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Crystal-melt reaction within the lower oceanic crust II
Magma batches underplate, ascend, stall and erupt along oceanic spreading ridges. Igneous complexes are characterised by FC and MASH processes. You will run fractional crystallization experiments to determine the liquid line of descent of mantle-derived melt.
Keywords: Spreading ridges, magma mixing, reaction, experimental petrology, gabbro, basalt
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.
Trace elements are widely used in igneous geochemistry. They are employed in crystallization (e.g. dePaolo, 1981; Jagoutz, 2010) and melting (e.g. Kelemen et al., 1997 [PTRSL]) models, to estimate the chemistry of liquids from which crystals grew (e.g. Blundy & Shimizu, 1991; Kelemen et al., 1997 [EPSL]; Leuthold et al., 2014), to constrain magma source compositions (e.g. Pearce et al., 1984). Igneous rocks very frequently show cumulative texture and signature (i.e. zoned phenocrysts in cumulates or in lavas). The parental melt chemistry may be difficult to measure, but can be retrieved from experimentally-determined partition coefficients and crystal chemical analyses. Partition coefficients depend on melt chemistry and structure (e.g. Kohn and Schofield, 1994; Blundy et al., 1995, 1996; Gaetani, 2004; Prowatke and Klemme, 2006), crystal chemistry (e.g. Blundy and Wood, 1994, 2003; Gaetani and Grove, 1995; Mollo et al., 2016), temperature and pressure (e.g. Wood & Blundy, 1997; Hill et al., 2011; Sun and Liang, 2012) and water content (e.g. Green et al., 2000; Wood and Blundy, 2001; Gaetani, 2003), that will change upon fractional crystallization.
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. Trace elements are widely used in igneous geochemistry. They are employed in crystallization (e.g. dePaolo, 1981; Jagoutz, 2010) and melting (e.g. Kelemen et al., 1997 [PTRSL]) models, to estimate the chemistry of liquids from which crystals grew (e.g. Blundy & Shimizu, 1991; Kelemen et al., 1997 [EPSL]; Leuthold et al., 2014), to constrain magma source compositions (e.g. Pearce et al., 1984). Igneous rocks very frequently show cumulative texture and signature (i.e. zoned phenocrysts in cumulates or in lavas). The parental melt chemistry may be difficult to measure, but can be retrieved from experimentally-determined partition coefficients and crystal chemical analyses. Partition coefficients depend on melt chemistry and structure (e.g. Kohn and Schofield, 1994; Blundy et al., 1995, 1996; Gaetani, 2004; Prowatke and Klemme, 2006), crystal chemistry (e.g. Blundy and Wood, 1994, 2003; Gaetani and Grove, 1995; Mollo et al., 2016), temperature and pressure (e.g. Wood & Blundy, 1997; Hill et al., 2011; Sun and Liang, 2012) and water content (e.g. Green et al., 2000; Wood and Blundy, 2001; Gaetani, 2003), that will change upon fractional crystallization.
With this M.Sc. thesis project, you will describe and quantify the effect of fractional crystallization on mantle-derived melt at spreading ridges settings, using experimental petrology. The outcomes will offer an important database on trace element partitioning during MORB fractional crystallization (similar to Nandedkar et al., 2016 CMP for arc magmatism). Results will be used to calculate the range of trace element geochemical evolution along the normal liquid line of descent of mantle-derived melt and check whether shifts are due to crystal-melt reaction. In addition, I expect we will constrain the conditions for silicate-iron melts segregation and the petrogenesis of oxide-gabbro and plagiogranite (see Charlier and Grove, 2012 CMP for hot spot setting).
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 from one-atmosphere furnace, SEM and EPMA laboratories. You will also use a Linkam heating stage, where you can observe gabbro melting live! 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 project, you will describe and quantify the effect of fractional crystallization on mantle-derived melt at spreading ridges settings, using experimental petrology. The outcomes will offer an important database on trace element partitioning during MORB fractional crystallization (similar to Nandedkar et al., 2016 CMP for arc magmatism). Results will be used to calculate the range of trace element geochemical evolution along the normal liquid line of descent of mantle-derived melt and check whether shifts are due to crystal-melt reaction. In addition, I expect we will constrain the conditions for silicate-iron melts segregation and the petrogenesis of oxide-gabbro and plagiogranite (see Charlier and Grove, 2012 CMP for hot spot setting). 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 from one-atmosphere furnace, SEM and EPMA laboratories. You will also use a Linkam heating stage, where you can observe gabbro melting live! 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