Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle
Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle
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DOI：
10.1007/s00410-014-0989-3
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发表时间：
2014-03
影响因子：
3.5
通讯作者：
Bernhard Mayer;Bernhard Mayer;Stefan Jung;Rolf L. Romer;J. Pfänder;A. Klügel;Andreas Pack;E. Gröner
中科院分区：
文献类型：
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作者：
Bernhard Mayer;Bernhard Mayer;Stefan Jung;Rolf L. Romer;J. Pfänder;A. Klügel;Andreas Pack;E. Gröner
德国中欧省Rhön地区第三纪碱性玄武岩中的钾霞石-角闪石斑晶获得了24.0-24.1 Ma的新的高精度40 Ar/39 Ar年龄。主量和微量元素组成表明，这些高TiO 2（4-7重量%）角闪石与其寄主岩石处于平衡状态。这些角闪石斑晶在化学性质上类似于上地幔岩浆脉中的角闪石，而不同于橄榄岩中的浸染状角闪石。大多数角闪石与其碱性玄武质寄主岩具有相似的同位素特征（87 Sr/86 Sr 24 = 0.7035-0.736，ε Nd 24 = +3.8-+4.0，206 Pb/204 Pb 24 = 19.21- 19.37，207 Pb/204 Pb 24 = 15.58- 15.62，208 Pb/204 Pb 24 = 38.95-39.16），但两个样本的同位素组成（ε Nd 24 = −4.0和−2.9; 206 Pb/204 Pb 24 = 17.08和18.11; 207 Pb/204 Pb 24 = 15.51和15.58;~（208）Pb/~（204）Pb ~（24）= 37.41和37.99），表明在熔融过程中有一种古老的壳源成分参与。角闪石斑晶的氧同位素组成为5.4 ~ 7.5 ‰，反映了上地幔源区的氧同位素不均一性。角闪石和寄主岩石对的同位素组成对比进一步表明，斑晶记录了区域范围内和岩石圈地幔内不同深度的Rhön火山场火山历史的早期阶段。温度和压力估计值介于1，010至1，080 °C和0.7至1.0 GPa之间，与上地幔碱性岩石中角闪石稳定性的实验结果一致。主要和微量元素和同位素组成的地球化学表明，从碱性玄武岩的火成角闪石可能会保留同位素的不均匀性，这表明他们的样品在小规模上地幔岩性的不均匀性。
Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhön, Central European Province, Germany) yield new high-precision40Ar/39Ar ages of 24.0–24.1 Ma. Major and trace element compositions demonstrate that these high-TiO2(4–7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (87Sr/86Sr24= 0.7035–0.736, εNd24= +3.8–+4.0,206Pb/204Pb24= 19.21–19.37,207Pb/204Pb24= 15.58–15.62,208Pb/204Pb24= 38.95–39.16), but two samples show contrasting isotopic compositions (εNd24= −4.0 and −2.9;206Pb/204Pb24= 17.08 and 18.11;207Pb/204Pb24= 15.51 and 15.58;208Pb/204Pb24= 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 ‰, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhön volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 °C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale.