John T. Chesley

High-Magnesian Andesite from Mount Shasta: A product of Magma Mixing and Contamination, not a Primitive Mantle Melt

It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65–72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts.

RE-OS SYSTEMATICS OF MANTLE XENOLITHS FROM THE EAST AFRICAN RIFT : AGE, STRUCTURE, AND HISTORY OF THE TANZANIAN CRATON

In order to understand the effects of contractional and extensional tectonics on thick, mantle roots, we have undertaken a systematic study of mantle xenoliths from the Labait volcano, which lies within the East African Rift on the eastern boundary of the Archean Tanzanian craton. The Re-Os systematics of the Labait xenoliths show that ancient, refractory lithosphere is present to depths of ;140 km. Above this depth, the mantle section consists of harzburgitic xenoliths with whole rock 187 Os/ 188 Os between 0.1081 and 0.1140, corresponding to Re depletion (TRD) ages of 2.8 to 2.0 Ga. Chromites from these samples are generally less radiogenic than their corresponding whole rocks and have TRD ages between 2.5 to 2.9 Ga, yielding the best estimate for the age of this portion of the lithosphere. Coupled petrographic and isotopic data for some of these samples indicate they have been variably overprinted by recent addition of Re and/or radiogenic Os. Between 4.4 to 4.7 GPa (;140 to 150 km depth), peridotites are more fertile and yield younger TRD ages (1.0 Ga to future ages). The highest temperature sample has radiogenic 187 Os/ 188 Os (0.133), overlapping the range measured for metasomatic xenoliths and the Labait host melilitite (0.13 to 0.14). This range is taken to represent asthenospheric mantle beneath the Tanzanian craton, which has plume-like isotopic characteristics. The suite shows a good correlation on 187 Os/ 188 Os vs. temperature (hence depth) and 187 Os/ 188 Os vs. wt.% Al2O3 or CaO plots. These trends, which pass above primitive mantle compositions, may reflect mixing of recent plume-derived Os with ancient lithospheric Os, formation of the lowest portion of the lithosphere during successive melting events or a combination of both processes. Our data show that complete delamination of the lithospheric mantle has not occurred beneath the Tanzanian craton during its long tectonic history. However, if the Archean lithosphere was originally thicker than the ;140 km currently beneath Labait, then the lithosphere has been thinned, either by thermal erosion associated with the rift or by partial delamination during Proterozoic collision. Finally, we see no evidence for extensive lithospheric thinning associated with development of the East African Rift, although overprinting of the lithosphere by rift-related magmas has occurred. Copyright

Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot Islands: implications for Mesoproterozoic marine evolution

A 4‰ positive shift in the carbon isotopic composition of the oceans, recorded globally in marine carbonate rocks at ∼1.3 Ga, suggests a significant change in Mesoproterozoic carbon cycling. Enhanced burial fluxes of organic carbon, relative to inorganic carbon, implied by this isotopic shift may have resulted in increased oxygenation of the Earths biosphere, as has been suggested for similar Paleoproterozoic and Neoproterozoic carbon isotope events. This hypothesized Mesoproterozoic oxygenation event may be recorded in the geologic record by the appearance of the oldest preserved, laterally extensive, bedded marine CaSO4 evaporites in the ∼1.2 Ga Grenville and Bylot supergroups. Speculation that the appearance of extensively preserved marine gypsum and/or anhydrite reflects increased biospheric oxygenation has been challenged, however, by the hypothesis that CaSO4 precipitation prior to the Mesoproterozoic may have been inhibited by significantly higher marine carbonate saturation, which would have facilitated carbonate precipitation and effectively limited Ca2+ availability during seawater evaporation (Grotzinger, J.P., 1989. Controls on Carbonate Platform and Basin Development, vol. 44, SEPM, Tulsa, OK, pp. 79–106), regardless of O2 levels. The 1.2 Ga Society Cliffs Formation (Bylot Supergroup, northern Baffin Island) consists of ∼720 m of peritidal carbonates, evaporites, and minor siliciclastic rocks. Evaporites occur predominantly in the lowermost 300 m of the Society Cliffs Formation, where gypsum beds (1–250 cm thick) constitute up to 15% of the exposed strata. Stratigraphic and sedimentologic constraints, as well as isotopic (C, O, Sr) and elemental (Ca, Sr, Na, K, Ba) compositions of evaporites and associated carbonates, indicate a marine origin for Society Cliffs gypsum. An upsection increase in δ34S of Society Cliffs gypsum (from +22‰ to +32‰ VCDT) is therefore interpreted to reflect primary variation in Mesoproterozoic marine sulfate compositions, although the inferred rapidity of isotopic change requires a marine sulfate reservoir significantly smaller than that of the modern ocean. Examination of the maximum fractionation between coeval sulfide and sulfate reservoirs, however, indicates that Mesoproterozoic oceans were not sulfate-limited with respect to bacterial sulfate reduction either before or after the hypothesized 1.3 Ga oxygenation event. Although increased ocean-atmosphere oxygenation may have increased marine sulfate concentrations at this time, the exact role of a Mesoproterozoic oxygenation event cannot be ascertained. Furthermore, high Mg/Ca ratios measured in Society Cliffs gypsum suggest that elevated Mg2+ concentrations in Proterozoic marine systems may have helped sustain carbonate hypersaturation, and that Ca2+-limitation may have played a significant role in the Proterozoic record of evaporite deposition.

Crust–mantle interaction in large igneous provinces: Implications from the Re–Os isotope systematics of the Columbia River flood basalts

The source and evolution of magmas that form large igneous provinces is a controversial topic. At the center of the debate is whether the different reservoirs — continental crust, asthenosphere or sub-continental lithosphere — contribute to the formation and evolution of these provinces. The isotopic systematics of Re and Os offer significant new information to constrain this debate. Because Os is compatible and Re incompatible during mantle melting, the different possible melt reservoirs will develop distinct Os isotopic signatures over time. Therefore, the source of magmas and the different possible contaminants should be readily distinguishable using the Os isotopic system alone. We have focused our study on the Re and Os composition of the Columbia River basalt Group (CRBG), which is one of the youngest and best studied continental flood basalts provinces in the world. Samples from throughout the well documented stratigraphic column, that represent primitive and contaminated magmas, as well as samples emplaced in stable North American Craton and Mesozoic accreted terrane were analyzed. The initial 187Os/188Os ranges from the lowest value of ∼0.13 in the most primitive Imnaha Formation and increases to values as high as 0.4 in flows from formations emplaced in the accreted terrane. Samples located on the craton have initial 187Os/188Os isotopic ratios that vary from ∼1 to ∼3. The data conclusively demonstrate that continental crust, most likely mafic lower crust, played an important role in the evolution of this flood basalt province. These data also show that the sub-continental lithospheric mantle (SCLM) was not significantly involved in the formation or modification of these continental flood basalts.

Source contamination versus assimilation: an example from the Trans-Mexican Volcanic Arc

Abstract Volcanic samples representing a wide range of lithologies and compositions were collected from the Miocene to Quaternary age Michoacan-Guanajuato volcanic field (MGVF) in the Trans-Mexican volcanic belt (TMVB). The samples were analyzed for major and trace elements, and 87Sr/86Sr and 187Os/188Os in an effort differentiate the importance of source contamination and assimilation in continental arc magmatism. Re concentrations in the MGVF samples range from 0.03 to 0.13 ppb and Os concentrations range from 0.05 to 0.001 ppb. The 87Sr/86Sr of the samples vary little, ranging from 0.7037 to 0.7047, despite a wide range in major element composition. However, the 187Os/188Os vary greatly, from 0.135 to 0.410. Decreasing Os concentration and increasing 187Os/188Os show a clear relation with indicators of fractionation such as MgO or Ni. A plot of 187Os/188Os versus Ba/Nb for all samples from the MGVF show two distinct trends: (1) a wide variation in Ba/Nb (50–200) associated with minor variations in 187Os/188Os (∼0.135–0.145), and (2) increasing 187Os/188Os (0.145–0.40) associated with restricted Ba/Nb (35–70). These trends are best explained through a dynamic multi-component process. Fluids are released from the subducting slab, resulting in melting of the overlying asthenospheric wedge. The pristine fluids have high Ba and low Re and Os concentrations. The resulting melts have variable Ba/Nb, but unradiogenic 187Os/188Os. Superimposed upon these melts are both assimilation and fractional crystallization processes, which affect both the Ba/Nb and 187Os/188Os systems as they ascend into the lower crust.

Large-scale mantle metasomatism: a Re–Os perspective

There is a debate on the behavior of Re and Os during mantle metasomatism. Some argue that the mantle can acquire high 187Os/188Os (0.15 to >1.0) either directly from metasomatic events or by the growth of 187Os from 187Re over time. However, any suggestions of subduction-related Os metasomatism producing whole-scale elevation of the mantle to 187Os/188Os values greater than 0.15 need to be supported by comparisons of increased Re and Os concentrations and isotopic ratios, with like and consistent increases in elements (i.e., Ba, B, Rb) and isotopic ratios (i.e., Nd, Pb, Sr) known to be affected by metasomatic processes. All of the samples in the literature, either xenoliths or minerals (sulfides, pyroxene, phlogopite or amphibole) that are thought to represent the products of mantle metasomatism, follow a clear correlation of increasing Re/Os with decreasing Os concentration. This inverse correlation requires enrichment in Os concentrations in the metasomatizing agent by two to four orders of magnitude relative to the original subduction component in order to substantially elevate 187Os/188Os, eliminating sediment or slab melts as an effective metasomatic component. To date there is no evidence to support conclusions that Os metasomatism, either directly or by a two-stage process involving Re metasomatism and 187Os growth over time, will substantially change the 187Os/188Os of the mantle to values >0.15. When other isotopic systems are considered along with Os, in metasomatism-based models, these models are not plausible. Consequently, large-scale mantle melts should reflect near-chondritic values. The recognition of the robustness of the Re–Os system will allow for the discrimination of different mantle and crustal reservoirs involved in magmatic processes.

Isotopic ratios and release rates of strontium measured from weathering feldspars

Based upon dissolution of feldspars under controlled laboratory conditions, we conclude that Sr release, at pH 3, is neither consistently stoichiometric nor constant for the feldspars measured. Bytownite, microcline, and albite all initially release Sr at rates which are 5 (bytownite) to 160 (microcline) times faster than steady-state release rates. The Sr/Si ratios in the early effluents are significantly elevated compared to the bulk mineral values. The 87Sr/86Sr measured in effluent early in dissolution is higher than the bulk mineral 87Sr/86Sr for bytownite, but lower than bulk mineral ratios for microcline and albite. 87Sr/86Sr ratios for the feldspar powders also changed markedly during dissolution of the three phases. In part, nonstoichiometric release of Sr can be explained by the presence of secondary phases (exsolution lamellae or minute quantities of accessory phases) or by surface leaching. Although we infer that these feldspars eventually release Sr with isotopic composition roughly equal to that of the bulk mineral at steady-state, the feldspars dissolve at extremely different rates (bytownite releases Sr at a steady-state rate ∼102 to 103 times faster than albite and microcline, at pH 3). Therefore, a mixture of these feldspars, or of other minerals exhibiting vast differences in dissolution rate, will release 87Sr/86Sr ratios distinctly different from the bulk whole rock. In addition, initial Sr release rates of the minerals (bytownite > microcline > albite) differ from steady-state release rates (bytownite > albite > microcline), complicating analysis of weathering solutions. Log (rate constants) for bytownite, albite, and microcline decrease from −13.5 to −16.4 to −17.2 (mol Sr cm−2 s−1). Interpretation of catchment scale riverine 87Sr/86Sr ratios on the basis of whole-rock Sr isotopes is, therefore, problematic at best, and would require normalization of bulk isotopic ratios by relative rates of dissolution of Sr-contributing phases. We also argue that abraded feldspar particles formed naturally, for example, during glaciation, will show this initial transient nonstoichiometric release. However, once the transient release is completed (perhaps 102 to 103 yr after abrasion), as long as the solution chemistry remains relatively constant, stoichiometric release of cations from feldspars, including Sr, is expected. The most likely way, therefore, to increase riverine fluxes of major cations or radiogenic 87Sr/86Sr is to create highly flushed (and, therefore, far from equilibrium) water-rock systems such as glacial sediments and soils, with reactive minerals (e.g., carbonate, plagioclase, or biotite) containing significant radiogenic strontium.

A detrital model for the origin of gold and sulfides in the Witwatersrand basin based on Re-Os isotopes

Abstract The Re-Os systematics of gold and sulfides from the Witwatersrand basin were utilized to determine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin. Gold from a gravity concentrate from the Western Areas Gold Plant and gold from the Vaal Reef have very high Os concentrations of approximately 73 to 10000 ppb and 3 to 32 ppb Re, resulting in 187Re/188Os ratios of 0.010 to 0.185. The gold has subchondritic 187Os/188Os ratios between 0.1056 to 0.1099 and an average value of 0.1067. Rhenium depletion ages (TRD) range from 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga. Pyrite from the Vaal Reef have Os concentrations ranging from 0.26 to 0.68 ppb, Re concentrations of 1.7 to 2.8 ppb and187Re/188Os ratios of approximately 14 to 87. The pyrite samples have measured 187Os/188Os ratios of 0.84 to 4.7 and define an isochron with an age of 2.99 ± 0.11 Ga (MSWD = 0.77). The Os isotopic data from the direct measurement of gold preclude introduction of gold to the Witwatersrand basin from crustally derived metamorphic or hydrothermal fluids between 2.7 to 2.0 Ga. The unradiogenic 187Os/188Os ratios, old TRD ages of the Western Areas and Vaal Reef gold samples, as well as the contemporaneously old age of the Vaal Reef pyrite are consistent with detrital deposition of gold during the formation of the Witswatersrand basin. The Os data will allow for minor hydrothermal remobilization and/or overprinting of hydrothermal gold on preexisting detrital gold grains but does not support the introduction of gold solely by hydrothermal fluids.

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare‐up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo‐depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion‐related trench sediment was shuffled from the trench to the sub‐arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high‐pressure/low‐temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite‐facies metamorphism and limited partial melting.

Geologic origins of salinization in a semi-arid river: The role of sedimentary basin brines

Semi-arid and arid rivers typically exhibit increasing salinity levels downstream, a trend often attributed to irrigated agriculture, primarily due to evapotranspiration. In contrast, the results of our investigations in one salinized river suggest that geological sources of salt added by groundwater discharge are more important than agricultural effects. We performed detailed synoptic sampling of the Upper Rio Grande– Rio Bravo, an arid-climate river with signifi cant irrigated agriculture, and identifi ed a series of salinity increases localized at the distal ends of sedimentary basins. Using Cl/Br, Ca/Sr, 87 Sr/ 86 Sr, and 36 Cl/Cl ratios and δ 234 U values as environmental tracers, we show that these increases result from localized discharge of high-salinity groundwater of a sedimentary brine source. These groundwater fl uxes, while very small (<1 m 3 s –1 ), are the dominant solute input and, combined with downstream evapotranspirative concentration, result in salinization. Furthermore, 36 Cl/Cl ratios and δ 234 U values for these brines are close to secular equilibrium, indicating brine ages on the order of millions of years. The recognition of a substantial geologic salinity source for the Rio Grande implies that alternative salinity management solutions, such as interception of saline groundwater, might be more effective in reducing salinity than changes in agricultural practices.