James R. Budahn

Geochemical evidence for diversity of dust sources in the southwestern United States

Several potential dust sources, including generic sources of sparsely vegetated alluvium, playa deposits, and anthropogenic emissions, as well as the area around Owens Lake, California, affect the composition of modern dust in the southwestern United States. A comparison of geochemical analyses of modern and old (a few thousand years) dust with samples of potential local sources suggests that dusts reflect four primary sources: (1) alluvial sediments (represented by Hf, K, Rb, Zr, and rare-earth elements, (2) playas, most of which produce calcareous dust (Sr, associated with Ca), (3) the area of Owens (dry) Lake, a human-induced playa (As, Ba, Li, Pb, Sb, and Sr), and (4) anthropogenic and/or volcanic emissions (As, Cr, Ni, and Sb). A comparison of dust and source samples with previous analyses shows that Owens (dry) Lake and mining wastes from the adjacent Cerro Gordo mining district are the primary sources of As, Ba, Li, and Pb in dusts from Owens Valley. Decreases in dust contents of As, Ba, and Sb with distance from Owens Valley suggest that dust from southern Owens Valley is being transported at least 400 km to the east. Samples of old dust that accumulated before European settlement are distinctly lower in As, Ba, and Sb abundances relative to modern dust, likely due to modern transport of dust from Owens Valley. Thus, southern Owens Valley appears to be an important, geochemically distinct, point source for regional dust in the southwestern United States.

Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and “North America” blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78–1.69 Ga) and “North America” (1.71–1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace- element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the “North America” block are characterized by depleted eNd values (3.4–3.9) and younger Nd model ages (1800–1740 Ma) and have lower K 2 O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted eNd (0.6–2.6) and older Nd model ages (2070–1880 Ma). Despite the subtle differences, granitoids from both the Caborca and “North America” blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave– Yavapai transition zone, whereas rocks of the “North America” block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave–Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

Geochemical evidence for airborne dust additions to soils in Channel Islands National Park, California

There is an increasing awareness that dust plays important roles in climate change, biogeochemical cycles, nutrient supply to ecosystems, and soil formation. In Channel Islands National Park, California, soils are clay-rich Vertisols or Alfi sols and Mollisols with vertic properties. The soils are overlain by silt-rich mantles that contrast sharply with the underlying clay-rich horizons. Silt mantles contain minerals that are rare or absent in the volcanic rocks that dominate these islands. Immobile trace elements (Sc-Th-La and Ta-Nd-Cr) and rare-earth elements show that the basalt and andesite on the islands have a composition intermediate between upper-continental crust and oceanic crust. In contrast, the silt fractions and, to a lesser extent, clay fractions of the silt mantle have compositions closer to average upper-continental crust and very similar to Mojave Desert dust. Island shelves, exposed during the last glacial period, could have provided a source of eolian sediment for the silt mantles, but this is not supported by mineralogical data. We hypothesize that a more likely source for the silt-rich mantles is airborne dust from mainland California and Baja California, either from the Mojave Desert or from the continental shelf during glacial low stands of sea. Although average winds are from the northwest in coastal California, easterly winds occur numerous times of the year when “Santa Ana” conditions prevail, caused by a high-pressure cell centered over the Great Basin. The eolian silt mantles constitute an important medium of plant growth and provide evidence that abundant eolian silt and clay may be delivered to the eastern Pacifi c Ocean from inland desert sources.

Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite, and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, among these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on the basis of distinctive rare-earth-element (REE) patterns: melilitite-related, melanephelinite-related, meymechite-related, trachybasalt-related, and ankaramite-related. The REE ratios and patterns that distinguish the groups have not evolved by fractionation, because they display no relation to MgO content. Judging from the isotopic data, crustal contamination had little influence on magma evolution. All rock groups, despite their geochemical dissimilarities, show close geochemical linkages among themselves, and significant geochemical similarity to kimberlites of the Yakutian province and ocean-island basalts (OIB). Thus, all of these continental and oceanic magmas may have originated in the same part of the mantle. Geochemical distinctions among the five rock groups could have been caused by various degrees of partial melting and differing amounts of dissolved volatiles.

Paleoclimatic Significance of Chemical Weathering in Loess-Derived Paleosols of Subarctic Central Alaska

ABSTRACT Chemical weathering in soils has not been studied extensively in high-latitude regions. Loess sequences with modern soils and paleosols are present in much of subarctic Alaska, and allow an assessment of present and past chemical weathering. Five sections were studied in detail in the Fairbanks, Alaska, area. Paleosols likely date to mid-Pleistocene interglacials, the last interglacial, and early-to-mid-Wisconsin interstadials. Ratios of mobile (Na, Ca, Mg, Si) to immobile (Ti or Zr) elements indicate that modern soils and most interstadial and interglacial paleosols are characterized by significant chemical weathering. Na2O/TiO2 is lower in modern soils and most paleosols compared to parent loess, indicating depletion of plagioclase. In the clay fraction, smectite is present in Tanana and Yukon River source sediments, but is absent or poorly expressed in modern soils and paleosols, indicating depletion of this mineral also. Loss of both plagioclase and smectite is well expressed in soils and paleosols as lower SiO2/TiO2. Carbonates are present in the river source sediments, but based on CaO/TiO2, they are depleted in soils and most paleosols (with one exception in the early-to-mid-Wisconsin period). Thus, most soil-forming intervals during past interglacial and interstadial periods in Alaska had climatic regimes that were at least as favorable to mineral weathering as today, and suggest boreal forest or acidic tundra vegetation.

Relation of peralkaline magmatism to heterogeneous extension during the middle Miocene, southeastern Nevada

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre-middle Miocene calcalkaline magmas. eNd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208Pb/204Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.

Geochemical, Isotopic, and SHRIMP Age Data for Precambrian Basement Rocks, Permian Volcanic Rocks, and Sedimentary Host Rocks to the Ore-bearing Intrusions, Noril'sk-Talnakh District, Siberian Russia

Petrographic, geochemical, and isotopic data have been obtained for 33 samples selected to provide constraints on contamination models for the volcanic and intrusive components of the Late Permian to Early Triassic, Siberian flood-volcanic province. Twenty-one of these samples were carried from great depth in an explosive diatreme of Triassic age, whereas 12 were collected from drill core from depths of tens to 2000 m. The studied diatreme xenoliths are: (1) fragments of the crystalline basement; and (2) fragments of a basaltic-to-rhyolitic volcanic suite. Prompted by an unexpected, Late Paleozoic, Rb-Sr isochron age for this compositionally diverse volcanic suite, a SHRIMP U-Pb zircon age of ∼270 Ma was obtained for a rhyodacite xenolith. Previously, a SHRIMP zircon U-Pb age of ∼910 Ma had been determined for a leucogranite xenolith from the crystalline basement; this sample also contains substantial amounts of inherited, Early Proterozoic and Archean zircon. The presence of this volcanic suite, only ∼20 m.y. older than the 251 Ma, flood-volcanic sequence, is an extremely provocative result, inasmuch as hundreds of exploration drill holes in the Norilsk area, and throughout the Siberian platform, have encountered only Tungusskaya Series coal-bearing sedimentary rocks in this stratigraphic/time interval. These data support arguments that subduction/underthrusting from the West Siberian Lowland under the northwest margin of the Siberian craton took place in Late Permian time. The isotopic data obtained for the xenolith suite indicate that the upper part of the crystalline basement under the northwest margin of the Siberian craton is composed of Late Proterozoic (Riphean) rocks-alkaline granites, trondhjemites, crystalline schists, gneisses, and amphibolites-with much in common with rocks of the Central zone of the Taymyr folded area, which has been interpreted as an accretionary block formed and joined to Siberia in Late Riphean to Vendian time. Measured isotopic characteristics for the Precambrian crystalline basement, and the Paleozoic sedimentary rocks that host the ore-bearing intrusions in the Norilsk region, provide parameters for quantitative modeling of crustal contamination during evolution of the Siberian flood-volcanic rocks and related intrusions, both while en route to the surface and at the site of intrusion emplacement.

A comprehensive survey of faults, breccias, and fractures in and flanking the eastern Española Basin, Rio Grande rift, New Mexico

A comprehensive survey of geologic structures formed in the Earth’s brittle regime in the eastern Española Basin and flank of the Rio Grande rift, New Mexico, reveals a complex and protracted record of multiple tectonic events. Data and analyses from this representative rift flank-basin pair include measurements from 53 individual fault zones and 22 other brittle structures, such as breccia zones, joints, and veins, investigated at a total of just over 100 sites. Structures were examined and compared in poorly lithified Tertiary sedi ments, as well as in Paleozoic sedimentary and Proterozoic crystalline rocks. Data and analyses include geologic maps; field observations and measurements; orientation, kinematic, and paleostress analyses; statistical examination of fault trace lengths derived from aeromagnetic data; mineralogy and chemistry of host and fault rocks; and investigation of fault versus bolideimpact hypotheses for the origin of enigmatic breccias found in the Proterozoic basement rocks. Fault kinematic and paleostress analyses suggest a record of transitional, and perhaps partitioned, strains from the Laramide orogeny through Rio Grande rifting. Normal faults within Tertiary basin-fill sediments are consistent with more typical WNW-ESE Rio Grande rift extension, perhaps de coupled from bedrock structures due to strength contrasts favoring the formation of new faults in the relatively weak sediments. Analyses of the fault-length data indicate power-law length distributions similar to those reported from many geologic settings globally. Mineralogy and chemistry in Proterozoic fault-related rocks reveal geochemical changes tied to hydro thermal alteration and nearly isochemical transformation of feldspars to clay minerals. In sediments, faulted minerals are characterized by mechanical entrainment with minor secondary chemical changes. Enigmatic breccias in rift-flanking Protero zoic rocks are autoclastic and isochemical with respect to their protoliths and exist near shatter cones believed to be related to a previously reported pre-Pennsylvanian impact event. A weak iridium anomaly is associated with the breccias as well as adjacent protoliths, thus an impact shock wave cannot be ruled out for their origin. Major fault zones along the eastern rift-flank mountain front are discontinuous and unlikely to impede regional groundwater flow into Española Basin aquifers. The breccia bodies are not large enough to constitute aquifers, and no faultor breccia-related geochemical anomalies were identified as potential contamination sources for ground or surface waters. The results of this work provide a broad picture of structural diversity and tectonic evolution along the eastern flank of the central Rio Grande rift and the adjacent Española Basin representative of the rift as a whole and many rifts worldwide.