He, Sr and Nd isotopic variations in lavas from the juan fernandez archipelago, SE pacific
He, Sr and Nd isotopic variations in lavas from the juan fernandez archipelago, SE pacific
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太平洋东南部胡安·费尔南德斯群岛熔岩中的 He、Sr 和 Nd 同位素变化
DOI：
10.1007/bf00712980
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发表时间：
1993
影响因子：
3.5
通讯作者：
H. Craig
中科院分区：
文献类型：
--
作者：
K. Farley;A. Basu;H. Craig
在智利胡安费尔南德斯群岛板块内火山的一套熔岩上测量了氦、锶和钕的同位素比率以及主要和微量元素的组成。来自Mas Afuera和Mas a Tierra岛以及Monte Alpha和Friday海山的熔岩的Sr和Nd同位素比值位于地幔阵列的低143Nd/144Nd侧（87Sr/86Sr: 0.7034-0.7037;143Nd/144Nd: 0.51281-0.51289）。这些示踪剂的同质性表明，在胡安费尔南德斯火山活动中，地幔源成分的范围有限。与之形成鲜明对比的是3he /4He的大范围，从7.8 RA到18.0 RA。ma A Tierra的双峰he /4He分布与两个不同的火山谱系有关；3He/4He比值为14.5 ~ 18.0 RA (n=15)，而后盾状玄武岩为11.2 ~ 13.6 RA （n=12）。元素和同位素系统分析表明，盾构熔岩中的富集（Loihi-like）羽流源向后盾构火山中的MORB-like源转变。这些来源之间的转变在3he /4He中比在其他同位素示踪剂中更为明显。ma Afuera的拉斑玄武岩具有异常均匀的同位素和元素特征；Sr和Nd的比值与ma和Tierra相似，但3he /4He在8.3±0.5 RA （n=17）时较低且更均匀。考虑到这两个岛屿在其他地球化学特征上的巨大相似性，阿夫拉岛和马斯蒂拉岛之间的巨大差异令人惊讶。Mas Afuera和Mas a Tierra的结果都表明，氦记录的系统过程不容易从其他同位素或元素指标中显现出来。岩浆房的脱气和局部的交代事件都不可能是原因。我们认为观测到的变化可能归因于羽流和软流圈源的混合，其中羽流成分的特征是挥发物的不均匀分布，或者在混合之前遭受了小程度部分熔体的提取。
Helium, Sr, and Nd isotopic ratios and major and trace element compositions have been measured on a suite of lavas from the intra-plate volcanos of the Juan Fernandez Archipelago, Chile. Lavas from the islands of Mas Afuera and Mas a Tierra and from Monte Alpha and Friday seamounts have Sr and Nd isotopic ratios lying on the low143Nd/144Nd side of the mantle array (87Sr/86Sr:0.7034–0.7037;143Nd/144Nd:0.51281–0.51289). The homogeneity of these tracers suggests the involvement of a restricted range of mantle source compositions throughout Juan Fernandez volcanism. In marked contrast is the large range in3He/4He, from 7.8 to 18.0 RA. A bimodal3He/4He distribution on Mas a Tierra is associated with two distinct volcanic lineages;3He/4He ratios of 14.5–18.0 RA (n=15) occur in alkalic and tholeiitic shield basalts, whereas post-shield basanites range from 11.2 to 13.6 RA (n=12). Elemental and isotopic systematics demonstrate a transition from an enriched (Loihi-like) plume source in the shield lavas to a more MORB-like source in the post-shield volcanics. The transition between these sources is much more pronounced in3He/4He than in the other isotopic tracers. The predominantly tholeiitic basalts of Mas Afuera have exceptionally uniform isotopic and elemental characteristics; Sr and Nd ratios are similar to those of Mas a Tierra, but3He/4He ratios are lower and more uniform at 8.3±0.5 RA (n=17). The dramatic distinction between Mas Afuera and Mas a Tierra helium is surprising given the great similarity between the two islands in other geochemical characteristics. Both the Mas Afuera and Mas a Tierra results demonstrate that helium records systematic processes not readily apparent from other isotopic or elemental indicators. Neither magma chamber degassing nor local metasomatic events are likely to be responsible. We suggest that the observed variations may be attributed to mixing of plume and asthenospheric sources in which the plume component is characterized by a heterogeneous distribution of volatiles or has suffered extraction of small degree partial melts prior to mixing.