Introduction to a virtual special issue on crustal melting

This virtual special issue represents a collection of papers concerning Crustal Melting selected by the Editor from those published on various aspects of this theme in the JMG since 1982. The papers are grouped into sequences that address topics that have been prominent in the JMG during the last 30 years concerning the origin, evolution and tectonic role of migmatites and migmatitic granulites in crustal evolution. These topics are: Open‐ and closed‐system processes in the formation of migmatites and migmatitic granulites; thermobarometry, P–T paths, phase equilibria modelling and retrograde processes in formerly melt‐bearing rocks; geochronology in partially melted rocks; and, microstructures, deformation and tectonics of melt bearing rocks. About one‐third of the papers published in the JMG since its inception concern the origin, evolution and tectonic significance of migmatites and migmatitic granulites in crustal evolution, including the first special issue published by the JMG concerning ‘Studies in the genesis and deformation of migmatites’ edited by Tracy & Day ( 1988; Volume 6, Issue 4, Pages 385‐543 ). Three subsequent Special Issues of the JMG include papers relevant to the theme of this virtual special issue; they are ‘Metamorphic processes: a celebration of the career contribution of Ron Vernon’, ‘Processes in granulite metamorphism’ and ‘Granulites, partial melting and the rheology of the lower crust’, edited by Brown & Clarke ( 2002; Volume 20, Issue 1, Pages 1‐213 ), Brown & White ( 2008; Volume 26, Issue 2, Pages 121‐299 ) and Brown et al. ( 2011; Volume 29, Issue 1, Pages 1‐166 ), respectively. The selection of papers in this virtual special issue is by no means comprehensive, but it is intended as a representative selection of what has been published in the JMG over 30 years to give the reader a broad overview of crustal melting. Furthermore, although many papers address more than one topic, each is included only once and has been placed within the most appropriate section.     

An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model.     

EXPORTS OF ORGANIC CARBON IN BRITISH RIVERS

This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7–103·5 kg ha⁻¹ year⁻¹, with a median flux of 31·9 kg ha⁻¹ year⁻¹ in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two–thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change. © 1997 by John Wiley & Sons, Ltd.     

伽师地震区地壳细结构及发震断层的初步研究

199年至1998年伽师地区共出现9次震级为6.1-6.8级的强震. 在一个非常短的时间间隔内和非常小的地区范围内接连出现这么多次震级非常接近的地震,确实非常罕见. 为研究伽师强震区的深部构造背景和孕震机制,本文对伽师地震区的余震观测资料进行了分析处理. 利用联合反演技术同时得到了地震震源位置和地震区地壳三维速度结构. 余震震中沿一北北东向条带分布,与强震分布的两个条带中的北东向条带位置基本重合. 三维反演得到的速度结构结果表明,在地下12 km以下存在一条北北东向和一条北北西向的低速条带. 上述两低速条带与强震分布的两个条带位置很接近. 初步推测,低速条带对应了地壳深部的两条断裂. 在我们观测期间,北北东向断裂有微震活动,北北西向断裂相对平静.     

MEASUREMENT OF FROST-INDUCED SNOWMELT RUNOFF IN A FOREST SOIL

Snow interception in a coniferous stand leads to considerable short-range variability in snowcover depth, which in turn affects the water and heat regime of the soil. To study the coupling between snow accumulation, frost penetration, and hydrological response, plot-scale experiments were conducted in a subalpine spruce forest. The stony, sandy–loamy Spodosol was highly permeable and had an organic layer of 5–15 cm thickness. Within two plots, one underneath a tree crown and one in a canopy gap, we measured near-surface runoff, soil temperature, and liquid water content. Snow and frost depths varied more in space than between two winter periods at given locations. Frost penetration was greater near the trunk, where a higher portion of snowmelt water drained downslope close to the surface than in the gap due to frost-induced reduction of infiltration. In both years, the spring snowmelt occurred over two distinct periods. During the first snowmelt, the water percolated primarily through the frozen layer and part of it probably refroze within the frozen layer, thereby raising the total water and ice content. During the second event, near-surface runoff was more pronounced.     

Changes in Silicic Melt Structure Between the Two Bandelier Caldera-Forming Eruptions, New Mexico, USA: Evidence from Zirconium and Light Rare Earth Elements

The Cerro Toledo Rhyolite is a group of high-silica rhyolitedomes and tephras which were erupted during the period betweenthe Lower Bandelier Tuff (LBT) at 1?45 Ma and the Upper BandelierTuff (UBT) at 1?12 Ma. The tephra sequence reflects the changingcomposition of the most fractionated liquids at the top of themagma chamber or chambers owing to crystallization during this0?33-Ma interval, over which it is possible to trace magma evolutionin detail. Incompatible elements such as Cs and Lu generallyincrease in concentration upsection through the tephra sequence.By contrast, Zr initially decreases upsection from 153 ppm to134 ppm in the middle, then increases upward to 320 ppm in themost evolved rhyolites. The Zr/Cs and Hf/Cs ratios initiallydecline, then become constant. The dramatic rise in Zr at nearlyconstant Zr/Cs suggests suppression of zircon crystallizationand a change from a high degree of zircon oversaturation toone of marginal oversaturation in the most evolved magmas. Theobserved Zr trends in the Cerro Toledo Rhyolite are oppositeto those (1) predicted by experimental studics of Zr solubilityin silicic magmas and (2) observed in the Bishop Tuff. LREEshow broadly similar relations to Zr and Hf. We have examined two parameters—iron content and volatilecontent of the magma—which may have controlled the crystallizationbehavior of zircon and the LREE-rich phase(s). The dramaticincrease in Zr and LREE in the most evolved Cerro Toledo Rhyolitesand basal UBT plinian tephras is accompanied by similar increasesin iron and halogen contents. If changa in the halogen contentsof matrix glasses are indicative of changes in overall volatilelevels at the top of the magma chamber(s), the increase in volatilesand iron may have modified the structural state of the magmaand increased the solubilities of zircon and the LREE-rich phase(s),thereby raising the saturation levels of Zr and LREE and stabilizingthese elements in the melt. Disruption of the melt structuremay have resulted from (1) Fe as a network modifier or quasi-molecularcomplex, (2) breaking of bridging oxygens by anions such asOH^– and F^–, and/or (3) complexing of Al and alkalisby OH, F, and/or Cl.     

Performance evaluation and work-load estimation for geographic information systems

Abstract

Agencies acquiring GIS hardware and software are faced with uncertainty at two levels: over the degree to which the proposed system will perform the functions required, and over the degree to which it is capable of doing so within proposed production schedules. As the field matures the second concern is becoming more significant. A formal model of the process of acquiring a GIS is presented, based on the conceptual level of defining GIS sub-tasks. The appropriateness of the approach is illustrated using performance data from the Canada Land Data System. It is possible to construct reasonably accurate models of system resource utilization using simple predictors and least squares techniques, and a combination of inductive and deductive reasoning. The model has been implemented in an interactive package for MS-DOS systems.     

Tactile-map output from geographical information systems: the challenge and its importance

Abstract

The incorporation of the technology of geographic information systems (GIS) into a wide range of planning decisions makes it important that the visually handicapped have access to GIS-based map products to participate more fully in community decision making. Fortunately, GIS contain many procedures useful to the design of tactile maps, including changes of scale and simplification. Developing research opportunities are outlined within the context of the general problem. Tactile maps using microcapsule paper can be produced quickly, but they must be tested using visually handicapped subjects to ensure that suitable symbolization and specifications are developed.     

PUBLIC MEMORY AND POLITICAL POWER IN GUATEMALA'S POSTCONFLICT LANDSCAPE

ABSTRACT. Landscape interpretation, or “reading” the landscape, is one of cultural geography's standard practices. Relatively little attention, however, has been paid to reading landscapes transformed by insurgency movements or civil wars. Those landscapes can tell us a great deal about past and present political and social relationships as well as continuing power struggles. Guatemala presents a complicated postwar landscape “text” in which the struggle for power continues by many means and media, including how the war is portrayed on memorials, and in which the Catholic Church and the military/state are the two main competing powers. This essay explores some of the images and the text presented in Guatemala's postconflict landscape through contrasting landmarks and memorials associated with the country's thirty‐six‐year‐long civil war that formally ended in 1996.     

Bromide distribution characteristics of different Zechstein 2 rock salt sequences of the Southern Permian Basin: a comparison between bedded and domal salts

The bromide profile of the bedded Zechstein 2 rock salts in the Southern Permian Basin is characterized by a continuous increase in the bromide contents from base to top, indicating progressive evaporation of sea water. Former studies have suggested that the bromide distribution in domal salts is more irregular, raising such questions as whether there is a causal relationship between bromide distribution and deformation‐related processes or whether various halite types with different primary bromide contents account for such irregularities. In this study, the bromide distribution of a defined stratigraphic section (Hauptsalz) of the Zechstein 2 was investigated in three salt deposits, with bedded salt (Teutschenthal) compared with domal salts (Morsleben and Gorleben). Furthermore, two different halite types (Kristallbrocken, matrix) were distinguished during sampling. Comparison between the locations reveals that: (i) the characteristic trend of the bromide profile was generally preserved in both the bedded and the more intensely deformed domal salts; (ii) the dispersion of bromide contents along the running average curve is lower in domal salt, especially in the lower half of the Hauptsalz; and (iii) the Kristallbrocken are absent in the lower half of the Hauptsalz of Morsleben and Gorleben. The absence of the Kristallbrocken and the lower dispersion of the bromide contents in the domal salts are clear indications for the influence of salt migration‐related processes on the bromide distribution characteristics. It shows that these processes are associated with a redistribution of bromide, which eventually results in a homogenization of the originally varying bromide contents. On the other hand, preservation of the characteristic trend in the bromide profiles indicates that large‐scale brecciation, folding processes or circulating bromide‐rich fluids played only a minor role during the formation of the salt domes. Selective sampling of the Kristallbrocken and matrix halite displays a disparity in bromide content that is not consistent between the studied locations. These specific phenomena might be related to different depositional locations in the Zechstein Basin or to post‐depositional processes.