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火山省(BSVP)、白令海和俄罗斯东北部的OPP-98-13609超镁铁捕虏体套为研究和描述欧亚大陆和北美两大板块交界处地幔岩石圈物质的地球化学演化提供了一个很好的机会。存在着一套记录充分、种类繁多的包体，但人们对这些岩石及其形成的岩石圈地幔知之甚少。这项研究的主要目的是：1)模拟裂谷过程中影响上地幔岩石圈的岩浆和交代过程；2)确定在岩石圈演化过程中不同时期侵入岩浆的来源；3)确定在BSVP之下的岩石圈地幔中大离子亲石元素中亏损了多少物质。了解在伸展过程中控制岩石圈演化的过程将为裂谷作用的热力学模型和大陆之下地幔的化学演化提供重要的约束。在努尼瓦克岛上，许多粗粒超镁铁质包体含有角闪石，少数含有金云母和磷灰石。这些包体在稀土元素模式和主岩的钕、锶同位素组成上有相似之处，表明这两种包体之间有很强的联系。它们还显示1)轻稀土元素和重稀土元素，2)归一化丰度比大于1，3)钾、锶、锶、钡和轻稀土元素丰度超过原始地幔的估计值。这些特征支持努尼瓦克岛包体中的角闪石具有交代成因的观点。然而，目前尚不清楚这种现象是普遍存在于BSVP以下的地幔岩石圈，还是仅限于少数地方，表明俯冲交代作用。为了加深我们对地幔如何通过熔体和交代流体的引入和排出而演化的理解，首席研究员将确定：1)X射线荧光全岩样品的主要元素浓度和电子探针矿物的主要元素浓度；2)薄片上的微量元素浓度；3)多收集器电感耦合等离子体质谱仪测定橄榄岩块状样品的铂族元素丰度模式；4)电子探针和穆斯堡尔光谱测定尖晶石中的氧逸度；5)单斜辉石、玻璃、角闪石和金云母的初始锶、钕、铅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.