V51E-01 08:00h
Permeability Constraints for the Sub-Ridge Mantle Inferred from Field Relations, Microstructures, and Geochemistry of Dunites in Oman
The segregation and transport of basaltic melt from its source deep beneath mid-ocean ridges is both efficient and rapid. Although mid-ocean ridge basalt (MORB) is an mixture of near-fractional melts derived from a range of pressures, the major element composition of primitive basalts, which form the bulk of the oceanic crust, implies that these primitive liquids traverse the uppermost 20-30 km of the mantle without significant chemical interaction with the rocks through which they pass. Additionally, $^{230}$Th excesses observed in recently erupted lavas indicate that some melts are segregated from their source at depths greater than 75 km and ascend to the surface with rates on order of a meter per year. Geochemical and petrologic data from abyssal and ophiolitic peridotites suggest that dunites accommodate the chemically-isolated transport of primitive melts to the surface via porous flow. Measured dunite size/frequency statistics coupled with observed systematic grain size variations in Oman peridotites provide a framework to determine the melt capacity of a network of porous dunite conduits and their ability to preserve major element disequilibrium with the shallow mantle as well as the excess radiogenic $^{230}$Th observed in MORB. Four permeability models are examined with respect to their ability to satisfy these chemical and volumetric constraints. A model in which both porosity and grain size vary systematically with dunite width produces both an unequilibrated melt flux sufficient to generate 6 km of oceanic crustal gabbro and the high melt velocities required to maintain $^{230}$Th excesses with sustainable maximum porosities consistent with independent microstructural constraints. The dunite width and grain size distributions observed in Oman imply that the permeability of dunites scales with their size and ranges from less than 10$^{-16}$ m$^{2}$ for dunites narrower than 100 cm to as high as 10$^{-11}$ m$^{2}$ in dunites 100 m wide. In an interconnected network of dunites which conserves flux, the bulk of the melt flux remains chemically unequilibrated and is accommodated in conduits with permeabilities sufficient to sustain average melt velocities on order of 0.1-1 m/yr. Although there is considerable uncertainty in the estimations of dunite abundance and grain size while in the melting region, the inferred variations in melt flux are consistent with melt-rock ratios calculated from the observed geochemical variations with dunite width and primitive lava compositions from Oman.
V51E-02 INVITED 08:15h
Diffusive Fractionation of U-Series Nuclides During MORB Production
Experimental studies indicate that the distribution of U-series nuclides among minerals and liquids during mantle melting is unlikely to reflect equilibrium partitioning if solid-state diffusion is the rate-limiting process. We have developed a numerical model that considers the diffusion-controlled partitioning of parent and intermediate daughter nuclides between multiple minerals and liquid during near-fractional adiabatic decompression melting, and apply it here to the $^{238}$U-$^{230}$Th-$^{226}$Ra and $^{235}$U-$^{231}$Pa systems in mid-ocean ridge settings. Diffusion coefficients are derived either from experimental data or, where no data exist, from an elastic model (Van Orman et al., 2001, CMP 141:687-703). Before melting begins, the solid mantle is assumed to be in bulk secular equilibrium. Parent nuclides are distributed in chemical equilibrium among the minerals, and intermediate daughters have a steady-state distribution, calculated numerically, that reflects a balance between production, decay and diffusive exchange. During melting, a small fraction of melt remains and interacts with the upwelling solid, while the rest is instantaneously removed from the system and pooled with other extracted melts. A primary control on the composition of the melt is the solid upwelling rate, which sets the time available for diffusive exchange and ingrowth of daughters. With increasing upwelling rate, the [$^{230}$Th]/[$^{238}$U] activity ratio decreases because less time is available for ingrowth and because diffusion of U is slightly faster than Th in high-Ca pyroxene at mantle solidus temperatures. The [$^{226}$Ra]/[$^{230}$Th] ratio increases with upwelling rate as the effective partition coefficients for U and Th increase while the effective partition coefficient for Ra remains small due to its significantly higher diffusivity. The [$^{231}$Pa]/[$^{235}$U] ratio increases with melting rate when melting begins in the garnet facies and has the opposite trend when melting begins in the spinel facies. The resulting correlations between [$^{230}$Th]/[$^{238}$U] and [$^{226}$Ra]/[$^{230}$Th], and between [$^{230}$Th]/[$^{238}$U] and [$^{231}$Pa]/[$^{235}$U], resemble the data for MORB. However, disequilibrium dynamic melting of a single homogeneous source cannot account for the observed correlations of $^{230}$Th excess and $^{226}$Ra excess with the degree of enrichment of the basalt. We will discuss other possibilities for these correlations.
V51E-03 INVITED 08:30h
Physical and Temporal Controls on Lower Crustal Melting and Mixing: Mass and Enthalpy Transport in Actively Growing Arcs
The growth of continental crust in arc settings, as well as the thermal and compositional character of the crust, is ultimately dictated by the flux of basaltic magma from the mantle and the interaction between crustal and basaltic material. We present a quantitative assessment of the thermal and dynamic response of the lower crust to the intrusion of basaltic dike swarms in a two-dimensional, stochastic computational framework. We will examine the physical and temporal controls on crustal melting, mingling, and mixing as well as some of the major element, trace element, and U-series consequences of these lower crustal interactions. Distinct melting and mixing environments are predicted as a result of the crustal thickness, flux of basalt, and age of the arc system. Shallow crustal (approx. 30 km) environments and arc settings with low fluxes of mantle basalt are likely repositories of isolated pods of mantle and crustal melts in the lower crust, both converging on dacitic to rhyodacitic composition. These may be preferentially rejuvenated in subsequent intrusive episodes. Mature arc systems with thicker crust (approx. 50 km) produce higher crustal and residual basaltic melt fractions reaching approx. .4 for geologically reasonable basalt fluxes. The basaltic to basaltic-andesite composition of both crustal and mantle melts will readily mix as the network of dikes collapses and Reynolds numbers reach 10$^{-4}$ to 1.0 in the interiors of dikes that have been breached by ascending crustal melts. This may provide one mechanism for MASH-like processes. Residual mineral assemblages of the crust thickened by repeated intrusion are predicted to be garnet pyroxenitic, which are denser than mantle peridotite and also generate convective instabilities where some of the crustal material is lost to the mantle. This reconciles the thinner than predicted crust in regions that have undergone flux of mantle basalt for a prolonged period of time, and helps explain the enrichment of incompatible elements such as K$_{2}$O typical of mature arc settings without the associated mass balance problem. Sr/Y and La/Yb ratios in the associated melts are predicted to be elevated and accompanied by significant $^{226}$Ra-excesses.
V51E-04 INVITED 08:45h
Incremental Assembly and the Nature of Granitic Plutons
Plutonic rocks are generally mapped on the basis of observable petrographic features such as mineralogy and texture. It is commonly assumed that these features distinguish magma bodies and that contacts between mappable phases separate different magma bodies; indeed, this assumption tacitly underlies the last century of research on how plutons form and are emplaced. However, recent work demonstrates that many superficially homogeneous, well-defined plutons (e.g., Half Dome Granodiorite of the Tuolumne Intrusive Suite, California) grew over millions of years by amalgamation of numerous small intrusive increments, many of which solidified before the next increment was emplaced. Within such intrusive suites, contacts between plutons may range from sharp to highly gradational; mappable contacts may bound different intrusive increments along their lengths, are likely diachronous, and can represent time gaps much smaller than the growth times of the plutons that they bound. The characteristic properties (texture, mineralogy, composition) of a pluton formed by protracted assembly of small intrusive increments thus cannot reflect in situ processes within a single magma body, but rather must be produced by repeated formation of the same magma type (e.g., in the lower crust) before injection, and repeated thermal history thereafter. This is clear in the Sierra Nevada, where the "Tuolumne triad" of nested plutons (outer foliated, medium-grained, mafic granodiorite; medial equigranular, coarse-grained granodiorite; inner megacrystic granodiorite/granite) was repeated several times in the Late Cretaceous along $>$400 km of arc length. These intrusive suites cannot have been derived from a single magma body yet comprise identical sequences of nearly identical rocks. A pluton is thus analogous to a stratigraphic lithofacies; the processes by which parts of a pluton formed were fundamentally similar across the scale of an entire arc, but different parts of any given pluton were likely added independently and as much as millions of years apart. Mappable contacts thus do not necessarily distinguish intrusive pulses but rather may mark changes in petrogenetic processes.
V51E-05 INVITED 09:00h
Geochronologic and Thermochronologic Evidence for Incremental Assembly of Large Zoned Intrusions
Geochronologic and thermochronologic data from rocks of the Tuolumne Intrusive Suite (TIS) of California preclude the possibility that the suite represents a differentiated blob (or even several differentiated blobs) of magma. Rather, the data are best interpreted to reflect amalgamation of many small intrusions into the large, zoned intrusive suite. We suggest that the results from the Tuolumne are generally applicable, and rethinking of current models for the emplacement and differentiation of such plutons is required. Zircon U-Pb geochronologic data indicate that the TIS was assembled over a period of at least 10 m.y. between 95 and 85 Ma. Zircon ages for individual units (such as the Half Dome Granodiorite) yield ranges of up to 4 m.y. The zircon ages are characterized by little inheritance - consistent with the observation that the rocks yield uniformly low zircon saturation temperatures - and minor Pb-loss, allowing for precise determination of crystallization ages. Existing hornblende Ar -Ar dates for TIS units (R.W. Kistler and R. Fleck) are 4-6 m.y. younger than U-Pb zircon ages for the same unit, suggesting either protracted cooling or later reheating. Simple two-dimensional thermal modeling of TIS intrusion as either a single blob or as discrete blobs corresponding to the mapped plutonic units fails to reproduce either the observed range of zircon ages, or the observed discrepancy between zircon and hornblende cooling ages. Consequently, even "homogeneous" map units such as the Half Dome were incrementally assembled. These results contradict the conventional interpretation that mapped pluton contacts outline discrete magma bodies. Instead, the plutons represent amalgamations of many small batches of magma, and must include internal contacts that may be cryptic. Although the geometries of intrusive pulses into the TIS are not yet known, we propose that detailed mapping combined with regional Ar-Ar thermochronology and 3-D thermal modeling of a variety of intrusive histories and geometries will provide a clear picture of pluton construction.
V51E-06 INVITED 09:15h
Construction of the Vinalhaven Intrusive Complex, Maine, USA: the Plutonic Record of Evolving Magma Chambers Affected by Multiple Episodes of Replenishment, Rejuvenation, Crystal Accumulation and Eruption
Increasingly, the plutonic roots of volcanic systems can be shown to contain temporal records of events inferred from the study of volcanic rocks. The Vinalhaven intrusive complex preserves evidence for multiple episodes of silicic and mafic replenishments, rejuvenation of granite, and probable eruptive events over a nominal time-span of 1.7 Ma (Hawkins and Wiebe, this volume). The complex is about 12 km in diameter and consists mainly of cg granite, a thick section of arcuate, inward-dipping gabbro-diorite sheets in the southeastern half of the complex, and a circular core of fg granite. Field relations demonstrate that the base of the intrusion is along the southeastern margin of the complex, and the top is along the northwestern margin where it intrudes coeval volcanic rocks. Aphyric basaltic and granitic dikes fed this essentially bimodal intrusion. When basaltic dikes intersected a silicic chamber, basalt spread across a floor of silicic crystal mush to form gabbro-diorite sheets in granite. Several extensive layers of angular blocks of country rock occur within the mafic rocks. Granitic dikes and the fg granitic core of the complex have sharp to gradational contacts with cg granite, and, locally, both granites are intimately mixed and commingled. These relations indicate that new silicic injections mixed into partly crystallized resident magma. Several irregular bodies of porphyry (0.2 to 0.5 km in average dimension) intrude cg granite with sharp, gradational, or commingled contacts. The porphyry has 5 to 40% corroded phenocrysts, identical in composition to crystals in the granite, and a variably quenched matrix. Some of these bodies formed when late injections of basalt remelted largely solid portions of cg granite. New silicic input may have contributed to other porphyry bodies. The matrix probably quenched because of a sudden decrease in pressure, possibly due to eruption of magma from the chamber. The cg granite and inter-layered mafic rocks preserve a stratigraphic section that records magma chamber evolution during the early growth of the Vinalhaven intrusion. Near the base of this section, mafic sheets flowed across almost the entire width of intrusion. The large volume and comparable extent of country rock blocks at this level suggest a major collapse of the roof of a large tabular chamber, and this collapse probably records a major eruption of silicic magma from the chamber. At higher levels of this section, mafic sheets and country rock blocks gradually become restricted to the eastern third of the complex and are entirely absent in the upper half of the intrusion. The less extensive layers of country rock blocks may record smaller eruptions from more restricted chambers. The lower western and entire upper parts of the complex lack mafic rocks and consist of homogeneous, cg granite with a wide variety of schlieren structures that demonstrate granite formed largely by crystal accumulation on a chamber floor. Preliminary measurements of schlieren orientations suggest that the evolving chamber floors were highly irregular and consisted of basin forms with diameters that were much smaller than the width of the plutonic complex - perhaps as little as 1 to 2 km. Although the granite appears homogeneous and to lack any sharp internal contacts, it may have accumulated largely in small, semi-isolated chambers that waxed and waned due to crystallization, replenishment, rejuvenation and accumulation.
V51E-07 09:30h
Age and Thermal History of the Bushveld Complex, South Africa
The Bushveld Complex (BC) is one of the largest, most economically important and well-studied layered mafic intrusions in the world. Despite plentiful radioisotopic studies over the past 30 years, the age and emplacement chronology of the BC are not well-constrained. Biotite $^{40}$Ar/$^{39}$Ar data from the UG-2 chromitite layer yield consistent plateau ages around 2042 Ma (IUGS 1977 constants; 28.02 Ma for FCs here and throughout), implying either a slow cooling rate or systematic error when compared with the available Pb/Pb ages of 2059 to 2061 Ma (Nomade {\it et al.}, 2004, J. Geol. Soc. Lond., 161: 411-420). We are acquiring $^{40}$Ar/$^{39}$Ar and U/Pb data to evaluate the rapid emplacement and cooling suggested by petrological and heat-conduction studies (Cawthorn and Walraven, 1998, J. Petrol. 39: 1669-1687). Biotite and hornblende are present as intercumulus phases in gabbros and also in ubiquitous pegmatoid veins cutting the mafic and ultramafic rocks. Preliminary $^{40}$Ar/$^{39}$Ar results from both the eastern and western limbs of the BC show biotite integrated ages clustering between 2030 and 2050 Ma, slightly older than hornblende plateau ages (2030-2040 Ma). Biotites are locally subject to discordance suggestive of $^{39}$Ar recoil redistribution with an interlayer alteration phase; as in other such cases the integrated ages are more consistent and sensible whereas plateau ages are in some cases impossibly old. Biotite from an Fe-rich ultramafic pegmatoid in the western limb (Karee Mine) yields duplicate ~100% concordant plateaux spectra that average 0.8% older than the average of 4 hornblende plateaux. The cause of this apparent discordance (biotite age > hornblende age) is not understood although it is possible that the biotites have unusually high closure temperatures due to large diffusion radii related to the coarse (~5 mm) grain size. Initial ID-TIMS U/Pb single-zircon analyses indicate an age of 2058 Ma for the late-stage Nebo Granite, as displayed by concordant ages on crystals pre-treated with annealing/chemical abrasion (Mattinson, in review). Slightly younger, discordant ages indicating Pb loss were observed as well. Hornblende from the same sample yields 4 $^{40}$Ar/$^{39}$Ar plateaux averaging ~2035 Ma for single crystals. The new data are consistent with the previously inferred rapid emplacement and cooling chronology and confirm a ca. 1% systematic bias between current calibrations of the $^{40}$Ar/$^{39}$Ar and U/Pb chronometers (Min {\it et al.}, 2000, GCA 64: 73-98). The implied rapidity of injection of the multiple magma pulses is reminiscent of the emplacement timescale of most continental flood basalts (CFB). We concur with previous suggestions that the 3-6 x 10$^{5}$ km$^{3}$ BC represents the intrusive component of a massive CFB province whose eruptive component is largely eroded but locally represented by the Rooiberg Group volcanics. The age of the BC paleomagnetic pole used to anchor Paleoproterozoic Kaapvaal Craton apparent polar wander paths is well established, but it should be considered that this pole may not average geomagnetic secular variation.
V51E-08 INVITED 09:45h
On the Itinerant History of Crystals in Magma Reservoirs
The storage times of magma systems have been imaged by a variety of geophysical and geochemical approaches, each of which provides different insights because each is necessarily biased in some fashion. Perhaps the most fundamental bias is the predominance of magma storage records based on extrusive rocks. This, in turn, implies some bias towards imaging of the most-fluid portions of a magma reservoir. Factors that may affect the probability of eruption and therefore apparent storage intervals are the frequency and interplay between magma replenishment and magma arrest in the crust, the volatile content of the magma, and the tectonic regime of magmatic activity. In situ Pb and Th isotopic analyses of the accessory phases zircon and allanite from rhyolites show 1) that successive eruptions can apparently sample the same crystal populations and 2) that crystal growth may occur intermittently, separated by up to tens of k.y. These results provide evidence for discontinuous crystal growth and for the rejuvenation of growth at least in part by magma mixing and magma replenishment. Chemical analyses suggest that these same observations also broadly apply to the major mineral phases but the chronological details could differ if crystals are selectively preserved during magma ascent and/or mixing, and/or due to differential buoyancy between phases. Our work on the age and compositional zoning of allanite might be particularly revealing in this respect since the buoyancy of allanite is similar to those of major phases. Radiometric methods generally give older crystallization ages than those determined by kinetic considerations (e.g., CSD, diffusional relaxation). Accepting the kinetic ages at face value, crystallization appears to typically require $<$100 y. with a maximum duration of $\sims$1 k.y. for major phenocryst phases. In apparent corroboration of these timescales, many magmas have $^{226}$Ra excesses that are difficult to reconcile with magma storage times of $>$few k.y. $^{230}$Th-$^{226}$Ra ages for mineral {\it separates} are generally indicative of crystallization on timescales that are an order of magnitude greater than those based on kinetic considerations, while $^{238}$U-$^{230}$Th ages may be even another order of magnitude greater still. These observations can collectively be reconciled if "phenocryst" populations include some older crystals thatare not easily distinguished on petrographic grounds. Accessory phase dating indicates that "old" crystals may be derived from earlier intrusions as well as from country rocks and/or source areas. Eruptions may only evacuate a fraction of a magma reservoir. At the same time, magma reservoirs are rarely close to a steady-state balance between influx and efflux nor are they well-mixed. Thus crystals might carry-over from one eruption to next if they are suspended in the most-mobile liquid portions of the chamber or if they are re-entrained in liquid as the liquid-mush transition zone migrates in response to the thermal effects of recharge and/or eruption. The almost ubiquitous evidence for complex and protracted crystal records is especially notable if nucleation occurs largely in solidification fronts: in this case the crystals most susceptible to recycling would represent only the most-recent intervals of crystal growth. The duration of the radiometric crystal record, in contrast, appears to require more dynamic reservoir processes, involving active crystal suspension, and rapid and large migrations in the mush-liquid transition.