David A. Wark

Pre-eruption recharge of the Bishop magma system

The 650 km 3 rhyolitic Bishop Tuff (eastern California, USA), which is stratigraphically zoned with respect to temperatures of mineral equilibration, refl ects a corresponding thermal gradient in the source magma chamber. Consistent with previous work, application of the new TitaniQ (Ti-in-quartz) thermometer to quartz phenocryst rims documents an ~100 °C temperature increase with chamber depth at the time of eruption. Application of TitaniQ to quartz phenocryst cores, however, reveals lower temperatures and an earlier gradient that was less steep, with temperature increasing with depth by only ~30 °C. In many late-erupted crystals, sharp boundaries that separate low-temperature cores from high-temperature rims cut internal cathodoluminescent growth zoning, indicating partial phenocryst dissolution prior to crystallization of the high-temperature rims. Rimward jumps in Ti concentration across these boundaries are too abrupt (e.g., 40 ppm across a distance of <10 µm) to have survived magmatic temperatures for more than ~100 yr. We interpret these observations to indicate heating-induced partial dissolution of quartz, followed by growth of high-temperature rims (made possible by lowering of water activity due to addition of CO 2 ) within 100 yr of the climactic 760 ka eruption. Hot mafi c melts injected into deeper parts of the magma system were the likely source of heat and CO 2 , raising the possibility that eruption and caldera collapse owe their origin to a recharge event.

Contributions to precision and accuracy of monazite microprobe ages

Abstract We examine the factors controlling accuracy and precision of monazite microprobe ages, using a JEOL 733 Superprobe equipped with 4 PET crystals, and both 1-atm gas flow Ar X-ray detectors and sealed Xe X-ray detectors. Multiple PET crystals allow for simultaneous determination of Pb concentration on up to 3 detectors, and the effects of different detector gases on spectral form can be addressed. Numerous factors in the X-ray production, detection, and counting sequence affect spectral form, including: choice of accelerating voltage, changes in d-spacing of the diffraction crystal, use of X-ray collimation slits, and type of detector gas. The energy difference between ArKα X-rays and XeLα X-rays results in, for 1-atm Ar detectors, escape peaks of second-order LREE L line X-rays that cannot be filtered using differential mode PHA. The second-order LREE energies are passed to the counter and produce, for a 140 mm Rowland circle, several problematic interferences in the Pb region of a monazite wavelength-dispersive (WD) spectrum. WD monazite spectra produced with Xe detectors are free from second-order LREE interferences in the Pb region; escape peaks of the secondorder LREE are filterable with differential mode PHA if Xe detectors are employed. Silicon, Ca, Y, Ce, P, Th, U, and Pb (2 spectrometers) are measured as part of the monazite microprobe dating protocol; ±2σ variations in elements fixed for ZAF corrections do not affect the age outside of analytical uncertainty. ThMα, UMβ, and PbMα are the analyzed lines of the age components. Corrections for interference of ThMζ1,2 and YLγ2,3 on PbMα are significant, but can be done precisely, and reduce the precision of theMα analysis by a trivially small amount. ThMγ, M3-N4, and M5-P3 interferences on UMβ can be corrected, as well, but ThM5 and M4 absorption edges in high-Th samples make estimation of UMβ background problematic. Background fits for UMβ peaks show that linear vs. exponential fits for UMβ do not, in general, produce statistically significant differences in microprobe ages. However, linear vs. exponential background fits for PbMα peaks do produce significantly different ages, most likely because of (1) low Pb concentrations relative to U; (2) ThMζ1 interference on backgrounds between ThMζ1 and PbMβ; and (3) SKα and Kβ interference in S-bearing monazite. For 6-min analyses (3 min peak, 3 min background) at 25 keV and 200 nA, 1σ Pb precisions are approximately 3.4% at 1700 ppm and 9.5% at 750 ppm; at 15 keV, precision decreases by roughly 25% of the 25 keV value. These precisions are constant for fixed current, analysis time, and concentration, but the statistical precision of distinct populations of monazite grains (domains) is a function of the total number of analyses within the domain. Instrumental errors (current measurement, dead time, pulse shift, d-spacing change) add 1.10% to random errors, but errors caused by pulse shift and d-spacing changes can be accounted for and corrected. Decreasing accelerating voltage from 25 to 15 keV decreases ZAF correction factors by as much as 50% relative, but replicate age analyses of Trebilcock monazite at 15 and 25 keV are statistically indistinguishable. Grain orientation, miscalculated background intensity, uncorrected interferences, and surface effects also introduce systematic errors. Accurate background interpolation and interference correction reduces systematic error to approximately 5.20% in addition to random (counting) error. Microprobe ages (~420 Ma) and 208Pb/232Th SIMS ages (~430 Ma) of monazite from Vermont are in agreement to within ~10 m.y. The discrepancy between U-Th-total Pb microprobe ages and 208Pb/232Th ages is removed when the high background measurement for PbMα is shifted to the short-wavelength side of PbMβ, removing a possible ThMζ1 interference.

Accessory mineral behavior during differentiation of a granite suite: monazite, xenotime and zircon in the Sweetwater Wash pluton, southeastern California, U.S.A.

Abstract Compositional and textural characteristics of the accessory minerals monazite, xenotime and zircon in the Sweetwater Wash granites and related aplites indicate that these phases not only controlled much of the trace-element geochemistry of the suite, but that they also record the melt compositional changes that occurred during magmatic differentiation. Fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare-earth element (REE) depletion. This was accompanied by increasing heavy-REE concentrations until xenotime joined the crystallizing assemblage; subsequently, combined monazite-xenotime fractionation resulted in lowering of the entire REE budget. Zircon, which contains inherited cores that were apparently resorbed and rounded during initial anatexis, was saturated throughout the differentiation history of the Sweetwater Wash suite. Accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales. The differences are best displayed by monazite: on average, crystals in more differentiated rocks (aplites) are relatively depleted in light REEs and contain higher concentrations of the substituting elements U and Th. Zircon displays complimentary increases in Hf and Y, while xenotime shows a slight increase in Th and in Gd/Ho ratios; both phases also exhibit higher U concentrations in aplites than in granites. These differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites, but also within individual thin sections, due to in situ fractionation. On an even smaller scale, strong compositional variations are present within single crystals, possibly due to diffusion-controlled melt-compositional gradients in the regions (a) adjacent to growing major phases, and (b) adjacent to the growing accessory crystal itself. Our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes, but that the scale of observed variations must be carefully considered.

The incorporation of Pb into zircon

Abstract The incorporation of Pb into zircons grown from Pb-rich solutions was evaluated using three different approaches: (1) high-temperature growth of large crystals from Pb-silicate melts; (2) hydrothermal overplating of thin epitaxial layers on substrates of natural zircon; and (3) growth of small, homogeneously nucleated crystals from aqueous fluids. The melt-grown zircons (50–400 μm) were crystallized from PbOSiO2ZrO2 (±P2O5) liquid at atmospheric pressure by cooling from 1430° to 1350°C. In the P2O5 free system, despite 66 wt% PbO in the melt, these zircons contain 3 atom% Pb, with apparent zircon/fluid partition coefficients of 4.2 and 2.6, respectively, for Pb4+ and Pb2+. In contrast to the case of melt-grown zircons, available P is excluded from the aqueous epitaxial zircon, suggesting that charge balance is accomplished by H+ instead. Small (2–5 μm) zircons grown by cooling aqueous solutions (PbO + SiO2 + ZrO2 ± P2O5) from 800°C or 900°C contain ∼ 0.25–0.5 atom% Pb (∼ 2–4 wt% PbO), yielding apparent DPb values of ∼ 0.2–0.3. Available P5+ is incorporated in a 2:1 ratio with Pb2+, suggesting a specific charge-balance mechanism: [2P5+ + Pb2+] = [2Si4+ + Zr4+]. However, Pb enters the zircon even when P is unavailable, so H+ may again play a charge-balancing role. Because of the rapid, polythermal modes of zircon growth and the high Pb content of the experimental systems, the apparent partition coefficients should not be viewed as equilibrium values, but as qualitative indicators of Pb compatibility under various growth circumstances. The overall results are consistent with the low but variable levels of non-radiogenic (common) Pb in natural zircons. The increased compatibility of Pb in fluid-grown, low-temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet-metamorphic rocks, i.e., high concentrations of common Pb.

Origin of mantle (rapakivi) feldspars: experimental evidence of a dissolution- and diffusion-controlled mechanism

Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO2) at 0.8 GPa, 850°C, orthoclase (Or93) and sanidine (Or30) partially dissolved and were mantled by sodic plagioclase (∼An25–30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward —mostly in the form of parallel blades — toaard the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static “boundary zone” of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultancously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as the Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of ∼dacitic composition. Mantled feldspars in some intrusive systems (i.e., “rapakivi granites”) may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes.

Lateral variation in slab orientation beneath Toba Caldera, northern Sumatra

The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earths largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub-crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large-scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab.

Nonlinear pressure diffusion in a porous medium: Approximate solutions with applications to permeability measurements using transient pulse decay method

Transient pulse decay has been widely used to measure permeability of tight rocks and synthetic materials. When the pore fluid is a gas (e.g., dry air, Ar, or N2) as used in a gas permeameter, the pressure diffusion equation governing the pulse decay problem is nonlinear due to a pressure-dependent gas compressibility and molecular slippage effect (also known as the Klinkenberg effect). To simplify data analysis in permeability measurement using a gas permeameter, an approximate solution to the nonlinear diffusion equation was obtained using a regular perturbation method. This solution, which is similar to the original exponential solution of Brace et al. [1968] for a case when the compressibility of the pore fluid is a constant, is valid in the limit when the volume of the interconnected pore fluid is much smaller than the volume of the upstream reservoir. Applications of the approximate solution to laboratory measured pulse decay data show that the estimated sample permeability can be overestimated by as much as a factor of two if the transient gas pressure decay experiment is conducted at low pressures and if molecular slippage is not taken into account. The molecular slippage can be effectively eliminated if the pulse decay measurement is conducted at a mean pressure at least 5 times higher than the Klinkenberg slip factor, which is on the order of 1 bar for texturally equilibrated marble and quartzite used in the permeability study of Wark and Watson [1998].

Effect of grain size on the distribution and transport of deep‐seated fluids and melts

Because permeability increases with the square of the grain size for a given fluid fraction, it is commonly assumed that porous flow of fluids and melts is more effective in coarse-grained domains of deep-seated rocks. This relative behavior is accurate, however, only as a description of two systems operating in isolation. We demonstrate that if coarse- and fine-grained domains are in chemical communication, then equilization of pore-wall curvature across the system results in concentration of fluid or melt in domains of finer grain size. Because of this localization of fluid, the permeability of the finer-grained domains will match or exceed that of domains with coarser grains. Fluid focussing due to differences in grain size has potentially important implications for grain-scale transport of metamorphic fluids — including those released from subducting slabs into the sheared mantle wedge — and also for partial melts in the mantle and deep crust.

Oligocene ash flow volcanism, northern Sierra Madre Occidental: Role of mafic and intermediate-composition magmas in rhyolite genesis

Field, geochemical, and isotopic data from the Tomochic volcanic center in Chihuahua, Mexico, are interpreted to indicate a genetic relationship between large-volume rhyolite ash-flow tuffs and associated more mafic lithologies. These lithologies include (1) porphyritic, two-pyroxene andesite (>35 Ma) that was extruded mostly before ash-flow volcanism, and (2) crystal-poor basaltic andesite that was erupted mostly after ash-flow activity (∼30 Ma) but which was also extruded earlier (∼34 Ma) with hybrid intracaldera lavas. Major silicic units at Tomochic include the Vista (∼34 Ma) and Rio Verde (∼32 Ma) rhyolite ash-flow tuffs; also present are ash-flow tuffs (∼38, 36, and 29 Ma) erupted from other sources. A model of rhyolite genesis by closed-system crystal fractionation of andesite is consistent with geochemical and isotopic data. The least evolved Vista rhyolite was formed by fractionation of ∼65% the original mass of andesite; an additional ∼55% fractionation of plagioclase, alkali feldspar, quartz, biotite, hornblende, FeTi oxides, and sphene generated the most evolved Vista sample. Rio Verde rhyolites were generated from andesite by ∼50% mass fractionation of an assemblage dominated by plagioclase, pyroxene, and FeTi oxides. Initial Nd and Sr isotope ratios of andesite-dacite lavas (eNd = −2.3 to −5.2; 87Sr/86Sr = 0.7060 to 0.7089) and of rhyolites (eNd = +0.5 to −2.7; 87Sr/86Sr = 0.7053 to 0.7066) partly overlap and extend from values near the mantle array toward values typical of old continental crust on an eNd-87Sr/86Sr diagram. These isotope ratios, which do not correlate with indices of differentiation, are interpreted to indicate that parental andesite already contained a crustal component (possibly >20%) before fractionation to rhyolite. The isotopic and geochemical signatures of andesites apparently reflect the incorporation of crust by subduction-related, mafic melts represented by (but more primitive than) exposed basaltic andesites, which have isotope ratios (eNd = +1.0 to −0.1; 87Sr/86Sr = 0.7044 to 0.7053) near “bulk earth”. The pattern of volcanic evolution at the Tomochic center, specifically the transition from andesitic to rhyolite dominated, with late extrusion of basaltic andesite, also occurred in other parts of the volcanic field, and roughly coincided with a sharp decrease in the rate of Farallon plate subduction. This change in subduction rate apparently resulted in a decreased flux of mafic melts into the crust from below, and was associated with the onset of crustal extension and hence, shorter residence times for mafic melts formerly ponded in the deep crust These, in turn, resulted in (1) the change from andesitic to rhyolite-dominated volcanism as ascending intermediate-composition magmas stalled, coalesced, and differentiated to produce rhyolite, (2) extrusion of basaltic andesite upon brittle failure of the shallow crust, and (3) subsequent termination of calc-alkalic volcanism throughout the Sierra Madre Occidental.

Plagioclase mantles on sanidine in silicic lavas, Clear Lake, California: Implications for the origin of rapakivi texture

At Clear Lake, California, mixed dacite lavas contain sanidine mantled by oligoclase that closely resemble rapakivi feldspars in granites. Resorption and mantling of sanidine by plagioclase is one of many disequilibrium textures in these lavas resulting from maficfelsic magma interaction. Mantling of sanidine by plagioclase may occur directly in mixed magmas due to shifts in composition and temperature, or indirectly in crystal-rich silicic magmas influenced by thermal under-plating, or mafic-felsic hybridization in adjacent regions of the magmatic system. Other silicic laves containing mantled sanidine also show evidence for mafic-felsic magma interaction. Textural and compositional similarities between rapakivi texture in extrusive and intrusive rocks suggest that they have a common origin. Moreover, modification of early-formed textures due to protracted crystallization, subsolidus reactions, and metasomatic-hydrothermal alteration in the plutonic environment accounts for most of the observed differences between extrusive and intrusive examples. The association of coeval mafic rocks and evidence for mafic-felsic magma interaction in many plutons containing rapakivi exture confirm that magma mixing plays a key role in its formation in some intrusive texamples. Evidence for a compositional control and multiple episodes of mantle formation are also compatible with repeated cycles of recharge and mixing. Regardless of the exact cause of K-feldspar instability, textural evidence from volcanic rocks and experiments indicates that development of rapakivi texture is controlled by cation diffusion in a dissolution boundary layer developed on K-feldspar. Mantles appear to form in a two-stage process that involves initial epitaxial nucleation of plagioclase on sanidine, followed by simultaneous dissolution of sanidine and inward growth of plagioclase.