Geochemistry, Geophysics, Geosystems

doi: 10.1002/ggge.20557pmid: N/A

Moitra, P.; Gonnermann, H. M.

doi: 10.1002/2014GC005554pmid: N/A

Erupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid‐particle suspensions of dynamical similarity to crystal‐bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel‐Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction‐based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal‐rich magmas must take the shear‐rate dependence of viscosity into account. Shear‐rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution.

Ni, Huaiwei

doi: 10.1002/2014GC005574pmid: N/A

Rather than indicating formation/peak temperature, oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures (Tae). The isotopic fractionations and Tae values deliver information about cooling history, as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate. Despite that several models, such as the Fast Grain Boundary (FGB) model, have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling, extraction of cooling rate remains far from straightforward. On the other hand, there is a well‐defined quantitative relationship between the Dodson closure temperature (Tc) and the cooling rate, but Tc cannot be directly measured. Based on simulation results of existing models for a variety of rock systems, including open systems (with an infinite fluid reservoir), closed systems (with negligible fluid participation) and semi‐open systems (with moderate fluid participation), this study demonstrates that Tae of the mineral pair with the largest equilibrium isotope fractionation (PLEIF) is always bounded by their Tc values, regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange. If the two Tc values happen to be similar, Tae will serve as a good approximation of both Tc, provided that the equilibrium fractionation factor has been precisely determined as a function of temperature. One such pair is quartz‐magnetite. By contrast, a mineral pair with similar Tc but relatively small fractionation is susceptible to the disturbance from other minerals, hence does not always have Tae confined within their Tc range. The relationship of Tae‐Tc correspondence for PLEIF with similar Tc can be used to constrain either cooling rate (i.e., as a speedometry method) or oxygen isotope diffusivity if one of them has been independently determined. In the latter case, the inferred oxygen diffusivity may be an index of water fugacity (i.e., as a hygrometry method) when compared with experimental diffusivity values measured under different fluid conditions.

Emry, E. L.; Nyblade, A. A.; Julià, J.; Anandakrishnan, S.; Aster, R. C.; Wiens, D. A.; Huerta, A. D.; Wilson, T. J.

doi: 10.1002/2014GC005588pmid: N/A

Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high‐quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single‐station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 ± 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 ± 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

Fox, B. R. S.; Wartho, J.; Wilson, G. S.; Lee, D. E.; Nelson, F. E.; Kaulfuss, U.

doi: 10.1002/2014GC005534pmid: N/A

Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51 ± 0.24 and 23.38 ± 0.24 Ma (2σ). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi‐1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo‐single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long‐term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene.

Huang, Wentao; van Hinsbergen, Douwe J. J.; Dekkers, Mark J.; Garzanti, Eduardo; Dupont‐Nivet, Guillaume; Lippert, Peter C.; Li, Xiaochun; Maffione, Marco; Langereis, Cor G.; Hu, Xiumian; Guo, Zhaojie; Kapp, Paul

Zan, Jinbo; Fang, Xiaomin; Yang, Shengli; Yan, Maodu

doi: 10.1002/2014GC005616pmid: N/A

Recent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the Chinese Loess Plateau. So far, however, similar work has not been undertaken on the loess deposits in Central Asia. In this paper, 17 loess and paleosol samples from three representative loess sections in Central Asia were separated into four grain size fractions, and then systematic rock magnetic measurements were made on the fractions. Our results demonstrate that the content of the <4 μm fraction in the Central Asian loess deposits is relatively low and that the samples generally have a unimodal particle distribution with a peak in the medium‐coarse silt range. We find no significant difference between the particle size distributions obtained by the laser diffraction and the pipette and wet sieving methods. Rock magnetic studies further demonstrate that the medium‐coarse silt fraction (e.g., the 20–75 μm fraction) provides the main control on the magnetic properties of the loess and paleosol samples in Central Asia. The contribution of pedogenically produced superparamagnetic (SP) and stable single‐domain (SD) magnetic particles to the bulk magnetic properties is very limited. In addition, the coarsest fraction (>75 μm) exhibits the minimum values of χ, χARM, and SIRM, demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the Central Asian loess deposits.

Usui, Yoichi; Shibuya, Takazo; Sawaki, Yusuke; Komiya, Tsuyoshi

doi: 10.1002/2014GC005508pmid: N/A

Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse‐grained, magnetically unstable oxides. In this study, the first example of tiny, needle‐shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First‐order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μT. Local metamorphic condition suggests that those magnetite may carry remanence of ∼3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence.

Wanless, V. D.; Shaw, A. M.; Behn, M. D.; Soule, S. A.; Escartín, J.; Hamelin, C.

doi: 10.1002/2014GC005517pmid: N/A

Here we present volatile, major, and trace element concentrations of 64 olivine‐hosted melt inclusions from the Lucky Strike segment on the mid‐Atlantic ridge. Lucky Strike is one of two locations where a crustal melt lens has been seismically imaged on a slow‐spreading ridge. Vapor‐saturation pressures, calculated from CO2 and H2O contents of Lucky Strike melt inclusions, range from approximately 300–3000 bars, corresponding to depths of 0.5–9.9 km below the seafloor. Approximately 50% of the melt inclusions record crystallization depths of 3–4 km, corresponding to the seismically imaged melt lens depth, while an additional ∼35% crystallize at depths > 4 km. This indicates that while crystallization is focused within the melt lens, significant crystallization also occurs in the lower crust and/or upper mantle. The melt inclusions span a range of major and trace element concentrations from normal to enriched basalts. Trace element ratios at all depths are heterogeneous, suggesting that melts are not efficiently homogenized in the mantle or crust, despite the presence of a melt lens. This is consistent with the transient nature of magma chambers proposed for slower‐spreading ridges. To investigate the petrogenesis of the melt inclusion compositions, we compare the measured trace element compositions to theoretical melting calculations that consider variations in the melting geometry and heterogeneities in the mantle source. The full range of compositions can be produced by slight variations in the proportion of an Azores plume and depleted upper mantle components and changes in the total extent of melting.

Dang, Haowen; Jian, Zhimin; Kissel, Catherine; Bassinot, Franck

doi: 10.1002/2014GC005550pmid: N/A

Within the precession band, an interhemispheric antiphase pattern in the tropical hydroclimate is supported by many paleorecords, and optimally explained by the forcing of precessional insolation change. However, scenarios within the western equatorial Pacific (WEP), which plays the role of the ascending center of atmospheric convection, remain poorly determined. In this study, a marine sediment core from the Halmahera Sea, East Indonesia, was analyzed with high‐resolution XRF scanning, quantitative discrete XRF, and ICP‐AES/MS measurements. The terrigenous fractions in this core are constrained by their trace elemental characteristics to be locally sourced from Halmahera Island, and hence reflect variations in the local riverine runoff and precipitation. On this basis, a continuous record of precipitation changes of the western equatorial Pacific was reconstructed with multidecadal resolution over the last ∼240 ka, using an age model established by the correlation between an adjusted ice volume model and benthic δ18O constrained by 14C dating. The records of terrigenous input show a dominant ∼23 kyr periodicity with a 90°∼100° phase lag to the boreal summer (i.e., in‐phase with the boreal autumn) insolation change. This pattern can be explained by the variability in the convective activity over the WEP, which might be primarily controlled by precessional changes in the El Niño and Southern Oscillation (ENSO) system. A dynamic linkage is implied between the precessional variations in the convective activity in the WEP and the East Asian and Australia‐Indonesian summer monsoons (EASM and AISM), in the sense of their distinct stable phase relationship to precession.