Ilya N. Bindeman

Ion Microprobe Study of Plagioclase-Basalt Partition Experiments at Natural Concentration Levels of Trace Elements

We present here a study of plagioclase/melt partitioning of trace elements at their natural concentration levels, using sample charges from the widely cited plagioclase/melt partitioning experiments of Drake and Weill (1975). In these experiments, sample charges were doped to ∼1 wt% with Sr, Ba, rare earth elements (REE) and Y, but each charge was only doped with one to four elements. Thus, these samples provide an opportunity to compare partition coefficients (Dι) at natural concentration levels with those for doped concentration levels for the same composition of plagioclase, melt and temperature. Plagioclase-glass pairs of seventeen runs at four different plagioclase compositions and temperatures were analyzed by electron microprobe for major elements and some of the doped trace elements and by ion microprobe for undoped Li, Be, B, F, Mg, P, Cl, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. Partitioning of the homovalent substituting ions Sr2+ and Ba2+ show no differences between doped and natural concentration levels. Ion microprobe measured Dι heterovalent substituting ions REE3+ and Y3+ at natural concentration levels (0.3–3 ppm) in samples doped with wt% levels of Sr or Ba are up to three times higher than at doped concentration levels and cannot be explained by analytical artifacts. We discuss possible reasons for this. All trace element Dι data show linear relationships of the forms ln (Dι) = a∗ XAn + b∗ and RT ln (Dι) = a XAn + b in 0.4 < XAn < 0.8 range. Alkalies, alkaline earths, and lanthanides exhibit the same type of compositional dependence within each group of elements. Slopes a∗ and a vary with the increase of the ionic radius within each valence group. The smaller ions of each of these groups exhibit no or positive slopes a∗ and a; the larger ions show negative slopes. The magnitudes of the slopes increase linearly with ionic radii within the same valence group. This relationship allows extrapolation and prediction of the compositional dependence of elements of the same group whose concentrations could not be measured in this work. We present best fit approximation parameters for the RT ln (Dι) = a XAn + b relationship. These can be used in various petrologic applications to reconstruct the primary trace elemental composition of the parental melt from which plagioclase crystallized.

Trace element partitioning between plagioclase and melt: investigation of dopant influence on partition behavior

Abstract We present results from an ion microprobe study of REE-doped and natural concentration plagioclase-basalt run products of Drake (1972) that carries on from our earlier study (Bindeman et al., 1998) . The goals of this work are (1) to determine plagioclase/melt partition coefficients for all REE for four analyzed plagioclase compositions (An40–80); and (2) to determine whether doping with REE influences partition coefficients of other REE and other trace elements. In combination with our analyses of Sr-doped runs (Bindeman et al., 1998) , the new data allow us to compare partition coefficients (Dι’s) of trace elements at natural concentrations with those in REE-doped and Sr-doped runs. In these comparisons, runs have the same run temperature and compositions, but different doping element(s). We find that ln(DREE) decreases as a linear function of REE atomic number, in contrast to most plagioclase-melt partition experiments. The slopes of the ln(Dι) vs. %An and RT ln(Dι) vs. %An dependencies of REE and monovalent and divalent cations increase from smaller to larger ions of each valence group. DLa/DY and DLa/DLu increase by more than 5 times as plagioclase composition changes from An80 to An40. Within each valence group, slopes on ln(Dι) vs. %An and RT ln(Dι) vs. %An plots increase linearly with ionic radius, with the trivalent REE showing the steepest slope vs. ionic radius dependence. Application of the elastic modulus model of Blundy and Wood (1994) to our results yielded good results, confirming that elastic properties determine partition behavior. We also present Dι’s for U. We find no detectable effects of trace element doping on Dι’s of 30 trace elements of varying size and charge. This implies that coupling between different trace elements is not a significant process during partitioning in natural systems, even for the case of heterovalent substitutions (such as REE). However, DREE and DY in REE-doped runs are 30–100% higher than DREE in undoped runs and Sr-doped runs. Doping with thousands of ppm of three selected REE (it does not matter which three) affects Dι’s of Y and all REE in these runs, including those at natural concentration levels. We suggest that substituting trace cations largely compete for sites of substitution with the mineral-forming cations Na and Ca. We speculate that lower DREE in REE-doped runs are the result of a change in the REE substitution mechanisms at doped concentrations of all REE which leads to a different value of the Henry’s law constant.

Empirical calibration of oxygen isotope fractionation in zircon

New empirical calibrations for the fractionation of oxygen isotopes among zircon, almandine-rich garnet, titanite, and quartz are combined with experimental values for quartz-grossular. The resulting A-coefficients (‰K2) are: Zrc Alm Grs Ttn Qtz 2.64 2.71 3.03 3.66 Zrc 0.07 0.39 1.02 Alm 0.32 0.95 Grs 0.63 Full-size table Table options View in workspace Download as CSV for the relation 1000 ln αY-X = AY-X (106/T2). The fractionation of oxygen isotopes between zircon and coexisting minerals can provide otherwise unavailable evidence of magmatic processes, including crystallization, remelting, and assimilation-fractional crystallization.

Post-caldera volcanism: in situ measurement of U^Pb age and oxygen isotope ratio in Pleistocene zircons from Yellowstone caldera

The Yellowstone Plateau volcanic field, the site of some of the largest known silicic volcanic eruptions, is the present location of NE-migrating hotspot volcanic activity. Most volcanic rocks in the Yellowstone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km 3 of Lava Creek Tuff (LCT), have unusually low oxygen isotope ratios. Ion microprobe analysis of both U^Pb age and N 18 O in zircons from these low-N 18 O lavas reveals evidence of complex inheritance and remelting. A majority of analyzed zircons from low-N 18 O lavas erupted inside the Yellowstone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5^0.4 Ma). These ages and the high-N 18 O cores indicate that these lavas are largely derived from nearly total remelting of normal-N 18 O Huckleberry Ridge Tuff (HRT) and other pre-LCT volcanic rocks. A post-HRT low-N 18 O lava shows similar inheritance of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and U^Pb isotope ratios of individual crystals. fl 2001 Elsevier Science B.V. All rights reserved.

Formation of low-δ18O rhyolites after caldera collapse at Yellowstone, Wyoming, USA

We present a new model for the genesis of low-δ18O rhyolites of the Yellowstone caldera based on analyses of zircons and individual quartz phenocrysts. Low-δ18O rhyolites were erupted soon after the massive caldera-forming Lava Creek Tuff eruption (602 ka, ∼1000 km3) and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites. These zircons are isotopically zoned and out of equilibrium with their host low-δ18O melts and quartz. Diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron, in which the alteration-resistant quartz and zircon initially retained their δ18O values. These results link melting to caldera collapse, rule out rapid or catastrophic magma–meteoric water interaction, and indicate wholesale melting rather than assimilation or partial melting.

Voluminous low δ18O magmas in the late Miocene Heise volcanic field, Idaho: Implications for the fate of Yellowstone hotspot calderas

We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large caldera-forming ignimbrites and post-caldera lavas of the Heise volcanic field, a nested caldera complex in the Snake River Plain, that preceded volcanism in Yellowstone. Early eruption of three normal δ 18 O voluminous ignimbrites with δ 18 O quartz = 6.4‰ and δ 18 O zircon = 4.8‰ started at Heise at 6.6 Ma, and was followed by a 2‰–3‰ δ 18 O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km 3 Kilgore ignimbrite, and post-Kilgore intracaldera lavas with δ 18 O quartz = 4.3‰ and δ 18 O zircon = 1.5‰. The Kilgore ignimbrite represents the largest known low-δ 18 O magma in the Snake River Plain and worldwide. The post-Kilgore low δ 18 O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal δ 18 O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low δ 18 O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in δ 18 O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low δ 18 O signature from mantle or crustal sources. Instead, δ 18 O age trends indicate progressive remelting of low δ 18 O hydrothermally altered intracaldera rocks of previous eruptions. This trend may be generally applicable to older caldera complexes in the Snake River Plain that are poorly exposed.

Giant Kiruna-type deposits form by efficient flotation of magmatic magnetite suspensions

Kiruna-type iron oxide-apatite (IOA) deposits are an important source of Fe ore, and two radically different processes are being actively investigated for their origin. One hypothesis invokes direct crystallization of immiscible Fe-rich melt that separated from a parent silicate magma, while the other hypothesis invokes deposition of Fe-oxides from hydrothermal fluids of either magmatic or crustal origin. Here, we present a new model based on Fe and O stable isotopes and trace and major element geochemistry data of magnetite from the ~350 Mt Fe Los Colorados IOA deposit in the Chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits. We propose that concentration of magnetite takes place by the preferred wetting of magnetite, followed by buoyant segregation of these earlyformed magmatic magnetite-bubble pairs, which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults. Our data demonstrate an unambiguous magmatic origin, consistent with the namesake IOA analogue in the Kiruna district, Sweden. Further, our model explains the observed coexisting purely magmatic and hydrothermal-magmatic features and allows a genetic connection between Kiruna-type IOA and iron oxide-copper-gold deposits, contributing to a global understanding valuable to exploration efforts.

Fragmentation phenomena in populations of magmatic crystals

Abstract Fragmentation of crystals is an important mechanism, and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention. Recent advances in textural analysis, extraction techniques, digital imaging, and computer-assisted measurements enable rapid accumulation of 3D data on particle shapes and size distributions. This paper reviews fragment size distributions (FSD) that result from fragmentation: lognormal, fractal, loggamma, and Weibull; discusses their genesis mechanisms; and presents relevant examples of fragmentation from experimental physics. Next, the paper considers FSDs of feldspars on digitized images of thin sections and on published images, and quartz extracted from vesicular pumice by acid from eight well-known large-volume eruptive units. The acid solution of pumice enables examination of volume abundance, 3D shapes, proportions of fragmented crystals, and measurements of their CSDs and FSDs. FSDs were also measured in samples of welded tuff and a granite disaggregated by electric pulse. Products of syneruptive shock wave fragmentation, and fragmentation by an electric pulse are found to be fractal with large breakage probabilities, branching ratios, and fractal dimensions of 2 to 3. In contrast, most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism, which results in low breakage probability and small number (2−3) of fragments per breakage cycle. These results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal FSD. Measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions, and low survival rate for large inclusions, and inclusions with large radius/crystal size ratio. The modeling of periodic fragmentation of crystals with melt inclusions due to overheating and/or decompression, which may occur many times during the lifetime of a long-lived magma body, may explain concave-down, lognormal CSDs abundant in igneous rocks. The genesis of lognormality can be explained by the fragmentation algorithm of Kolmogorov (1941). Other algorithms may generate lognormal-like loggamma distributions. Fragmentation serves as an important size limiting factor, a nucleation aid, and it facilitates isotopic and trace elemental exchange.

Boron and oxygen isotope evidence for recycling of subducted components over the past 2.5 Gyr

Evidence for the deep recycling of surficial materials through the Earth’s mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent–daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that δ18O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in δ11B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth’s surface. Moreover, δ11B is negatively correlated with 187Os/188Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, 11B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from ∼3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands ∼3 Gyr later.

Crystal sizes in evolving silicic magma chambers

Crystal size distributions (CSDs) of quartz and zircon phenocrysts in individual pumice clasts from several voluminous ash-flow tuffs provide a quenched snapshot view of conditions in preclimactic magma chambers. A common feature of these CSDs is a concave-down, lognormal shape, in contrast to the reported linear CSDs in more mafic systems. This feature is interpreted to be a general result of surface-controlled, size-dependent growth by a layer nucleation in silicic magmas at low supersaturation. Specific CSDs may be important for interpreting nucleation and crystal-growth conditions and mechanisms in magmas erupted as large ash-flow tuffs and smaller-volume volcanic units, and for fingerprinting different magma batches (layers) in products of the same eruption.