James Wittke | Northern Arizona University (original) (raw)

Papers by James Wittke

Research paper thumbnail of Presence of all Three Allotropes of Impact-Diamonds in the Younger Dryas Onset Layer (YDB) Across N America and NW Europe

We report the discovery of all three diamond allotropes (cubic diamond, lonsdaleite, and n-diamon... more We report the discovery of all three diamond allotropes (cubic diamond, lonsdaleite, and n-diamond) in an extraterrestrial (ET) impact layer (the YDB), dating to the Younger Dryas onset at 12.9 ka. YDB diamonds are distributed broadly across N America and NW Europe at 15 sites spanning 9,000 km or 23 percent of Earth's circumference. N-diamonds and lonsdaleite, or hexagonal diamond,

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Research paper thumbnail of Impact-Shocked diamonds, Abrupt Ecosystem Disruption, and Mammoth Extinction on California's Northern Channel Islands at the Allerod-Younger Dryas Boundary (13.0-12.9 ka)

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Research paper thumbnail of Reply to Boslough et al.: They ignore Decades of Comet Research

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Research paper thumbnail of The Quaternary impact record from the Pampas, Argentina: earth and planetary. Sci Lett 219

Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since t... more Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since the late Miocene. The present contribution focuses on two localities containing Quaternary impact glasses: along the coastal sequences near Centinela del Mar (CdM) and from near Rio Cuarto (RC). These highly vesicular glasses contain clear evidence for an impact origin including temperatures sufficient to melt most mineral constituents (1700 ‡C) and to leave unique quench products such as L-cristobolite. The CdM glasses occur within a relatively narrow horizon just below a marine transgression expressed by a series of coastal paleo-dunes and systematic changes in the underlying sediments. Highresolution 40 Ar/ 39 Ar dating methods yielded an age of 445 þ 21 ka (2c). Glasses were also recovered from scattered occurrences lower in the section but were dated to 230 þ 40 ka. This inconsistency between stratigraphic and radiometric age is most likely related to a nearby outcrop of glass that h...

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Research paper thumbnail of A Journey to Tibet and Continental Collision

Journal of Geoscience Education, 1997

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Research paper thumbnail of Reply to van Hoesel, et al., regarding impact-related YDB nanodiamonds from the Netherlands

Pnas, 2013

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Research paper thumbnail of Unusual Metasomatic Rocks Associated with Subducted Mantle Peridotite in the Orocopia Schist at Cemetery Ridge, Southwest Arizona

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Research paper thumbnail of Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: initial report

Http Dx Doi Org 10 1080 00206814 2014 928916, Jun 24, 2014

ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generall... more ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

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Research paper thumbnail of Middle Miocene nepheline-bearing mafic and evolved alkaline igneous rocks at House Mountain, Arizona Transition Zone, north-central Arizona

The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona... more The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona Transition Zone, about 7 km SW of Sedona, AZ. Deep erosion has exposed internal structural and stratigraphic relationships of the volcano. Mapping documents two igneous suites: (1) alkali basalt to trachyte and alkali-feldspar syenite, and (2) olivine melanephelinite, nepheline monzodiorite, nepheline monzosyenite and nepheline

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Research paper thumbnail of Multi-Component Source for Muong Nong-Type Bediasite 30775-2

Meteoritics

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Research paper thumbnail of Significance of Newly Discovered Subduction Complex, Including Partially Serpentinized Mantle Peridotite, Southwest Arizona

The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists (PORS) of southern Cali... more The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists (PORS) of southern California and southwest Arizona are anomalous because they exhibit many characteristics of a subduction complex, yet occupy positions far inland from the continental margin. Prolonged debate has led to consensus that the PORS are tectonic correlatives of the Franciscan Complex, carried beneath southwest North America during low-angle, northeastward subduction. However, several important aspects of their origin remain unclear. Among the three units, Orocopia Schist is exposed the farthest inland, in southwest Arizona as much as a few hundred kilometers from the present or former continental margin. In 2012 we found an additional, unexpected exposure of Orocopia Schist even farther inland—90 km west of the outskirts of greater Phoenix. This body of Orocopia Schist (at a place called Cemetery Ridge), is extraordinary because it includes numerous blocks of serpentinized mantle peridotite. These...

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Research paper thumbnail of Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: initial report

International Geology Review, 2014

ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generall... more ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

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Research paper thumbnail of Hornblende phenocrysts record a pressure gradient in and contamination of the Taylor Creek Rhyolite magma reservoir

The Taylor Creek Rhyolite of southwestern New Mexico comprises 20 coeval porphyritic lava domes e... more The Taylor Creek Rhyolite of southwestern New Mexico comprises 20 coeval porphyritic lava domes erupted from a large vertically zoned reservoir of silicic magma. The rhyolite is high-silica, subalkaline, and is nearly constant in major-element composition. Trace elements and [sup 87]Sr/[sup 86]Sr[sub i] (0.705 to 0.713) define vertical zoning that records a downward-decreasing imprint of minor (<1 wt%) partial assimilation of Proterozoic roof rocks. Consistent with the major-element homogeneity, electron-microprobe analyses of hornblende phenocrysts show little or no measurable variation in principal constituents. The hornblende is edenite whose mean composition and standard deviation of 110 analyses are SiO[sub 2], 44.66 [+-] 0.64; TiO[sub g], 1.27 [+-] 0.13; Al[sub 2]O[sub 3], 6.80 [+-] 0.31; FeO, 21.00 [+-] 1.60; MnO, 1.19 [+-] 0.16; MgO, 9.94 [+-] 1.09; CaO, 10.51 [+-] 0.22; Na[sub 2]O, 2.22 [+-] 0.13; K[sup 2]O, 0.98 [+-] 0.08; F, 2.04 [+-] 0.35; Cl, 0.20 [+-] 0.03. Except for FeO, MnO, and MgO, compositional variations are non systematic and mostly within analytical uncertainty. FeO and Mno exhibit strong negative correlation with MgO. Individual hornblende crystals are zoned to relatively MgO-rich and FeO-MnO-poor rims, opposite what might be expected if the Taylor Creek Rhyolite magma reservoir evolved chemically isolated from its surroundings. Hornblende with rims richest in MgO occurs in domes fed from the uppermost part of the reservoir. Calculated pressures based on Al in hornblende range from 1.6 to 2.0 kb, [+-] 0.5 kb. Though the range of calculated P is encompassed within the uncertainty, the lowest hornblende pressure is for a dome fed from, or near, the top of the reservoir, whereas the chemically defined vertical zoning.

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Research paper thumbnail of Roof-rock contamination of Taylor Creek Rhyolite, New Mexico, as recorded in hornblende phenocrysts and biotite xenocrysts

American Mineralogist, 1996

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Research paper thumbnail of New Lunar Meteorite Northwest Africa 773: A Tholeiite from the Moon?

Olivines and pyroxenes in gabbroic rock fragments from the recently found meteorite Northwest Afr... more Olivines and pyroxenes in gabbroic rock fragments from the recently found meteorite Northwest Africa 773 (NWA773) show evidence of Fe-enrichment broadly similar to the tholeiitic trend defined for terrestrial rocks. The meteorite consists of two main lithologies: a two-pyroxene olivine gabbro and a fragmental, heterolithic breccia. The olivine gabbro lithology consists of olivine (Fo68), pigeonite (Wo11En65), augite (Wo36En50), and plagioclase

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Research paper thumbnail of Nanodiamonds and Carbon Spherules from Tunguska, the K/T Boundary, and the Younger Dryas Boundary Layer

Agu Fall Meeting Abstracts, Dec 1, 2009

More than a dozen markers, including nanodiamonds (NDs) and carbon spherules (CS), occur in a sed... more More than a dozen markers, including nanodiamonds (NDs) and carbon spherules (CS), occur in a sedimentary layer marking the onset of the Younger Dryas (YD) cooling episode at ~12.9 ka. This boundary layer, called the YDB, has been found at nearly forty locations across North America, Europe, and Asia, although not all markers are present at any given site. Firestone et al. (2007) and Kennett et al. (2008, 2009) proposed that these markers resulted from a cosmic impact/airburst and impact-related biomass burning. Here we report features common to the YDB event, the Cretaceous-Tertiary (K/T) impact, and the Tunguska airburst of 1908. In sediments attributed to each event, we and other researchers have recovered NDs either inside or closely associated with CS, which appear to be the high-temperature by-products of biomass burning. CS range in diameter from about 500 nanometers to 4 millimeters with a mean of ~100 microns, and they typically contain NDs, including lonsdaleite (hexagonal diamonds), in the interior matrix and in the crust. To date, CS and NDs have been found in the K/T layer in the United States, Spain, and New Zealand. Similarly, CS and NDs have been found in the YDB layer in the United States, Canada, United Kingdom, Belgium, the Netherlands, Germany, and France. Thus far, every site examined contains NDs and/or CS in the K/T and YDB layers; conversely, we have yet to detect CS associated with NDs in any non-YDB sediments tested. Five allotropes of NDs have been identified in association with CS: cubic diamonds, lonsdaleite, n-diamonds, p-diamonds, and i-carbon, which are differentiated by slight variations in their crystalline structure. All allotropes have been identified using scanning electron microscopy (SEM), high-resolution electron microscopy (HREM), and transmission electron microscopy (TEM) with confirmation by selected area diffraction (SAED). Lonsdaleite is found on Earth only in three instances: (1) in the laboratory, where it is produced by shock synthesis under a high-temperature-high-pressure regime (~1000°C to 1700°C at 15 GPa) or by carbon vapor deposition (CVD) under a very-high-temperature-low-pressure regime (~13,000°C at 300 Torr) (Maruyama et al., 1993); (2) after arrival on Earth inside extraterrestrial material; and (3) as a result of high-temperature cosmic impact/airbursts. Lonsdaleite associated with CS has been found in sediments only at the K/T, the YDB, and Tunguska, consistent with the hypothesis that all three events have cosmic origins, although the nature of the impactors may have been different.

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Research paper thumbnail of Unique Polymict Breccia Northwest Africa 7531 Composed of Recrystallized LL Clasts Associated with CR Metachondrite Material: Evidence for Highly Equilibrated Ordinary Chondritic Impactors onto the CR Chondrite Parent Body

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Research paper thumbnail of Northwest Africa 2824: Another Eucrite-like Sample from the Ibitira Parent Body?

Meteoritics and Planetary Science Supplement, Sep 1, 2009

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Research paper thumbnail of Northwest Africa 5131: Another Tafassasset-Like Metachondrite Related to the CR Chondrite Parent Body

Meteoritics and Planetary Science Supplement, Sep 1, 2011

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Research paper thumbnail of Ctenacanthiform sharks from the Permian Kaibab Formation, northern Arizona

Http Dx Doi Org 10 1080 08912963 2012 683193, Aug 1, 2012

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Research paper thumbnail of Presence of all Three Allotropes of Impact-Diamonds in the Younger Dryas Onset Layer (YDB) Across N America and NW Europe

We report the discovery of all three diamond allotropes (cubic diamond, lonsdaleite, and n-diamon... more We report the discovery of all three diamond allotropes (cubic diamond, lonsdaleite, and n-diamond) in an extraterrestrial (ET) impact layer (the YDB), dating to the Younger Dryas onset at 12.9 ka. YDB diamonds are distributed broadly across N America and NW Europe at 15 sites spanning 9,000 km or 23 percent of Earth's circumference. N-diamonds and lonsdaleite, or hexagonal diamond,

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Research paper thumbnail of Impact-Shocked diamonds, Abrupt Ecosystem Disruption, and Mammoth Extinction on California's Northern Channel Islands at the Allerod-Younger Dryas Boundary (13.0-12.9 ka)

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Research paper thumbnail of Reply to Boslough et al.: They ignore Decades of Comet Research

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Research paper thumbnail of The Quaternary impact record from the Pampas, Argentina: earth and planetary. Sci Lett 219

Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since t... more Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since the late Miocene. The present contribution focuses on two localities containing Quaternary impact glasses: along the coastal sequences near Centinela del Mar (CdM) and from near Rio Cuarto (RC). These highly vesicular glasses contain clear evidence for an impact origin including temperatures sufficient to melt most mineral constituents (1700 ‡C) and to leave unique quench products such as L-cristobolite. The CdM glasses occur within a relatively narrow horizon just below a marine transgression expressed by a series of coastal paleo-dunes and systematic changes in the underlying sediments. Highresolution 40 Ar/ 39 Ar dating methods yielded an age of 445 þ 21 ka (2c). Glasses were also recovered from scattered occurrences lower in the section but were dated to 230 þ 40 ka. This inconsistency between stratigraphic and radiometric age is most likely related to a nearby outcrop of glass that h...

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Research paper thumbnail of A Journey to Tibet and Continental Collision

Journal of Geoscience Education, 1997

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Research paper thumbnail of Reply to van Hoesel, et al., regarding impact-related YDB nanodiamonds from the Netherlands

Pnas, 2013

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Research paper thumbnail of Unusual Metasomatic Rocks Associated with Subducted Mantle Peridotite in the Orocopia Schist at Cemetery Ridge, Southwest Arizona

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Research paper thumbnail of Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: initial report

Http Dx Doi Org 10 1080 00206814 2014 928916, Jun 24, 2014

ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generall... more ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

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Research paper thumbnail of Middle Miocene nepheline-bearing mafic and evolved alkaline igneous rocks at House Mountain, Arizona Transition Zone, north-central Arizona

The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona... more The Middle Miocene House Mountain shield volcano is located on the northern margin of the Arizona Transition Zone, about 7 km SW of Sedona, AZ. Deep erosion has exposed internal structural and stratigraphic relationships of the volcano. Mapping documents two igneous suites: (1) alkali basalt to trachyte and alkali-feldspar syenite, and (2) olivine melanephelinite, nepheline monzodiorite, nepheline monzosyenite and nepheline

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Research paper thumbnail of Multi-Component Source for Muong Nong-Type Bediasite 30775-2

Meteoritics

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Research paper thumbnail of Significance of Newly Discovered Subduction Complex, Including Partially Serpentinized Mantle Peridotite, Southwest Arizona

The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists (PORS) of southern Cali... more The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists (PORS) of southern California and southwest Arizona are anomalous because they exhibit many characteristics of a subduction complex, yet occupy positions far inland from the continental margin. Prolonged debate has led to consensus that the PORS are tectonic correlatives of the Franciscan Complex, carried beneath southwest North America during low-angle, northeastward subduction. However, several important aspects of their origin remain unclear. Among the three units, Orocopia Schist is exposed the farthest inland, in southwest Arizona as much as a few hundred kilometers from the present or former continental margin. In 2012 we found an additional, unexpected exposure of Orocopia Schist even farther inland—90 km west of the outskirts of greater Phoenix. This body of Orocopia Schist (at a place called Cemetery Ridge), is extraordinary because it includes numerous blocks of serpentinized mantle peridotite. These...

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Research paper thumbnail of Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: initial report

International Geology Review, 2014

ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generall... more ABSTRACT The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

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Research paper thumbnail of Hornblende phenocrysts record a pressure gradient in and contamination of the Taylor Creek Rhyolite magma reservoir

The Taylor Creek Rhyolite of southwestern New Mexico comprises 20 coeval porphyritic lava domes e... more The Taylor Creek Rhyolite of southwestern New Mexico comprises 20 coeval porphyritic lava domes erupted from a large vertically zoned reservoir of silicic magma. The rhyolite is high-silica, subalkaline, and is nearly constant in major-element composition. Trace elements and [sup 87]Sr/[sup 86]Sr[sub i] (0.705 to 0.713) define vertical zoning that records a downward-decreasing imprint of minor (<1 wt%) partial assimilation of Proterozoic roof rocks. Consistent with the major-element homogeneity, electron-microprobe analyses of hornblende phenocrysts show little or no measurable variation in principal constituents. The hornblende is edenite whose mean composition and standard deviation of 110 analyses are SiO[sub 2], 44.66 [+-] 0.64; TiO[sub g], 1.27 [+-] 0.13; Al[sub 2]O[sub 3], 6.80 [+-] 0.31; FeO, 21.00 [+-] 1.60; MnO, 1.19 [+-] 0.16; MgO, 9.94 [+-] 1.09; CaO, 10.51 [+-] 0.22; Na[sub 2]O, 2.22 [+-] 0.13; K[sup 2]O, 0.98 [+-] 0.08; F, 2.04 [+-] 0.35; Cl, 0.20 [+-] 0.03. Except for FeO, MnO, and MgO, compositional variations are non systematic and mostly within analytical uncertainty. FeO and Mno exhibit strong negative correlation with MgO. Individual hornblende crystals are zoned to relatively MgO-rich and FeO-MnO-poor rims, opposite what might be expected if the Taylor Creek Rhyolite magma reservoir evolved chemically isolated from its surroundings. Hornblende with rims richest in MgO occurs in domes fed from the uppermost part of the reservoir. Calculated pressures based on Al in hornblende range from 1.6 to 2.0 kb, [+-] 0.5 kb. Though the range of calculated P is encompassed within the uncertainty, the lowest hornblende pressure is for a dome fed from, or near, the top of the reservoir, whereas the chemically defined vertical zoning.

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Research paper thumbnail of Roof-rock contamination of Taylor Creek Rhyolite, New Mexico, as recorded in hornblende phenocrysts and biotite xenocrysts

American Mineralogist, 1996

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Research paper thumbnail of New Lunar Meteorite Northwest Africa 773: A Tholeiite from the Moon?

Olivines and pyroxenes in gabbroic rock fragments from the recently found meteorite Northwest Afr... more Olivines and pyroxenes in gabbroic rock fragments from the recently found meteorite Northwest Africa 773 (NWA773) show evidence of Fe-enrichment broadly similar to the tholeiitic trend defined for terrestrial rocks. The meteorite consists of two main lithologies: a two-pyroxene olivine gabbro and a fragmental, heterolithic breccia. The olivine gabbro lithology consists of olivine (Fo68), pigeonite (Wo11En65), augite (Wo36En50), and plagioclase

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Research paper thumbnail of Nanodiamonds and Carbon Spherules from Tunguska, the K/T Boundary, and the Younger Dryas Boundary Layer

Agu Fall Meeting Abstracts, Dec 1, 2009

More than a dozen markers, including nanodiamonds (NDs) and carbon spherules (CS), occur in a sed... more More than a dozen markers, including nanodiamonds (NDs) and carbon spherules (CS), occur in a sedimentary layer marking the onset of the Younger Dryas (YD) cooling episode at ~12.9 ka. This boundary layer, called the YDB, has been found at nearly forty locations across North America, Europe, and Asia, although not all markers are present at any given site. Firestone et al. (2007) and Kennett et al. (2008, 2009) proposed that these markers resulted from a cosmic impact/airburst and impact-related biomass burning. Here we report features common to the YDB event, the Cretaceous-Tertiary (K/T) impact, and the Tunguska airburst of 1908. In sediments attributed to each event, we and other researchers have recovered NDs either inside or closely associated with CS, which appear to be the high-temperature by-products of biomass burning. CS range in diameter from about 500 nanometers to 4 millimeters with a mean of ~100 microns, and they typically contain NDs, including lonsdaleite (hexagonal diamonds), in the interior matrix and in the crust. To date, CS and NDs have been found in the K/T layer in the United States, Spain, and New Zealand. Similarly, CS and NDs have been found in the YDB layer in the United States, Canada, United Kingdom, Belgium, the Netherlands, Germany, and France. Thus far, every site examined contains NDs and/or CS in the K/T and YDB layers; conversely, we have yet to detect CS associated with NDs in any non-YDB sediments tested. Five allotropes of NDs have been identified in association with CS: cubic diamonds, lonsdaleite, n-diamonds, p-diamonds, and i-carbon, which are differentiated by slight variations in their crystalline structure. All allotropes have been identified using scanning electron microscopy (SEM), high-resolution electron microscopy (HREM), and transmission electron microscopy (TEM) with confirmation by selected area diffraction (SAED). Lonsdaleite is found on Earth only in three instances: (1) in the laboratory, where it is produced by shock synthesis under a high-temperature-high-pressure regime (~1000°C to 1700°C at 15 GPa) or by carbon vapor deposition (CVD) under a very-high-temperature-low-pressure regime (~13,000°C at 300 Torr) (Maruyama et al., 1993); (2) after arrival on Earth inside extraterrestrial material; and (3) as a result of high-temperature cosmic impact/airbursts. Lonsdaleite associated with CS has been found in sediments only at the K/T, the YDB, and Tunguska, consistent with the hypothesis that all three events have cosmic origins, although the nature of the impactors may have been different.

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Research paper thumbnail of Unique Polymict Breccia Northwest Africa 7531 Composed of Recrystallized LL Clasts Associated with CR Metachondrite Material: Evidence for Highly Equilibrated Ordinary Chondritic Impactors onto the CR Chondrite Parent Body

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Research paper thumbnail of Northwest Africa 2824: Another Eucrite-like Sample from the Ibitira Parent Body?

Meteoritics and Planetary Science Supplement, Sep 1, 2009

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Research paper thumbnail of Northwest Africa 5131: Another Tafassasset-Like Metachondrite Related to the CR Chondrite Parent Body

Meteoritics and Planetary Science Supplement, Sep 1, 2011

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Research paper thumbnail of Ctenacanthiform sharks from the Permian Kaibab Formation, northern Arizona

Http Dx Doi Org 10 1080 08912963 2012 683193, Aug 1, 2012

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