Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands
Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands
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佛得角地幔柱采样：佛得角群岛圣安涛熔体成分的演化
DOI：
10.1093/petrology/egi071
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发表时间：
2006
影响因子：
3.9
通讯作者：
M. Runge
中科院分区：
文献类型：
--
作者：
P. Holm;J. Wilson;B. Christensen;L. Hansen;S. L. Hansen;K. M. Hein;A. Mortensen;R. Pedersen;S. Plesner;M. Runge
佛得角群岛西北部圣安唐岛的火山活动历史包括在750万年至30万年之间喷发的碧玄岩 - 响岩系列岩浆，以及在70万年至10万年之间喷发的（黄长石）霞石岩 - 响岩系列岩浆。最原始的火山岩为含橄榄石单斜辉石斑晶的岩石，而演化程度更高的岩石含有单斜辉石、铁 - 钛氧化物、钛闪石、蓝方石、榍石、透长石的斑晶；斜长石存在于一些中间岩石中。所分析的样品氧化镁含量范围为19% - 0.03%；最原始的样品二氧化硅含量为37% - 46%，二氧化钛含量为2.5% - 7%，并且在高度不相容元素方面相对于原始地幔富集50 - 200倍；碧玄岩系列的富集程度低于霞石岩系列。每个系列的地球化学趋势都可以通过碧玄岩和霞石岩母岩浆中斑晶组合的分离结晶来模拟。几乎没有矿物 - 熔体不平衡的证据，因此岩浆混合在控制全岩成分方面并不重要。地幔熔融过程是利用经过分馏校正的岩浆成分来模拟的；这些模型表明，在90 - 125千米深处，非均质地幔橄榄岩源发生了1% - 4%的部分熔融。最原始岩浆类型中不相容元素的富集是高μ（HIMU）型洋岛玄武岩（OIB）的典型特征。圣安唐岛火山序列的锶、钕和铅同位素组成以及地球化学特征随时间发生系统性变化。较古老的火山岩（750万年 - 200万年）在两种主要地幔源成分之间变化，其中一种是年轻的高μ型，其铅同位素比值（\(^{206}Pb/^{204}Pb\)）为19.88，\(\Delta7/4\)为5，\(\Delta8/4\)为0，锶同位素比值（\(^{87}Sr/^{86}Sr\)）为0.7033，钕同位素比值（\(^{143}Nd/^{144}Nd\)）为0.51288，而另一种的放射性成因锶和铅含量稍低，放射性成因钕含量较高。中等年龄的火山岩（200万年 - 30万年）显示源区变化为与碳酸岩有关的年轻高μ型源（\(^{206}Pb/^{204}Pb\) = 19.93，\(\Delta7/4\) = 5，\(\Delta8/4\) = 38，\(^{87}Sr/^{86}Sr\) = 0.70304）和类似亏损地幔（DM）源之间的双组分混合。在年轻火山岩（30万年 - 10万年）中，\(\Delta7/4\) > 0（古老高μ型）的更富集不相容元素的组分较为突出。在佛得角群岛南部很重要的EM1组分似乎在圣安唐岛岩浆的岩石成因中没有起作用。认为原生岩浆是由佛得角地幔柱中的部分熔融产生的；成分的时间变化被认为反映了地幔柱管道中的分层现象。
The volcanic history of Santo Antao, NW Cape Verde Islands, includes the eruption of basanite–phonolite series magmas between 7 5 and 0 3 Ma and (melilite) nephelinite–phonolite series magmas from 0 7 to 0 1 Ma. The most primitive volcanic rocks are olivine clinopyroxene-phyric, whereas the more evolved rocks have phenocrysts of clinopyroxene Fe–Ti oxide kaersutite hauyne titanite sanidine; plagioclase occurs in some intermediate rocks. The analysed samples span a range of 19–0 03% MgO; the most primitive have 37–46% SiO2, 2 5–7% TiO2 and are enriched 50–200 · primitive mantle in highly incompatible elements; the basanitic series is less enriched than the nephelinitic series. Geochemical trends in each series can be modelled by fractional crystallization of phenocryst assemblages from basanitic and nephelinitic parental magmas. There is little evidence for mineral–melt disequilibrium, and thus magma mixing is not of major importance in controlling bulk-rock compositions. Mantle melting processes are modelled using fractionation-corrected magma compositions; the models suggest 1–4% partial melting of a heterogeneous mantle peridotite source at depths of 90–125 km. Incompatible element enrichment among the most primitive magma types is typical of HIMU OIB. The Sr, Nd and Pb isotopic compositions of the Santo Antao volcanic sequence and geochemical character change systematically with time. The older volcanic rocks (7 5–2 Ma) vary between two main mantle source components, one of which is a young HIMU type with Pb/Pb 1⁄4 19 88, D7/4 1⁄4 5, D8/4 0, Sr/ Sr 1⁄4 0 7033 and Nd/Nd 1⁄4 0 51288, whereas the other has somewhat less radiogenic Sr and Pb and more radiogenic Nd. The intermediate age volcanic rocks (2–0 3 Ma) show a change of sources to two-component mixing between a carbonatite-related young HIMU-type source ( Pb/Pb 1⁄4 19 93, D7/4 1⁄4 5, D8/4 1⁄4 38, Sr/Sr 1⁄4 0 70304) and a DM-like source. A more incompatible element-enriched component with D7/4 > 0 (old HIMU type) is prominent in the young volcanic rocks (0 3– 0 1 Ma). The EM1 component that is important in the southern Cape Verde Islands appears to have played no role in the petrogenesis of the Santo Antao magmas. The primary magmas are argued to be derived by partial melting in the Cape Verde mantle plume; temporal changes in composition are suggested to reflect layering in the plume conduit.