Showing papers by "Nelia W. Dunbar published in 2002"
TL;DR: In this paper, the 230Th-238U disequilibria were used to evaluate the methods previously proposed to explain melt generation beneath northern Tibet, and the diversity of mechanisms that could be responsible for these relatively small degree melts suggests that the cause of melt production may have varied over time and/or over space in northwestern Tibet and therefore that timing of volcanism may not directly related to any single tectonic event.
Abstract: [1] 238U-230Th disequilibria and Sr, Nd, and Pb isotopic and chemical data for young ( 10−3 kg m−3 yr−1) are permitted in the case of an eclogitic residue; hydrous metasomatic phases, if present, would lower these limits further. The source of the AKB magmas has probably been enriched in incompatible elements relative to bulk Earth since at least the middle Proterozoic (>1 Ga), likely via metasomatism by a relatively dry silicate melt, and, if the ultimate source of the lavas is mantle lithosphere, concentrations of incompatible trace elements in the source could be similar to those of xenoliths and massifs. The 230Th-238U disequilibria provide additional information to evaluate the methods previously proposed to explain melt generation beneath northern Tibet. The measured 230Th enrichments are uncharacteristic of melts generated by subduction but could potentially be produced during shear heating of the uppermost lithospheric mantle, by convective removal of the lower lithosphere and heating of the remaining lithospheric mantle, or by decompression during extension across a releasing bend of a strike-slip fault. The diversity of mechanisms that could be responsible for these relatively small-degree melts suggests that the cause of melt production may have varied over time and/or over space in northwestern Tibet and therefore that timing of volcanism may not be directly related to any single tectonic event.
47 citations
TL;DR: In this article, a system for measuring a vertical strain-rate profile in the firn on polar ice sheets using a readily available video camera to detect metal bands inserted in an air-filled hole was developed.
Abstract: We have developed a system for measuring a vertical strain-rate profile in the firn on polar ice sheets using a readily available video camera to detect metal bands inserted in an air-filled hole. We used this system in 1995 and 1996 at Taylor Dome, Antarctica. We use density measurements combined with our strain rates to infer vertical velocities. From our velocities we calculate a steady-state depth age scale for the firn at Taylor Dome. The age of a visible ash layer from 79.1 m is 675 ± 25 years; this ash can be correlated with ash found at 97.2 m in a recent ice core at Siple Dome, West Antarctica.
22 citations
Journal Article•
TL;DR: In the Ashikule Basin (AKB) in northwestern Tibet as discussed by the authors, trachyandesites from the AKB have been used to provide constraints on the origin of magmas produced within this region of continental collision.
Abstract: [1] U-Th disequilibria and Sr, Nd, and Pb isotopic and chemical data for young (<120 ka) trachyandesites from the Ashikule Basin (AKB) in northwestern Tibet provide constraints on the origin of magmas produced within this region of continental collision. Compared to lavas from both continental and oceanic settings, the AKB samples show large excesses of Th with respect to U (up to (Th)/(U) = 1.36). Partial melting of garnet-bearing lithologies (garnet peridotite, garnet pyroxenite, or eclogite) could be responsible for these Th excesses and could plausibly occur either in the lithospheric mantle or in the lower crust. Small porosities (<0.4%) and slow melting rates (<10 4 kg m 3 yr ) are required in the case of a garnet peridotite residue, although larger porosities (up to 18%) and melting rates (>10 3 kg m 3 yr ) are permitted in the case of an eclogitic residue; hydrous metasomatic phases, if present, would lower these limits further. The source of the AKB magmas has probably been enriched in incompatible elements relative to bulk Earth since at least the middle Proterozoic (>1 Ga), likely via metasomatism by a relatively dry silicate melt, and, if the ultimate source of the lavas is mantle lithosphere, concentrations of incompatible trace elements in the source could be similar to those of xenoliths and massifs. The Th-U disequilibria provide additional information to evaluate the methods previously proposed to explain melt generation beneath northern Tibet. The measured Th enrichments are uncharacteristic of melts generated by subduction but could potentially be produced during shear heating of the uppermost lithospheric mantle, by convective removal of the lower lithosphere and heating of the remaining lithospheric mantle, or by decompression during extension across a releasing bend of a strike-slip fault. The diversity of mechanisms that could be responsible for these relatively small-degree melts suggests that the cause of melt production may have varied over time and/or over space in northwestern Tibet and therefore that timing of volcanism may not be directly related to any single tectonic event.
13 citations
TL;DR: McLEMORE et al. as mentioned in this paper investigated the geology and geochemistry of the Redrock Granite and anorthosite xenoliths in the northern Burro Mountains, Grant County, New Mexico, USA.
Abstract: McLEMORE, VIRGINIA T., DUNBAR, NELIA, KOSUNEN, PAULA J., RAMO, O. TAPANI, HEIZLER, MATT, and HAAPALA, ILMARI 2002. Geology and geochemistry of the Redrock Granite and anorthosite xenoliths (Proterozoic) in the northern Burro Mountains, Grant County, New Mexico, USA. Bulletin of the Geological Society of Finland 74, Parts 1–2, 7–52. Mineral ages from the A-type granites and anorthosite xenoliths in the Redrock area in the northwestern Burro Mountains in southwestern New Mexico cluster around ~1220–1225 Ma and provide yet another example of bimodal igneous activity during this time period in the southwestern United States. The metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Granite exhibits the textural, mineralogical, and geochemical features of A-type granite that was emplaced at a relatively high crustal level. Field relationships, whole rock and mineral geochemical and isotopic trends suggest that the four phases of the Redrock Granite are genetically related, with the miarolitic biotite/alkali feldspar granite being the youngest phase. Spatial relationships and geochemical data suggest that the anorthosite xenoliths were coeval with the Redrock Granite, which is consistent with the anorthosite being derived from the upper mantle, possibly due to deep mantle upwellings, and the Redrock Granite from the lower crust. The process involved melting in the upper mantle, emplacement of anorthosite in the crust resulting in partial crustal melting and thinning, and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Redrock Granite and anorthosite were presumably derived from sources characterized by subtle, long-term LREE depletion, with eNd (at 1220 Ma) values on the order of +1 to +2.
3 citations
01 Jan 2002
TL;DR: Tanos and Blackshare formations as discussed by the authors were identified from late Oligocene to late Miocene sedimentation along the northeastern margin of the Albuquerque Basin and Hagan embayment.
Abstract: Over a kilometer of sedimentary and volcanic rocks of the Santa Fe Group are exposed in the Hagan embayment, a structural re-entrant between the Albuquerque and Espanola Basins. We identify two new lithostratigraphic units, the Tanos and Blackshare Formations, that resulted from late Oligocene to late Miocene sedimentation along the northeastern margin of the Albuquerque Basin and Hagan embayment. We designate exposures in Arroyo de la Vega de los Tanos as the type section and the San Felipe Pueblo NE quadrangle as the type area for these two formations. The Tanos Formation is 279 m (915 ft) thick at the type locality where it disconformably overlies the Espinaso Formation. The Tanos Formation consists of a basal conglomerate overlain by mudstone and sandstone, representing deposition in a closed basin. The overlying Blackshare Formation is over 1,000 m (3,281 ft) thick and contains dominantly sandstone with interbeds of lenticular conglomerate, conglomeratic sandstone, and minor mudstone, commonly arranged in fining-upward sequences. Paleocurrent measurements, gravel composition, and field relationships indicate derivation from the neighboring Ortiz Mountains. The Blackshare Formation conformably overlies and interfingers with the Tanos Formation and represents a general westward progradation of the Ortiz Mountains piedmont during Miocene time. Stratal tilts of the Tanos‐Blackshare succession decrease upsection, indicating that subsidence and deposition occurred concurrently. Subhorizontally bedded conglomerate and sandstone of the Pliocene‐Pleistocene Tuerto formation* overlie the Blackshare Formation in angular unconformity. A basaltic flow near the base of the Tanos Formation yielded an 40Ar/39Ar age of 25.41 ± 0 .32 Ma.
2 citations