Thermal model of successive dike injections and implications for the development of intraplate volcanoes

Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (N1050°) to avoid plagioclase crystallization. Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (N1 month), high-temperature conditions (N1000 °C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand,magma flow times of one to several months, combined with short time intervals between dike injections (b25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allowthe production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton. Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation forwhy lavas ofmafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde Archipelago). In extreme cases, when temperatures over 1000–1050 °C in the central part of the feeding zone are maintained for years, the remaining magma in the conduit does not solidify but is preserved in a mushy state. New pulses of magma would not be able to cross this zone butwould rather amalgamate in the incipientmagma reservoir. The present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface. Depending on the time interval between dike injections and the duration ofmagma flow through the crust, magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs. This process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity

Magma Differentiation in Vertical Conduits Revealed by the Complementary Study of Plutonic and Volcanic Rocks from Fuerteventura (Canary Islands)

Fractional crystallization is a widely accepted mechanism to account for magmatic differentiation, but the location and geometry of the locations where this process takes place are still largely unknown. This is mainly due to the scarcity of occurrences where direct links between deep-seated cumulate rocks and their associated differentiation products are observed. The island of Fuerteventura in the Canary Archipelago offers such an opportunity. A dismembered Miocene volcano reveals a complex plutonic root zone, named PX1, consisting of a network of feeder conduits that can be related to remnants of contemporaneous lava flows. The lavas reveal a typical alkaline differentiation trend, whereas PX1 is composed of vertically layered cumulate lithologies ranging from olivine-rich wehrlites to clinopyroxenites and gabbros. Clinopyroxene phenocrysts in lavas (73·2–86·5 Mg#) exhibit the same zoning features and compositional ranges in major and trace elements as the plutonic clinopyroxenes, suggesting a direct genetic link between plutonic and effusive rocks. We interpret the PX1 rocks as resulting from the accumulation in volcanic conduits of phenocrysts segregated from ascending crystal-bearing magmas. Semi-quantitative modeling shows that fractional crystallization can indeed explain the evolution of the erupted lavas from basalt to basaltic trachyandesite. Progressive fractionation of the mineral assemblages olivine → olivine + minor clinopyroxene → clinopyroxene → clinopyroxene + plagioclase → plagioclase + kaersutite + apatite, with variable amounts of Fe–Ti oxides, can reproduce the major element compositional trend revealed by the Fuerteventura lavas. This calculated sequence coincides with the sequence of crystallization deduced from the textures of the PX1 rocks and could a priori generate the mineral assemblages that constitute the plutonic cumulates. However, complex core–mantle chemical zoning observed in PX1 clinopyroxenes suggests more complicated differentiation processes than simple crystal segregation and accumulation in vertical conduits. Primitive cores in PX1 clinopyroxenes (Mg# 80–88) display resorption features and sieve textures; they are interpreted as pre-existing crystals entrained from deeper levels or crystallized during the early stages of magma evolution. Clinopyroxene mantles display more evolved compositions (Mg# 74–80) with reverse zoning and external resorption features. Asymmetry in chemical zoning suggests crystallization in a confined environment such as a crystal mush. All these observations point to progressive mineral coarsening in the crystal cumulates, induced by the pervasive percolation of uprising melts on their way to the surface. The main factors controlling the variability of the cumulate lithologies are the crystal–melt segregation efficiency, the degree of differentiation and modal proportions of the magmas at their time of intrusion, and the extraction efficiency of the residual melts from the cumulates. The proposed model suggests that a differentiation process could take place in the feeder conduits and does not require storage of significant magma volumes (or magma chambers) in the crust to explain the observed evolution from mafic to intermediate magmas. Nevertheless, the existence of such large magma reservoirs is not excluded, in particular to explain the generation of significant volumes of differentiated lavas, as observed in other Canary Islands. Magma differentiation in vertical conduits opens up new perspectives in the understanding of fractional crystallization processes associated with the evolution of alkaline mafic magmas in ocean islands and continental intraplate settings

Interface modification to optimize charge separation in cyanine heterojunction photovoltaic devices

We investigate heterojunction photovoltaic devices using the carbocyanine 1,1'-diethyl-3,3,3',3'-tetramethylcarbocyanine perchlorate (Cy5) as donor and buckminsterfullerene (C60) as acceptor. We find that photocurrent generation occurs at the interface between CY5 and indium tin oxide (ITO) as well as at the organic heterointerface. By analyzing the spectral dependence of the photocurrent as a function of applied voltage, we were able to demonstrate that poly(3,4-ethylenedioxythiophene) (PEDOT) inhibits electron injection from the cyanine into ITO. Since the photocurrent generation at the ITO electrode is opposite to the one generated at the organic heterojunction, the use of PEDOT leads to increased short-circuit current and open-circuit voltage. [All rights reserved Elsevier

Emotions and motivation in mathematics education: theoretical considerations and empirical contributions

Emotions and motivation are important prereq-uisites, mediators, and outcomes of learning and achieve-ment. In this article, we first review major theoretical approaches and empirical findings in research on students\u27 emotions and motivation in mathematics, including a dis-cussion of how classroom instruction can support emo-tions and motivation. Based on this review, we encourage researchers from mathematics education and other dis-ciplines of educational research to combine their efforts. Second, we provide an overview of the contributions in this special issue, most of which reflect such a combination of efforts by considering perspectives from both mathematics education and other fields of educational research. Finally, we consider the neglect of intervention studies and outline directions for future research. We identify intervention studies that target emotions and motivation as one promis-ing but so far underrepresented line of research in mathe-matics education and review results from existing interven-tion studies. For future research, we suggest that researchers should implement fine-grained concepts, assessment instru-ments, theoretical hypotheses, and methods of analysis tai-lored to the specific features of the mathematical domain to adequately investigate students\u27 emotions and motivation in this domain