Crystal-Chemical. Petrological, Geochemical and Geodynamical Notes and an Extensive Bibliography. Eclogite and Eclogite-Facies Rocks, 1988, Amsterdam: Elsevier 1 206

[45] Smith D. C., Cheeney R. F.. Orientated Needles of Quartz in Clinopyroxene: Evidence for Exsolution of SiO₂ from a Non-Stoichiometric Supersilicic “Clinopyroxene”. 26th International Geol. Congress (Abstract), Paris, 1980, 1 14

[46] Smyth J. R.. Cation Vacancies and the Crystal Chemistry of Breakdown Reactions in Kimberlitic Omphacites. Am. Mineral., 1980, 65: 1185-1191.

[47] Spengler D., Van-Roermund H. L. M., Drury M. R., . Deep Origin and Hot Melting of an Archaean Orogenic Peridotite Massif in Norway. Nature, 2006, 440(7086): 913-917.

[48] Tinker, D., Lesher, C. E., 2001. Solubility of TiO₂ in Olivine from 1 to 8 GPa. EOS Transactions of the American Geophysical Union, Fall Meeting 2001, Abstract #V51B-1001

[49] Tribaudino M., Nestola F., Camara F., . The High-Temperatur P2₁/c-C2/c Phase Transition in Fe-Free Pyroxene (Ca_0.15Mg_1.85Si₂O₆): Structural and Thermodynamic Behavior. Am. Mineral., 2002, 87: 648-657.

[50] Tribaudino M., Prencipe M., Bruno M., . High-Pressure Behaviour of Ca-Rich C2/c Clinopyroxenes along the Join Diopside-Enstatite (CaMgSi₂O₆-Mg₂Si₂O₆). Phys. Chem. Minerals, 2000, 27(9): 656-664.

[51] Tsai C. H., Liou J. G.. Eclogite-Facies Relics and Inferred Ultrahigh-Pressure Metamorphism in the North Dabie Complex, Central-Eastern China. Am. Mineral., 2000, 85: 1-8.

[52] Van-Ark E. M., Ulmer P., Risold A. C., . TiO₂ Solubility in Mantle Olivine as a Function of Pressure, Temperature, a(SiO₂) and f(H₂). EOS Trans. Am. Geophys. Union, 1998, 79 17 164

[53] Van Roermund H. L. M., Drury M. R.. Ultra-high Pressure (P>6 GPa) Garnet Peridotites in Western Norway: Exhumation of Mantle Rocks from >185 km Depth. Terra Nova, 1998, 10(6): 295-301.

[54] Van Roermund H. L. M., Drury M. R., Barnhoorn A., . Relict Majoritic Garnet Microstructures from Ultra-deep Orogenic Peridotites in Western Norway. J. Petrology, 2001, 42(1): 117-130.

[55] Wenk E.. Zur Regionalmetamorphose und Ultrametamorphose im Lepontin Regional Metamorphism and Ultrametamorphism in the Lepontine Alps. Fortschr. Miner., 1970, 47(1): 34-51.

[56] Wenk E., Keller F.. Isograde in Amphibolitserien der Zentralalpen Isograds in Amphibolite Series of the Central Alps. Schweiz. Mineral. Petrogr. Mitt., 1969, 49(1): 157-198.

[57] Wenk E., Schwander H., Stern W.. On Calcic Amphiboles and Amphibolites from the Lepontine Alps. Schweiz. Mineral. Petrogr. Mitt., 1974, 54(1): 97-149.

[58] Woodland A. B., Angel R. L.. Reversal of the Orthoferrosilite-High-Pressure Clinoferrosilite Transition, a Phase Diagram for FeSiO₃ and Implications for the Mineralogy of the Earth’s Upper Mantle. Eur. J. Mineral., 1997, 9 245

[59] Yamaguchi Y., Akai J., Tomita K.. Clinoamphibole Lamellae in Diopside of Garnet Lherzolite from Alpe Arami, Bellinzona, Switzerland. Contrib. Mineral. Petrol., 1978, 66(3): 263-270.

[60] Zhang L. F., Song S. G., Liou J. G., . Relict Coesite Exsolution in Omphacite from Western Tianshan Eclogites, China. Am. Mineral., 2005, 90: 181-186.

[61] Zhang R. Y., Hirajima T., Banno S., . Petrology of Ultra-high Pressure Rocks from the Southern Sulu Region, Eastern China. J. Metamorphic Geology, 1995, 13(6): 659-675.

[62] Zhang Z. M., Shen K., Xiao Y. L., . Mineral and Fluid Inclusions in Zircon of UHP Metamorphic Rocks from the CCSD-Main Drill Hole: A Record of Metamorphism and Fluid Activity. Lithos, 2006, 92(3–4): 378-398.

[63] Zhu Y. F., Ogasawara Y.. Phlogopite and Coesite Exsolution from Super-silicic Clinopyroxene. International Geology Review, 2002, 44(9): 831-836.