High-pressure transformations in CaSiO 3

Wollastonite transforms to a triclinic high-pressure polymorph, wollastonite II, at pressures > 25 kb. The equilibrium boundary [P(bars)=-4.7× T °C+32.810] has a rather flat negative P-T slope. The rapid reactivity of the transition over at least a 1000° C range, its insensitivity to T and its location around 30 kb are indicative of its potential for a pressure calibration curve at high T .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47317/1/410_2004_Article_BF00383442.pd

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H 2 O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two-pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg-rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73522/1/j.1525-1314.2003.00479.x.pd

Valley receives 2003 N.L. Bowen award

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94640/1/eost14650.pd

Feldspar and oxide thermometry of granulites in the Adirondack Highlands

Thermometry of regionally metamorphosed granulites of the Adirondack Highlands has been undertaken using feldspar and iron-titanium-oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An 18–30 ) and microcline perthite (Or 57–87 ) pairs from charnockites and granitic gneisses give K D [Na/(Na+Ca+K] plag /[Na/(Na+Ca+K)] or = 2–3 yielding temperatures of 650 ° to 750 ° C in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pressures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magnetite (ulvöspinel ss 16–45) and ilmenite (hematite ss 4.7–6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ° to 800 ° C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10 −20 and 10 −16 and always lie within 10 +1 of the f buffered by fayalite-magnetite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulvöspinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically reasonable results. The inferred 750 ° and 700 ° C isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790 ± 50 ° C) occur between Saranac Lake and Tupper Lake, New York.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47321/1/410_2004_Article_BF00372874.pd

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr 2 O 3, and 20 to 220 ppm H 2 O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T  ≈ 600 °C and P  = 15 kbar) to 95 km (where T  ≈ 800 °C and P  = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM 21 O 38 , where A  = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi 2 O 6 ), and a new oxide mineral, carmichaelite (MO 2−x (OH) x , where M  = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb 2 O 5 ), Cr (up to ∼6 wt% Cr 2 O 3 ), and OH (up to ∼0.9 wt% H 2 O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42226/1/410-135-2-3-164_91350164.pd

Petrology of a margarite-bearing meta-anorthosite from Seljeneset, Nordfjord, western Norway: Implications for the P-T history of the Western Gneiss Region during Caledonian uplift

The Western Gneiss Region of Norway contains relics of eclogite and high-pressure granulite that have been extensively overprinted by amphibolite-facies assemblages. A meta-anorthosite from Seljeneset. Nordfjord, contains fine-grained margarite+quartz+plagioclase+/-muscovite clots set in a zoisite+quartz+plagioclase+/-clinozoisite matrix. In some clots relict kyanite is preserved. Equilibria modeled in the CASH system indicate a retrograde P-T path that begins in the kyanite+zoisite field (P>8.2 kbar), passes through reaction (1), kyanite+zoisite+quartz[right harpoon over left]anorthite+H2O, between 610[deg]C and about 800[deg]C, and finally through reaction (2), kyanite+anorthite+H2O[right harpoon over left]margarite+quartz. The sodic composition of plagioclase in reaction (1) suggests that pressures in excess of 15-17 kbar (at 750-800[deg]C) may have been experienced during Silurian eclogite-facies metamorphism. The simplified CASH reaction (2) occurs between invariant points at 4.3 kbar,520[deg]C and 8.2 kbar,610[deg]C, but polyvariance, due to variable Na solid solution, broadens the reaction into a band about 150[deg]C wide. Cooling through 500[deg]C occurred about 410 Ma ago. This study provides the first well-defined P-T path for Caledonian retrograde metamorphism at amphibolite-facies conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27410/1/0000444.pd

Closure temperatures of the Sm---Nd system in metamorphic garnets

Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm---Nd ages obtained from Archean garnets 0.1-0.5 cm in length are 30-110 Ma younger than the U---Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm---Nd ages obtained from Proterozoic garnets with a diameter of 0.1-5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm---Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm---Nd system in garnets analyzed in this study is ca. 600 +/- 30[deg]C. Only garnets with radii much larger than 5 cm may record Sm---Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29817/1/0000163.pd

Erratum

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47330/1/410_2005_Article_BF01164528.pd

Reinvestigation and application of olivine-quartz-orthopyroxene barometry

Experiments in a piston-cylinder apparatus have been carried out at 700-1050[deg]C, 10-16 kbar to determine the stability of ferrosilite (FeSiO3) relative to fayalite + quartz. Reaction reversals within 0.1-kbar intervals locate the equilibrium at 10.5, 11.0, 11.5, 12.0, 12.6, 13.3, 14.1 and 14.8 kbar at 700, 750, 800, 850, 900, 950, 1000, and 1050[deg]C, respectively, reflecting the intercept with the [alpha]-[beta] quartz transition at about 880[deg]C. The tight reversals severely constrain the reaction slope, providing a basis for limited extrapolation and calculations. However, the lack of accurate activity and cation-distribution data for orthopyroxene and olivine generates substantial uncertainties when considering the effects of large proportions of additional components such as MgO. Experiments and calculations indicate that additional components dramatically extend the pyroxene stability field and that pressures that have been inferred from ferrosilite-rich pyroxenes in natural assemblages are 1-3 kbar too high.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23295/1/0000232.pd