Frederick J. Ryerson

Lead diffusion in apatite and zircon using ion implantation and Rutherford Backscattering techniques

Abstract Ion implantation was used to introduce Pb into the minerals zircon and apatite. Diffusion profiles were obtained using Rutherford Backscattering, and the results were fitted with a model to determine the diffusion coefficients. This approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times. Results for apatite over the temperature range 600–900°C are in good agreement with earlier results obtained for higher temperatures ( WATSON et al., 1985) and are described by the following Arrhenius Law: D = 1.27 × 10 −4 cm 2 s −1 exp( −54.6 ± 1.7 Kcl mol −1 RT . This suggests that radiation damage induced by ion implantation has little effect on Pb diffusion in this case due to rapid annealing of induced damage at low temperatures. The activation energy stated above is somewhat smaller than that previously determined (70 kcal mol−1), reflecting the larger range in 1/T sampled by the present results. Closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data. Diffusion coefficients for Pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of Pb diffusion based on geochronologic data and U/ Pb zoning in natural zircons. The differing results for zircon and apatite obtained in this study appear to be related to each minerals ability to anneal the radiation damage induced by ion implantation.

The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: experiments and models

Abstract The low atomic mass elements B, Be, and Li are viewed as sensitive tracers of the involvement of subducted materials in the genesis of island arc magmas. In order to better assess the role of dense aqueous fluids in the slab-to-mantle transfer of these elements during subduction, measurements have been made of partition coefficients for B, Be, and Li between aqueous fluid and minerals likely to be present in the basaltic portion of the downgoing slab, namely clinopyroxene and garnet. Experiments at 900°C and 2.0 GPa reveal that the average clinopyroxene-fluid partition coefficient for Be (∼2) exceeds that for either Li (∼0.2) or B (∼0.02) and values are 100× (B,Li) to 1000× (Be) larger than partition coefficients for garnet. Clinopyroxene-fluid partition coefficients were found to vary with the alumina content of run-product clinopyroxenes, but this variation is interpreted to reflect the specific exchange reaction that governs the incorporation of these elements into the pyroxene structure, and not mineral-fluid disequilibrium. The element pairs B-Be, B-Nb, and Li-Yb are considered to be essentially unfractionated during the partial melting process, as evidence by their coherent behaviour in apparently cogenetic lavas and the similarity in their measured mineral-melt partition coefficients. A comparison of clinopyroxene-fluid partition coefficients for these elements with clinopyroxene-silicate melt values reveals that B/Be, B/Nb, and Li/Yb ratios will be significantly fractionated in coexisting aqueous fluid with respect to the residual solid. The elevated ratios of B/Be, B/Nb, and Li/Yb in island arc lavas relative to MORB are thus considered to be consistent with an origin by fluid-mediated slab-to-mantle transport. A quantitative model of slab dehydration accompanied by progressive water loss and changes in residual mineral mode reveals that source regions with B/Be and B/Nb appropriate for producing the Izu and Kurile IAB suites can be generated using available estimates for the composition of altered oceanic crust, although B abundances at the high end of published values are required. Because the highest values of B/Be and B/Nb are produced in the mantle wedge at relatively shallow depths, some additional process, such as subduction-induced flow of a hydrated mantle wedge, is required in order to transfer enriched material to depths appropriate for the formation of magmas beneath the volcanic front. Calculations indicate that by the time the slab reaches a depth of 200 km, B/Be and B/Nb in the dehydration residue has been reduced to ∼5–12% of initial values. Thus, the preferential loss of B during dehydration is viewed as a viable mechanism to prevent the excess B acquired during near-surface alteration of oceanic crust from being cycled into the mantle, thereby maintaining the distinction in B/Be and B/Nb for mantle and crustal reservoirs.

Determination of oxygen self-diffusion in åkermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions

Oxygen self-diffusion coefficients have been measured for three natural diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel, and a synthetic akermanite for oxygen fugacities ranging from the NNO to IW buffers. The experiments employed a gas-solid isotopic exchange technique utilizing 99% 18O-enriched CO-CO2 gas mixtures to control both the oxygen fugacity and the isotopic composition of the exchange reservoir. Diffusion profiles of the 18O tracer were obtained by in-depth analysis with an ion microprobe. The experimental results, fit to the Arrhenius relation D = D0e(−QRT), yield the following: Do (m2 s−1)Q (kJ mol−1)diopside4.3− 3.8+32.6× 10 − 4457 ± 26akermanite4.7− 4.4+83.5× 10 − 7457 ± 26278 ± 33spinel2.2− 1.8+8.7× 10 − 7404 ± 21anorthite8.4− 8.0+174× 10 − 13162 ± 36 At a given temperature, oxygen diffuses about 100 times more slowly in diopside than indicated by previous bulk-exchange experiments (Connolly and Muehlenbachs, 1988). Our data for anorthite, spinel, and akermanite agree well with prior results obtained by gas-solid exchange and depth profiling methods (Elphick et al., 1988; Reddy and Cooper, 1981; Yurimoto et al., 1989, respectively). Since these other experiments were conducted at different oxygen fugacities, this agreement indicates that diffusion of oxygen in these nominally Fe-free minerals is not greatly affected by fO2 in the range between pure oxygen and the iron-wustite buffer. However, our diffusion coefficients for anorthite, melilite, and spinel are also uniformly lower than those obtained by bulk analysis of crushed powders at similar temperatures (Muehlenbachs and Kushiro, 1974; Hayashi and Muehlenbachs, 1986; Ando and Oishi, 1974). The oxygen diffusion data are used to evaluate the effects of three different types of thermal histories upon the oxygen isotopic compositions of minerals found in Type B Ca-Al-rich inclusions (CAIBs) in carbonaceous chondrites: 1. (1) gas-solid exchange during isothermal heating, 2. (2) gas-solid exchange as a function of cooling rate subsequent to instantaneous heating, and 3. (3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. With the assumptions that the mineral compositions within a CAIB were uniformly enriched in 16O prior to any thermal processing, that effective diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyroxene, none of the above scenarios can reproduce the relative oxygen isotopic anomalies observed in CAIBs without improbably long or unrealistically intense thermal histories relative to current theoretical models of nebular evolution. The failure of these simple models, coupled with recent observations of “disturbed” magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicative of alteration and recrystallization, suggests that the oxygen isotopic compositions of these phases may have also been modified by alteration and recrystallization possibly interspersed with multiple melting events. Because the modal abundance of spinel remains relatively constant for plausible melting scenarios, and its relatively sluggish diffusion kinetics prevent substantial equilibration, Mg-Al spinel is the most reliable indicator of the oxygen isotopic composition of precursor material which formed Type B CAIs.

Compositional controls on the partitioning of U, Th, Ba, Pb, Sr and Zr between clinopyroxene and haplobasaltic melts: implications for uranium series disequilibria in basalts

The partitioning of U, Th, Pb, Sr, Zr and Ba between coexisting chromian diopsides and haplobasaltic liquids at oxygen fugacities between the iron-wustite buffer and air at 1285°C has been characterized using secondary ion mass spectrometry. The partition coefficients for Th, U and Zr show a strong dependence on the Al and Na content of the clinopyroxene. A good correlation between IVAl and DTh exists for all recent Th partitioning studies, providing a simple explanation for the two order of magnitude variation in DTh observed in this and previous studies [1,2]. Because mantle clinopyroxenes generally have greater than 5 wt% Al2O3, we suggest that the relevant partition coefficients for U and Th are between 0.01 and 0.02. While variations in Al and Na in clinopyroxene affect the absolute value of the Th and U partition coefficients, they have no effect on their ratio, DThDU. Our results reinforce the inference that equilibrium partitioning of U and Th between clinopyroxene and melt cannot explain the observed 230Th excesses in basalts. Indeed, under the oxygen fugacities relevant to MORB petrogenesis, clinopyroxene has little ability to fractionate U from Th (DThDU < 2), implying that chemical disequilibrium between melt and wall rock during transport is not required to preserve 230Th excesses generated in the garnet stability field. If the Ba partition coefficient serves as an analog for Ra and the partition coefficient of U5+ serves as an analog for Pa5+, then 226Ra and 231Pa excesses can be generated by clinopyroxene-melt partitioning. Using compositionally dependent partition coefficients, a melting model is used to show that equilibrium porous flow can explain variations in uranium series activities from the East Pacific Rise by varying the depth of melting.

Oxide solution mechanisms in silicate melts: Systematic variations in the activity coefficient of SiO2

Abstract Phase equilibria and spectroscopic data are used to develop a simple model for the interaction of various oxide components and molten SiO 2 . Network modifying oxides, M x O y produce nonbridging oxygens thereby depolymerizing the SiO 2 network. The energetics of nonbridging oxygen formation are least favorable when the field strength of the metal cation is high. This produces relatively strong M-O and Si-O-Si bridging bonds at the expense of weaker Si-O-M bonds ( De Jong et al. , 1980). This relationship is manifested by an increase in positive deviations from ideality with increasing cation field strength in M x O y -SiO 2 systems; the activity coefficient of SiO 2 is inversely correlated with Si-O-M bond strength. Network forming oxides (aluminates, phosphates, titanates. zirconates, etc.) may copolymerize with the SiO 2 network. Mixing on the same quasi-lattice produces solutions which approach ideality. Deviations from ideality in such solutions can be linked to distortions in the SiO 2 network. Discrete anion formers (phosphates, titanates, chromates, zirconates) complex with metal oxides other than SiO 2 to form discrete structural units which do not copolymerize with SiO 2 . The SiO 2 network is essentially shielded from the high charge density cations in such systems and unmixing is common. As a result, the relative deviations from ideality in such melts are high. It is important to recognize that oxides such as P 2 O 5 , TiO 2 and ZrO 2 may act as either network-formers or discrete anion formers depending upon melt composition, and are probably distributed between these two “sites” in most geologically important liquids. The latter structural role is favored in more basic compositions.

Pressure-temperature-time path discontinuity in the Main Central thrust zone, central Nepal

Metapelites collected in central Nepal reveal a discontinuity in metamorphic pressuretemperature-time (P-T-t) paths near the base of the Main Central thrust zone, despite an absence of obvious structural breaks. Garnets in the structurally lowest rocks grew with increasing T and P (loading), whereas garnets 1‐3 km upsection grew with increasing T, but decreasing P (exhumation). Monazite grains in structurally lower rocks yield ionmicroprobe Th-Pb ages of 8‐9 Ma. Structurally higher monazite grains range from 10 to 22 Ma. The P-T-t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca. 8 Ma, but also reflect thermal relaxation following older (20 Ma or older) thrust movement. The Main Central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate.

Postglacial left slip rate and past occurrence of M≥8 earthquakes on the Western Haiyuan Fault, Gansu, China

High-resolution (HR) air photographs and a 1-m horizontal and 2-m vertical resolution digital elevation model derived from them by stereophotogrammetry provide new constraints on the behavior of the western stretch of the active Haiyuan fault, in Gansu province, China. The photographs cover three swaths along the fault, each about 2-km-long and at least 500-m-wide, near the village of Songshan, at 103.5oE. This high-resolution data set is used to map and measure cumulative horizontal offsets of alluvial terraces and risers that range between 115 and la 135 m, and 70 and 90 m, at two sites. Dating these terraces with 14C yields minimum and maximum ages of 8400 and 7600, and 14,200 years B.P., respectively. This leads to a postglacial slip rate of 12±4 mm/yr, with a most likely minimum value of 11.6±1.1 mm/yr. The smallest stream offsets observed on the HR photographs range between 8 and 16 m and are interpreted as coseismic displacements of the last few earthquakes with M ≥ 8 that ruptured the 220-km-long Tianzhu gap of the fault, west of the Yellow River. Earthquakes of that size within this gap, which has been quiescent for at least 800 years, would recur at intervals of 1050±450 years.

Terrestrial accretion under oxidizing conditions.

Earths Ingredients What was the composition of the earliest terrestrial starting blocks? The answer lies in understanding how Earths interior separated into mantle and core components. Siebert et al. (p. 1194, published online 10 January) performed a series of high pressure and temperature experiments to track how chromium and vanadium, which have a slight affinity for iron, partition into metal and silicate fractions. Combined with accretionary models, the data suggest that Earth accreted under the same relatively oxidizing conditions under which the most common types of meteorites formed. Transferring oxygen in the form of FeO from the mantle to the core could have gradually reduced the mantle to its present-day oxidation state. Earths core formed under conditions similar to those that formed the most common meteorites. The abundance of siderophile elements in the mantle preserves the signature of core formation. On the basis of partitioning experiments at high pressure (35 to 74 gigapascals) and high temperature (3100 to 4400 kelvin), we demonstrate that depletions of slightly siderophile elements (vanadium and chromium), as well as moderately siderophile elements (nickel and cobalt), can be produced by core formation under more oxidizing conditions than previously proposed. Enhanced solubility of oxygen in the metal perturbs the metal-silicate partitioning of vanadium and chromium, precluding extrapolation of previous results. We propose that Earth accreted from materials as oxidized as ordinary or carbonaceous chondrites. Transfer of oxygen from the mantle to the core provides a mechanism to reduce the initial magma ocean redox state to that of the present-day mantle, reconciling the observed mantle vanadium and chromium concentrations with geophysical constraints on light elements in the core.

Long‐term slip rate of the southern San Andreas Fault from 10Be‐26Al surface exposure dating of an offset alluvial fan

We determine the long-term slip rate of the southern San Andreas Fault in the southeastern Indio Hills using ^(10)Be and ^(26)Al isotopes to date an offset alluvial fan surface. Field mapping complemented with topographic data, air photos and satellite images allows precise determination of piercing points across the fault zone that are used to measure an offset of 565 ± 80 m. A total of 26 quartz-rich cobbles from three different fan surfaces were collected and dated. The tight cluster of nuclide concentrations from 19 samples out of 20 from the offset fan surface implies a simple exposure history, negligible prior exposure and erosion, and yields an age of 35.5 ± 2.5 ka. The long-term slip rate of the San Andreas Fault south of Biskra Palms is thus 15.9 ± 3.4 mm/yr. This rate is about 10 mm/yr slower than geological (0–14 ka) and short-term geodetic estimates for this part of the San Andreas Fault, implying changes in slip rate or in faulting behavior. This result puts new constraints on the slip rate of the San Jacinto and on the Eastern California Shear Zone for the last 35 kyr. Our study shows that more sites along the major faults of southern California need to be targeted to better constrain the slip rates over different timescales.

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

An iron spin transition has no effect on the seismologic properties of lower-mantle minerals. Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.