Eric W. James

Geochronology of late Pleistocene to Holocene speleothemsfrom central Texas: Implications for regional paleoclimate

A detailed chronology for four stalagmites from three central Texas caves separated by as much as 130 km provides a 71 000-yr record of temporal changes in hydrology and climate. Mass spectrometric 238 U230 Th and 235 U231 Pa analyses have yielded 53 ages. The accuracy of the ages and the closedsystem behavior of the speleothems are indicated by interlaboratory comparisons, concordance of 230 Th and 231 Pa ages, and the result that all ages are in correct stratigraphic order. Over the past 71 000 yr, the stalagmites have similar growth histories with alternating periods of relatively rapid and slow growth. The growth rates vary over more than two orders of magnitude, and there were three periods of rapid growth: 71‐60 ka, 39‐33 ka, and 24‐12 ka. These growth-rate shifts correspond in part with global glacial-interglacial climatic shifts. Paleontological evidence indicates that around the Last Glacial Maximum (20 ka), climate in central Texas was cooler and wetter than at present. This wetter interval corresponds with the most recent period of increased growth rates in the speleothems, which is consistent with conditions necessary for speleothem growth. The temporal shift in wetness has been proposed to result

Petrogenesis of Mid-Proterozoic granitic magmas: examples from central and west Texas

Circa 1.1 Ga granitic magmatism in Texas was manifested as two compositional groups: (1) the 1.12 Ga Red Bluff granitic suite in west Texas; and (2) 1.12-1.07 Ga granites of the Llano uplift of central Texas. Both suites share some characteristics typical of ‘anorogenic’ granites (e.g. potassium- and iron-rich bulk compositions, Fe-rich hydrous silicates, emplacement conditions involving low oxygen fugacities and water contents) and exhibit similar isotopic characteristics. However, rock associations, mineral chemistries, and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible. The Red Bluff granitic suite includes cogenetic syenites, quartz syenites and granites; transitional ferrobasaltic dikes are also present. In contrast, syenitic and mafic rocks are not associated with the Llano granites. The Llano granites contain biotite and calcic amphibole with lower Fe(Fe + Mg) ratios compared to those occurring in the Red Bluff rocks. Alkali amphiboles (e.g. arfvedsonite) occur in the Red Bluff granites but not in the Llano granites. The Red Bluff granitoids are characterized by high FeOTMgO ratios, high (Na2O + K2O), high concentrations of HFSE and rare earth elements (REE), and other features typical of A-type, ‘within-plate’ granites [e.g. the Pikes Peak batholith (PPB)]. The Llano granites are geochemically distinct with generally higher P2O5 and Sr, lower Na2O, FeOTMgO, Zr, Y and REE, and much lower Ta and Nb. Nd isotopic data overlap between the two granite suites and have ‘juvenile’ signatures. However, trace element data suggest different petrogeneses for the two suites. The Red Bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas, with minor crustal assimilation. The Llano granites appear to represent anatectic melts derived from slightly older, juvenile crustal sources; some melts underwent fractional crystallization controlled by feldspar and accessory minerals. The petrology and geochemistry of ∼ 1.1 Ga granites in Texas indicate that they should not be considered as part of a single ‘anorogenic’ magmatic event. The Red Bluff granitic suite was emplaced into a shelf sequence, north of the Grenville Front, within a broad zone characterized by mild extension. In contrast, Llano granites are late-stage intrusions emplaced into multiply deformed and metamorphosed crust, south of the Grenville Front, during or after waning stages of Grenville orogenesis.

Mid‐Cenozoic stress evolution and magmatism in the Southern Cordillera, Texas and Mexico: Transition from continental arc to intraplate extension

Orientations of dikes, veins, faults, and slickenlines reveal the evolution of stress during Eocene to Miocene magmatism in the southern Cordillera. Where most thoroughly studied by us in Trans-Pecos Texas, magmatism began at about 48 Ma shortly after the cessation of Laramide folding. Dikes and veins that formed from then until about 32 Ma strike dominantly east-northeast. This indicates that the least principal stress (σ3) was north-northwest; additional data suggest that the maximum principal stress (σ1) was east-northeast. The stress field changed to σ1 vertical and σ3 east-northeast (i.e., east-northeast extension) at least by 28 Ma and probably by 31 Ma. Dikes and veins that formed between 31 and 17 Ma, when all magmatism ceased in Texas, strike north-northwest. This change marks the beginning of regional, Basin and Range extension; however, major normal faulting, exclusively of high-angle type, did not begin until about 24 Ma. A similar stress change, marked by a similar change in dike and vein orientations, occurred throughout the southwestern United States and northern Mexico. The time of change is not well constrained in Texas, but available information allows it to have occurred at the same time thoughout the southern Cordillera. We suggest the earlier stress field is related to east-northeast convergence between the Farallon and North American plates. The change in stress is approximately coincident with collision of the East Pacific Rise and paleotrench. Extension may be related to the change from a convergent to a transform margin along the western edge of North America. The changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in Texas. Contemporaneity of the changes in stress and magmatism indicates that they are related. Combined with regional age patterns, paleostress and geochemical data indicate that pre-31 Ma magmatism in the southern Cordillera occurred in a subduction-related, continental volcanic arc. Subsequent magmatism occurred in an environment of intraplate extension of the Basin and Range province.

High-resolution 87Sr/86Sr chemostratigraphy of the Silurian: Implications for event correlation and strontium flux

Analyses of 87Sr/86Sr in Silurian conodonts recovered from localities in North America and Europe representing 13 of the 14 defined Silurian conodont zones provide a high-resolution record of seawater chemistry for the Silurian Period. These data, which are characterized by little or no scatter, depict several high-frequency cycles superimposed on a gradual longer term rise in 87Sr/86Sr for the Silurian. High-frequency cycles have a duration of about one conodont zone, and many correlate with sequence boundaries recognized around the world. These data provide a much higher resolution image of secular changes in 87Sr/86Sr during the Silurian and may require a rethinking of models of strontium isotope flux in marine basins.

A global model for cave ventilation and seasonal bias in speleothem paleoclimate records

Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave-air CO2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave-air CO2. Elevated CO2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure, and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium, and CO2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not.

Southeastern extent of the North American craton in Texas and northern Chihuahua as revealed by Pb isotopes

The geographic patterns of Pb isotopic compositions of Eocene to Miocene igneous rocks appear to delineate the southeastern edge of the North American Precambrian craton in Trans-Pecos Texas and northern Chihuahua, Mexico. The boundary parallels and lies southeast of the buried fault contact between Ouachita facies thrust sheets and Paleozoic cratonic sedimentary rocks. Lead isotopic data for basalts and granulite xenoliths suggests that the basalts contain a mixture of Pb from mantle and lower-crustal sources. On the northwest side of the boundary, this mixing involves 1.35 to 1.1 Ga cratonic lithosphere and yields linear arrays of 206Pb/204Pb versus 207Pb/204Pb and 208Pb/204Pb. Basaltic magmas on the southeast side of the boundary apparently interacted with more-radiogenic lithosphere accreted during the Ouachita orogeny. Basalts in a 50-km-wide central zone between the northwestern and southeastern provinces have Pb isotopic compositions that are intermediate between those of the northwestern and southeastern sources. In the northwestern province, lead isotopic compositions of intermediate to felsic rocks are less radiogenic than those of associated basalts, whereas in the southeast province they have more radiogenic compositions than the basalts. This isotopic divergence reflects assimilation of contrasting crust of each province. Most felsic Tertiary rocks of the northwestern province have lower 207Pb/204Pb versus 206Pb/204Pb and somewhat elevated 208Pb/204Pb versus 206Pb/204Pb, suggesting assimilation of relatively high Th/U Precambrian crust. The central province also contains low 206Pb/204Pb and 207Pb/204Pb Precambrian components, but few igneous rocks have elevated 208Pb/204Pb. Southeastern-province crust appears dominantly Phanerozoic, although Precambrian basement with Pb isotopic compositions near model average crust may be present. The Pb isotopic zonation observed in West Texas is similar to that found in the Appalachians of the northern United States and Canada and in the Caledonides of Europe: that is, a progression from more-radiogenic outboard terranes to less-radiogenic inboard terranes. Lead isotopic ratios from Tertiary igneous rocks of the northwestern part of Trans-Pecos Texas are similar to those of adjacent New Mexico and southeastern Arizona, although regional basement ages and isotopic characteristics differ. Extending the West Texas isotopic terranes to the southwest holds promise for testing and improving the understanding of the Sonora-Mojave megashear; Precambrian assembly of Gondwana; and correlation of Grenville-Caledonian terranes between the Appalachians, Texas, and eastern to southern Mexico.

Compositional Changes in Trans-Pecos Texas Magmatism Coincident with Cenozoic Stress Realignment

The composition and inferred sources of Cenozoic magmas in Trans-Pecos Texas changed abruptly between 32 and 28 Ma, coincident with a fundamental change in stress regime. Magmas emplaced between 48 and 32 Ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas. These rocks have low Nb and Ta compared to Zr, Hf, La, Ba, and K, characteristics typical of continental volcanic arcs. However, the Texas rocks are more alkalic, have higher concentrations of incompatible elements, and have higher ratios of Nb and Ta to Zr, Hf, La, Ba, and K than is typical of rocks from arcs near trenches. These differences reflect relative position within the Cordilleran arc. Trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios. Trans-Pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone. Magmas emplaced after 31 Ma were either bimodal, alkali basalt-rhyolite, or exclusively basaltic. They have high absolute abundances of incompatible trace elements and high ratios of Nb and Ta to Zr, Hf, La, Ba, and K. The mafic rocks are typical of continental rift or ocean island basalts. Trace element models using Nb, Ta, Y, Yb, Hf, and Zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting, variable amounts of fractional crystallization, but little or no assimilation. The change in magma compositions and sources is best illustrated by a drop in Zr/Nb, from between 18 and 6.25 in the earlier, arc rocks to between 6.25 and 2 in the later, rift-related rocks. Stress regime appears to play an important role in the process of magma generation and evolution. The change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 Ma occurred in a continental volcanic arc, whereas later activity occurred in a setting of intraplate extension. The magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3–4 m.y.

Geochemical logging in the Cajon Pass Drill Hole and its application to a new, oxide, igneous rock classification scheme

A new elemental oxide classification scheme for crystalline rocks is developed and applied to geochemical well logs from the Cajon Pass drill hole. This classification scheme takes advantage of measurements of elements taken by a geochemical logging tool string. It uses K_2O versus SiO_2/Al_2O_3 to distinguish between granites, granodiorites, tonalites, syenites, monzonites, diorites, and gabbros. Oxide measurements from cores are used to calibrate the elemental abundances determined from the well logs. From these logs, a detailed lithologic column of the core is generated. The lithologic column derived from the well log classification scheme is compared with a lithologic column constructed from core samples and well cuttings. In the upper 1295 m of the well, agreement between the two columns is good. Discrepancies occur from 1295 to 2073 m and are believed to be caused by the occurrence of rock types not distinguished by the classification scheme and/or the occurrence of secondary minerals. Despite these discrepancies, the well log-based classification scheme helps to distinguish changes in rock type and shows potential as an aid to the construction of lithologic columns in boreholes of crystalline rocks.

Trade winds drive pronounced seasonality in carbonate chemistry in a tropical Western Pacific island cave—Implications for speleothem paleoclimatology

Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in mid-latitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it is has been proposed caves will ventilate on daily timescales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8-year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 - 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave effect the interpretation of speleothem-based paleoclimate records. This article is protected by copyright. All rights reserved.