Cooper George F, Blundy Jon D, Macpherson Colin G, Humphreys Madeleine C S, Davidson Jon P
1Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE UK.
2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ UK.
Contrib Mineral Petrol. 2019;174(5):39. doi: 10.1007/s00410-019-1576-4. Epub 2019 May 6.
Quantifying the storage conditions and evolution of different magmatic components within sub-volcanic plumbing systems is key to our understanding of igneous processes and products. Whereas erupted magmas represent a portion of the eruptible volcanic system, plutonic xenoliths provide a complementary record of the mushy roots of the plumbing system that cannot be mobilised easily to form lavas and consequently offer a unique record of magma diversity within the sub-volcanic plumbing system. Here, we present a detailed petrological and geochemical study of erupted plutonic xenoliths from the island of Sint Eustatius (Statia), in the northern Lesser Antilles volcanic arc. The plutonic xenoliths are predominantly gabbroic, but vary in texture, mineral assemblage and crystallisation sequence. We report major, trace and volatile (HO and CO) concentrations of xenolith-hosted melt inclusions (MIs) and interstitial glass. The MIs have a very large range in major element (49-78 wt% SiO and 0.1-6.1 wt% MgO) and trace element concentration (72-377 ppm Sr, 32-686 ppm Ba, 39-211 ppm Zr). Their chemistry varies systematically with host phase and sample type. Significantly, it shows that (1) plutonic xenoliths record a complete differentiation sequence from basalt to rhyolite (2) apatite, but not zircon, saturation was reached during crystallisation, (3) amphibole breakdown reactions play a role in the genesis of shallow gabbronorite assemblages, and (4) mixing between crystal cargos and multiple discrete bodies occurred. Residual melt volatile contents are high (≤ 9.1 wt% HO and ≤ 1350 ppm CO), returning volatile saturation pressures of 0-426 MPa. Multiple reaction geobarometry and experimental comparisons indicate that equilibration took place in the upper-middle crust (0-15 km). We infer that the Statia plutonic xenoliths represent portions of a large heterogeneous crystal mush within which a great diversity of melts was stored and mixed prior to eruption. Our data show that compositional variations in magmatic plumbing systems exceed those observed in volcanic products, a likely consequence of the blending that occurs prior to and during eruption.
量化次火山管道系统中不同岩浆成分的储存条件和演化，是我们理解火成岩过程和产物的关键。虽然喷发的岩浆代表了可喷发火山系统的一部分，但深成捕虏体提供了管道系统软泥状根部的补充记录，这些根部不易被调动形成熔岩，因此提供了次火山管道系统内岩浆多样性的独特记录。在此，我们展示了对来自小安的列斯群岛北部火山弧圣尤斯特歇斯岛（斯塔蒂亚岛）喷发的深成捕虏体进行的详细岩石学和地球化学研究。深成捕虏体主要为辉长岩质，但在结构、矿物组合和结晶顺序上有所不同。我们报告了捕虏体中熔体包裹体（MIs）和填隙玻璃的主量、微量元素及挥发分（H₂O和CO₂）浓度。熔体包裹体的主量元素（49 - 78 wt% SiO₂和0.1 - 6.1 wt% MgO）和微量元素浓度（72 - 377 ppm Sr、32 - 686 ppm Ba、39 - 211 ppm Zr）范围非常大。它们的化学组成随寄主相和样品类型系统变化。重要的是，研究表明：（1）深成捕虏体记录了从玄武岩到流纹岩的完整分异序列；（2）结晶过程中达到了磷灰石而非锆石的饱和；（3）角闪石分解反应在浅部辉长苏长岩组合的成因中起作用；（4）晶体货物与多个离散体之间发生了混合。残余熔体挥发分含量很高（≤ 9.1 wt% H₂O和≤ 1350 ppm CO₂），对应的挥发分饱和压力为0 - 426 MPa。多种反应地质压力计和实验对比表明，平衡发生在中上地壳（0 - 15 km）。我们推断，斯塔蒂亚岛深成捕虏体代表了一个大型非均质晶体软泥的部分，在喷发之前，大量不同成分的熔体在其中储存并混合。我们的数据表明，岩浆管道系统中的成分变化超过了火山产物中观察到的变化，这可能是喷发前和喷发期间混合作用的结果。
