宾川苦橄岩的水含量:对峨眉山大火成岩省成因的启示

近几年来对西伯利亚、哥伦比亚河、塔里木等大火成岩省原始岩浆水含量的分析表明,其源区的显著水化是造成地 幔熔融异常的重要原因之一。峨眉山大火成岩省是全球二叠纪大火成岩省的重要组成之一,前期对其位于宾川剖面底部的 大理苦橄岩水含量的分析表明地幔的水化在峨眉山大火成岩省形成中起到了重要作用。然而大理苦橄岩只能代表峨眉山大 火成岩省初始阶段的熔融条件,而在其后的喷发历史中水是否也起到了关键作用还不清楚。定量分析后阶段喷发的苦橄岩 水含量有助于更完整评估水在大火成岩省形成中的作用。本文报道了位于峨眉山大火成岩省西部岩带宾川剖面中上部的宾 川苦橄岩的水含量,结果表明原始岩浆水含量可高达2.8 wt%,比大理苦橄岩的水含量略低,但仍显著高于MORB 和OIB, 达到了岛弧玄武岩的水含量范围。这表明水对整个大火成岩省的形成中都扮演了重要角色。     

科马提岩与苦橄岩的区别及对若干晚古生代“科马提岩”的质疑

以往学术界更多的关注科马提岩和苦橄岩的相似性,忽略其差异。通过全数据模式,采集数据库内全球的太古宙科马提岩、后太古宙低/高钛苦橄岩数据,对比三者之间的差异发现,科马提岩更富MgO、Cr、Ni、Cs、Pb、Co和Zn,其次为低钛苦橄岩(除Co和Zn),其余主量、微量元素的含量由高至低依次为高钛苦橄岩、低钛苦橄岩、科马提岩。依据元素间的差异(如Cr/Ga、MgO/Ga、MnO/Zr、Cr/Zr等),采用密度分布函数(Density Distribution)在Matlab软件中绘制出可有效区分3类岩石的等密度判别图,并用该图对若干晚古生代"科马提岩"的岩性重新厘定。结合岩相学和地球化学特征研究表明,晚古生代"科马提岩"中,印度东部为高钛苦橄岩,越南为化学成分与科马提岩类似的低钛苦橄岩,印度拉达克地区为低钛苦橄岩。     

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Picritic lava flows near Lijiang in the late Permian Emeishanflood basalt province are associated with augite-phyric basalt,aphyric basalt, and basaltic pyroclastic units. The dominantphenocryst in the picritic flows is Mg-rich olivine (up to 91·6%forsterite component) with high CaO contents (to 0·42wt %) and glass inclusions, indicating that the olivine crystallizedfrom a melt. Associated chromite has a high Cr-number (73–75).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 1630–1690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected Nd–Sr–Pbisotope ratios of the picritic and basaltic lavas are indistinguishableand cover a relatively small range [e.g. _Nd(t) = –1·3to +4·0]. The higher _Nd(t) lavas are isotopically similarto those of several modern oceanic hotspots, and have ocean-island-likepatterns of alteration-resistant incompatible elements. Heavyrare earth element characteristics indicate an important rolefor garnet during melting and that the lavas were formed byfairly small degrees of partial melting. Rough correlationsof isotope ratios with ratios of alteration-resistant highlyincompatible elements (e.g. Nb/La) suggest modest amounts ofcontamination involving continental material or a relativelylow-_Nd component in the source. Overall, our results are consistentwith other evidence suggesting some type of plume-head originfor the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian     

大火成岩省是地幔柱作用引起的吗?

自Morgan在1971年提出地幔柱假说来解释大洋岛链的火山迁移规律以来,对于地幔柱是否存在一直存在着争论。大火成岩省以短时间内的巨量喷发为特征,这种特殊现象一般被认为是地幔柱作用的结果,原因是它可以解释大火成岩省中的许多现象:(1)短时间内的巨量幔源岩浆;(2)喷发前>500 m的隆起;(3)高温苦橄岩和科马提岩的存在;(4)热点轨迹;(5)在没有挥发分和压力降低的条件下地幔发生熔融;(6)高的3He/4He比值。但是并不是所有大火成岩省都具有上述特征,而且上述现象有的也可以用其他机制来解释,如边缘对流、大规模岩石圈拆沉、裂谷减压和陨石撞击等。但是目前还没有一个模式可以解释所有大火成岩省的所有现象,而总体上大火成岩省的许多地质现象与地幔柱模式较为吻合。     

Rapid Change of Lava Composition from 1998 to 2002 at Piton de la Fournaise (Reunion) Inferred from Pb Isotopes and Trace Elements: Evidence for Variable Crustal Contamination

After an unusually long quiet period of nearly 6 years, in 1998the Piton de la Fournaise volcano started a new cycle of intensevolcanic activity. We report geochemical data on the first nineevents (53 samples), from the long-lived initial eruption (sixand a half months) of 1998 to the high-flux picritic eruptionof January 2002. Pb isotopes and trace elements display systematic,coupled variations, which are mostly confined to the beginningand the end of the period. Two well-defined binary mixing trendsare shown by Pb–Pb and Pb–trace element relationships.These trends indicate a change of end-member components betweenMarch and June 2001 that coincides with the transition fromsteady-state basalts to picrites. A three-component mixing modelinvolving a homogeneous plume and two contaminants successfullyexplains the data. The Pb–Pb relationship requires thattwo mixing processes occur successively: plume-derived magmainteracts first with altered oceanic crust, and the resultinghybrid product then interacts at shallower level with the oldlavas constituting the base of the volcanic edifice. Assimilationof edifice material decreased continuously from 1998 to 2002,whereas assimilation of oceanic crust drastically increasedduring the late-stage picritic eruption. These results suggestthat picrites may have resided for an unusually long time atan oceanic crustal level before ascending rapidly through thevolcanic edifice with little interaction with channel walls. KEY WORDS: assimilation; lead isotopes; picrites; Piton de la Fournaise; trace elements     

Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle?

The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K₂O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging ⁸⁷Sr/⁸⁶Sr= 0·70308 (SD = 0·00004) and ¹⁴³Nd/¹⁴⁴Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a _Os value of +13·6, typical forOIB. This contrasts with published ¹⁸⁷Os/¹⁸⁸Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negative_Os values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo₈₉, has Na₂O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; T_p 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at T_f 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (T_p 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer     

峨眉山大火成岩省中高Os苦橄岩的发现及地质意义

本文对峨眉山大火成岩省中苦橄岩及其共生的玄武岩进行了铂族元素(PGE)分析,结果表明苦橄岩比玄武岩的PGE含量要高至少一个数量级,并且具有明显高的Os含量,不仅比熔融程度最高的科马提岩要高,而且比原始地幔还要高,另外,还显示出超球粒陨石的Os/Ir比值(2.84～3.88)。其高的Os/Ir比值可能与岩浆上升过程中混入黑色页岩有关。部分熔融计算表明,含有0.01%硫化物的原始地幔 0.5%的外核在7%的熔融程度下,然后又被约10%的黑色页岩混染可以模拟原始岩浆的PGE含量。其Os含量及其他地球化学特征与其同时代的西伯利亚暗色岩系的相似性可能暗示了这两个大火成岩省来自于同一个起源于核-幔边界的超级地幔柱。另外,还根据苦橄岩和玄武岩PGE的含量估算了该地区PGE的成矿潜力。