Estimation of thermal structure in the mantle wedge of northeastern Japan from seismic attenuation data

Abstract Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H2O. The MSP olivine is too forsterite-rich (Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima.

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Exhumation of subducted mafic rocks in a dynamically evolving thermal structure: constraints from phase equilibria modelling

10.5194/egusphere-egu21-13868 ◽

2021 ◽

Author(s):

Rilla C. McKeegan ◽

Victor E. Guevara ◽

Adam F. Holt ◽

Cailey B. Condit

Keyword(s):

Phase Equilibria ◽

Subduction Zone ◽

Subduction Zones ◽

Mantle Wedge ◽

Dynamic Models ◽

Thermal Structure ◽

Temporal Dependence ◽

Subduction Channel ◽

Mafic Rocks ◽

Point Of No Return

The dominant mechanisms that control the exhumation of subducted rocks and how these mechanisms evolve through time in a subduction zone remain unclear. Dynamic models of subduction zones suggest that their thermal structures evolve from subduction initiation to maturity. The series of metamorphic reactions that occur within the slab, resultant density, and buoyancy with respect to the mantle wedge will co-evolve with the thermal structure. We combine dynamic models of subduction zone thermal structure with phase equilibria modeling to place constraints on the dominant controls on the depth limits of exhumation. This is done across the temporal evolution of a subduction zone for various endmember lithologic associations observed in exhumed high-pressure terranes: sedimentary and serpentinite m&#233;langes, and oceanic tectonic slices.Initial modeling suggests that both serpentinite and sedimentary m&#233;langes remain positively buoyant with respect to the mantle wedge throughout all stages of subduction (up to 65 Myr), and for the spectrum of naturally constrained ratios of mafic blocks to serpentinite/sedimentary matrix. In these settings, exhumation depth limits and the &#8220;point of no return&#8221; (c. 2.3 GPa) are not directly limited by buoyancy, but potentially rheological changes in the slab at the blueschist-eclogite transition stemming from: the switch from amphibole-dominated to pyroxene-dominated rheology and/or dehydration embrittlement. These mechanisms may increase the possibility of brittle failure and hence promote detachment of the slab top into the subduction channel. For the range of temperatures recorded by exhumed serpentinite m&#233;langes, the locus of dehydration for altered MORB at the slab top coincides with the point of no return (2.3 GPa) between 35 and 40 Myr, suggesting a strong temporal dependence on deep exhumation in the subduction channel.&#160;Tectonic slices composed of 50% mafic rocks and 50% serpentinized slab mantle show a temporal dependence on the depth limits of positive buoyancy. For the range of temperatures recorded by exhumed tectonic slices, the upper pressure limit of positive buoyancy is ~2 GPa, and is only crossed between ~30 and 40 Myr after subduction initiation. Some exhumed tectonic slices record much higher pressures (2.5 GPa); thus, other mechanisms or lithologic combinations may also play a significant role in determining the exhumation limits of tectonic slices.&#160;Future work includes constraining how the loci of dehydration vary through time for different degrees of oceanic crust alteration, how exhumation limits and mechanisms may change with different subducting plate ages, and calculating how initial exhumation velocities may vary through time. Further comparison with the rock record will constrain the parameters that control the timing and limits of exhumation in subduction zones.

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Validation and Evaluation of an Estimation Method for Deep Thermal Structures Using an Activity Index in Major Geothermal Fields in Northeastern Japan

Energies ◽

10.3390/en13184684 ◽

2020 ◽

Vol 13 (18) ◽

pp. 4684

Author(s):

Yota Suzuki ◽

Hirofumi Muraoka ◽

Hiroshi Asanuma

Keyword(s):

Hot Springs ◽

Activity Index ◽

Thermal Structure ◽

Estimation Error ◽

Estimation Method ◽

Maximum Temperature ◽

Brittle Ductile Transition ◽

Geothermal Fields ◽

Northeastern Japan ◽

Isothermal Layer

A considerable number of rock bodies with varying percentages of supercritical fluid exist around the brittle–ductile transition (BDT) zone at a depth of several kilometers from the surface of the Earth, in northeastern Japan. As the BDT zone in the granitic basement of the continental crust is estimated to occur at about 380 °C, the identification of the depth corresponding to 380 °C is important to utilize the thermal energy inside the “supercritical geothermal systems”. In this study, we focused on an estimation method to determine the depth of the isothermal layer corresponding to 380 °C, using the activity index (AI) obtained from the maximum-temperature data of the geothermal wells and hot springs. The thermal profiles of deep and hot exploration boreholes and the hypocentral distribution of natural earthquakes were used to evaluate the characteristics and accuracy of the deep thermal structure, using the activity index. The estimated depth corresponding to 380 °C tended to be higher than the actual depth, with a maximum possible estimation error of approximately 9.7 km. Distribution maps showing the depth of the isothermal layer corresponding to 380 °C were created for six major geothermal fields in northeastern Japan, using the results from this study.

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