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Crystal-melt reaction within the lower oceanic crust I
Magma batches underplate, ascend, stall and erupt along oceanic spreading ridges. Igneous complexes are characterised by FC and MASH processes. You will examine crystal-scale records of partial melting in lower crust gabbroic cumulates from the Pacific fast-spreading ridge.
Keywords: Spreading ridge, magma mixing, gabbro, clinopyroxene, chemical zoning, microtexture, geochemistry, MORB
For the last four decades, igneous petrologists, geochemists and structuralists have studied active spreading ridges. Their goal was to understand the mantle composition, melting processes and melt migration mechanisms leading to the formation of Mid-Ocean Ridge Basalt (MORB), representing ≈75% of the lavas annual budget. The primitive MORB derive from magmas formed by partial melting of the upwelling upper mantle peridotites and are commonly accepted to provide direct information into mantle processes. These magmas show a large scatter in major, trace and isotopic compositions, due to 1) variable degree of partial melting of the source, 2) the source heterogeneity (lherzolite, harzburgite, pyroxenite), 3) source metasomatism and water content, 4) pressure of melting, 5) porous flow reaction between ascending melts and the mantle, 6) fractional crystallization and 7) magma mixing. Additionally, it has recently been suggested that reactions may occur between the gabbroic lower crust and migrating melts from a new asthenospheric diapir. This process is efficient and quick, but is usually neglected or treated in a simplistic way in global geochemical models.
In crystallizing magma chambers, co-existing crystals and interstitial liquid are ideally in chemical and textural equilibrium. However, if permeability enables it, a newly injected reactive melt could percolate along grain boundaries or within melt channels. The percolating melt will dissolve pre-existing crystals in an attempt to attain equilibrium, mix with interstitial liquid, saturate, and crystallize new or secondary minerals. Thus, its physical and chemical characteristics will be progressively modified, resulting in a hybrid product that will differ from the one produced by simple crystallisation along a liquid line of descent. This complex process is named reactive liquid flow (Lissenberg et al., 2013 EPSL; Leuthold et al., 2014 CMP; Coumans et al., 2016 GCA). Melt-crystals reactions recognition is critical in lavas and magma chambers worldwide to avoid incorrect petrogenetic models stemming from incorrect equilibrium premises. The assimilation of mafic crystals/cumulates by mafic melts is difficult to detect because of the lack of contrasts (mineralogy, chemistry, isotopes) between the reactants. Thus, it requests a detailed study, using a multi-disciplinary approach and state-of-the-art technology.
For the last four decades, igneous petrologists, geochemists and structuralists have studied active spreading ridges. Their goal was to understand the mantle composition, melting processes and melt migration mechanisms leading to the formation of Mid-Ocean Ridge Basalt (MORB), representing ≈75% of the lavas annual budget. The primitive MORB derive from magmas formed by partial melting of the upwelling upper mantle peridotites and are commonly accepted to provide direct information into mantle processes. These magmas show a large scatter in major, trace and isotopic compositions, due to 1) variable degree of partial melting of the source, 2) the source heterogeneity (lherzolite, harzburgite, pyroxenite), 3) source metasomatism and water content, 4) pressure of melting, 5) porous flow reaction between ascending melts and the mantle, 6) fractional crystallization and 7) magma mixing. Additionally, it has recently been suggested that reactions may occur between the gabbroic lower crust and migrating melts from a new asthenospheric diapir. This process is efficient and quick, but is usually neglected or treated in a simplistic way in global geochemical models. In crystallizing magma chambers, co-existing crystals and interstitial liquid are ideally in chemical and textural equilibrium. However, if permeability enables it, a newly injected reactive melt could percolate along grain boundaries or within melt channels. The percolating melt will dissolve pre-existing crystals in an attempt to attain equilibrium, mix with interstitial liquid, saturate, and crystallize new or secondary minerals. Thus, its physical and chemical characteristics will be progressively modified, resulting in a hybrid product that will differ from the one produced by simple crystallisation along a liquid line of descent. This complex process is named reactive liquid flow (Lissenberg et al., 2013 EPSL; Leuthold et al., 2014 CMP; Coumans et al., 2016 GCA). Melt-crystals reactions recognition is critical in lavas and magma chambers worldwide to avoid incorrect petrogenetic models stemming from incorrect equilibrium premises. The assimilation of mafic crystals/cumulates by mafic melts is difficult to detect because of the lack of contrasts (mineralogy, chemistry, isotopes) between the reactants. Thus, it requests a detailed study, using a multi-disciplinary approach and state-of-the-art technology.
With this M.Sc. thesis projects, you will describe and quantify the effect of melt-crystals reactions on the mafic lower crust cumulates and the percolating MORB, using complementary petrological and geochemical approaches. The outcomes will offer a new vision on the evolution of MORB, the most abundant magma on Earth, with important consequences for spreading ridges processes and mantle geochemistry and geodynamic.
During this project, you will learn how to read papers and use published data, how to write a scientific proposal, present and defend your ideas and write a final report. You will learn how to acquire, reduce and interpret data acquired using thin section microscopy and SEM, EPMA, LA-ICP-MS laboratories. I have strong experience with those techniques and will provide all the necessary assistance and supervision.
I offer two Master thesis research projects to motivated students, on igneous petrology and geochemistry (1) and on experimental petrology (2). They can be done in parallel or individually (there is no competition and no overlap, but possible collaboration).
Contact me directly to discuss those projects and your ideas!
With this M.Sc. thesis projects, you will describe and quantify the effect of melt-crystals reactions on the mafic lower crust cumulates and the percolating MORB, using complementary petrological and geochemical approaches. The outcomes will offer a new vision on the evolution of MORB, the most abundant magma on Earth, with important consequences for spreading ridges processes and mantle geochemistry and geodynamic. During this project, you will learn how to read papers and use published data, how to write a scientific proposal, present and defend your ideas and write a final report. You will learn how to acquire, reduce and interpret data acquired using thin section microscopy and SEM, EPMA, LA-ICP-MS laboratories. I have strong experience with those techniques and will provide all the necessary assistance and supervision. I offer two Master thesis research projects to motivated students, on igneous petrology and geochemistry (1) and on experimental petrology (2). They can be done in parallel or individually (there is no competition and no overlap, but possible collaboration). Contact me directly to discuss those projects and your ideas!
Dr. Julien Leuthold
Inst. of Geochemistry and Petrology
Office NO F 74
Clausiusstrasse 25
8092 ETH Zürich
Switzerland
+41 (0)44 632 43 19
julien.leuthold@erdw.ethz.ch
https://www.erdw.ethz.ch/en/people/profile.html?persid=210267
Dr. Julien Leuthold Inst. of Geochemistry and Petrology Office NO F 74 Clausiusstrasse 25 8092 ETH Zürich Switzerland +41 (0)44 632 43 19 julien.leuthold@erdw.ethz.ch https://www.erdw.ethz.ch/en/people/profile.html?persid=210267