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Pyroxenite xenoliths from the Rio Puerco volcanic field, New Mexico: Melt metasomatism at the margin of the Rio Grande rift

¹ Department of Earth & Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA

View larger version (56K): [in this window] [in a new window]   Figure 1. Generalized map and satellite image showing location of the Rio Puerco volcanic (RPV) necks relative to the Colorado Plateau, the Rio Grande rift, and the Jemez Lineament (satellite image excerpted from Resource Map 23 from the New Mexico Bureau of Mining and Mineral Resources, 2000).

View larger version (176K): [in this window] [in a new window]   Figure 2. Photos from Cerrito Negro. (A) Prominent banding reflects different pyroclastic eruptions at the neck. (B) Lherzolite and spinel pyroxenite xenoliths enclosed within pyroclastic breccia. (C–D) Composite lherzolite-pyroxenite xenoliths.

View larger version (153K): [in this window] [in a new window]   Figure 3. Thin section scans of representative lherzolite and pyroxenite xenoliths. Long dimension of each thin section is 45 mm (cpx—clinopyroxene).

View larger version (63K): [in this window] [in a new window]   Figure 4. Backscattered-electron (BSE) images of matrix and a pyroxene megacryst in lherzolite sample CTON-57. Numbers show locations of representative microprobe analyses listed in the table below. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, ol—olivine, sp—spinel.

View larger version (67K): [in this window] [in a new window]   Figure 5. Backscattered-electron (BSE) image and microprobe analyses from carbonate-bearing lherzolite sample CTON-53. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, oliv—olivine, cc—calcite.

View larger version (26K): [in this window] [in a new window]   Figure 6. Clinopyroxene compositions as a function of host rock type. Diamonds—spinel lherzolites, squares—Cr-diopside pyroxenites, circles—spinel ± garnet pyroxenites; the four garnet-bearing samples are shown in reds and oranges. COP—Colorado Plateau lherzolites for comparison (data from Smith, 2000). RGR—Rio Grande rift lherzolites and pyroxenites for comparison (data from Baldridge, 1979; Wilshire et al., 1988; Smith, 2000).

View larger version (50K): [in this window] [in a new window]   Figure 7. Backscattered-electron (BSE) image and microprobe analyses from Cr-diopside pyroxenite sample CTON-67. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, incls—inclusions, sp—spinel.

View larger version (53K): [in this window] [in a new window]   Figure 8. Backscattered-electron (BSE) image and microprobe analyses from intergrown pyroxenes in garnet pyroxenite sample CTON-81. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene.

View larger version (71K): [in this window] [in a new window]   Figure 9. Backscattered-electron (BSE) image and microprobe analyses from garnet pyroxenite sample CTON-17, showing relict garnet surrounded by wide symplectic reaction zone. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene.

View larger version (101K): [in this window] [in a new window]   Figure 10. Backscattered-electron (BSE) image, X-ray element maps, and microprobe analyses from garnet reaction zone in sample CTON-81. Red box indicates location of Figure 11. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, gar—garnet, plag—plagioclase.

View larger version (204K): [in this window] [in a new window]   Figure 11. Detailed X-ray element maps of interface between garnet reaction zone and spinel reaction zone in sample CTON-81. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, gar—garnet, pl—plagioclase, sp—spinel.

View larger version (59K): [in this window] [in a new window]   Figure 12. Backscattered-electron (BSE) image and microprobe analyses from carbonate-bearing, spinel-absent pyroxenite CTON-7. Note inclusions of orthopyroxene within calcite, which is itself included within clinopyroxene. Also note Mn zoning in calcite. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene.

View larger version (53K): [in this window] [in a new window]   Figure 13. Backscattered-electron (BSE) image and microprobe analyses from spinel pyroxenite sample CG-30. Calcite is intimately associated with orthopyroxene and surrounded by clinopyroxene. Both silicate and carbonate glass inclusions surround the calcite + orthopyroxene, and clinopyroxene is zoned toward lower Al, Ti, and Na contents in association with melt inclusions. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, incls—inclusions, cc—calcite, FIs—fluid inclusions.

View larger version (33K): [in this window] [in a new window]   Figure 14. (A) Temperature ranges calculated for spinel lherzolites and pyroxenites from Rio Puerco necks, using the two-pyroxene thermometer of Brey and Köhler (1990). Cr-diopside pyroxenites overlap lherzolite temperature range, most spinel ± garnet pyroxenites record temperatures around 1050 ± 50 °C, and carbonate-bearing pyroxenites record temperatures in excess of 1100 °C. Quadrilateral inset into pyroxenite field indicates range of pressure-temperature conditions calculated from garnet-bearing pyroxenites CTON-6 and CTON-77. (B) Pressure-temperature conditions of Rio Puerco xenoliths compared to xenoliths from the Colorado Plateau (COP; Smith and Barron, 1991; Riter and Smith, 1996; Smith, 2000) and the Rio Grande rift (RGR) and Arizona Transition Zone (ATZ; Baldridge, 1979; McGuire, 1994; Smith, 2000). Pyroxenite solidus is from Pertermann and Hirschmann (2003); carbonated lherzolite solidus is from Gudfinnsson and Presnall (2005).

View larger version (30K): [in this window] [in a new window]   Figure 15. Major-element compositions of Rio Puerco basalts (Hallett, 1994) and lherzolites, pyroxenites, silicate (sil) melt inclusions, and carbonate (carb) melt inclusions from this study. Dashed lines connect basalt with 8% MgO, average lherzolite, and most calcic carbonate melt inclusion as reference for three-phase mixing estimates.

View larger version (73K): [in this window] [in a new window]   Figure 16. Cartoons illustrating setting and probable origin of Rio Puerco pyroxenite xenoliths. (A) Tectonic setting of Rio Puerco volcanic (RPV) field on margin of Rio Grande rift (RGR) and within the Jemez Lineament. Source region for pyroxenite xenoliths is more extensively modified by interaction with silicate + carbonatite melts than xenolith source areas beneath the Colorado Plateau (COP) or the Rio Grande rift, suggesting that Jemez Lineament may localize metasomatizing melts at depth. (B) Lherzolite is converted to pyroxenite via melt-rock interaction adjacent to melt conduits, probably less than 1 m.y. before xenolith entrainment. Garnet (gar) forms in areas of extensive melt infiltration and subsequently breaks down in response to continued heating and decompression during rifting. Spinel (sp) also breaks down in response to heating ± decompression, either in situ or during ascent in host basalt. Abbreviations: opx—orthopyroxene, cpx—clinopyroxene, ol—olivine, an—anorthite.