Peter Holden

Evidence for long residence times of rhyolitic magma in the Long Valley magmatic system: the isotopic record in precaldera lavas of Glass Mountain

The high-silica rhyolite lavas of Glass Mountain, California, provide a detailed record of the evolution of the Long Valley magmatic system during the 1.4 m.y. prior to the catastrophic eruption of the voluminous zoned rhyolitic Bishop Tuff at 0.73 Ma. The older lavas of Glass Mountain are extremely evolved, and were erupted from 2.1 to 1.2 Ma with87Sr/86Sr of 0.707–0.739 and eNd of −3 to −4, whereas the younger lavas are slightly less evolved and were erupted between 1.2 and 0.79 Ma with87Sr/86Sr of 0.706–0.707 and eNd close to −1, essentially identical to the Sr and Nd isotopic compositions of the Bishop Tuff. Neither the older nor younger lavas display a relationship between the isotopic compositions of Sr and Nd. The Pb isotopic compositions are effectively uniform at206Pb/204Pb= 19.13–19.17. A group of older lavas that outcrop in a northwest-trending band near the topographic rim of the caldera collectively define an apparent Rb—Sr isochron age(t)of2.09 ± 0.06Ma with an intercept of87Sr/86Sr= 0.7060 ± 3, where analyzed older lavas outcropping outboard of this band define an isochron of1.90 ± 0.02Ma(87Sr/86Sr)t = 0.7063 ± 4) These isochron ages are identical to the K—Ar ages for oldest erupted rhyolites in the two regions, respectively. The younger lavas away from the caldera define an apparent Rb—Sr isochron age(t)of1.14 ± 0.08Ma with the same(87Sr/86Sr)t as the older lavas. The agreement between the87Sr/86Sr ratios of the rhyolite magmas when emplaced in the Long Valley magmatic system (0.706) and that of basaltic lavas in the region (0.7059–0.7062) suggests that the Sr isotopic compositions of these Sr-poor rhyolites may be totally dominated by Sr from mantle-derived components. K—Ar ages and stratigraphic relations for both the older and younger lavas seem to rule out the possibility that the Rb—Sr isochron ages record the times of eruption. The very low Sr concentrations of all the lavas (down to 0.1 ppm) cannot be produced simply by crustal fusion; the extremely high and variable Rb/Sr ratios (up to 2000) and low Sr concentrations must be due to extensive fractional crystallization. We interpret the isochrons as dating this process. To produce the observed volume of highly evolved rhyolite requires differentiation of a sizeable body of magma; thus the older lavas may have been derived from an evolved roof zone of a differentiated magma chamber that was present in the Long Valley region as early as 2.1 Ma. The similarity of the isotopic compositions of the younger Glass Mountain lavas and the Bishop Tuff, as well as the isochronous relationship of most of the younger lavas, indicate that the chamber containing the magma later to erupt as the younger lavas of Glass Mountain and the Bishop Tuff was already formed, isotopically homogenized, and had developed an evolved roof zone by about 1.1 Ma. There is no evidence for significant assimilation of crustal rocks or input of mantle-derived magma into the upper reaches of the system after this time. The preservation of isochronous relationships between lavas (and their constituent glasses) implies that most of the crystallization of the magma (i.e., that which affected Rb/Sr ratios) took place in discrete dateable events. The fact that the three isochrons have the same(87Sr/86Sr)t requires that the magmas did not exist for long as highly evolved liquids prior to the differentiation events that established the range of Rb/Sr ratios and the isochrons, as such magmas would increase in87Sr/86Sr by 0.0002–0.005 per 100,000 years (depending on Rb/Sr ratio) due to in-situ decay. Comparing the isochron ages of the Glass Mountain lavas with times of their eruption yields residence times of silicic magma as great as 0.7 m.y. To prevent the rhyolitic magma from cooling and crystallizing during these intervals, heat must have been supplied to the system, presumably in the form of new additions of magma. This magma must not have reached the upper regions of the magma reservoir; otherwise the isochrons would have been destroyed by mixing in these Sr-poor rocks. Our interpretation of the isotopic data for Glass Mountain is at variance with recent models that invoke repeated melting of the deep crust rather than episodic tapping of magma chambers as mechanisms for producing repeated rhyolitic eruptions spanning periods of 105–106 years. The evidence for Long Valley is rather that major differentiation accompanies fresh inputs of magma into the system and that magma is maintained in stable zones in the chamber for hundreds of thousands of years.

δ18O values, 87Sr86Sr and Sr/Mg ratios of Late Devonian abiotic marine calcite: Implications for the composition of ancient seawater

Abstract Late Devonian (Frasnian) abiotic marine calcite has been microsampled and analyzed for 87 Sr 86 Sr ratios, δ18O and δ13C values, and minor element concentrations. Portions of marine cement crystals from the Alberta and Canning Basins have escaped diagenetic alteration and preserve original marine δ18O values (−4.8%. ± 0.5, PDB), δ13C values (+2.0 to +3.0%., PDB), 87 Sr 86 Sr ratios (0.70805 ± 3), and Sr/Mg weight ratios (0.04 to 0.05). Marine 87 Sr 86 Sr ratios are globally consistent and can be correlated within the Alberta Basin, and among the Alberta, Canning, and Williston Basins. Correlation of isotopic and chemical data strengthen the conclusion that marine cements from the Leduc Formation preserve original marine δ18O values which are 3 to 4%. lower than those of modern marine cements. These low δ18O values are best explained by precipitation from 18O-depleted seawater and not by elevated seawater temperature or diagenetic alteration. For comparison with Devonian data, analogous data were collected from Holocene Mg-calcite and aragonite marine cements from Enewetak Atoll, Marshall Islands. Mg-calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater, and Mg-calcite cements are homogeneous with respect to Sr and Mg contents. Empirically derived homogeneous distribution coefficients for Mg and Sr in modern, abiotic Mg-calcite from Enewetak Atoll are 0.034 and 0.15, respectively. An equation describing the dependence of DSr on Mg content was based on a compilation of Sr and Mg data from Holocene abiotic marine calcite (DSr = 3.52 × 10−6 (ppm Mg) + 6.20 × 10−3). Unlike that derived from experimental data, this Sr-Mg relation is consistent over a range of 4 to 20 mol% MgCO3 and may represent precipitation phenomena which are minimally controlled by kinetic effects. Comparison of Sr and Mg contents of analogous Devonian and Holocene marine cements suggests that the Mg/Ca ratio of Late Devonian seawater was significantly lower and that the Sr/Ca ratio was significantly higher than that of modern seawater.

Modelling the petrogenesis of high Rb/Sr silicic magmas

Halliday, A.N., Davidson, J.P., Hildreth, W. and Holden, P., 1991. Modelling the petrogenesis o1 high Rh/Sr silicic magmas. In: A. Peccerillo (Guest-Editor), Geochemistry of Granitoid Rocks. Chem. Geol., 92: 107-114. Rhyolites can be highly evolved with Sr contents as low as 0.1 ppm and Rb/Sr > 2,0(t0. In contrast, granite batholiths are commonly comprised of rocks with Rb/Sr 100. Mass-balance modelling of source compositions, differentiation and contamination using the trace-element geochemistry of granites are therefi)re commonly in error because of the failure to account for evolved differentiates that may have been erupted from the system. Rhyolitic magmas with very low Sr concentrations ( ~ 50. Evidence from phenocryst/glass/ whole-rock concentrations supports high Sr partition coefficients in feldspars from high silica rhyolites. However, the low modal abundance of plagioclase commonly observed in such rocks is difficult to reconcile with such simple fractionation models of the observed trace-element trends. In certain cases, this may be because the apparent trace-element trend defined by the suite of cogenelic rhyolites is the product of different batches of magma with separate differentiation histories accumulating in the magma chamber roof zone.

Chemical and isotopic evidence for major mass transfer between mafic enclaves and felsic magma

Abstract In both the Strontian and Criffell zoned granitoid plutons, certain chemical elements (notably K, Rb, Ca, Sr and Pb) display a correlation between their concentration in microdiorite enclaves and their concentration in the immediately enclosing host granitic rock. This appears to represent the effects of post-incorporation mixing and exchange between the enclave and its host magma. Enclave initial 87 Sr 86 Sr ratios are indistinguishable from those of the host, while enclave initial 143 Nd 144 Nd ratios are systematically more radiogenic establishing that these enclaves are not melt segregations nor restite. The isotopic data are in agreement with the chemical data in indicating that Sr is almost entirely derived from the host Despite clear evidence of major mass transfer of alkali metals, alkali earths and transition metals, a comparison of the concentrations and ratios of HFSE and REE, and Nd isotopic ratios with those of a number of potential starting protoliths, allows the enclave precursors to be positively discriminated. In this regard enclave HFSE and REE compositions may allow for models of granitoid production by the injection of hot, mantle-derived magmas into the crust to be tested.

Eclogites with oceanic crustal and mantle signatures from the Bellsbank kimberlite, South Africa, part 2: Sr, Nd, and O isotope geochemistry

The Sr, Nd, and O isotopic compositions of garnet and clinopyroxene mineral separates from nine eclogite xenoliths from the Bellsbank kimberlite (erupted at 120 Ma) define three groups. Group A eclogites, considered to be mantle cumulates, are characterized by 6180, and 87Sr/Srsr values typical of mantle-derived materials (+5.1 to +5.6%o and 0.7042-0.7046, respectively), and very low Sm/Nd ratios, (apparent) Rb/Sr ratios and ~NaLlZ01 values (0.057-0.078, 0.00005-0.00136 and - 14 to - 16 respectively). The REE and isotopic data for these eclogites can be modelled in terms of crystallization from a Group II kimberlite magma at - 1-1.5 Ga. Group B and C eclogites, believed to be the metamorphosed products of ancient subducted oceanic crust, are characterized by low 3180 (+2.9 to +4.7), extremely high ~Nd[a20] (-- +40 to +219), and radiogenic 87Sr/S6Sr ratios (0.708-0.710). The Sm/Nd ratios of the Group B eclogites are very high (up to 1.6). The data for Group B and C eclogites define a linear correlation on Sm/Nd and 1/Nd vs. ~Nal~201 diagrams. These relationships are consistent with mixing of the Bellsbank kimberlite (CNO [1201 = --10; Sm/Nd = 0.10) with a depleted eclogite end-member (eNd[120 ] + 219; Sm/Nd = 1.6) during a cryptic metasomatic event. The Sr isotopic variations in Group B and C eclogites cannot be generated by simple two-component mixing. The St, Nd, and O isotope data for Group B and C eclogites probably reflect a complex sequence of depletion and enrichment events, in both crust and mantle settings. Enrichments which possibly affected the Group B and C eclogites include seawater-alteration of a MORB-like protolith, which lowered the 6180 and raised the SVSr/86Sr ratio, but left the Nd isotopic compositions unchanged, and cryptic metasomatism by the magmatism that produced the Bellsbank kimberlite. The high Sm/Nd ratio of the depleted eclogite end-member cannot be generated by extraction of a melt from a modern MORB composition. Rather, it is argued that such high Sm/Nd ratios are produced as a result of partitioning during the recrystallization of a MORB-like component to eclogite during subduction. Nd model ages suggest that this process occurred - 2.3-2.4 Ga.

Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents

The isotopic composition of Nd is uniform in metalliferous sediments formed at distances varying from >1000 km to within 10 km of the East Pacific Rise (EPR) palaeoridge. These data indicate that hydrothermal vent fluids, despite having concentrations more than 500 times greater, have no effect on the Nd isotopic composition of seawater. This implicates efficient scavenging of hydrothermal Nd by particulates, resulting in extremely short residence times (< 1 year) close to the hydrothermal vents. Therefore Nd isotopic studies of ancient seawater precipitates, particularly metalliferous sediments, cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates (or residence times) relative to those of modern oceans.

Sea-level-driven changes in ocean chemistry at an Upper Cambrian extinction horizon

A combined stratigraphic and isotopic study across a Late Cambrian mass-extinction horizon (Pterocephaliid-Ptychaspid biomere boundary) in cratonal and miogeoclinal sections of the western United States reveals evidence for a pattern of sea-level rise, decreasing 87 /Sr 86 Sr ratios, and increasing δ 13 C. The positive shift in δ 13 C, reaching maximum values a few metres above the extinction interval, is interpreted to reflect elevated primary productivity and an increase in the rate of burial of organic matter during expansion of the oxygen-minimum zone. The 87 /Sr 86 Sr decrease is interpreted to reflect some combination of increased sea-floor spreading rates and decreased continental weathering rates. The δ 13 C and 87 /Sr 86 Sr trends independently support the field-based hypothesis that a sea-level rise led to catastrophic oceanic overturn, possibly culminating in anoxic conditions, that ultimately proved lethal to a large fraction of the existing shelf fauna.

Mesozoic invasion of crust by MORB-source asthenospheric magmas, U.S. Cordilleran interior

Mafic and ultramafic xenoliths entrained in lavas of the Cima volcanic field have Nd and Sr isotopic ratios indicative of a source similar to that of mid-ocean ridge basalt (MORB). Nd and Sr internal isochrons demonstrate a Late Cretaceous intrusion age. These results, combined with evidence for emplacement in the lower crust and upper mantle, indicate invasion of the lower crust by asthenospheric magmas in the Late Cretaceous. Constituting the first prima facie evidence for depleted-mantle magmatism in the Basin and Range province prior to late Cenozoic volcanism, these results lend key support to models suggesting crustal heating by ascent of asthenosphere in the Mesozoic Cordilleran interior.

RB-SR AND SM-ND ISOTOPIC VARIATIONS IN DISSECTED CRUSTAL XENOLITHS

Abstract Four crustal xenoliths from Scotland have been dissected in order to evaluate the effects of magma-xenolith interaction on Rb-Sr and Sm-Nd isotopic systematics, and hence the use of the xenoliths for accurate determination of source compositions. Each xenolith was sampled from the edge to the center. The Nd isotopic compositions are virtually uniform across each xenolith; however, there are significant variations in Rb, Sr, and REE concentration, as well as Rb Sr and Sm Nd ratios and Sr isotopic composition. Most of this variation appears to be inherited from the protolith. However, one sample shows systematic variations in Rb and Sr concentration and Sr isotopic composition with distance from the xenolith edges due to interaction with the host magma. This sample displays petrographic evidence for grain boundary melt infiltration. These data are consistent with theoretical considerations of transport times and diffusivities that place limits on the amount of modification that is possible via diffusion alone. The calculated effects of transport of even large amounts of crustal xenoliths on the compositions of xenoliths and host magmas will be small, provided transport times are short. Overall, the isotopic variations within the xenolith suite mirror the source rocks from which they were extracted and are inferred to reflect variations in the deep crust of Scotland. However, small samples, such as most crustal xenoliths, may be of little use in defining meaningful crustal residence ages because of modal variations in metamorphic minerals and isotopic re-equilibration. The one xenolith studied has extremely variable Sm Nd ratios due to the presence of garnet and yields a broad range of geologically meaningless Nd model ages, but yields an Sm-Nd isochron age of 396 ± 49 Ma. The Loch Roag xenolith has a large range of Rb Sr ratio, and the Sr isotopic data define an isochron age of 416±28 Ma. Both ages are taken to represent the timing of metamorphism and/or cooling from 600°C at the end of the Caledonian Orogeny in Scotland.

Estimation of porphyrin concentration in the kerogen fraction of shales using high-resolution reflectance spectroscopy

The authors have derived and calibrated an interpretive model for estimating porphyrin concentration in bitumen and kerogen from spectral reflectance data in the visible and near-ultraviolet region of the spectrum. Preliminary results obtained using the model are consistent with concentrations determined from the bitumen extract and suggest that 40 to 60% of the total porphyrin concentration remains in the kerogen after extraction of bitumen from thermally immature samples. The reflectance technique will contribute to porphyrin and kerogen studies and can be applied at its present level of development to several areas of geologic and paleo-oceanographic research.