Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation,assimilation and mixing

The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in ⁸⁷Sr/⁸⁶Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO₂ content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH₂O, assimilation and magma mixing.     

Contrasted abyssal basalt liquidus trends: evidence for mantle major element heterogeneity

Natural submarine basalt glasses define liquidus trends which may be summarized as curvilinear regression lines in the normative plagioclase-pyroxene-olivine ternary. Individual basalt suites separated in time or space may differ in major element composition; these differences translate into systematic differences in normative plagioclase, leading to efficient discrimination of individual trends in this ternary. Comparison of two contrasted sets of trends to available experimental and petrographic data confirms the similarity of the natural basalt trends to those predicted by 1-atm experiments, in general agreement with earlier studies based on whole-rock data. These contasted liquidus trends are believed to reflect major element heterogeneity and varying degrees of melting of the mantle source. This major element heterogeneity is not simply correlated with heterogeneity in incompatible trace elements, and like incompatible trace element heterogeneity, there is growing evidence that it may vary abruptly in space and time. The existence of these compositionally contrasted basalt suites must be considered in petrogenetic modelling; specifically, they will introduce scatter in most generalized variation diagrams, and will increase the likelihood of “misfits” in fractionation calculations utilizing parent and residual compositions drawn from different suites.     

Exploring the geophysical and socio-economic determinants of land cover changes in Eastern Mau forest reserve and Lake Nakuru drainage basin,Kenya

Understanding the linkages between the biogeophysical and socio-economic processes that operate at different spatial and temporal scales is important for land cover change mitigation. This study analysed several factors that explained the forest-shrubland conversions, grassland conversions and cropland expansions in Lake Nakuru drainage basin and Eastern Mau forest reserve in Kenya from 1985 to 2011. Logistic regression models were developed using a combination of remote sensing-based land cover data, and geographical information systems-based geophysical and socio-economic data (i.e., temperature, rainfall, elevation, slope, aspect, topographic wetness, curvature, soil pH, soil cation exchange capacity (CEC), population density and distance to road, river and town). The results were varied; for example, in the period 1985–2000, forest-shrubland conversions were linked to distance to road), distance to town, soil pH, soil CEC, rainfall, topographic wetness, curvature and aspect. The same factors, in addition to slope and distance to river, also determined the likelihood of forest-shrubland conversions in the period 2000–2011. Overall, significance of the determining factors varied depending on time and nature of land cover change. For example, topographical factors influenced grassland conversions in the period 1985–2000, while soil-related factors did not. But in the period 2000–2011, the converse was true. Therefore, policies for restoration, conservation and sustainable management of critical ecosystems (e.g., forests) should be spatially targeted and time-specific. These results broaden our knowledge of land cover dynamics in this locality, and provide a base for effective environmental policy formulation, planning and management.     

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation,southeastern California

ABSTRACT

Stable isotopes combined with pre-existing ⁴⁰Ar/³⁹Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ¹⁸O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ¹⁸O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ¹⁸O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.     

Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano,Oregon

Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H₂O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na₂O content of 6.5 wt% at 67 wt% SiO₂. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H₂O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H₂O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H₂O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H₂O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H₂O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H₂O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H₂O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H₂O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H₂O, which may be used to look for “damp” fractionation sequences elsewhere.     

Evidence of hydrous differentiation and crystal accumulation in the low-MgO,high-Al2O3 Lake Basalt from Medicine Lake volcano,California

The late Pleistocene Lake Basalt of Medicine Lake volcano, California is comprised of variably porphyritic basalt and basaltic andesite flows and scoria. These eruptives are similar in composition and phenocryst abundance to the low-MgO, high-Al²O³ mafic magmas common in convergent margin settings. The petrogenesis of the magmas that produced the Lake Basalt has been inferred from field relations, melting experiments and subsequent major and trace element modeling. Their formation involved both hydrous differentiation and plagioclase accumulation and thus the Lake Basalt can be used to constrain the relative contributions of these processes to the production of high-Al²O³ arc basalt. Phenocryst-poor lavas of the Lake Basalt formed by hydrous differentiation; their compositions and observed phenocrysts were reproduced in 1 kbar, H²O-saturated melting experiments. Anorthite-rich plagioclase compositions of the lavas of the Lake Basalt necessitate crystallization from melts with between 4 and 6 wt% dissolved H²O. Phenocryst-rich lavas of the Lake Basalt, with 18 modal% phenocrysts and greater, formed by plagioclase accumulation in magmas similar to the phenocryst-poor lavas. This interpretation is supported by the depleted incompatible element abundances and enriched Sr/Zr ratio of the more porphyritic lavas relative to the phenocryst-poor lavas. We model the formation of the Lake Basalt as a two-stage process that combines a differentiation model and a plagioclase accumulation model. Stage one involved hydrous fractionation, granitic assimilation and mixing with undifferentiated parent magma. This process generated lavas with up to 19.2 wt% A1²O³ and 7 modal% phenocrysts. In stage two, plagioclase accumulated in these liquids and produced more aluminous and porphyritic lavas with up to 21.8 wt% A1²O³ and 33 modal% phenocrysts.     

Bestimmung absoluter Eigenbewegungen in einem KSZ-Feld mit zwei Astrographen

In the field No. 153 of the KSZ program relative proper motions of 144 stars up to 15^m.0 are presented as well as parameters for the reduction to absolute with reference to 6 galaxies. The external error of one proper motion is 0.014″/y. It is shown that in a small field the statistical method for the estimation of the magnitude error is not able to give significant results due to the variance of the proper motion. It is concluded that in practice the long focus and the wide angle astrographs of the Main Observatory of the Ukrainian Academy of Science in Kiev can be combined for the derivation of proper motions.