The electron microprobe is an ideal tool for ceramic provenience studies because it is fast, accurate, and relatively inexpensive. Most sherds are of adequate size for analysis, and samples can be curated for future study. Accurate chemical characterization of temper or clay used in vessel manufacture allows identification of potential raw material sources. Samples from the sources can be compared to vessels from production locales to model resource acquisition patterns. Electron microprobe characterization of volcanic ash temper in whiteware vessels from road construction projects in the Kayenta Anasazi region suggests that raw temper material was transported a surprizing distance.
A new species and genus of an actinolepid arthrodire, Aleosteus eganensis, is described from the Lower Devonian Sey Dolomite in the Egan Range of east-central Nevada. A cladistic analysis of the known members of the family shows that this genus is closely related to the other North American actinolepids, which are of similar age pointing to the establishment of this fauna in relative isolation. An Early Devonian distribution pattern is suggested by the appearance of the most derived genera of actinolepids in the Lochkovian.
Dikes of basanitic nephelinite at the predominantly basaltic House Mountain volcano host ocelli, dikes, pods and sheets of feldspar ijolite and nepheline monzosyenite that may be explained by differentiation according to the series: basanitic nephelinite -> feldspar ijolite -> nepheline monzosyenite. Titanaugite, nepheline, feldspar and olivine occur in all stages of this sequence, which shows systematic Fe enrichment in mineral chemistry and bulk composition. The nephelinite (mg = 76) contains Fo85-86 olivines with a thin Fo65-77 rim, and Wo48-52Fs8-14En36-44 titanaugite. The ijolite (mg in the range 51-61) contains titanaugite with a Wo48-53Fs14-18En30-35 core and a Wo48-52Fs20-24En28-31 rim. The monzosyenite (mg in the range 46-50) has Fo18-29 olivine, and titanaugite zoned from Wo48-50Fs15-27En29-34 to Wo48Fs21-42En10-31 with a green-brown rim of Wo48Fs37-45En7-15. Along the proposed sequence, LILE generally show a 2-3x enrichment and compatible elements show a strong depletion. A petrogenetic model for the association, based on field relationships, geochemistry, and petrography, includes buoyant rise of a heterogeneous body of ijolitic + monzosyenitic magma through a large dike of molten nephelinite to shallow depths (<0.5 km) in the volcano. Turbulence or shear along the boundary may have stirred small globules (ocelli) of felsic magma into the nephelinite. The large body of ijolite-monzosyenite spread as a sheet when it encountered the chilled top of the dike, and fractionated to form cm- to m-scale banding and layering. Large crystals, intergrowth and skeletal textures, and miarolitic cavities with zeolite resulted from rapid crystallization of undercooled, fluid-rich felsic magma. Post-intrusion differentiation of the nephelinite host produced wispy patches and schlieren of felsic liquid that were locally filter-pressed into small dikes of nephelinite monzosyenite.
The Taylor Creek Rhyolite, a group of coeval mid-Tertiary high-silica rhyolite lava domes in southwestern New Mexico, is notable for recording bulk-rock evidence of minor, yet easily measurable, contamination of its source magma reservoir, resulting from assimilation of Proterozoic roof rock. Most of the evidence is recorded in trace element concentrations and 87Sr/86Sri ratios which are far different in uncontaminated magma and roof rocks. Hornblende phenocrysts and biotite xenocrysts also record the effects of contamination. Electron microprobe analyses show that all hornblende grains are zoned to Mg-rich and Fe-, Mn-poor rims. Rim MgO typically is about 10 wt% greater than core MgO. Other hornblende constituents are not measurably variable. Biotite xenocrysts, a trace mineral constituent, are present only in the domes that are most contaminated, as judged by bulk-rock variations in trace-element concentrations and 87Sr/86Sri. Biotite grains are invariably partly to almost completely altered. Microprobe analyses of the cores of the least-altered grains show large variations in Fe and Mg, and that biotites contain 2 - 20 times as much Mg as fresh biotite phenocrysts in other high-silica-rhyolite lavas. Fe and Mg are negatively correlated in hornblende and biotite, consistent with mixing two end-member compositions. The mass ratio of contaminant to magma probably was less than 1:100, and major constituents, including Al, were not measurably affected in hornblende. Aluminum-in-hornblende barometry yields essentially a constant calculated pressure of about 1.5 kilobars, which is consistent with the interpretation that all contamination occurred in a boundary zone about 300 meters thick at the top of the magma reservoir.
Rocks of the Uvalde portion of the Balcones igneous province of south-central Texas include nephelinite, basanite, alkali basalt, and phonolite. The nephelinites and basanite represent primitive, potentially primary, magmas based on high Mg#, high Ni contents, and the presence of mantle xenoliths. Normalized trace-element patterns and diagnostic elemental ratios for these rocks are very similar to those of ocean-island basalts (OIB). Phonolites display OIB-like patterns strongly overprinted by the effects of fractional crystallization. Trace-element variations in the nephelinites, basanite, and alkali basalt result from variable degrees of partial melting of a grossly homogeneous mantle source. Initial 87Sr/86Sr, assuming an age of 75 Ma, ranges between 0.70313 and 0.70467 (<0.70369 for all except one sample). Initial eNd = 6.2 +/- 0.4 (1 sigma) for the mafic rocks excluding the alkali basalt (5.0); the phonolites have initial eNd = 5.7 +/- 0.1. Initial Pb ratios for all samples are 206Pb/204Pb = 18.87 +/- 0.14 and 207Pb/204Pb = 15.56 +/- 0.02. The mafic Uvalde rocks show strongly asthenospheric isotopic and trace-element signatures reflecting a source slightly less depleted than N-MORB and comparable to that of OIB. There is no evidence of significant crustal contamination or a lithospheric mantle signature in any of the mafic rocks other than the alkali basalt. Phonolitic rocks probably are derived by fractionation of nephelinitic parental magmas with negligible crustal interaction.
The magnetization of fresh natrocarbonatite lavas from Oldoinyo Lengai in Tanzania is dominated by small amounts of single- or pseudo-single-domain grains of a spinel in the solid solution series jacobsite (MnFe2O4) - magnetite (Fe3O4). Although this phase may aquire TRM before carbonatite lava crust has ceased being mobile, the Oldoinyo Lengai samples are good paleomagnetic recorders of the field they cooled in. In comparison, samples from older carbonatites in Tanzania have very different magnetic mineralogies and unstable behaviour of remanent magnetization. There are two possible explanations for the contrast in magnetic properties. Recrystallization of fresh carbonatites during weathering may destroy the original remanence and lead to the production of variuos authigenic magnetic minerals. Alternatively, the different magnetic mineralogies may derive from distinct types of carbonatite magmas. Some older calcitic carbonatites may have associated magnetic anomalies that could be useful in prospecting for economically valuable minerals off associated with carbonatites.
Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd, and Pb isotopic ratios and trace-element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. PRe-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, eNd = -9.2 to -10.7); alkali basalts (87Sr/86Sr = 0.7049 to 0.7054, eNd = -2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, eNd = -3.5 to -7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, eNd = -1.4 to -2.6). Post faulting lavas have lower 87Sr/86Sr (<0.7045) and eNd from -3.2 to 2.3. Pb isotopic data for both pre- and post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occured relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with the major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P, and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently increased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.
The Hubbert and Rubey fluid-pressure hypothesis for the mechanics of thrust faulting has been evaluated in a preliminary fashion for the active fold-and-thrust belt of northwestern Taiwan using CPC fluid-pressure data. A hydrostatic pressure-depth ratio (104N/m3) exists at depths shallower than about 1.5 to 4.1 km, depending upon the locality. Overpressured reservoirs exist below the hydrostatic zone and commonly exhibit pressure-depth ratios of about 1.7 x 104 N/m3, which is equivalent to a fluid-pressure to solid pressure ratio (l of Hubbert and Rubey) of about 0.7. The transition zone between hydrostatic and overpressure reservoirs is stratigraphically controlled and apparently reflects a lithologic change from interconnected permeable sands to isolated sands imbedded in or overlain by impermeable shales. The transition zone is commonly within the Chuhuangkeng Formation although it ranges between the Tungkeng Formation and Piling Shale. Active decollement within the fold-and-thrust belt only lie within the overpressured section, an observation in agreement with the fluid-pressure hypothesis. A calculation shows that approximately half as much work is required to push a thrust sheet along the top of the overpressured zone (l = 0.7) as along the base of the hydrostatic zone (l = 0.465). The position of the decollement within the overpressured zone apparently depends on factors other than fluid pressure. The principal decollement lies within the Miocene Wuhchihshan Formation, a transgressive sandstone almost 2 km below the fluid-pressure transition zone.