Mineralogical Constraints on the Magma Mixing Beneath the Iheya Graben, an Active Back-Arc Spreading Centre of the Okinawa Trough

2020 ◽  
Author(s):  
Yu-Xiang Zhang ◽  
Zhi-Gang Zeng ◽  
Glenn Gaetani ◽  
Le Zhang ◽  
Zhi-Qing Lai
Keyword(s):  
Trace Element ◽  
Major Element ◽  
Magma Mixing ◽  
Okinawa Trough ◽  
Nd Isotope ◽  
Magma Plumbing System ◽  
The Okinawa Trough ◽  
Back Arc ◽  
Magma Plumbing

Abstract The Iheya Graben is a back-arc spreading centre in the middle part of the Okinawa Trough. It is also located in the centre of an anomalous volcanic zone (volcanic arc migration phenomenon, or VAMP) and is characterized by bimodal volcanism, unusually high heat flow and active hydrothermal circulation. The subvolcanic magma plumbing system and the magmatic processes related to the formation of rare erupted intermediate lavas in this area remain uncertain. In this study, we conducted systematic mineralogical analyses (in situ major element, trace element and Sr isotopes) and whole rock geochemical analyses (major element, trace element and Sr–Nd isotopes) on an andesite (T5-2; type C andesite) and a rhyolite (C11; type 2 rhyolite), and present evidence for magma mixing in the origins of these lavas. Andesite T5-2 contains a mafic mineral assemblage and a silicic mineral assemblage, which are derived from a basaltic melt and a type 2 rhyolitic melt, respectively. A 4:6 mixture of basalt and type 2 rhyolite from the Iheya Graben reproduces the whole-rock major element, trace element, and Sr–Nd isotope compositions of T5-2. Rhyolite C11 contains a group of disequilibrium minerals that crystallized from a less evolved rhyolitic melt with relatively more enriched Sr–Nd isotope compositions, suggesting mixing of this melt with a more evolved and isotopically more depleted rhyolitic melt. This mixing process could produce a series of rhyolitic melts with a negative correlation between SiO2 concentrations and 87Sr/86Sr ratios (or a positive correlation for 143Nd/144Nd ratios), which are recorded by the whole group of type 2 rhyolites. The results from mineral-based thermobarometers suggest that the premixing storage temperatures of the basaltic and rhyolitic melts are ∼1100 °C and 870–900 °C, respectively. The hybrid andesitic melt has temperatures of ∼950 to ∼980 °C. The magma storage pressures are not well constrained, ranging from ∼400 MPa to ∼100 MPa. We show that magma mixing plays a significant role in the origins of diverse volcanism in the middle Okinawa Trough; more specifically, two of the three types of andesites (types B and C) and one of the two types of rhyolites (type 2) are associated with magma mixing. We thus propose a complex magma plumbing system with multichamber magma storage and frequent magma mixing beneath the Iheya Graben.

A highly diverse silicic end member of a bimodal magma suite formed in an active continental back arc, Okinawa Trough

2021 ◽  
Author(s):  
Arran Murch ◽  
Kenichiro Tani ◽  
Takashi Sano ◽  
Shigekazu Yoneda
Keyword(s):  
Magma Mixing ◽  
Okinawa Trough ◽  
Fractional Crystallisation ◽  
Chemical Diversity ◽  
Crustal Assimilation ◽  
Arc Volcanism ◽  
The North ◽  
The Okinawa Trough ◽  
Formation Conditions ◽  
Back Arc

<p>The Okinawa Trough (OT) is an incipient continental back-arc basin that extends from Kyushu in the north to Taiwan in the south. The Okinawa Trough can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT) with active back-arc volcanism restricted to volcanic centres located in en-echelon grabens the MOT and SOT. Previous studies have shown magmatism in the OT is bimodal (basaltic to rhyolitic), with at least two types of silicic melts inferred to form through pure fractional crystallisation from basalt and by fractional crystallisation along with minor crustal assimilation (Shinjo and Kato, 2000).</p><p>Here we present petrological descriptions, along with major, trace element and Sr–Nd isotopic data for 75 silicic end member samples recovered as both lava and pumice, collected during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. Samples were dredged from various seafloor knolls and ridges located in the Io and Iheya grabens and from Izena Hole in the MOT, and from a single volcanic ridge in the Yaeyama graben and a single isolated knoll in the SOT.</p><p>Results show a chemically highly diverse silicic end member magmas, with at least four identifiable groups based on differences in the degree of enrichment of incompatible elements (LREE, K, Rb, Ba, etc.). Each group contains at least one dense lava sample suggesting the chemical diversity is a primary feature of magmatism in the Okinawa Trough rather than a result of the floating in of pumiceous material from various locations.</p><p>Using petrological descriptions and the chemistry of samples along with MELTS modelling we plan to calculate magma formation conditions and identify any evidence of magma mixing or crustal assimilation. In doing so we hope to provide a model to explain the diversity of silicic magma chemistry in the MOT and SOT.</p><p> </p><p>Shinjo, R., and Kato, Y. (2000). Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. Lithos 54, 117–137. doi:10.1016/S0024-4937(00)00034-7.</p>

The influence of a subduction component on magmatism in the Okinawa Trough: Evidence from thorium and related trace element ratios

2017 ◽  
Vol 145 ◽  
pp. 205-216 ◽  
Author(s):  
Kun Guo ◽  
Zhi-Gang Zeng ◽  
Shuai Chen ◽  
Yu-Xiang Zhang ◽  
Hai-Yan Qi ◽  
...  
Keyword(s):  
Trace Element ◽  
Okinawa Trough ◽  
Element Ratios ◽  
The Okinawa Trough ◽  
Subduction Component

scholarly journals Moho Geometry of the Okinawa Trough Based on Gravity Inversion and Its Implications on the Crustal Nature and Tectonic Evolution

2021 ◽  
Vol 9 ◽  
Author(s):  
Liang Zhang ◽  
Xiwu Luan
Keyword(s):  
Early Stage ◽  
Okinawa Trough ◽  
High Heat ◽  
Crustal Thickness ◽  
Moho Depth ◽  
Gravity Inversion ◽  
The Okinawa Trough ◽  
Oceanic Spreading ◽  
Thickness Variations ◽  
Back Arc

The Okinawa Trough (OT) is an incipient back-arc basin, but its crustal nature is still controversial. Gravity inversion along with sediment and lithospheric mantle density modeling are used to map the regional Moho depth and crustal thickness variations of the OT and its adjacent areas. The gravity inversion result shows that the crustal thicknesses are 17–22 km at the northern OT, 11–19 km at the central OT, and 7–19 km at the southern OT. Because of the crust with a thickness larger than 17 km, the slow southward arc movement, and scarce contemporaneous volcanisms, the northern OT should be in the stage of early back-arc extension. All of the moderate crustal thickness, high heat flow, and intense volcanism at the central OT indicate that this region is probably in the transitional stage from the back-arc rifting to the oceanic spreading. A crust that is only 7 km thick, lithosphere strength as low as the mid-ocean ridge, and MORB-similar basalts at the southern OT demonstrate that the southern OT is at the early stage of seafloor spreading.

Timescales of magma mixing in the magma plumbing system of Bezymianny volcano: insights from diffusion chronometry

2018 ◽  
pp. 52-58
Author(s):  
V. O. Davydova ◽  
V. D. Shcherbakov ◽  
P. Yu. Plechov
Keyword(s):  
Magma Mixing ◽  
Mafic Magma ◽  
Plumbing System ◽  
Magma Evolution ◽  
Bezymianny Volcano ◽  
Magma Plumbing System ◽  
Diffusion Chronometry ◽  
Shallow Magma Chamber ◽  
Trigger Mechanisms ◽  
Magma Plumbing

Zoned crystals record changes in magma evolution, such as injection of mafic magma into a shallow magma chamber, which often triggered to eruption of arc volcanoes. We applied diffusion chronometry for reverse zoned crystals of orthopyroxene from 6 eruptions of Bezymianny volcano during 2006–2012 years and showed correlation between time of injection of new magma and recorded seismicity. We descripted two type of Bezymianny volcano eruptions, which driven by different trigger mechanisms: 1) orthopyroxene rims formed during up to 3 years previous to eruption; 2) orthopyroxene rims formed during 0–2 months previous to eruption.

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