Dry peralkaline felsic liquids and carbon dioxide flux through the Kenya rift zone
Dry peralkaline felsic liquids and carbon dioxide flux through the Kenya rift zone
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通讯作者：
Editor B. o. Mysen;D. K. Bailey;R. Macdonald
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作者：
Editor B. o. Mysen;D. K. Bailey;R. Macdonald
- 在奈瓦沙湖周围，肯尼亚裂谷的地形顶点，全新世爆发了大范围的过碱性长英质岩浆。玻璃质样品非常丰富。火山学和岩相学的证据表明，这些玻璃反映的特点是低浓度的H 2 0在熔体中，这种关系是由实验和化学信息证实。不同的岩浆类型，粗面岩，comendites和pantellerites，几乎完全从不同的中心喷发，即使这些足够接近的嵌入物从相邻的火山喷发的产品。这种个性在化学变化图中得以保持，其中来自不同中心的产品形成单独的组，没有分级组成。在一些图表中，较小的集群出现在主要群体中。这些独特的模式表明，裂谷下方的熔体没有连续的演化，每个中心都代表着不同熔体的流出。同一中心内的批次也可能代表不同的融化阶段。在区域背景下，这些熔体的产生可以最好地解释为富CO2流体流入期间不同来源的熔融。粗面岩，pantellerites和comendites，然后可能是一个通量/熔融循环的产品，因为它逐步爬升通过地幔，镁铁质深地壳，和硅铝质地壳。熔体中高浓度的钠和铁要求这些元素在源岩中的增强：这可能被过去23 Ma以来裂谷段长期活动导致的区域交代作用所证实。不相容的微量元素的变化不能归因于固-熔相互作用，但可以在熔化过程中由流体赋予。
-Around Lake Naivasha, at the topographic culmination of the Kenya rift, a wide range of peralkaline felsic magmas erupted in the Holocene. Glassy samples are abundant. Volcanological and petrographic evidence indicates that these glasses reflect characteristically low concentrations of H 2 0 in the melts, and this relationship is confirmed by experimental and chemical information. Distinct magma types, trachytes, comendites and pantellerites, erupted almost exclusively from different centres, even though these are sufficiently close for intercalation of the erupted products from adjacent volcanoes. This individuality is maintained in chemical variation diagrams where the products from different centres form separate groups, with no gradational compositions. In some diagrams smaller clusters appear within the main groups. These distinctive patterns indicate that there is no continuous evolution of the melts below the rift, and that each centre represents the outpouring of a different melt. Batches within the same centre could also represent different episodes of melt generation. Generation ofthese melts in a regional context can best be explained by melting ofdifferent sources during the influx of a CO 2 -rich fluid. Trachytes, pantellerites and comendites could then be the products of a flux/melting cycle as it climbs progressively through mantle, mafic deep crust, and sialic crust. The high concentrations of sodium and iron in the melts call for enhancement of these elements in the source rocks:this could be achievedby regional metasomatism resultingfrom prolonged activity through the rift segment over the past 23 Ma. Incompatible trace element variations cannot be attributed to solid-melt interactions but could be imparted by the fluid during melting.