【作者】 禹尧；

【导师】 徐夕生； Suzanne Y.O’Reilly； William L.Griffin；

【作者基本信息】 南京大学 ， 矿物学岩石学矿床学， 2012， 博士

【摘要】 大陆岩石圈地幔(SCLM)形成于地球壳幔分异过程,是地幔发生较大规模部分熔融、抽取后的残留体；这一过程也包括水(H2O/OH)的抽取和运移。除含水熔体/流体之外,岩石圈地幔的性质和演化过程,也受赋存于地幔矿物(名义上无水矿物,NAMs)中的结构水的强烈影响：比如,橄榄石中微量的结构水可以显著降低其粘滞度,从而降低岩石圈地幔的流变强度,影响其“稳定性”。同时,与LREE (La, Ce)和LIL (K)相似,在熔融体系中,H2O/OH表现为极不相容组分(Dperidotite/melt=ca0.007-0.009)，这使其成为岩石圈地幔熔融/抽取事件和地幔交代富集事件的敏感指示剂。因此,对岩石圈地幔中水的赋存状况(包括其含量和空间分布)的研究为我们深入探究岩石圈地幔的性质和演化历史提供了一条新思路。由该新角度切入,也有助于我们去进一步认识水在岩石圈地幔形成和演化过程中所扮演的“角色”。被碱性玄武岩浆携带至地表的橄榄岩包体是直接来自岩石圈地幔的样品,绝大部分岩石圈地幔的水便赋存于这些主要由NAMs(比如,橄榄石,辉石)组成的地幔岩包体中。本次研究对象是来自福建明溪、牛头山以及广东麒麟、安远新生代火山岩中的橄榄岩包体。除两件产自明溪的石榴子石相包体外,其余样品均为尖晶石相。利用傅里叶红外光谱(FTIR),我们测定了上述橄榄岩组成矿物的结构水含量。考虑到(1)辉石颗粒内部水含量均一分布；(2)不同样品间,单斜辉石与斜方辉石之间水的分配系数保持一致(Dcpx/opx=2.3),以及(3)单斜辉石中水含量与主量元素相关关系的保存,我们认为上述橄榄岩包体中的辉石保存了其在源区时的水含量。由于橄榄石水含量普遍很低(<30ppm),辉石通常才是上地幔水的主要载体。因此主要取决于辉石中的水含量的全岩水含量,也大致能够反映其在源区时水的赋存情况。根据各组成矿物水含量以及矿物百分含量计算得知：华夏陆块橄榄岩包体的全岩水含量为12-94ppm(平均值：60±20ppm),明显高于来自华北克拉通东部的橄榄岩包体的水含量(平均值：26±17ppm),而后者被认为是华北克拉通减薄后古老岩石圈地幔的残留。但上述同属中国东部岩石圈地幔的水含量值均低于其他大陆岩石圈地幔,无论是克拉通(122±54ppm)还是非克拉通(81±40ppm)地区。现今华南岩石圈地幔中等偏低的水含量与它为一个富集的古老岩石圈地幔的情况相一致,是其经历多次地质事件后的结果。自早侏罗纪开始,在古太平板块俯冲过程中析出的流体逐步积累于上覆地幔楔中,导致岩石圈地幔水化。机械上,这部分添加的水“软化”了岩石圈,使其在物理性质上更接近于软流圈地幔,实现了某种意义上的岩石圈减薄；热力学上,多余的水使得熔融温度降低,随后发生的燕山期大规模岩浆活动与“湿”的岩石圈地幔的存在息息相关。在此岩浆活动期,岩石圈地幔的不相容组分重新被抽取进入基性岩浆,此过程伴随着岩石圈地幔的脱水,使其成为“干”的岩石圈地幔。岩石圈减薄后,促使软流圈地幔上涌,并发生减压熔融,具软流圈地幔特征的新生代玄武质岩浆便是很好的佐证。源自软流圈地幔的含水熔体/流体自下而上水化岩石圈地幔,恰好解释了岩石圈地幔底部石榴石相二辉橄榄岩为何最富水(>80ppm),它们也代表了本次研究中起源最深的样品(～1.9Gpa)。这些饱满的石榴石相橄榄岩几乎未经过熔体抽取,可能是软流圈地幔顶部冷却后贴合于之前的岩石圈地幔底部形成的新生岩石圈地幔。这一岩石圈地幔的纵向增生,标志着岩石圈减薄时代的结束。橄榄岩包体水含量和其氧逸度的负相关关系仅见于牛头山包体样品。相较于其他三个远离主要断裂带的采样点,牛头山恰位于长乐—南澳断裂带上。此外,与其他三地的地幔岩相比(Mg#>90),牛头山地幔岩也普遍具有更低的Mg#(Mg#<90)和水含量(<60ppm)。由此可见,断裂带为古太平洋板块俯冲释放的氧化的含水熔体/流体提供了上升和渗透的通道,导致该区岩石圈地幔氧逸度增加,这使得水在地幔岩中溶解度降低,水含量也随之降低。在中国东部新生代玄武岩质火山岩中,除了含较丰富的超镁铁岩包体外,还含石榴石、辉石、刚玉等巨晶矿物。锆石巨晶,作为Lu-Hf同位素的重要载体,可能是研究岩石圈地幔重要的新样品。本文所述的山东昌乐、福建明溪、海南蓬莱等地区的新生代玄武岩中就产有锆石巨晶。它们通常无色透明-淡褐色,部分颗粒长达16mm。昌乐、明溪及蓬莱锆石巨晶的U-Pb年龄分别为19.2±0.7Ma,1.2±0.1Ma和4.1±0.2Ma,略早于其相应的寄主岩的喷发时代(分别为16.05-18.87Ma,0.9-2.2Ma,3Ma)。昌乐、明溪及蓬莱锆石巨晶的εHf(t)值分别为9.02±0.49,6.83±0.47,4.46±0.48,表明其地幔来源。我们认为锆石巨晶起源于交代富集岩石圈地幔,而后被寄主玄武岩浆快速携带至地表。单颗粒锆石巨晶自形的形态、均一的内部结构和化学组成说明其形成于稳定的环境中。显著的Ce正异常和Eu异常的缺失表明其源区环境为氧化,且不存在斜长石的分离结晶。不同地区锆石巨晶的Hf同位素组成差异明显,并与其相应的寄主玄武岩Sr-Nd-Pb同位素组成耦合。这说明玄武岩成分可能也受到锆石巨晶的源区——富集岩石圈地幔的影响,同时,也表明中国东部上地幔的物质组成在空间上的不均一。锆石巨晶可能为我们研究岩石圈地幔的性质和演化打开了新窗口。更多还原

【Abstract】 Refractory subcontinental lithospheric mantle (SCLM) is produced by the removal of partial melts from mantle rocks; this process includes the removal of H2O. The nature and evolution of the SCLM is also strongly influenced by hydrous melts and fluids, which affect the physical and chemical properties of mantle minerals and rocks. Like some LREEs (La, Ce) or LILs (K), H2O behaves as an extremely incompatible component (Dperidotite/melt=ca0.007-0.009) in a melt/solid system, which makes it a sensitive tracer of melt-extraction and metasomatism in the SCLM. Thus an understanding of the H2O inventory of the SCLM provides another tool to study the nature and the evolution of the SCLM, and the role H2O has played during lithospheric modification.Peridotite xenoliths in alkali basalts consist mainly of nominally anhydrous minerals (NAMs, e.g., olivine, pyroxene) and are direct samples that may reflect the actual H2O budget of SCLM. This study has focussed on determining the H2O contents of peridotite xenoliths from four localities (Mingxi, Anyuan, Niutoushan and Qilin) in the Cathaysia block, SE China using Fourier Transform Infrared Spectroscopy (FTIR). Considering (1) the homogeneity of water distribution within single pyroxene grains;(2) the partitioning of water between cpx and opx with an average Dcpx/opx of2.3; and (3) the correlations between the H2O contents and major element concentrations in cpx, it is suggested that the pyroxenes have largely preserved the water content of their mantle source.The whole-rock water contents calculated from mineral modes range from12to94ppm (average60±20ppm). This is much higher than the previously-reported water contents of xenoliths from the North China Craton (NCC)(average26±17ppm). However, it is still quite low compared to those of continental lithospheric mantle worldwide, as inferred from analyses of typical cratonic (122±54ppm) and off-craton (81±40ppm) peridotites. The present relatively low water budget has evolved through multiple geological events over the long history of this SCLM, e.g., hydration due to paleo-Pacific plate subduction, dehydration by melt extraction during Yanshanian magmatism and subsequent rehydration due to the fluxing by low-degree astheno spheric melts after lithospheric thinning. This is evidenced by young basaltic volcanism at the surface. Water itself plays an important role during the modification of the subcontinental lithospheric mantle. The garnet Iherzolites, which represent the deepest portion of the SCLM (-1.9GPa) sampled in this study, have the highest water contents (>80ppm). This fertile garnet-facies layer with almost no extraction of water at depth may be young lithosphere accreted at the bottom of the pre-existing lithosphere. It marks the completion of the lithospheric thinning episode and the upwelling of asthenosphere.A negative correlation between pyroxene water contents and the oxygen fugacity has been found only in xenoliths from Niutoushan (Mg#<90), which lies on the Changle-Nan’ao fault zone. In contrast with the xenoliths from the other three localities, the Niutoushan xenoliths also have lowest water content (<60ppm) and Mg#<90). The fault may have facilitated the infiltration of the Niutoushan peridotites by oxidized fluids (or hydrous melts) rising from the subducting Pacific plate.Besides the abundant peridotites in the Cenozoic basalts in eastern China, there are also garnet, pyroxene and corundum megacrysts. As a key holder of Lu-Hf isotopic system, zircon megacrysts might be another probe into the SCLM. Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown-maroon, and some of them are up to16mm long. U-Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are19.2±0.7Ma,1.2±0.1Ma and4.1±0.2Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05-18.87Ma,0.9-2.2Ma,3Ma). εHf(t) values of zircon megacrysts are9.02±0.49,6.83±0.47,4.46±0.48for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.更多还原