Comment on “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by P. H. Warren (MAPS 47:209–227)

Ureilites are carbon‐rich ultramafic (olivine + dominantly low‐Ca pyroxene) achondrites with poorly understood petrogenesis. One major problem concerns the origin of extensive variation in FeO content (olivine core Fo values ranging from approximately 75 to 95) among the individual ureilites. The two main competing hypotheses to explain this variation are: (1) equilibrium smelting, in which ureilite Fo values were established by pressure‐dependent (depth‐linked) carbon redox reactions on the ureilite parent body during partial melting; or (2) nebular inheritance, in which the variation in FeO contents was derived from ureilite precursors and was preserved during partial melting. The paper “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by Warren (2012) discusses a series of topics related to ureilite petrogenesis. In each case, an argument is presented within the context of smelting versus nonsmelting models. Collectively, these arguments create the impression that there are many valid arguments against smelting. The purpose of this comment is to point out flaws in some of these arguments, and/or to show that the issues they address are independent of smelting versus nonsmelting models. Both equilibrium smelting and nebular inheritance (simple anatexis) models face challenges in explaining all the properties of ureilites, but both remain viable.     

Radio,X-ray,and optical observations of the flare of June 13, 1980, at 6h22m UT

A subflare of importance Sf was observed on June 13, 1980 simultaneously by instruments aboard the Solar Maximum Mission (SMM) and various ground based observatories. We describe and compare different kinds of observations, with emphasis on the Hard X-Ray Imaging Spectrometer (HXIS) images and spectra, and on the one-dimensional microwave images with high time and spatial resolution, obtained with the Westerbork Synthesis Radio Telescope (WSRT). The fast electrons causing the X-ray and microwave impulsive bursts had a common acceleration source, but the burst were produced at the opposite footpoints of the loops involved, with microwaves emitted near to a sunspot penumbra. The flare (of a ‘compact’ type) was probably triggered by an emerging flux, and two possible interpretations of this process are briefly discussed.

     

Computation of ray integrals and ray amplitudes in radially symmetric media

Summary A new approximation of the velocity-depth distribution in radially symmetric media is suggested. This approximation guarantees the continuity of velocity and its first and second derivatives, and does not generate false low-velocity layers. It removes false anomalies from the amplitude-distance curve and considerably increases its stability. The evaluation of ray integrals and ray amplitudes using this velocity-depth approximation does not require the computation of any transcendental function and is, therefore, very fast. Numerical examples are presented.     

Modeling and distance determination of planetary nebulae

The monthly probability of occurrence of southward (B _z ^– ) component of IMF estimated independent of the sector polarity observed near earth is found to change with the magnitude of solar wind velocity. The above analysis is done for each month during two years around sunspot minima and maxima in cycle 21. The results will be interpreted in terms of association of southwardB _z events with solar wind flows of distinct solar origin such as low and high speed solar wind.     

Management of the old field vegetation in the Namaqua National Park, South Africa: conflicting demands of conservation and tourism

Springtime mass displays of wild flowers are especially prominent on old fields and other disturbed sites in Namaqualand, an arid region in the north–western part of South Africa. This springtime floral spectacle draws thousands of tourists annually. However, there is a potential conflict between biodiversity conservation and the mass flowering that is a valuable source of income to the region. To develop a management plan for the old field vegetation in the Namaqua National Park, the questions of whether to disturb or not, how often to disturb and what type of disturbance should be applied are of vital importance. The aim of this paper is to report on experimental work to determine the impact of disturbance on mass flowering displays and biodiversity. Results show that although species diversity increased with time since last cultivation, the mass effect of the flowering display diminished. The loss of the mass effect appears to be primarily caused by the increased abundance of a short–lived perennial species ( Leysera gnaphalodes ) that suppressed the flowering display of the prominent annual species. To maintain mass displays as a tourist attraction, regular disturbance by tilling therefore seems essential. Initially, a four–year rotational tilling programme is proposed to produce a mosaic of fields of different ages to ensure that there will always be some old fields producing a spectacular and uncluttered display. Frequency of disturbance should thereafter be determined by an adaptive management policy. If the management of the old field vegetation is seen in the context of conserving total landscape ecodiversity, this includes conserving cultural bio–landscapes and their intrinsic values. Viewed in this context, human–induced disturbance in the small area set aside for mass displays in the Namaqua National Park (less than 0.05% of the park) becomes justified.     

Late-Glacial climatic changes in Eastern France (Lake Lautrey) from pollen, lake-levels, and chironomids

High-temporal resolution analyses of pollen, chironomid, and lake-level records from Lake Lautrey provide multi-proxy, quantitative estimates of climatic change during the Late-Glacial period in eastern France. Past temperature and moisture parameters were estimated using modern analogues and ‘plant functional types’ transfer-function methods for three pollen records obtained from different localities within the paleolake basin. The comparison of these methods shows that they provide generally similar climate signals, with the exception of the Bölling. Comparison of pollen- and chironomid-based temperature of the warmest month reconstructions generally agree, except during the Bölling. Major abrupt changes associated with the Oldest Dryas/Bölling, Alleröd/Younger Dryas, and the Younger Dryas/Preboreal transitions were quantified as well as other minor fluctuations related to the cold events (e.g., Preboreal oscillation). The temperature of the warmest month increased by ∼5°C at the start of Bölling, and by 1.5°-3°C at the onset of the Holocene, while it fell by ca. 3° to 4°C at the beginning of Younger Dryas. The comparative analysis of the results based on the three Lautrey cores have highlighted significant differences in the climate reconstructions related to the location of each core, underlining the caution that is needed when studying single cores not taken from deepest part of lake basins.     

Sediment budget and tectonic evolution of the Meuse catchment in the Ardennes and the Roer Valley Rift System

The Meuse river system is located in the northeastern part of the Paris Basin, the Ardennes, and the Roer Valley Rift System (RVRS). The Meuse river system developed during the uplift of the Ardennes since the Eocene and it was affected by renewed rifting of the RVRS starting in the Late Oligocene. In response to the uplift of the Ardennes, the river system incised and a terrace sequence developed during the Plio–Pleistocene. The sediments generated by erosion in the catchment were transported into the RVRS and further to the north, into the Zuiderzee Basin and the North Sea Basin. Using a digital terrain model, the amount of eroded rock volume versus time for the Meuse catchment has been computed using the Paleogene and older planation surfaces and the fluvial terraces. Comparison of the amount of eroded material with the volume of sediment preserved in the RVRS for the early Middle Pleistocene shows that about 17.5% of the sediment volume transported into the RVRS remained there, the rest being transported further into the Zuiderzee Basin and the North Sea Basin. The Quaternary tectonic uplift of the Ardennes inferred from the incision history of the Meuse river system is characterized by a long-term uplift, on which a Middle Pleistocene acceleration is superimposed. The accelerated uplift is contemporaneous with an uplift event in the RVRS and in the neighbouring Eifel area, and with the onset of the youngest phase of volcanism in the Eifel area. The areal distribution of this uplift is characterized by a dome shape centered around the Eifel area.     

Origin of the recrystallisation front in the Ronda peridotite by km-scale pervasive porous melt flow

 It is well established that porous melt flow in the upper mantle may significantly affect partial mantle melt compositions. Less well established are the length-scale of porous flow and whether porous melt flow can be a volumetrically important magmatic process. The only source for observations concerning the length-scale and nature of pervasive porous melt flow are peridotite massifs. Here we present such observations in the form of structural, and major and trace element data from peridotites of the Ronda massif, southern Spain. Trace element concentrations were obtained with high analytical precision (ICP-MS) and include trace elements rarely analysed in peridotites, such as Rb, Th, Nb and Ta. The western portion of the Ronda massif can be divided into two structural facies. The first and oldest is composed of deformed, porphyroclastic spinel peridotites, the second of virtually undeformed granular spinel peridotites. They are separated by a recrystallisation front across which grain growth of all phases occurred. The granular domain can be further subdivided into three subfacies: coarse-granular, fine-granular, and layered-granular peridotites. According to structural facies, km-scale spatial variations unrelated to Ca and Al abundances have been recognised for mg-numbers [atomic Mg/(Mg±Fe)] and incompatible elements such as rare earth elements (REE), Th and high-field-strength elements (HFSE; including Ti). Such variations are reminiscent of those commonly ascribed to mantle metasomatism, but have never been documented on the km-scale. The origin of the recrystallisation front is related to km-scale pervasive melt percolation. Feed-back processes between grain growth and melt fraction could have led to important accumulation of melt at the recrystallisation front, accomplished mainly by melting/dissolution. Variation in melt fraction across the front explains the spatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances, and could account for many of the classical differences between basalts from convergent and extensional tectonic settings. Whereas the coarse-granular peridotites reflect a stage of steady-state pervasive porous melt flow, the fine- and layered-granular facies probably reflect the terminate stages of porous melt flow. Processes associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes, and progressive channelling of melt flow associated with olivine-producing reactions. Both processes show complex overprinting relationships in both time and space. Received: 10 January 1995/Accepted: 1 September 1995