Skip to main content
Article thumbnail
Location of Repository

Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

By Hilary Downes, R. MacDonald, B.G.J. Upton, K.G. Cox, J.L. Bodinier, P.R.D. Mason, D. James, P.G. Hill and B.C. Hearn


Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al<inf>2</inf>O<inf>3</inf> contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al<inf>2</inf>O<inf>3</inf> and TiO<inf>2</inf>) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0·5121 (close to the host minette values) to 0·5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0·5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas. The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in large ion lithophile elements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr–Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallization at mantle depths of minette magmas in Eocene times, that caused further metasomatic enrichment of the lithosphere

Topics: es
Publisher: Oxford Journals
Year: 2004
OAI identifier:

Suggested articles


  1. (1991). 7. Temperature and fO2 estimates from ultramafic xenoliths from the Bearpaw Mountains, using the method of Ballhaus et
  2. 8. Whole-rock compositional data for spinel peridotite tectonite xenoliths from the Bearpaw Mountains.
  3. (1997). A compilation of new and published major and trace element data for doi
  4. (1972). Abbreviations: C = cumulate; W = wehrlite; M = mica; P = pyroxenite; Wb = websterite; G = glimmerite Table 11 Trace element analyses of Bearpaw samples by solution ICP-MS Spinel peridotite tectonites
  5. (1975). Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. doi
  6. (1974). All values of Na2O were below detection limit.
  7. (1969). An accurate spectrographic method for the analysis of a wide range of geological samples. doi
  8. (1991). Ancient and enriched upper mantle beneath north-central Montana: evidence from xenoliths.
  9. (1997). and 21 others doi
  10. (1992). Application of the ICP-MS technique to trace-element analysis of peridotites and their minerals. doi
  11. (1989). Bearpaw Mountains, doi
  12. (1990). Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kb with applications. doi
  13. (1988). Chemical and isotopic evidence for potassic mafic magmas from an old K-enriched mantle source and for Eocene crustal melting in the central Montana alkalic province.
  14. (1989). Chemical and isotopic studies of ultramafic inclusions from the San Carlos Volcanic Field, Arizona: a bearing on their petrogenesis. doi
  15. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. doi
  16. (1999). Chemical and isotopic systematics of peridotite xenoliths from the Williams kimberlite, Montana: clues to processes of lithosphere formation, modification and destruction.
  17. (1986). Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from highpressure experiments and natural rocks. doi
  18. (1992). Comparison of the compositions of phlogopites in Bearpaw ultramafic xenoliths with the composition of phenocrysts in host minettes (Macdonald et al.,
  19. (1994). compilation of working values and sample description for 383 geostandards. doi
  20. (1999). Compositionally stratified cratonic lithosphere: petrology and geochemistry of peridotite xenoliths from the Labait volcano,
  21. (1996). Constraints from partitioning experiments on the composition of subductionzone fluids. doi
  22. (1990). Constraints on the composition of the continental lithospheric mantle. doi
  23. (1987). Crust and upper mantle beneath the northern plains; evidence from xenoliths.
  24. (1998). Data from Table 2. Field of non-cratonic mantle olivines from McDonough & Rudnick
  25. (1999). Dating mantle metasomatism; U-Pb geochronology of zircons in cratonic mantle xenoliths from Montana and Tanzania.
  26. (1998). Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting. doi
  27. (1994). Depletion and enrichment history of subcontinental lithospheric mantle: An Os, Sr, Nd and Pb isotopic study of ultramafic xenoliths from the northwestern Wyoming Craton. doi
  28. (1999). Determination of incompatible trace elements in mantle clinopyroxenes by LA-ICP-MS: a comparison of analytical performance with established techniques. Geostandards Newsletter: doi
  29. (1991). Eocene potassic magmatism in the Highwood Mountains, Montana: petrology, geochemistry and tectonic implications. doi
  30. (1988). Evidence for Archean subduction and crustal recycling, Wyoming Province. doi
  31. (1990). Experimental study of barium partitioning between phlogopite and silicate liquid at upper-mantle pressure and temperature. doi
  32. (1994). Ferric iron in peridotites and mantle oxidation states. doi
  33. (2001). Field for spinel peridotites from Downes
  34. (1998). Field of spinels and olivines in cratonic mantle spinel peridotites from Bernstein et al
  35. (1992). Fields of pyroxenes in mantle peridotites from McDonough & Rudnick
  36. (1996). Fluid-peridotite interactions in mantle wedge xenoliths.
  37. (1986). Fluids in equilibrium with peridotite minerals: implications for mantle metasomatism. doi
  38. (2001). Formation and modification of the shallow sub-continental lithospheric mantle; a review of geochemical evidence from ultramafic xenolith suites and tectonically emplaced ultramafic massifs of western and central Europe. doi
  39. (1984). Garnet peridotites from Williams kimberlites, north-central doi
  40. (1997). Genesis and evolution of low-Al orthopyroxene in spinel peridotite xenoliths, doi
  41. (1991). Geochemistry of igneous rocks of the Crazy Mountains, Montana, and tectonic models for the Montana alkalic province. doi
  42. (1991). High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contributions to Mineralogy and Petrology doi
  43. (1986). In search of a bulk-earth composition. doi
  44. (1989). Introduction - T346 Montana high-potassium igneous province. doi
  45. (1987). Kimberlite-transported nodules from Colorado-Wyoming; a record of enrichment of shallow portions of an infertile lithosphere. doi
  46. (1986). Kimberlites: Mineralogy, Geochemistry and Petrology. doi
  47. (1996). Laser ablation inductively coupled plasma – mass spectrometric transient signal data acquisition and analyte concentration calculation. doi
  48. (1980). Late Cretaceous-Paleocene-Eocene igneous activity in north-central Montana.
  49. (1991). Long term reproducibility of multicollector Sr and Nd isotope ratio analysis. doi
  50. (2003). Mantle domains in the lithosphere beneath the French Massif Central: trace element and isotopic evidence from mantle clinopyroxenes. doi
  51. (1997). Mantle upwelling and metasomatism beneath Central Europe: geochemical and isotopic constraints from mantle xenoliths from the Rhön (Germany). doi
  52. (1989). Mantle-normalised trace element abundances for clinopyroxenes from Bearpaw ultramafic xenoliths (LA-ICP-MS data). Normalising values for primitive mantle from Sun & McDonough
  53. (1996). Melt enrichment of shallow depleted mantle: a detailed petrological, trace element and isotopic study of mantle-derived xenoliths and megacrysts from the Cameroon Line. doi
  54. (1998). Melting and metasomatism in the continental lithosphere: Laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia. Contributions to Mineralogy and Petrology doi
  55. (1998). Melting of garnet peridotite and the origin of komatiite and depleted lithosphere. doi
  56. (1993). Melting study of peridotite KLB1 to 6.5 GPa, and the origin of basaltic magmas. doi
  57. (1997). Mexican peridotite xenoliths and tectonic terranes: correlations among vent location, texture, temperature, pressure, and oxygen fugacity. doi
  58. mica; Table 10. Whole-rock compositional data for mica clinopyroxenite and mica websterite xenoliths from the Bearpaw Mountains.
  59. (1998). Mineralogy and composition of the upper mantle.
  60. (1997). Minor elements in olivine from spinel lherzolite xenoliths: implications for thermobarometry. doi
  61. (1990). Montana Archean lithospheric mantle: chemical constraints from Euanomalous ultramafic xenoliths in minette.
  62. (1988). Multiple source components for alkalic igneous rocks in the Wyoming Province: isotopic and trace element evidence from central Montana.
  63. (1974). Normalising factors from Nakamura
  64. (1989). Normalising values for primitive mantle from Sun & McDonough
  65. (1994). Northern Tanzanian peridotite xenoliths: a comparison with Kaapvaal peridotites and inferences on metasomatic interactions.
  66. (1993). Partial melting of dry peridotites at high pressures: Determination of compositions of melts segregated from peridotites using aggregates of diamond. doi
  67. (1988). Pb, Sr, and Nd isotopic compositions of a suite of Late Archean igneous rocks, eastern Beartooth Mountains: implications for crust-mantle evolution. doi
  68. (1999). Peridotite xenoliths from Porcupine Dome,
  69. (1994). Peridotite xenoliths from western Grand Canyon and The Thumb: A probe into the subcontinental mantle of the Colorado Plateau. doi
  70. (1987). Peridotite xenoliths in Massif Central basalts, France: textural and geophysical evidence for asthenospheric diapirism.
  71. (1995). Petrology and geochemistry of mantle peridotite xenoliths from SE China. doi
  72. (1987). Petrology and geochemistry of the Cretaceous Independence volcanic suite, Absoroka Mountains, Montana: clues to the composition of the Archean subMontana mantle. doi
  73. (1987). Petrology of lamproites from Smoky Butte, doi
  74. (1997). Phase relations of natural phlogopite with and without enstatite up to 8 GPa: implications for mantle metasomatism. doi
  75. (1992). Potassic mafic lavas of the Bearpaw Mountains, Montana: mineralogy, chemistry, and origin. doi
  76. (1989). Primary and secondary mineralogy of carbonated peridotites from the MacDougal Springs diatreme.
  77. (1969). Quantitative electron microanalysis of rock-forming minerals. doi
  78. (1987). Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana. doi
  79. (2000). Relationships between geochemistry and structure beneath a palaeo-spreading centre: A study of the mantle section doi
  80. (1999). Selected major element oxides vs MgO for Bearpaw ultramafic xenoliths (data from Tables 8-10). Field of off-craton spinel peridotites from Downes (2001). Field of cratonic mantle spinel peridotites from Lee and Rudnick
  81. (1997). Spinel peridotite xenoliths from the AtsaginDush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle. Contributions to Mineralogy and Petrology doi
  82. (2001). Sr-Nd isotope diagram for ultramafic xenoliths from the Bearpaw Mountains (data from Table 12), compared with spinel peridotite xenoliths from non-cratonic lithospheric mantle of western Europe (Downes,
  83. (1985). Sr, Nd, and Pb isotope and minor-element geochemistry of lamproites and kimberlites. doi
  84. (1995). Strontium, neodymium, and lead isotopic evidence for the interaction of post-subduction asthenospheric potassic mafic magmas of the Highwood Mountains, doi
  85. (1989). The dependence of the Fe 2+-Mg cation-partitioning between olivine and basaltic liquid on pressure, temperature and composition. Contributions to Mineralogy and Petrology doi
  86. (1989). The dependence of the Fe2+-Mg cation-partitioning between olivine and basaltic liquid on pressure, temperature and composition. Contributions to Mineralogy and Petrology doi
  87. (1992). The oxidation state of subcontinental mantle: oxygen thermobarometry of mantle xenoliths from central Asia. Contributions to Mineralogy and Petrology doi
  88. (1974). The system MgO-Al2O3-SiO2: solubility of Al2O3 in enstatite for spinel and garnet peridotite compositions. doi
  89. (1996). Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones. doi
  90. (1978). Ultramafic inclusions from San Carlos, Arizona; petrologic and geochemical data bearing on their petrogenesis. doi
  91. (1998). Volcanic rocks from the southeast Greenland margin at 63°N: composition, petrogenesis and mantle sources. doi
  92. Wt. % MgO Mica websterites Mica pyroxenite cumulates Mica peridotite cumulates Peridotite tectonites LSC 225 field of spinel peridotites

To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.