Diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase

Residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals. We present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects. In particular, we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements (e.g., Mg), the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension. We show that contrary to intuition, diffusive fluxes of trace elements may be coupled to major element concentration gradients, and ignoring such coupling can lead to incorrect estimates of timescales. We illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a Holocene dacitic lava flow of Volcán San Pedro (Chilean Andes, 36°S). The inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 (30 to 148) yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods.     

Regional metamorphism of pelitic rocks around Kandra,Singhbhum, Bihar

A suite of pelitic rocks around Kandra, Singhbhum District, Bihar, displays a metamorphic gradient registered by the index minerals chlorite, biotite, garnet, staurolite and sillimanite in a Barrovian sequence. Metamorphism was by and large coeval with folding movements, and correlating the internal fabric of minerals and deformational characters, a regular sequence of the index minerals is derived. It is argued that the chronological order by itself is not sufficient to prove that metamorphism was progressive in time.Among the index minerals, garnet appears to have formed by the reaction chlorite+biotite_a+quartz garnet+biotite_b+H₂O. For the origin of sillimanite, a new reaction, 3 staurolite+muscovite+quartz=7 sillimanite+biotite+3H₂O, is suggested on the basis of significant textural features. Textural and petrological indications regarding the formation of staurolite are in discordance. Staurolite was either derived from the biotite zone phases, or should be taken to have formed, against textural evidences, from chloritoids of the garnet zone.Graphical analysis of the assemblages by Thompson's AFM projection reveals that chlorite and staurolite are excess phases owing to retrogression and incomplete reaction. Shifting of apices of triangular fields and intersection of garnet-biotite tie lines within a zone can be satisfactorily explained in terms of extra components CaO and MnO or their ratios. It is pointed out that if MgO/(MgO + FeO) between two phases show a linear relation, their tie lines will be concurrent on the AF side of the projection, the point of concurrence reflecting equilibrium and temperature of recrystallisation.     

Some primary structures in the chromitites of Orissa,India

Some important primary structures have been studied and described from the chromities of Sukinda Valley in Orissa, India. The structures include primary layering, Ball and Pillow structures, and cross laminations. From the similarity of the structures and texture of the chromitites with the sedimentary rocks, it is presumed that the chromities were formed by magmatic sedimentation under the influence of gravity.

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Zusammenfassung Aus den Chromititen von Sukinda Valley in Orissa, Indien, werden einige wichtige, primäre Gefüge untersucht und beschrieben. Die Gefüge zeigen primäre Bänderung, Ball and Pillow-Lagen und Kreuzschichtung. Aus der Ähnlichkeit der Gefüge der Chromitite mit denen sedimentärer Gesteine wird geschlossen, daß die Chromitite durch gravitative, magmatische Saigerung sedimentiert worden sind.

     

Impact of CAMEX-3 data on the analysis and forecasts of Atlantic hurricanes

Summary In the past, various field experiments were conducted using special aircrafts to enhance the observational database of hurricanes. Dropwindsondes (or “dropsondes”) are generally deployed to collect additional observations in the vicinity of the hurricane center. In addition to dropsondes, during the Third Convection and Moisture Experiment (CAMEX-3), which was conducted over the Atlantic Ocean and Gulf of Mexico during August–September 1998, LASE was also used to measure vertical moisture profiles. Four hurricanes: Bonnie, Danielle, Earl and Georges were targeted during this campaign. This paper describes the resulting impact of CAMEX-3 data, especially the LASE moisture profile data, on the hurricane analysis and forecast. The data were analyzed using a spectral statistical interpolation technique and the forecasts were made using the FSUGCM at T126 resolution with 14 σ-vertical levels. Results indicate that the LASE data had a significant impact on the moisture analysis. The reanalysis was slightly drier away from the hurricane center and wetter close to the center. Spiraling bands, both dry and wet, of moisture were clearly seen for hurricane Danielle. The LASE data did not affect the wind analysis significantly, however when it was used along with dropsonde observations the hurricane intensity and its structure were well represented and the forecast track produced from the reanalyzed initial condition had less forecast errors. The LASE and dropsonde observations were in good agreement. Received February 27, 2001 Revised July 31, 2001     

Estimating Deep Seafloor Interface and Volume Roughness Parameters Using the Multibeam-Hydrosweep System

Utilizing a hull-mounted, multinarrow beam echosounder onboard RV Polarstern, we measured variation of acoustic backscatter with incidence angles at two different sites in the Southern Oceans (Agulhas Plateau and the Riiser Larsen Sea). We modeled the data, using a composite roughness model, including water-sediment interface roughness and sediment volume roughness parameters. The model effectively uses the near normal incidence angle backscatter to determine the seafloor interface roughness parameters employing Helmholtz-Kirchhoff theory. Beyond 20° incidence angles, an application of Rayleigh-Rice theory is made by using a necessary splicing technique (combining both of the theories at 20° incidence angle). The estimated interface and volume roughness parameters are found to be in accordance with the known area geology.     

Urban Heat and Climate Justice: A Landscape of Thermal Inequity in Pinellas County,Florida

The combined effects of two global trends, urbanization and climate change, have generated considerable concern regarding their adverse and disproportionate impacts on the health of urban populations. This study contributes to climate‐justice research by determining whether elevated levels of urban heat, indicated by land surface temperature (LST), are distributed inequitably with respect to race/ethnicity, age, and socioeconomic status in Pinellas County, Florida. Our study utilizes 2010 MODIS and Landsat medium‐resolution, remotely sensed thermal data, census socio‐demographic information, and both conventional and spatial statistical methods. Results indicate that LST is significantly greater in census tracts characterized by higher percentages of certain racial/ethnic minorities and higher poverty rates, even after controlling for contextual factors and the effects of spatial autocorrelation. This reveals the presence of a landscape of thermal inequity: uneven distribution of heat within the built urban environment and a community structure with varying vulnerability.     

Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C

Experiments have been carried out to determine the temperature, oxygen fugacity (fO₂) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO₂^(1/3). This makes a knowledge of the T-fO₂ path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO₂ for Ca diffusion in olivine at a given oxygen fugacity (fO₂*) are given by:diffusion along [100]: log [Do (m²/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/moldiffusion along [010]: log [Do (m²/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/moldiffusion along [001]: log [Do (m²/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/molwhere Δ log fO₂ = log[fO₂*] − log[10⁻¹²] with fO₂* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo₈₃ and Fo₉₂. Diffusion in Fo₁₀₀ has a much higher activation energy than in Fe-bearing olivine and has a weaker fO₂ dependence.     

Some aspects of the role of intergranular fluids in the compositional evolution of metamorphic rocks

Minerals that react with each other during the progressive evolution of metamorphic terranes are not always in physical contact. As such, an “intergranular fluid” could play a major role in element transfer and chemical evolution. However, the nature of this fluid and its specific role remains somewhat elusive. Recent experiments in our laboratory shed some light on the behavior of such a fluid. Here we present a simple mathematical model which accounts for diffusion within crystals and fluid, solubility in the fluid and mass balance between the various reservoirs. The model elucidates the nature of element exchange between two minerals via the mediation of an intergranular fluid. It is shown that a coupling of thermodynamics and kinetics controls the evolution of the system and the concentration of an element in the intergranular fluid is a key parameter of interest. The results have important implications for standard tools of metamorphic petrology such as geothermometers and barometers, geospeedometry and the closure of isotopic systems. For example, homogeneity of mineral grains may be a poor criterion for equilibrium and the rim compositions of minerals showing diffusion zoning may be out of equilibrium with distant exchange partners, even in the presence of a fluid in which transport is fast.     

Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8–9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N–S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P–T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31–17 Ma): a given block within this region traversed the field of melting in 5–7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P–T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26–23 Ma) than the lower structural levels (~31–27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.     

The Mechanism of the 20°C Isotherm Depth Oscillations for the Bay of Bengal

The monthly variation of thermocline depth in terms of 20°C isotherm depth (Z20) in the Bay of Bengal has been studied using SODA and ARGO datasets. During a southwest monsoon, the Bay of Bengal is deepest in the western basin and shallowest in the eastern basin while it is the opposite during northeast monsoon. The Z20 oscillation is not only affected by semiannual reversing wind forcing but also influenced by coastal Kelvin waves from the eastern coast and remote effect from the Northern Indian Ocean and Malacca Strait. A linear relationship between SSHA and Z20 has been found from both SODA and observational studies.