Olivine-hosted MIs as recorders of processes and conditions of slab dehydration and magmatic differentiation in the subduction zones of northern Japan : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
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2021
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Massey University
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Abstract
Subduction zones are the controlling mechanisms of geochemical cycling between the Earth’s crust and mantle. Transfer of incompatible trace elements and volatile species from the subducted slab to the mantle wedge by an H₂O-rich component triggers hydrous melting of the metasomatized mantle. This mechanism produces the magmatism that gives rise to volcanic arcs across the globe. The analysis of basic volcanic products from arc settings thus provides a window to study the processes involved in arc magmatism, from the mass transfer of slab to mantle wedge, partial melting, transport, differentiation and storage.
In particular, olivine-hosted MIs (OHMIs) provide a window to assess for early magmatic processes without the interference of the crystal cargo, as they are potentially less affected by differentiation processes. OHMIs have the additional advantage of having the potential to retain the volatile signatures (H₂O, CO₂, S, Cl and F) which are usually lost through degassing during ascent and eruption.
The subduction zones of northern Japan make for an excellent natural laboratory for the study of arc variations as they have been well characterized by seismic tomography work, making for more accurate estimations of the geometry and PT conditions of the subducted slab. There is a significant variation in the distance of the volcanic front to the trench, and in addition there is back-arc magmatism, with one volcano located extremely far from thetrench. This produces a broad distribution of PT conditions of the slab below each volcano, making it feasible to study across-arc variations related to progressive subduction.
This thesis addresses the problem of post-entrapment processes that can make the recovery of initial OHMI compositions difficult. In particular, the identification of complete Fe-Mg re-equilibration of long stored antecrystic olivine hosts impedes the recovery of the syn-entrapment melt composition from traditional reverse crystallization and diffusion models. The MushPEC algorithm is developed, a novel tool to recover original MI compositions from a set of cogenetic re-equilibrated OHMIs that evolved through simple fractional or equilibrium crystallization.
Next, the storage conditions and differentiation process recorded by MI populations from five arc-front samples of northern Japan are addressed. Homogeneous olivine compositions with a wide range of SiO₂ contents indicate that most olivine-hosts in arc front systems of northern Japan are antecrystic and have been completely re-equilibrated. Application of the MushPEC algorithm shows that the differentiation trends that the MI follow within each sample are incompatible with simple fractional/equilibrium crystallization and require boundary layer fractionation, where interstitial differentiated melts formed in highly crystallized solidification fronts are progressively extracted and incorporated into the main un-differentiated magmatic body. The estimated P-T-H₂O conditions of the MIs agree with boundary layer fractionation processes, promoted in hydrous melts stored at >100 MPa, as proposed by previous studies.
Further, the processes of material release from the slab to the source mantle are addressed, and how they vary with increased P-T conditions across the arc. Across-arc variations of large ion lithophile (LILE), rare earth (REE), high field strength elements (HFSE) and volatile ratios are correlated with the estimated PT conditions of the slab and the expected metamorphic reactions that release components enriched in trace and volatile elements. Constant LILE/HREE and volatile/HREE ratios in the arc front indicate homogeneous slab component compositions along the arc. Progressive enrichment of these ratios towards the back-arc indicates a new liquid source enriched in most LILEs and volatiles. This increase coincides with the expected initiation of the antigorite breakdown reaction in lithospheric serpentinites, providing enriched liquids involved in back-arc magmatism. Constant (Pb, Sr)/HREE from arc to back-arc points to lawsonite breakdown occurring at all depths. Progressive LREE/HREE increase towards the back-arc is explained by increasing LREE mobility with increasing T, which controls the solubility of LREE-rich accessory minerals (e.g., allanite). Finally, the composition of the slab liquid depleted of the LILE and halogen components released by serpentinites when serpentine is exhausted at c. 8 GPa, and strong enrichment of HFSE elements indicates the participation of supercritical liquids at these extreme depths of the slab.
This thesis highlights the importance of assessing the re-equilibration of OHMIs during long storage times, a process that is seldom discussed in the literature, the widespread occurrence of boundary layer fractionation in arc magmatic systems that needs to be taken into account when modelling differentiation processes in hydrous magmas, as application of simple fractional crystallization models will result in estimations of inaccurate magmatic conditions. This work also highlights the importance of the hydrated lithospheric mantle on the production of back-arc magmatism in island arc systems and the potential of halogen volatile phases on the tracking of slab fluid compositions.
Description
The following Figures have been removed for copyright reasons: 2.2 (=Martin, 2011 Fig 7); 2.3 (=Kita et al., 2014 Fig 1); 2.4 (=Kita et al, 2012 Fig 14); 2.5 (=Van Horne et al., 2017 Fig 2); 2.9 (Kimura & Yoshida, 2006 Fig 5(b); 2.10 (=Wallace, 2005 Fig 2); 2.11 (=Wallace et al., 2021 Fig 3(a); 2.12 (=Danushevsky et al., 2000 Figs 3(a) & (b); 2.13 (=Rowe et al., 2009 Figs 10(a) & (b); 2.14 & 2.15 (=Newman & Lowenstern, 2002 Figs 1(b) & (c) respectively. Figure 2.1 is re-used under a CC Attribution 4.0 International (CC BY 4.0) license. Figures copyrighted to The Geological Society remain because they are within the limits allowed provided that the source is properly acknowledged.
Keywords
Magmatism, Olivine, Subduction zones, Japan