新疆布尔根蛇绿混杂岩的发现及其大地构造意义

新发现的出露于新疆北部额尔齐斯板块缝合带的布尔根蛇绿混杂岩带位于西伯利亚板块与哈萨克斯坦—准噶尔板块的结合带,呈北西向展布。主要表现为糜棱岩化的基质中混杂着大小不一、性质各异的蛇绿岩各组分的岩块。基质主要有糜棱岩化的火山岩、凝灰岩和破碎强烈的火山碎屑岩。蛇绿岩岩块主要有碳酸盐化的超镁铁岩(?)、玄武岩、辉长岩和硅质岩岩块等。玄武岩具有OIB和IAB特征。拉斑玄武岩SHRIMP锆石年龄352Ma,说明蛇绿岩的形成可持续到晚泥盆世—早石炭世。这一发现对于认识中国准噶尔盆地以北地区的晚古生代蛇绿岩带及其向境外的延伸具有重要的意义。     

全球海山玄武岩数据挖掘研究

海山是一个地貌术语,通常分为出露于海平面以上和淹没于以下的两类。海山具有复杂的成因,可产于各种不同的构造环境,其出露的岩性主要有：洋岛玄武岩（OIB）、大洋中脊玄武岩（MORB）、弧后盆地玄武岩（BABB）、岛弧玄武岩（IAB）和大陆边缘玄武岩（CMB）等。本文的研究表明,CMB 和OIB 的地球化学性质大体相似,但是,二者的成因可能既有相似性,也存在某些差异性。OIB 产于板块内部,属于板内岩浆活动的产物,通常认为与“热点”或“地幔柱”有关;而CMB 则可能是古大陆岩石圈与年轻洋壳发生浅部再循环的结果。所以,除“热点”理论外,古大陆岩石圈和年轻洋壳的浅部再循环在海山和洋岛火山形成过程中也扮演了重要的角色。来自IAB 的样品明显亏损Nb、Ta和富集K、Pb、Cs、Rb等大离子亲石元素,表明IAB 的形成与俯冲作用有关。研究表明,全球可能存在3 种类型的热点：第一类是原生的热点,来自深部地幔;第二类是次生的热点,可能形成在地幔柱的浅部,来自超级地幔柱的上部;第三类来自上地幔,可能是大洋岩石圈伸展的产物。因此,海山的成因不可能用地幔柱一种模式予以解释,还应当考虑板块活动中其他各种因素（洋壳再循环、古老陆壳再循环、消减带物质以及水的加入,部分熔融程度、岩浆混合作用、不同地幔端元混合等）的影响。     

全球岛弧玄武岩数据挖掘——在玄武岩判别图上的表现及初步解释

MORB(洋中脊玄武岩)、OIB(洋岛玄武岩)和IAB(岛弧玄武岩)是学术界最关心的3种玄武岩类型,其中尤以与板块消减作用有关的岛弧岩浆活动备受关注。岛弧可分为洋内岛弧和大陆边缘岛弧(活动陆缘弧)2类。对IAB进行讨论,重点探讨IAB的识别。IAT(岛弧拉斑玄武岩)和IAB是前弧、岛弧和后弧岩浆作用的产物,其中,后弧组分更具多样性,它不同于弧后玄武岩,前者属于弧的范围,而后者形成的动力学过程与俯冲系统有关,但其是独立的构造单元,尽管其岩浆作用可能仍受到俯冲流体的影响。前人对IAB进行了大量研究,提出了多种构造环境判别图解,并得到广泛应用。尝试应用全球玄武岩数据来验证上述判别图的可信度,研究发现,可信度高的判别图不多,且大多与Th、Ta(Nb)和Ti元素有关的,如Hf-Th-Ta(Nb)、TiZr-Sr和Th/Yb-Ta/Yb图,其余判别图的判别效果可信度低且具多解性,建议谨慎使用。IAB与MORB和OIB的区别主要体现在Nb-Ta亏损的特征上,是否受到俯冲流体的影响是区分IAB与MORB和OIB最重要的标志。     

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.