Lithospheric Mantle Evolution Beneath the Bering Sea Volcanic Province: An Isotopic, Elemental and Oxygen Fugacity Study of Peridotite Xenoliths

来自阿拉斯加西北部、白令海和俄罗斯东北部白令海火山省（BSVP）的超镁铁质捕虏岩套件为研究和描述欧亚大陆和北美两大板块边界地幔岩石圈物质的地球化学演化提供了绝佳的机会。存在着一套记录良好、种类繁多的捕虏体，但人们对这些岩石及其形成的岩石圈地幔知之甚少。本研究的主要目的是：1)模拟裂陷过程中影响上地幔岩石圈的岩浆和交代过程；2)确定岩石圈演化过程中不同时期侵入岩石圈的岩浆的来源；3)确定BSVP下岩石圈地幔中大离子亲石元素的物质枯竭程度。了解在伸展过程中控制岩石圈演化的过程将为裂谷的热力学模型和大陆下地幔的化学演化提供重要的约束。在努尼瓦克岛上，许多粗粒超镁质捕虏体含有角闪洞，少数含有绿云母和磷灰石。这些捕虏体在稀土元素模式和钕锶同位素组成上与寄主岩石相似，表明这两种类型的捕虏体之间存在很强的联系。1)轻稀土元素和重稀土元素，2)标准化丰度比大于1,3)钾、铷、锶、钡和轻稀土元素丰度超过原始地幔的估计。这些特征支持了Nunivak岛捕虏体中角闪洞具有交代成因的观点。然而，这种现象是普遍存在于BSVP以下的地幔岩石圈中，还是仅局限于几个地方，表明是俯冲交代作用，目前尚不清楚。为了进一步了解地幔是如何通过熔体和交代流体的引入和排出而演化的，首席研究员将通过x射线荧光测定整个岩石样品的主要元素浓度，通过电子显微探针测定矿物的主要元素浓度；2)利用二次电离质谱法测定薄片微量元素浓度；3)利用多收集器电感耦合等离子体质谱分析橄榄岩样品的铂族元素丰度；4)利用电子探针和穆斯堡尔谱分析尖晶石中的氧逸度；(5)斜辉石、玻璃、角闪石和云母上锶、钕、铅的初始同位素组成和相应的元素浓度。

AbstractOPP-98-13609Ultramafic xenolith suites from the Bering Sea Volcanic Province (BSVP) of northwest Alaska, the Bering Sea, and northeast Russia provide an excellent opportunity to examine and describe the geochemical evolution of mantle lithospheric materials at the boundary of two major plates, Eurasia and North America. A well-documented, diverse suite of xenoliths exists, but little is known about these rocks and the lithospheric mantle from which they were generated. The principal objectives of this research are to: 1) model the magmatic and metasomatic processes that affect upper mantle lithosphere during rifting; 2) determine the provenance of magmas which intrude the lithosphere at various times during its evolution, and 3) ascertain how much material is depleted in large ion lithophile elements in the lithospheric mantle beneath the BSVP. Understanding the processes that control lithospheric evolution during extension will provide important constraints on thermo-mechanical models of rifting and on the chemical evolution of the mantle beneath continents.On Nunivak Island, many coarse-grained ultramafic xenoliths contain amphibole and a small number contain phlogopite and apatite. These xenoliths have similarities in rare earth elements' patterns and neodymium and strontium isotopic compositions to the host rocks, suggesting a strong link between the two types of xenoliths. They also show 1) light rare earth elements and heavy rare earth elements, 2) normalized abundance ratios of greater than one, and 3) their potassium, rubidium, strontium, and barium and light rare earth element abundances exceed the estimates for primitive mantle. These characteristics support the idea that amphibole in Nunivak Island xenoliths has a metasomatic origin. However, it is not known whether this phenomenon was pervasive in the mantle lithosphere below the BSVP or limited to a few localities suggesting metasomatism by subduction.To advance our understanding of how the mantle evolves through introduction and expulsion of melts and metasomatic fluids, the Principal Investigator will determine the: 1) major element concentrations of whole rock samples by X-Ray fluorescence and of minerals by electron microprobe; 2) trace element concentrations by secondary ionization mass spectrometry on thin sections; 3) platinum-group element abundance patterns for peridotite bulk samples by the multi-collector inductively coupled plasma mass spectrometer; 4) oxygen fugacities in spinel by electron microprobe and Mossbauer spectroscopy; and 5) initial strontium, neodymium, and lead isotopic compositions and corresponding elemental concentrations on clinopyroxenes and glass, and amphibole and phlogopite when present.