Oxygen and Multiple S Isotope Investigation of Magma-Country Rock Interaction During Early Stages of Magmatism in the Midcontinent Rift System: Constraints From the Eagle Deposit

中大陆裂谷系统岩浆作用早期岩浆-围岩相互作用的氧和多硫同位素研究：来自 Eagle 沉积物的约束

• 批准号: 0710910
• 负责人: Chusi Li
• 金额: $ 15.36万
• 依托单位: Indiana University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0710910&HistoricalAwards=false
• 关键词: Oxygen; Multiple; Isotope; Investigation; Magma

Intellectual merit: The Eagle deposit, located in the Baraga Basin of northern Michigan, consists of semi-massive and massive Fe-Ni-Cu sulfides found within a dike-like peridotite body. The rocks are associated with the Proterozoic Midcontinent Rift System (MRS), and are thought to represent feeders to early-stage, rift-related volcanics. Country rocks include Proterozoic metasedimentary rocks that are locally graphitic and sulfidic; these rocks appeared to be likely sources for S contained in the igneous rocks, However, sulfides in the igneous rocks are characterized by d34S values less than 4 per mil, whereas sulfides in the Proterozoic rocks show a d34S range from ~10 to 40 per mil. Preliminary S isotopic analyses indicate a range of d33S values from 0.02 to -0.7 per mil, indicating significant mass independent fractionation of S isotopes that to date has only been documented in Archean rocks. Thus, most of the S contained in the Eagle deposit must be derived from Archean rocks, rather than from the sulfide-rich Proterozoic rocks. To verify and understand this surprising result, a detailed investigation of d33S variations is proposed for the Eagle deposit, as well as both Proterozoic and Archean country rocks. Multiple S isotope studies have proven to be important to the continuing evaluation of Earth's early atmosphere. However, recent studies have shown that mass independently fractionated S isotopes (MIF-S) may not be restricted to rocks greater that ~2 Ga in age. The proposed stratigraphically controlled study at Eagle will provide information on the distribution of d33S values in the igneous rocks and related sulfide mineralization, in the Proterozoic (two sequences, one of ~ 2.2-2.4 Ga and the other of ~1.8-1.9 Ga, which will cover important intervals in Earth history with respect to MIF-S), and in the Archean rocks. We also plan to analyze gabbroic rocks with associated Cu-Ni sulfide mineralization found in the Duluth Complex of Minnesota, located on the north side of the rift. In these samples d34S values of the sulfide accumulations are similar to the values of iron sulfides in carbonaceous, 1.8 to 1.9 Ga country rocks. This association is much different from that at Eagle, and presents an opportunity to contrast multiple S isotope systematics in mineralized systems that formed at different times in the development of the MRS. Oxygen isotope analyses of olivine and plagioclase in the rocks at Eagle will also be undertaken to investigate the relative importance of devolatilization and partial melting of country rocks in transferring MIF-S to the magmatic system. Results of the proposed study will increase our understanding of magmatic sulfide ore formation in the MRS, will shed light on the nature of interaction between mantle-derived magmas and crustal rocks at various times in the developing rift, and provide insight into S mass independent fractionation on Earth.Broader impacts. The proposed research represents a collaboration between academia and the minerals industry. Kennecott will fund a graduate student fellowship as part of the project, as well as limited geochronologic, mineralogical and geochemical studies. In addition, reserves of Ni and platinum-group elements in the United States are low and studies of this type are needed to evaluate known resources and to develop exploration models. Both graduate and undergraduate students will benefit from the collaboration with mining industry, and will receive training in the utilization of state-of-the-art analytical instrumentation and in methods of scientific investigation.

知识价值：Eagle矿床位于密歇根州北部的巴拉加盆地，由半块状和块状的Fe-Ni-Cu硫化物组成，这些硫化物位于一个脉状的橄榄岩体中。这些岩石与元古宙中大陆裂谷系（MRS）有关，被认为是早期裂谷相关火山的觅食者。乡村岩石包括元古代变质沉积岩，局部为石墨质和硫化物；然而，火成岩中硫化物的d34S值小于4 / mil，而元古代岩石中的硫化物的d34S值在~10 ~ 40 / mil之间。初步的S同位素分析表明，d33S值在0.02 ~ -0.7 / mil之间，这表明迄今为止仅在太古宙岩石中记录到的显著的质量独立分异S同位素。因此，Eagle矿床中所含的大部分硫必须来自太古代岩石，而不是来自富含硫化物的元古代岩石。为了验证和理解这一惊人的结果，建议对Eagle矿床以及元古代和太古代的岩石进行d33S变化的详细调查。多项S同位素研究已被证明对持续评估地球早期大气非常重要。然而，最近的研究表明，质量独立分馏S同位素（MIF-S）可能并不局限于年龄大于~2 Ga的岩石。在Eagle进行的地层控制研究将提供d33S值在火成岩和相关硫化物成矿作用、元古代（两个层序，一个为~ 2.2-2.4 Ga，另一个为~1.8-1.9 Ga，这将覆盖地球历史上重要的MIF-S层序）和太古宙岩石中的分布信息。我们还计划分析在位于裂谷北侧的明尼苏达州德卢斯杂岩中发现的带有伴生铜镍硫化物矿化的辉长岩。在这些样品中，硫化物堆积的d34S值与1.8 ~ 1.9 Ga的碳质岩石中硫化铁的值相似。这种联系与Eagle的大不相同，并提供了一个机会来对比在mrs发展过程中不同时间形成的矿化系统中的多个S同位素系统，Eagle岩石中的橄榄石和斜长石的氧同位素分析也将进行，以研究乡村岩石的脱挥发和部分熔融在将mfs -S转移到岩浆系统中的相对重要性。研究结果将增加我们对MRS岩浆硫化物矿形成的认识，揭示裂谷发育不同时期幔源岩浆与地壳岩石相互作用的性质，并为地球上的S质量独立分选提供新的见解。更广泛的影响。拟议的研究是学术界和矿产业之间的合作。作为该项目的一部分，Kennecott将资助一名研究生奖学金，以及有限的地质年代学、矿物学和地球化学研究。此外，美国镍族元素和铂族元素的储量很低，需要对这类元素进行研究，以评估已知资源和开发勘探模式。研究生和本科生都将受益于与采矿业的合作，并将接受使用最先进的分析仪器和科学调查方法的培训。