From Binary Mixing to Magma Chamber Simulator
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Abstract:
The interactions of magmas with their surroundings are important in the evolution of igneous systems and the crust. In this chapter, we conceptually distinguish assimilation from other modes of magmatic interaction and discuss a range of geochemical assimilation models. We define assimilation in its simplest form as an end-member mode of magmatic interaction in which an initial state ( t 0 ) that includes a system of melt and solid wall rock evolves to a later state ( t n ) where the two entities have been homogenized. In complex natural systems, assimilation can refer more broadly to a process where a mass of magma wholly or partially homogenizes with materials derived from wall rock that initially behaves as a solid. The first geochemical models of assimilation used binary mixing equations and then evolved to take account mass balance and fractional crystallization. Most recent tools, such as the Magma Chamber Simulator, treat open systems thermodynamically in order to simulate geochemical changes in crystallizing magma and partially melting wall rock. Such thermodynamic considerations are a prerequisite for understanding the consequences of assimilation. The geochemical signatures of magmatic systems—although dominated for some elements (particularly major elements) by crystallization processes—may be considerably influenced by simultaneous assimilation of partial melts of compositionally distinct wall rock.Keywords:
Fractional crystallization (geology)
Igneous differentiation
Magma chamber
Assimilation (phonology)
Igneous differentiation
Phenocryst
Fractional crystallization (geology)
Silicic
Trace element
Pigeonite
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Magma chamber
Silicic
Fractional crystallization (geology)
Phenocryst
Igneous differentiation
Layered intrusion
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Abstract Magma differentiation in arc settings has usually been attributed to an interplay of processes (fractional crystallization, assimilation, and magma mixing). Homogeneous fractional crystallization has been widely used to model the magmatic evolution of volcanic systems in arc settings due to its simplicity, even though boundary layer fractionation (BLF) has been proposed as a preponderant process of differentiation in hydrous magmatic systems. Both models produce distinct compositional paths and the application of the wrong model yields erroneous estimates of parameters like pressure–temperature-H2O conditions and primary melt compositions. Melt inclusion (MI) populations corrected for post-entrapment processes have the potential to help discriminate between these two types of fractional crystallization, as their compositions are not affected by crystal accumulation and should capture the magmatic evolution as crystallization occurs. In this study, olivine-hosted MIs are used to assess the differentiation trends of basic arc magmas in northern Japan. Differentiation trends from five arc volcanic systems in northern Japan show that BLF is ubiquitous. Homogeneous fractionation models are unable to explain the liquid lines of descent of minor elements, like TiO2 and P2O5. To reproduce these differentiation trends, the presence of accessory phases like titanomagnetite or apatite are required, which in many cases are not equilibrated by the melt or need to be fractionated in amounts that are incompatible with homogeneous fractionation. The prevalence of BLF in all studied arc magmas of northern Japan indicates that solidification fronts are key environments in the crustal evolution of some hydrous subduction zone magmas.
Igneous differentiation
Fractional crystallization (geology)
Melt inclusions
Magma chamber
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PetroGram is an Excel© based magmatic petrology program that generates numerical and graphical models. PetroGram can model the magmatic processes such as melting, crystallization, assimilation and magma mixing based on the trace element and isotopic data. The program can produce both inverse and forward geochemical models for melting processes (e.g. forward model for batch, fractional and dynamic melting, and inverse model for batch and dynamic melting). However, the program uses a forward modeling approach for magma differentiation processes such as crystallization (EC: Equilibruim Crystallization, FC: Fractional Crystallization, IFC: Imperfect Fractional Crystallization and In-situ Crystallization), assimilation (AFC: Assimilation Fractional Crystallization, Decoupled FC-A: Decoupled Fractional Crystallization and Assimillation, A-IFC: Assimilation and Imperfect Fractional Crystallization) and magma mixing. One of the most important advantages of the program is that the melt composition obtained from any partial melting model can be used as a starting composition of the crystallization, assimilation and magma mixing. In addition, PetroGram is able to carry out the classification, tectonic setting, multi-element (spider) and isotope correlation diagrams, and basic calculations including Mg#, Eu/Eu∗, εSr and εNd widely used in magmatic petrology.
Fractional crystallization (geology)
Igneous differentiation
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Fractional crystallization (geology)
Magma chamber
Igneous differentiation
Scoria
Silicic
Phenocryst
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Magma chamber
Igneous differentiation
Fractional crystallization (geology)
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Igneous differentiation
Fractional crystallization (geology)
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Here we present a perspective on the evolution of thought on the origin of compositional diversity in igneous rocks, starting with the pioneer Norman Levi Bowen. In pursing this question of diversity, which was first clearly identified by Daly (1914), Bowen established the utility of experimentally determined phase equilibria as an aid to understanding geologic processes. His work ultimately led him to attribute igneous rock diversity to a singular path of fractional crystallization. We summarize the evolution of understanding acquired by petrologists during and after Bowen9s time. Experimentalists beyond Bowen were crucial in furthering the understanding of the origin of the diversity of igneous rocks by discovering that more than one fractional crystallization path can occur in nature: at a minimum, differentiation can either be dry (tholeiitic) or hydrous (calc-alkaline). We also reassess the five alternative igneous processes that may give rise to compositional diversity that Bowen considered, but found to be wanting. These are magma mixing, liquid immiscibility, Soret diffusion, compositional gradients in liquids, and contamination of magma by foreign material (assimilation). These processes play important roles in igneous petrogenesis, that is, roles larger than Bowen envisioned, yet fractional crystallization remains fundamentally important.
Petrogenesis
Fractional crystallization (geology)
Igneous differentiation
Large igneous province
Igneous petrology
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Fractional crystallization (geology)
Magma chamber
Igneous differentiation
Caldera
Silicic
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Fractional crystallization (geology)
Magma chamber
Incompatible element
Igneous differentiation
Crystal (programming language)
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