Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA{chi}FC)

Significant petrogenetic processes governing the geochemicalevolution of magma bodies include magma Recharge (includingformation of ‘quenched inclusions’ or enclaves),heating and concomitant partial melting of country rock withpossible ‘contamination’ of the evolving magma body(Assimilation), and formation and separation of cumulates byFractional Crystallization (RAFC). Although the importance ofmodeling such open-system magma chambers subject to energy conservationhas been demonstrated, the effects of concurrent removal ofmagma by eruption and/or variable assimilation (involving imperfectextraction of anatectic melt from wall rock) have not been considered.In this study, we extend the EC-RAFC model to include the effectsof Eruption and variable amounts of assimilation, A. This model,called EC-E'RAFC, tracks the compositions (trace elements andisotopes), temperatures, and masses of magma body liquid (melt),eruptive magma, cumulates and enclaves within a composite magmaticsystem undergoing simultaneous eruption, recharge, assimilationand fractional crystallization. The model is formulated as aset of 4 + t + i + s coupled nonlinear differential equations,where the number of trace elements, radiogenic and stable isotoperatios modeled are t, i and s, respectively. Solution of theEC-E'RAFC equations provides values for the average temperatureof wall rock (T_a), mass of melt within the magma body (M_m),masses of cumulates (M_ct), enclaves (M_en) and wall rock () and the masses of anatectic melt generated () and assimilated (). In addition, t trace element concentrations and i + s isotopic ratios inmelt and eruptive magma (C_m, _m, _m), cumulates (C_ct, _m, _m), enclaves(C_en, , ) and anatectic melt (C_a, , ) as a function of magma temperature (T_m) are also computed. Input parametersinclude the (user-defined) equilibration temperature (T_eq),a factor describing the efficiency of addition of anatecticmelt () from country rock to host magma, the initial temperatureand composition of pristine host melt (, , , ), recharge melt (, , , ) and wall rock (, , , ), distribution coefficients (D_m, D_r, D_a) and their temperaturedependences (H_m, H_r, H_a), latent heats of transition (meltingor crystallization) for wall rock (h_a), pristine magma (h_m)and recharge magma (h_r) as well as the isobaric specific heatcapacity of assimilant (C_p,a), pristine (C_p,m) and recharge(C_p,r) melts. The magma recharge mass and eruptive magma massfunctions, M_r(T_m) and M_e(T_m), respectively, are specified apriori. M_r(T_m) and M_e(T_m) are modeled as either continuous orepisodic (step-like) processes. Melt productivity functions,which prescribe the relationship between melt mass fractionand temperature, are defined for end-member bulk compositionscharacterizing the local geologic site. EC-E'RAFC has potentialfor addressing fundamental questions in igneous petrology suchas: What are intrusive to extrusive ratios (I/E) for particularmagmatic systems, and how does this factor relate to rates ofcrustal growth? How does I/E vary temporally at single, long-livedmagmatic centers? What system characteristics are most profoundlyinfluenced by eruption? What is the quantitative relationshipbetween recharge and assimilation? In cases where the extractionefficiency can be shown to be less than unity, what geologiccriteria are important and can these criteria be linked to fieldobservations? A critical aspect of the energy-constrained approachis that it requires integration of field, geochronological,petrologic, and geochemical data, and, thus, the EC-ERAFC ‘systems’approach provides a means for answering broad questions whileunifying observations from a number of disciplines relevantto the study of igneous rocks. KEY WORDS: assimilation; energy conservation; eruption; open system; recharge     

Magmatic Evolution of the Skye Igneous Centre, Western Scotland: Modelling of Assimilation, Recharge and Fractional Crystallization

The Skye igneous centre, forming part of the British Tertiarymagmatic province, developed over a 7 Myr period (61–54Ma) and is characterized by a complex suite of lavas, hypabyssaland intrusive rocks of picritic to granitic composition. Theintrusion of magma from mantle to crust at 2x10^–3km³/yr(6 Mt/yr) advected magmatic heat of roughly 0·2 GW averagedover the period of magmatism supporting an ‘excess’heat flux of about 130 mW/m², or about twice the present-dayaverage continental heat flow. The volume of new crust generatedat Skye (15000 km³) spread over the present-day area of Skyecorresponds to 9 km of new crust. The geochemical evolutionof the Skye magmatic system is constrained using the Energy-ConstrainedRecharge, Assimilation, and Fractional Crystallization (EC-RAFC)model to understand variations in the Sr- and Pb-isotopic andSr trace-element composition of the exposed magmatic rocks withtime. The character (composition and specific enthalpy) of bothassimilant and recharge magma appears to change systematicallyup-section, suggesting that the magma reservoirs migrated toprogressively shallower levels as the system matured. The modelof the magma transport system that emerges is one in which magmabatches are stored initially at lower-crustal levels, wherethey undergo RAFC evolution. Residual magma from this stagethen migrates to shallower levels, where mid-crustal wall rockis assimilated; the recharge magma at this level is characterizedby an increasingly crustal signature. For some of the stratigraphicallyyoungest rocks, the data suggest that the magma reservoirs ascendedinto, and interacted with, upper-crustal Torridonian metasediments. KEY WORDS: assimilation; EC-RAFC model; geochemical modelling; magma recharge; Skye magmatism     

Spectral characteristics of iron-bearing phyllosilicates: Comparison to Orgueil (CI1), Murchison and Murray (CM2)

Abstract— Phyllosilicate alteration minerals are commonly found in low petrologic types of carbonaceous chondrites. Previous spectral studies have examined Mg-bearing phyllosilicates with limited success in matching the spectral properties of CM and CI chondrites. Transmission electron microscope and other analytical techniques suggest that Fe-bearing clays are more abundant in CI and CM chondrites than magnesian varieties. Here, we present the results of an examination of the reflectance spectra of Fe-phyllosilicates, including serpentines and berthierines, of which the latter were formerly known as septechlorites. We have measured the diffuse reflectance spectra of powdered samples from 0.3 to 25 μn. We find that these minerals provide a better spectral match to many of the features seen in CI and CM chondrites, and simple linear combinations of the spectra of both Fe- and Mg-phyllosilicates closely approximate the spectra of CM and CI chondrites.     

A disappearing biome? Reconsidering land-cover change in the Brazilian savanna

The Cerrado , the tropical savanna covering 22% of Brazil's territory, or approximately 1.783 million km², has suffered significant human impacts during the past three decades. This paper re-examines estimates of Cerrado vegetation change dynamics using high-resolution satellite remote sensing data from an area of interest extracted from eastern Mato Grosso State. This region has undergone a high degree of typical agricultural development since the early 1970s. Results indicate significant loss of original vegetation as well as high levels of regeneration, suggesting Cerrado vegetation may be more resilient to human impacts than catastrophic estimations suggest. The paper concludes with a critical review of Cerrado land-cover change studies and the implications of evidence for vegetation regeneration, land-cover dynamism and land-use intensification, paying particular attention to spatial scale and research methods. The discussion concludes that Cerrado land-cover change studied at a higher resolution and larger scales (smaller area) is required to represent more effectively the complexity of land conversion for better assessment of human impacts and environmental policy.     

"Power Geometries

Feminist contributions to the geographical literature on capitalist restructuring have tended to emphasize the importance of examining women's experiences vis-a-vis those of men. This research in progress report on the telecommunications service industry in New Zealand suggests that assuming the relative homogeneity of ‘women’ as an analytical category, then assessing the experience of women against those of men using national data, may no longer be satisfactory in a context in which production, finance and labour markets have been internationalised.     

Establishing chronologies from acid-insoluble organic 14C dates on antarctic (Ross Sea) and arctic (North Atlantic) marine sediments

To compare north and south polar marine paleoenvironments over the last 30,000 years, comparable chronological (radiocarbon) records must be developed and refined. Many areas in the polar regions do not preserve marine carbonates (foraminifera, mollusks), and thus age determinations, of necessity, are based on the acid-insoluble organic (AIO) fraction of the sediment. Although AIO ages are problematic and rarely used in the Arctic, they provide reasonable and consistent chronologies for the Ross Sea, Antarctica. AIO dates are meaningful in the Ross Sea because there are relatively high levels of productivity, good preservation of marine biogenic material in the sediment, and little input of terrigenous sediment and old/dead carbon. Event stratigraphy based upon proxy records of biogenic silica and δ¹³C can be used to assess the reliability of the AIO dates and surface age corrections. Reconstructed time-series of changes in the biogenic silica content of cores from the western Ross Sea show apparent similarities with the 'classic'deglacial climate sequence of the northern North Atlantic. Once the absolute ages of the antarctic AIO dates are constrained by independently dated records to validate surface age corrections, it will be possible to directly compare the timing of events such as ice-rafting events in the sedimentary record.     

基于逻辑回归模型的泥石流易发性评价与检验：以金沙江上游奔子栏—昌波河段为例

基于地理信息系统(ArcGIS100)平台和小流域单元,采用逻辑回归(LR)模型对金沙江上游(奔子栏—昌波河段)干热河谷区进行泥石流易发性评价,并对预测结果进行总体检验与随机个案检验。评价与检验结果表明,得到的最优指标组合下LR评价模型的AUC值为827%;预测的极高易发区、高易发区面积合占全区面积的3598%,实发泥石流面积占泥石流总面积的6503%;在个案检验中,位于各等级分区的检验组样本实发泥石流比例随着分区易发性等级降低,依次为917%(极高)、750%(高)、364%(中等)、167%(低)、0(极低),表明评价效果良好。研究区泥石流集中发育于金沙江沿岸的东北部、中部和西南部,主导性的评价指标依次为距主干道路距离、岩性、距断裂带距离、雨季月平均降雨量。人类活动与季节性降雨为研究区干热河谷泥石流的主要诱发条件。基于逻辑回归模型的泥石流易发性评价方法提高了泥石流发生可能性的预测精度,可为干热河谷区泥石流预测预警和防治提供参考依据。