西藏丁青宗白蛇绿混杂岩地球化学特征及其洋中脊叠加洋岛的成因

宗白蛇绿混杂岩位于班公湖—怒江缝合带东段丁青地区，是丁青蛇绿岩的重要组成部分。为确定该蛇绿混杂岩的构造属性，探讨其成因，对其进行了岩石学和全岩地球化学研究。结果表明，宗白蛇绿混杂岩存在两套成因不同的岩石。混杂岩中的玄武岩和辉长岩具有与洋岛玄武岩（OIB）相似的地球化学特征，其成因可能与来自地幔深部的“热点”作用有关。而混杂岩中的辉绿岩的地球化学特征与MORB相似，形成于洋中脊（洋内弧后盆地扩张中心）环境，并有受“热点”作用影响的印记；具有强烈亏损的地球化学特征的方辉橄榄岩则代表了洋内岛弧的基底残片，是已经亏损的地幔再次熔融产生玄武质岩浆后的残留物。这两套不同成因的岩石在板块汇聚过程中一起构造侵位于班公湖-怒江缝合带中。     

青藏高原东南木里地区二叠纪苦橄岩的Os-Sr-Nd同位素地球化学研究

本文报道了在青藏高原东南木里地区发现的二叠纪苦橄岩和与其共生玄武岩的主微量元素地球化学特征以及Os-Sr-Nd同位素组成。苦橄岩和与其共生玄武岩受地壳混染作用影响较小。根据苦橄岩的Ti/Y比值和初始的Os同位素组成,将木里苦橄岩分为两类:高Ti/Y型苦橄岩和低Ti/Y型苦橄岩,其中高Ti/Y型苦橄岩具有高的γ_(Os)= 5.3～ 10.7和ε_(Nd)= 5.9～ 6.4,与全球典型洋岛玄武岩的Os和Nd同位素组成接近,代表了地幔柱源区的同位素特征;而低Ti/Y型苦橄岩具有低的γ_(Os)=-4.1～ 1.2和ε_(Nd)= 3.2～ 5.0,可能表明受到了SCLM(大陆岩石圈地幔)源区物质的混染。与其共生的玄武岩具有低的γ_(Os)=-3.5～-1.6和ε_(Nd)=-0.6～ 0.7,表明其来自于不同于低Ti/Y型苦橄岩也有异于高Ti/Y型苦橄岩的地幔源区,但是也可能受到了SCLM物质的混染。基于Nd-Os同位素的地幔柱与SCLM的二端元混合模型显示:低Ti/Y型苦橄岩可能是SCLM物质组分与地幔柱起源的苦橄质原始岩浆混合形成的;与苦橄岩共生的玄武岩可能是由地幔柱来源的玄武质岩浆与SCLM小比例熔融的熔体混合形成的。     

确定南澜沧江缝合带的火山岩地球化学证据

本文着重根据火山岩的主量元素、稀土元素和微量元素特征厘定了我国西南“三江”地区昌宁－孟连带Ｃ１—Ｐ２火山岩和南澜沧江带Ｐ—Ｔ３火山岩的构造－岩浆类型。昌宁－孟连火山岩带为洋脊／准洋脊型－洋岛型玄武岩、蛇绿混杂岩带,代表澜沧江洋或古特提斯的残迹;南澜沧江火山岩带是一个陆缘火山弧带,是澜沧江洋盆向东俯冲消减、碰撞以及陆内俯冲过程的产物。两带火山岩是成对分布的古特提斯缝合带的产物。     

峨眉山大火成岩省西部苦橄岩及其共生玄武岩的地球化学：地幔柱头部熔融的证据

丽江地区的苦橄岩位于峨眉山大火成岩省的西部，其与辉斑玄武岩、无斑玄武岩和玄武质火山碎屑岩共生。苦橄岩中的斑晶主要为富镁橄榄石，其F0含量最高达91．6％，CaO含量最高达0．42％，其内含有少量玻璃包裹体，指示了橄榄石是在熔体中结晶形成的。苦橄岩中的铬尖晶石具有高的Cr#值（73-75）。计算的初始岩浆的MgO含量大约为22wt％，初始熔融的温度为1630-1680℃。研究结果表明，玄武质岩石是苦橄质岩浆通过橄榄石和单斜辉石分离结晶形成的。苦橄岩和玄武岩的Nd-Sr-Pb同位素比值差别不大，只落在一个很小的范围内（如εNd（t）=-1．3 to＋4．0）。高的εNd（t）值以及抗蚀变不相容元素的原始地幔标准化图解与洋岛玄武岩相似，并且其重稀土元素特征指示了源区有石榴子石的残余，而且是低部分熔融的产物。同位素比值与抗蚀变不相容元素比值（如Nb／La）的相关性表明，岩浆形成过程中有少量的大陆地壳物质或者相对低εNd（t）组分的大陆岩石圈地幔的混染。因此，总体上，苦橄岩的地球化学特征的研究结果支持了峨眉山大火成岩省是地幔柱头部熔融的成因模型。     

“三江”哀牢山带蛇绿岩特征研究

哀牢山带蛇绿岩由变质橄榄岩、堆晶杂岩和基性熔岩组成。其中二辉橄榄岩近似原始地幔岩,方辉橄榄岩为残留地幔岩。辉长岩－辉绿岩－辉石玄武岩系列及辉石岩－辉长闪长岩－钠长玄武岩－苦橄玄武岩系列分别为原始二辉橄榄岩经部分熔融产生的拉斑玄武岩浆及苦橄玄武岩浆结晶或结晶分异演化而成;前者具有洋脊玄武岩特征,后者具有准洋脊玄武岩特征,它们形成于大洋中脊环境。其形成时代不晚于早石炭世（Ｃ１）,侵位在晚三叠世一碗水组（Ｔ３ｙ）之前。     

滇西南曼信夭折堆积岩的确认及其意义

产于滇西南孟连曼信的苦橄岩,前人定为苦橄玢岩岩脉。经研究发现该苦橄岩普遍发育堆积结构,同时又普遍发育枕状构造且基质具淬火结构,并且与石炭纪洋脊型－准洋脊型玄武岩具有成因演化关系。这表明该苦橄岩不是岩脉,而是水下喷出相的夭折堆积岩,是因洋底扩张速度改变而导致岩浆房状态改变的结果,这也为澜沧江洋的扩张作用提供了佐证。     

阿尔泰造山带南缘中泥盆世苦橄岩及其大地构造和岩石学意义

阿尔泰造山带南缘中泥盆世苦橄岩位于北塔山组地层的下部, 其上依次为玄武岩和安山岩.3种岩性共同的特点是贫钛、富铁, 具Nb和Ta的负异常以及高场强元素的丰度与MORB相当, 具有典型的岛弧火山岩系的特点, 是准噶尔洋板块向南西俯冲的结果.苦橄岩和玄武岩的Zr/Nb和Sm/Nd比值与MORB相当, 表明其源区为亏损的MORB源.然而玄武岩的Ti/V和Zr/Sm比值均高于苦橄岩, 而且玄武岩的稀土元素配分曲线呈平缓型, 而苦橄岩则显示出低的稀土总量以及弱富集轻稀土型, 指示了玄武岩是被从俯冲的洋壳释放的流体交代的含角闪石的尖晶石橄榄岩的地幔源区低程度部分熔融形成的, 苦橄岩则是在高温条件下被流体交代过的石榴石橄榄岩高程度熔融的产物.安山岩则可能是榴辉岩部分熔融形成的.      

滇西北羊拉铜矿区两套玄武岩的成因及其对古特提斯洋演化的约束

西南三江地区地处冈瓦纳大陆与欧亚大陆结合带,是古特提斯构造域的东延部分。伴随古特提斯洋的闭合,区域内自西向东形成了澜沧江缝合带、昌宁-孟连缝合带、金沙江缝合带及甘孜-理塘缝合带,其中昌宁-孟连缝合带被认为是古特提斯洋主洋盆闭合后的残余。作为古特提斯洋的重要分支,金沙江洋的形成与演化长期受     

云南宾川-永胜-丽江地区低钛玄武岩和苦橄岩的岩石成因与源区性质

云南宾川-永胜-丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量摘要云南宾川—永胜—丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量的苦橄质玄武岩、苦橄岩和麦美奇岩。大部分火山岩的岩石化学组成属于拉斑玄武岩系列,少数低钛玄武岩属碱性玄武岩系列。它们不同程度地富集大离子亲石元素和轻稀土元素,相对亏损重稀土元素,稀土元素分馏明显,显著亏损相容元素(Co,V,Cr,Ni)。陆壳物质对低钛玄武岩浆的混染程度明显大于对苦橄质岩浆的影响程度。而且混染作用对于Sr同位素和大离子亲石元素的影响程度明显大于对Nd同位素和稀土元素的影响程度。Nd和Sr同位素证明,混染物主要是下地壳变质岩,也有少量上部陆壳物质。未受混染的样品具有适度亏损的Nd、Sr同位素组成。低钛玄武岩和苦橄岩类岩石是不同原生岩浆分异演化的产物。低钛玄武岩的原生岩浆是高镁拉斑玄武岩浆,原生苦橄质岩浆以EM-55为代表(MgO= 16.56%)。此外,还有一种比EM-55更富镁的原生岩浆。高镁拉斑玄武岩浆分异过程中的主要分离结晶相/堆晶相是单斜辉石,并有少量斜长石。苦橄岩浆分异过程中的主要分离结晶相/堆晶相是橄榄石,并有少量单斜辉石。参考相关的实验岩石学成果,可以证明,地幔柱源区由两种岩石组成:一种是50%榴辉岩和50%橄榄岩反应形成的石榴石辉石岩,另一种是橄榄岩。在地幔柱绝热上升过程中,位于其轴部的石榴石辉石岩的熔融作用始于≈165km,主要的熔融作用发生于165～128km,持续到66km,熔融产物为苦橄岩浆。橄榄岩的部分熔融始于≈150km,持续到66km,熔融产物是比EM-55更富镁的岩浆。地幔柱头部的熔融作用始于≈100km,终止于66km,主要的熔融作用发生于尖晶石稳定域,熔融产物为低钛玄武岩浆。     

西藏雅鲁藏布江缝合带西段东波蛇绿岩的构造背景特征

西藏东波蛇绿岩位于雅鲁藏布江缝合带西段,由地幔橄榄岩、辉石岩和辉长岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和少量二辉橄榄岩。岩体的边界出露玄武岩和硅质岩等。地幔橄榄岩中有少量辉石岩和辉长岩的脉岩,宽约1 m,走向北西,与岩体的构造线方向基本一致。各岩相岩石地球化学研究结果表明,东波蛇绿岩的岩相存在较大的差异,玄武岩具有与洋岛玄武岩(OIB)相似的地球化学特征,而地幔橄榄岩中辉石岩、辉长岩脉与洋中脊玄武岩(MORB)相似,形成于洋中脊环境,并受后期俯冲流体作用的改造。东波岩体中二辉橄榄岩具有与深海地幔橄榄岩较一致的轻稀土亏损特征,而方辉橄榄岩和纯橄岩的地球化学特征显示出岩体形成于MOR环境,后受到SSZ环境的改造。东波蛇绿岩的岩石地球化学特征显示其洋中脊叠加洋岛的构造背景。     

中国西南特提斯构造演化—幔柱构造控制

基于对中国西南特提斯巨型造山系的时空结构和构造－岩浆事件分析研究提出．泥盆－石炭纪时期出现于昌都－思茅陆块两侧的热幔柱导致了金沙江洋和澜沧江洋成对打开，热幔柱岩浆作用沿洋脊产出苦橄玄武岩和洋岛玄武岩，并造成区域地球化学异常。二叠纪末期出现于昌都－思茅－印支中央陆块下的冷幔柱导致了两大洋向该陆块下俯冲消减，陆块两缘发育沟－弧－盆体系，构成冷幔柱的洋壳板片在２００Ｍａ时期堆积沉落，诱发板块后继俯冲，产生滞后型孤火山－岩浆岩。发育于冈瓦纳大陆北缘的德干热幔柱在株罗纪导致怒江洋和雅鲁藏布江洋相继打开，在白垩纪末期（６６Ｍａ）形成德干玄武岩省。发育于劳亚大陆南缘的峨眉热幔柱在二叠纪，导致峨眉火成岩省的形成，在早中三叠世使甘孜－理塘断裂带扩张成洋。冷幔柱的持续发生，决定了雅鲁藏布江洋和甘孜－理塘向昌都－思茅陆块方向的俯冲消减，以及来自冈瓦纳大陆和劳亚大陆陆块分别向昌都－思茅陆块南北两侧拚贴和碰撞。     

Geochemistry,petrogenesis and tectonic significance of the basic volcanic units of the Zildat ophiolitic mélange,Indus suture zone,eastern Ladakh (India)

Abstract

Basic volcanic rocks within the Zildat ophiolitic mélange of Indus suture zone in eastern Ladakh are medium to fine grained with partially preserved primary texture and mineralogy. These rocks are predominantly alkaline basalt with high Nb/Y and enriched incompatible trace element characteristics, similar to those of the oceanic island basalt (OIB). The minor sub-alkaline basaltic rocks resemble N-type mid ocean ridge basalt (N-MORB) but with much lower abundances of incompatible trace element including REE. The alkaline rocks probably generated through variable, but low degrees of partial melting of enriched mantle source and evolved through high pressure olivine and clinopyroxene fractionation. Low pressure plagioclase and Fe- Ti oxide do not appear to be major fractionating phases. Limited data on the sub-alkaline rocks suggest that their parental melts were derived from mantle sources some what similar to that of N- MORB. Significant role of added cumulates of olivine, clinopyroxene and Fe- Ti oxides is also indicated in their genesis. Ophiolitic mélanges all along the Indus suture zone appear to have formed due to the accumulation of mélange material in the upper part of the subduction zone where they suffered glaucophanitic (blueschist) metamorphism and retrograded partially to greenschist grade as these were subsequently obducted to its present position probably during the Cenozoic Himalayan orogeny due to collision of Indian and Eurasian plates.     

Compositional Variation and Mineral Chemistry of the Jinshajiang and Lancangjiang Serpentinites,Yunnan Province,SW China

We present the whole rock and the mineral chemical data for upper mantle peridotites from the San-Jiang region in Yunnan, SW China. These peridotites are a part of a Paleo-Tethyan ophiolite belt occurring along the Jinshajiang and Lancangjiang suture zones. All samples of the Jinshajiang and Lancangjiang ultramafic rocks are completely serpentinized. The Jinshajiang serpentinites are characterized by no relict of olivine and pyroxene, and the Cr# content of spinels is 0.32–0.49. The Lancangjiang serpentinites were collected from two different locations; the northern location which has some relict of Opx(Al2O3 is 0.13–2.2 wt%, TiO 2 is 0.004–0.057 wt% and Mg# content is 0.895–0.933) and the Cr# content of spinel is 0.26-0.55; the southern location, which has some relict of Olivine(Fo = 90–92.5 and NiO = 0.12–0.26 wt%), and spinel Cr# ranging from 0.41 to 0.57. The whole rock geochemical and the mineral chemistry data imply that the Jinshajiang and Lancangjiang serpentinites represent abyssal peridotites residues after ~15–20% partial melting for the Jinshajiang and Lancangjiang serpentinites(south location), and ~11–19% partial melting for the Lancangjiang serpentinites(north location). In addition, the compositional trends of the spinel analyses of the Lancangjiang serpentinites imply that the MORB melt-peridotite interaction process played a significant role during their evolution. These processes are evidenced by an increase in Cr# with an increase in TiO 2, whereas the spinel analyses of the Jinshajiang serpentinites display an increase in Cr# with a decrease in Ti O2, indicating that the Jinshajiang serpentinites were subjected to a simple partial melting process.     

Isotope geochemistry of Jeongok basalts,northernmost South Korea: Implications for the enriched mantle end-member component

South Korea separates two mantle source domains for Late Cenozoic intraplate volcanism in East Asia: depleted mid-ocean-ridge basalt (MORB) mantle-enriched mantle type 1 (DMM-EM1) in the north and DMM-EM2 in the south. We determined geochemical compositions, including Sr, Nd, Pb, and Hf isotopes for the Jeongok trachybasalts (∼0.51 to 0.15 Ma K–Ar ages) from northernmost South Korea, to better constrain the origin and distribution of the enriched mantle components. The Jeongok basalts exhibit light rare earth element (LREE)-enriched patterns ([La/Yb]_N = 9.2–11.6). The (La/Yb)_N ratios are lower than that of typical oceanic island basalt (OIB). On a primitive mantle-normalized incompatible element plot, the Jeongok samples show OIB-like enrichment in highly incompatible elements. However, they are depleted in moderately incompatible elements (e.g., La, Nd, Zr, Hf, etc.) compared with the OIB and exhibit positive anomalies in K and Pb. These anomalies are also prime characteristics of the Wudalianchi basalts, extreme EM1 end-member volcanics in northeast China. We have compared the geochemistry of the Jeongok basalts with those of available Late Cenozoic intraplate volcanic rocks from East Asia (from north to south, Wudalianchi, Mt. Baekdu and Baengnyeong for DMM-EM1, and Jeju for DMM-EM2). The mantle source for the Jeongok volcanics contains an EM1 component. The contribution of the EM1 component to East Asian volcanism increases toward the north, from Baengnyeong through Jeongok to Mt. Baekdu and finally to Wudalianchi. Modeling of trace element data suggests that the Jeongok basalts may have been generated by mixing of a Wudalianchi-like melt (EM1 end-member) and a melt that originated from a depleted mantle source, with some addition of the lithospheric mantle beneath the Jeongok area. In Nd–Hf isotope space, the most enriched EM1-component-bearing Jeongok sample shows elevation of ¹⁷⁶Hf/¹⁷⁷Hf at a given ¹⁴³Nd/¹⁴⁴Nd compared with OIB. Recycled pelagic sediments may explain the EM1-end-member component of northeastern Asian volcanism, possibly from the mantle transition zone.     

南澜沧江结合带火山岩岩浆成因——洋脊洋岛与弧岩浆作用的性质

在对滇西南昌宁—孟连带石炭—二叠纪火山岩和南澜沧江带二叠—三叠纪火山岩的微量元素和稀土元素研究的基础上,划分了岩浆作用类型和岩浆演化系列,探讨了岩浆源区成分特点。昌宁—孟连带玄武质岩浆的演化主要受部分熔融和分离结晶作用控制;这些岩浆可以划分为３个岩浆演化系列：（１）稀土曲线平坦型的洋脊型拉斑玄武岩浆演化系列,其岩浆起源于亏损型地幔;（２）稀土曲线中等富集型的准洋脊型拉斑玄武岩浆演化系列,其岩浆起源于过渡型地幔;（３）稀土曲线强烈富集型的洋岛型碱性玄武岩浆演化系列,其岩浆起源于富集型地幔。南澜沧江带弧火山岩的岩浆成因主要受分离结晶作用控制,也可以划分为３个岩浆演化系列：（１）二叠纪低钾拉斑—中钾钙碱性岩浆演化系列;（２）中南段的晚三叠世低钾拉斑—中钾钙碱性岩浆演化系列;（３）北段的晚三叠世钾玄岩—高钾钙碱性岩浆演化系列     

大洋中脊、洋岛、岛弧玄武岩中橄榄石的对比研究

在岩浆结晶演化过程中,橄榄石是最早结晶的矿物之一,其化学成分不仅受到母岩浆成分的控制,而且还受到岩浆的结晶分异、部分熔融程度、压力、岩浆混合作用以及橄榄石与残余岩浆相互作用等因素的影响。学术界基本的认识是,橄榄石的成分主要与源区、分离结晶作用以及岩浆演化程度有关,是鉴别玄武岩是否为原始岩浆的重要证据,根据橄榄石的Fo值可以恢复岩浆是富镁还是富铁的。之前的学术界很少认为橄榄石与其形成的构造环境有关。本文在对全球大洋中脊玄武岩（MORB）、洋岛玄武岩（OIB）、岛弧玄武岩（IAB）中橄榄石主量元素特征进行了研究和对比后发现,橄榄石也有判断构造环境的功能,尤其OIB 橄榄石在许多元素含量上与MORB 和IAB 的橄榄石不同,可以通过Ni、P、Mg、Fe、K 和Mn等元素之间的关系进行较好的区分。目前的情况是,OIB 与IAB 中的橄榄石在某些情况下也能够区分开,但MORB 与IAB 中的橄榄石很难区分,这是需要进一步研究的。全球全体数据研究表明,OIB 的橄榄石富Ni是一个很重要的现象,本文认为,MORB 和IAB 来源于亏损的上地幔,故其橄榄石的Fo值高;OIB来源于富集的下地幔,可能Fe-Ni地核对OIB橄榄石化学成分有比较明显的影响,故导致OIB的橄榄石贫Fo和富Ni。     

大陆板内玄武岩数据挖掘:成分多样性及在判别图中的表现

通常认为,大陆溢流玄武岩(CFB)、裂谷玄武岩(CRB)、板内玄武岩(WPB)均产于板内构造环境,其地球化学特征与OIB类似,源于富集的下地幔,与地幔柱的活动有关。本文利用GEOROC数据库对全球CFB、CRB和WPB数据进行挖掘,发现上述三类玄武岩判别图投图几乎落入了全部的构造环境域,有些甚至主要落入MORB和IAB区,而不是落入WPB区。结果表明原先的玄武岩判别图的判别功能值得商榷,尤其对大陆玄武岩来说,许多判别图都存在问题。全体CFB、CRB和WPB的地球化学成分变化巨大,暗示其源区具有强烈的不均一性:部分CFB、CRB和WPB来自富集的地幔柱,仍然具有经典的OIB的特征;部分来自MORB的源区,与MORB的再循环作用有关;部分来自岛弧岩石圈之下的亏损地幔源区,以强烈亏损Nb-Ta为特征,类似岛弧玄武岩的地球化学特征。许多地区的大陆玄武岩可分为低钛和高钛两类,低钛玄武岩大多是亏损或强烈亏损的,而高钛玄武岩通常是富集型的。本文的研究表明,富集型大陆玄武岩可能来自富集的下地幔,而亏损的和强烈亏损的玄武岩可能来自具有MORB或岛弧特征的软流圈地幔。进一步指出,源区性质可能是大陆玄武岩多样性的主控因素,其次为部分熔融程度、熔融深度、结晶分离、陆壳混染以及AFC过程。     

阿尔金红柳沟蛇绿岩研究进展

实测了2条阿尔金地区红柳沟蛇绿混杂带的地质剖面.地球化学的研究发现混杂带中含有2种基性火山岩:MORB型和OIB型,其稀土配分型式分别为平坦型和轻稀土富集型,结合同位素ε_Nd值,推断混杂带中变基性火山岩来自于不同的岩浆源.变质橄榄岩稀土总量低,具有亏损型和平坦型2种不同的稀土配分型式.这种MORB和OIB的组合代表了地幔柱物质与洋壳物质的混合作用.     

Petrology,geochemistry and Ar-Ar geochronology of eclogites in Jinshajiang orogenic belt,Gonjo area,eastern Tibet and restriction on Paleo-Tethyan evolution

As one of the important Paleo-Tethys suture zones in eastern Tibet, the Jinshajiang orogenic belt is of great significance to study the tectonic evolution of the main suture zone of Paleo-Tethys. In this paper, eclogites developed in the Jinshajiang suture zone in Gonjo area, eastern Tibet, are selected as specific research objects, and petrological, geochemical and Ar-Ar geochronological analyses are carried out. The major element data of the whole rock reveals that the eclogite samples have the characteristics of picritic basalt-basalt and belong to the oceanic low potassium tholeiites. The results of rare earth elements and trace elements of the samples show that the protoliths of eclogites have characteristics similar to oceanic island basalt (OIB) or normal mid ocean ridge basalt (N-MORB). Muscovite (phengite) from two eclogite samples yield the Ar-Ar plateau ages of 247±2 Ma and 248±2 Ma respectively, representing the peak metamorphic age of eclogite facies and the timing of complete closure of the Jinshajiang Paleo-Tethys Ocean. Muscovite and biotite selected from the hosting rocks of eclogite yield the Ar-Ar plateau ages are 238±2 Ma and 225±2 Ma respectively, reflecting the exhumation age of eclogites and their hosting rocks. Combined with the zircon U-Pb dating data (244 Ma) of eclogites obtained in previous work, it can be concluded that the Jinshajiang Paleo-Tethys ocean was completely closed and arc-continent collision was initiated at about 248–244 Ma (T₂¹). Subsequently, due to the large-scale arc (continent)-collision orogeney between Deqin-Weixi continental margin arc and Zhongza block (T₃¹–T₃²), the eclogites were rapidly uplifted to the shallow crust.©2023 China Geology Editorial Office.     

Stirring geochemistry in mantle convection models with stiff plates and slabs

Numerical models of mantle convection are presented that readily yield midocean ridge basalt (MORB) and oceanic island basalt (OIB) ages equaling or exceeding the apparent ∼1.8-Ga lead isotopic ages of trace-element heterogeneities in the mantle. These models feature high-viscosity surface plates and subducting lithosphere, and higher viscosities in the lower mantle. The formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component. The models are run at the full mantle Rayleigh number and take account of faster mantle overturning and deeper melting in the past. More than 97% of the mantle is processed in these models. Including the expected excess density of former oceanic crust readily accounts for the depletion of MORB source relative to OIB sources. A novel finding is of gravitational settling of dense tracers within the low-viscosity upper mantle, as well as at the base of the mantle. The models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle. These results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle.