Iron Isotope Fractionation During Magmatism And Metamorphism In Mid-ocean Ridge And Subduction Zone:Implications For Geodynamic Processes

The rapid technological development in mass spectrometry has allowed determination of non-traditional stable isotopes of many elements,which have been used to“trace”earth processes.However,without fully understanding fractionation mechanisms of these stable isotopes,we are not yet ready to use them to“trace”or“constrain”any process.Among elements of these non-traditional stable isotopes,iron(Fe)is a major element in most rocks with mature analytical methods.In this thesis,I focus on Fe isotopes on a number of rock suites of global significance to study possible mechanisms of Fe isotope fractionation,which in turns offers new perspectives on the petrogenesis of these rocks with general significance:(1)Using ODP Hole 735B“magma chamber”rocks,I have demonstrated that the Fe isotope contrast between mid-ocean ridge basalts(MORB)and abyssal peridotites(AP)is a consequence of fractional crystallization recorded in these rocks.The?⁵⁶Fe values of these gabbroic“magma chamber”rock samples define a significant inverse curvilinear correlation with bulk rock Mg O/Fe O^T,demonstrating that?⁵⁶Fe in the evolving MORB melt increases as fractional crystallization proceeds,and contrary to expectation,?⁵⁶Fe continues to increase even when oxides begin to crystallize.(2)Using the well-characterized northern(17 to 23°N)Mariana Trough basalts(MTB),I have demonstrated that,from north to south,the along-trough-axis MTB?⁵⁶Fe variation,which correlates with incompatible element abundances and Sr-Nd-Pb-Hf radiogenic isotopes,represents a tectonic response to the back-arc basin development with time from initial arc-magmatism dominated by sub-arc lithospheric mantle melting with significant slab material contributions to back-arc rifting and to back-arc spreading center magmatism characterized by increasing asthenospheric mantle melting,decreasing slab material contribution and diminishing sub-arc lithospheric mantle participation.This study may be of global significance.(3)Using the plagiogranite samples of the classic Troodos ophiolite,I have demonstrated the significant Fe isotope fractionation during magmatic fluid metasomatism recorded in epidote of magmatic fluid origin.Detailed petrography and mineral compositional analysis on these epidote plagiogranites show that the epidote plagiogranite are crystallized from a highly dissolving agent that transforms existing minerals to precipitate new minerals like epidote.The Fe isotope composition of epidote is much heavier than plagioclase and bulk-rock.The heavy Fe isotope composition of epidote suggests that the epidote inherits the Fe³⁺and heavy Fe isotope composition from residual hydrous melt or silicic fluids.