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Melt depletion and subsequent metasomatism in the shallow mantle beneath Koolau volcano, Oahu (Hawaii)

机译:夏威夷瓦胡岛库劳火山下面的浅层地幔中的融化耗竭和随后的交代作用

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摘要

Xenoliths from Pali (Oahu, Hawaii) include samples of the mantle lithosphere underlying Koolau shield volcano. Most such xenoliths are spinel peridotites, the remainder being plagioclase-spinel peridotites, and garnet-free pyroxenites. Clinopyroxene separates from Pali peridotite xenoliths have relatively depleted 87Sr/86Sr (0.70309–0.70346) and 143Nd/144Nd (0.512967–0.513206). The mantle lithosphere beneath the Koolau volcano has a range of present-day epsilonNd = 6.9–11. On a 143Nd/144Nd versus 147Sm/144Nd diagram, they define a 61 ± 20 Ma errorchron, within error of the estimated age of ~80–85 Ma of the Pacific lithosphere at Oahu, and an intercept of epsilonNd = 6.8. We interpret the Pali spinel peridotites as samples of the Pacific lithosphere residual to melt extraction of the Pacific crust. These rocks were not metasomatized by melts having isotopic composition similar to the shield-building Koolau volcano, but they could have been metasomatized by melts related to the Honolulu Volcanic Series. Plagioclase mineral separates from two peridotites are in 87Sr/86Sr isotopic disequilibrium with coexisting clinopyroxenes. The plagioclases are light rare earth element depleted and have “enriched” 87Sr/86Sr ratios (0.70436 and 0.70443). The delta18O of olivine (5.09–5.12‰) and clinopyroxene (5.32–5.33‰) from spinel peridotites are typical for oceanic upper mantle rocks. In contrast, delta18O of olivine, pyroxene, and plagioclase in plagioclase-bearing peridotites are enriched by ~0.5‰ relative to the spinel peridotites (e.g., delta18O ratios of olivines from plagioclase peridotites are 5.4–5.5‰). It is here interpreted that the plagioclases represent phases that formed by reaction with or precipitation from a melt that intruded the Pacific lithospheric mantle. The Pacific lithospheric isotopic composition is reflected in the clinopyroxene separates. The high delta18O and 87Sr/86Sr of the plagioclase resemble Koolau lavas and suggest that the melt passing through the lithosphere was plume-related. The elevated delta18O of the plagioclase indicates that the melt from which it fractionated had a component of recycled crustal material not acquired from local crustal contamination. It is also estimated here that the duration of the porous melt flow of the Koolau-like melt though the mantle lithosphere was short-lived, ~10^2–10^4 years.
机译:来自Pali(夏威夷瓦胡岛)的Xenoliths包括位于Koolau盾构火山下面的地幔岩石圈样品。大多数此类异石是尖晶石橄榄石,其余是斜长石-尖晶石橄榄石和不含石榴石的辉石。从巴利橄榄岩异种岩中分离出的次氯环比相对耗竭的87Sr / 86Sr(0.70309–0.70346)和143Nd / 144Nd(0.512967–0.513206)。 Koolau火山下面的地幔岩石圈当前范围为εNd = 6.9-11。在143Nd / 144Nd与147Sm / 144Nd的关系图中,他们定义了61±20 Ma的误差时脉,在瓦胡岛太平洋岩石圈的估计年龄约80-85 Ma的误差范围内,并且epsilonNd的截距= 6.8。我们将Pali尖晶石橄榄岩解释为太平洋岩石圈残余物以熔融提取太平洋地壳。这些岩石没有被同位素组成类似于建造盾构的库拉火山的熔体交代,但是它们可能被与檀香山火山系列有关的熔体交代了。从两个橄榄岩中分离出斜长石矿物与共存的斜辉石存在于87Sr / 86Sr同位素不平衡中。斜长柄是贫的轻稀土元素,并且具有“丰富的” 87Sr / 86Sr比(0.70436和0.70443)。尖晶石橄榄岩中的橄榄石(5.09–5.12‰)和斜辉石(5.32–5.33‰)的δ18O是海洋上地幔岩石的典型特征。相反,斜长石橄榄石中橄榄石,辉石和斜长石的delta18O相对于尖晶石橄榄石富集约0.5‰(例如,斜长石橄榄石中橄榄石的delta18O比率为5.4-5.5‰)。这里解释为,斜长岩代表通过与侵入太平洋岩石圈地幔的熔体反应或从其沉淀而形成的相。太平洋岩石圈同位素组成反映在斜柏环烯分离物中。斜长石的高δ18O和87Sr / 86Sr类似于库劳熔岩,表明穿过岩石圈的熔体与羽流有关。斜长石的delta18O升高表明,从中分离出的熔体具有一部分不是从当地地壳污染获得的循环地壳材料。在此还估计,穿过地幔岩石圈的库鲁样熔体的多孔熔体流动持续时间很短,约10 ^ 2–10 ^ 4年。

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