Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii
Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii
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夏威夷科哈拉火山哈维熔岩的地球化学
DOI：
10.1007/bf00399369
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发表时间：
1988
影响因子：
3.5
通讯作者：
Michael O. Garcia
中科院分区：
文献类型：
--
作者：
S. Spengler;Michael O. Garcia
Hawi熔岩形成了科哈拉火山上的晚期碱性帽，其成分范围从夏威夷岩到粗面岩。新的，详细的现场测绘科哈拉和重新解释以前公布的年龄数据表明，有没有显着的喷发间隙之间的Hawi和下面的波洛卢盾熔岩先前建议。矿物和全岩化学数据是一致的结晶分馏起源的夏威夷岩粗面岩成分变化内观察到的Hawi熔岩。斜长石，单斜辉石，钛磁铁矿，橄榄石和磷灰石分馏需要解释这种变化。单斜辉石分馏可能发生在中等压力下，因为它实际上是不存在于这些熔岩，并不是一个近液相在压力小于8千磅。斜长石在Hawi夏威夷岩浆中是有浮力的，因此需要像动态流动结晶这样的机制来对其进行分馏，并解释熔岩中几乎没有斑晶的原因。Hawi熔岩在Sr和Nd同位素比值和/或不相容元素比值方面与Pololu熔岩不同。因此，它们是从成分上不同的来源派生出来的。与其他夏威夷碱性帽熔岩相比，Hawi熔岩具有非常高的磷和稀土元素浓度。这些差异可能是由于Hawi熔岩源中磷灰石含量较高。
Hawi lavas form the late stage alkalic cap on Kohala Volcano and range in composition from hawaiite to trachyte. New, detailed field mapping of Kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the Hawi and underlying Pololu shield lavas as was previously suggested. Mineral and whole-rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, olivine and apatite fractionation are needed to explain this variation. The clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 Kb. Plagioclase would be buoyant in the Hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas. Hawi lavas are distinct in Sr and Nd isotopic ratios and/or incompatible element ratios from the Pololu lavas. Thus they were derived from compositionally distinct sources. Compared to other suites of Hawaiian alkalic cap lavas, Hawi lavas have anomalously high concentrations of phosphorus and rare earth elements. These differences could be due to greater apatite content in the source for the Hawi lavas.