碩士 / 國立臺灣大學 / 海洋研究所 / 106 / Regarding lithium has high solubility in liquid phase and its two stable isotopes, 6Li and 7Li, significantly fractionate in the near-surface water-rock interaction, lithium isotopic ratios of arc volcanic rocks have the potential as a tracer of fluid in crust-mantle recycling. However, recent studies suggested that the input of sediments into the mantle wedge can also play a critical role in the Li abundances of arc volcanic rocks. The Coastal Range is the northernmost part of the Northern Luzon arc which has long been demonstrated as one of the arc systems with involvement of sediments in their magma sources, and thus its volcanic rocks are studied here to further examine the effects of sediments on behavior of Li in arc volcanic rocks.
Remarkable crustal contamination in the Coastal Range volcanic rocks has long been known. The integration of zircon U-Pb ages and Sr-Nd isotopic data of the Coastal Range volcanic rocks is first proposed in this study to successfully divide samples into two groups and to rule out samples in the group with relatively higher 87Sr/86Sr comparing with Nd isotopic values, resulted from crustal contamination. Then, Li concentrations and isotopic ratios, represented as δ7Li, of the remaining samples were analyzed by ICP-QMS (Agilent 7700x) and MC-ICP-MS (Nu Plasma II), respectively.
The variations of both Li/Y and δ7Li reflect the geochemistry of magma sources in the mantle wedge because these two parameters are not significantly modified during fractional crystallization and partial melting. Li/Y (0.4 ~ 1.4) and δ7Li (-1.3 ~ +7.8‰) in the Coastal Range volcanic rocks are significantly variable and different from those of MORB (Li/Y = 0.2; δ7Li = +3.7 ± 1.0). Although Li/Y ratios conventionally are treated as a tracer of fluid in arc magmatism, they show no correlation with other fluid proxies (e.g., Ba/Th, Sr/Th, and U/Th) in our samples. On contrary, Li/Y ratios become higher not only with time, but also increasing values of sediment-melt proxies (e.g., Th/Ce, Th/Nb, and Th/La) and decreasing ℇNd(T). This demonstrates the involvement of sediment melt, with higher Li/Y than that of MORB, in magma source of the Coastal Range arc volcanic rocks. Li isotopic values of our samples do not vary with their corresponding Li contents. Mixing models of Li-Nd isotopes suggest that δ7Li values in samples with higher ℇNd(T) were essentially influenced by slab fluids derived from sediments and altered oceanic crust, while those in samples with lower ℇNd(T) show a relatively limited δ7Li range and were dominated by melt of subducted sediments.
The results of this study can also be applied to those of the arc magmatism in the Martinique island arc, the only reported case that so far has the input of sediments to the mantle wedge and also has Li isotopic data reported. We further suggest that both the melt and fluids from subducted sediments can play key roles in the evolution of Li in subduction zone.
| Identifer | oai:union.ndltd.org:TW/106NTU05279025 |
| Date | January 2018 |
| Creators | Chia-Chi Hsiao, 蕭家其 |
| Contributors | Mei-Fei Chu, 朱美妃 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 75 |
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