Melting versus contamination effects on 238U-230Th-226Ra and 235U-231Pa disequilibria in lavas from São Miguel, Azores
Melting versus contamination effects on 238U-230Th-226Ra and 235U-231Pa disequilibria in lavas from São Miguel, Azores
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亚速尔群岛圣米格尔熔岩中熔化与污染对 238U-230Th-226Ra 和 235U-231Pa 不平衡的影响
DOI：
10.1016/j.chemgeo.2014.04.030
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发表时间：
2014
期刊：
影响因子：
3.9
通讯作者：
Prytulak J
中科院分区：
文献类型：
--
作者：
Prytulak J
亚速尔群岛的圣米格尔岛因其熔岩的长寿命放射性同位素比率范围惊人地大而引起了地球化学的兴趣。在该岛东部发现的熔岩中观察到的“富集”特征（例如，放射性 87 Sr/86 Sr 和非放射性 143 Nd/144 Nd）被认为源自其地幔来源中回收的地壳成分。这种肥沃的岩性应该比橄榄岩地幔具有更高的熔体生产率（压力每降低所产生的熔体量），并且这种遗产应该在源自此类材料的熔体的 U 系列特征中很明显。具体来说，喷发熔岩中的 238 U-230 Th 和 235 U-231 Pa 不平衡相结合应能识别其来源的熔化速率和/或矿物组成（例如石榴石的量）的显着差异。为此，针对十六个镁铁质熔岩（< 50 wt.% SiO 2）提出了新的 238 U–230 Th–226 Ra 和 235 U–231 Pa 不平衡，涵盖了圣米格尔岛长寿命放射性同位素变异的全部范围。然而，镁铁质熔岩的主要 U 系列特征可能会受到更多差异化产品同化的影响。值得注意的是，在圣米格尔玄武岩中发现了正长石结核和堆积的碱长石异晶。我们认为，玄武岩的二次污染在变化巨大的 230 Th-226 Ra 不平衡中最为明显（226 Ra 不足 18%，超过 300%），与 Ba/Th 比率密切相关。寿命较长的 U 系列核素在许多富集熔岩中仅受到轻微扰动，导致数据集中的噪声增加。在明显未受污染的样本中，来自圣米格尔西部的同位素贫化熔岩显示出比来自东部的富集样本略高的 230 Th 和 231 Pa 过量值。富集熔岩的 U 系列特征可以通过结合较高的 D´ U U 和 D´ Th 绝对值以及与枯竭的西部熔岩源相比更高的熔化速率来进行建模。这些参数与富含更易熔岩性（例如东部源中的榴辉岩和/或辉石岩）的橄榄岩源一致，但并非直接源自此类易熔岩性。 U系列同位素的唯一细微变化加上放射性同位素组成的巨大系统变化表明富集成分和/或其熔体与地幔橄榄岩充分混合，从而掩盖了它们独特的熔化行为。最后，圣米格尔西部 Sete Cidades 火山的玄武岩与亚速尔群岛皮科岛的玄武岩具有非常相似的长寿命同位素组成和 U 系列不平衡性。来自群岛中心和外围岛屿的熔岩的地球化学相似性反驳了与对底层羽流结构的不同部分进行采样相关的熔化行为的系统差异。
The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’signatures (eg, radiogenic 87 Sr/86 Sr and unradiogenic 143 Nd/144 Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238 U–230 Th and 235 U–231 Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (eg, the amount of garnet). To this end, new 238 U–230 Th–226 Ra and 235 U–231 Pa disequilibria are presented for sixteen mafic lavas (< 50 wt.% SiO 2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230 Th–226 Ra disequilibria (~ 18% deficits to> 300% excesses of 226 Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230 Th and 231 Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D¯ U U and D¯ Th, coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.
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DOI：
--
发表时间：
2003
期刊：
影响因子：
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作者：
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DOI：
--
发表时间：
1989
期刊：
影响因子：
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DOI：
--
发表时间：
1982
期刊：
影响因子：
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DOI：
--
发表时间：
1968
期刊：
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