上地幔高导物质的连通性和电导率的实验研究进展

矿物/岩石的电学性质是认识地球内部物质与结构的重要依据.高导物质(熔体、流体、石墨、磁铁矿等)能否显著影响上地幔的电性结构,取决于这些物质在上地幔中的连通性、电学性质和体积分数.近年来的研究表明,挥发分(水、二氧化碳)能够有效的降低橄榄岩的固相线,并提高玄武岩熔体的电导率.因此我们可以尝试用少量的熔体去解释上地幔某些区域的高导异常.但目前在高温高压下对这些熔体连通性的研究仍不足,我们并不清楚少量熔体在三维条件下是以何种形式连通的?俯冲带中的水主要储存在孔隙和含水矿物中.地慢矿物的蚀变,含水矿物脱水的过程中将产生含高浓度电解质的流体.二面角的观测证明了在高温高压下流体能在橄榄石间保持连通,因此我们可以认为这些高导流体对俯冲带的电性结构有很大程度的影响.石墨在橄榄岩体系中具有很高的二面角,不大可能以连通薄膜的形式在上地慢广泛存在,但石墨能否以网状形式连通?与机械的将蛇纹石和磁铁矿混合实验相比,绿泥石脱水产生的磁铁矿展示了更好的连通性.这意味着今后研究固相高导物质的连通性时不能忽略矿物在自然条件下的结构特征.     

中国东部大地幔楔形成时代和华北克拉通岩石圈减薄新机制——深部再循环碳的地球动力学效应

地震P波速度成像显示,西太平洋俯冲板片滞留在地幔过渡带,在中国东部形成"大地幔楔"结构.中国东部大陆玄武岩的Mg同位素调查揭示了该俯冲滞留板片携带大量碳酸盐交代地幔过渡带上覆的对流地幔,形成了碳酸盐化橄榄岩,它是中国东部晚白垩世和新生代大陆强碱性玄武岩的源岩.该玄武岩的Mg-Sr同位素组成与年龄关系显示自106Ma以来,该地幔源区的碳酸盐种属为菱镁矿+少量白云石,发生初熔的深度为300~360km.因此,该地幔源区的碳酸盐化交代作用应发生在大于360km的深度,即在106Ma以前俯冲板片已经开始滞留地幔过渡带,形成了大地幔楔结构.这一时代支持大地幔楔的俯冲板片后撤形成机制.根据高温高压实验结果,碳酸盐化橄榄岩熔融产生的含碳酸盐硅酸盐熔体在到达至华北克拉通初步减薄的岩石圈底部(180~120km)时仍可以有高达25~18wt%的CO_2含量,其SiO_2和含量及类似于强碱性的霞石岩和碧玄岩,并具有较高的εNd值(2~6).该熔体向上渗透并交代底部岩石圈地幔可形成碳酸盐化橄榄岩.由于克拉通地热增温线与碳酸盐化橄榄岩固相线相交于130km深度,则华北克拉通岩石圈底部的碳酸盐橄榄岩将发生部分熔融导致其物理性质类似软流圈且较容易被对流上地幔置换.其新生成的碳酸盐-硅酸盐熔体又可以向上渗透交代上覆的岩石圈地幔.如此重复这一碳酸盐化交代-熔融过程,可以使岩石圈减薄,而碳酸盐-硅酸盐熔体也转变呈较富硅和具有较低εNd值(低至-2)的碱性玄武岩.随着华北克拉通岩石圈的减薄,其地热增温线逐步向大洋岩石圈靠近,岩石圈碳酸盐化橄榄岩的初熔深度可小于130km和逐步接近70km.因此,富碳酸盐熔体与岩石圈相互作用是华北克拉通岩石圈减薄过程在晚白垩世和新生代的一种可能机制.据中国东部低玄武岩的年龄统计,106~25Ma区间岩石圈的碳酸盐化交代-熔融引发的减薄过程仅零星存在,而在25Ma以后才大规模发生.因此,华北克拉通岩石圈的减薄可能存在两个峰期:与克拉通破坏峰期(135~115Ma)同时发生的岩石圈减薄,和25Ma以后由富碳酸盐硅酸盐熔体与岩石圈相互作用引发的岩石圈进一步减薄,它使得华北克拉通东部岩石圈减薄至现今的约70km左右.     

鲁南地区大地电磁测深结果

本文根据10大个地电磁测深点的探测资料,研究了鲁南地区深部电性结构特征。结果表明,在测区上地幔内存在两个高导层,第一高导层出现在上地幔较浅部位,厚度为3—7公里,电阻率值为8一30Ω·m,该层的埋藏深度横向变化较大,从50公里到80公里;第二高导层出现在230到280公里,电阻率值仅为几Ω·m,深度变化比较小,该区未发现壳内高导层的存在。本文还初步探讨了地壳,上地幔电性结构与区域构造和地震活动之间的关系。     

大别山超高压变质带深部电性结构及其动力学意义初步研究

通过在大别造山带东部横穿超高压变质带的一条NNE向剖面大地电磁测深资料的分析解释,获得了关于沿剖面的地壳上地幔二维电性结构,显示北淮阳与大别地块是电性差异显著的构造单元,它们之间的界面与晓天—磨子潭断裂对应;晓天—磨子潭断裂倾向北,在中上地壳层位出现错动解耦现象;从地表向深处可划分出4个主要电性层：地表风化层、中上地壳高阻层、壳内相对高导层以及上地幔层;大别地块内中、上地壳层位以高阻层为主,与高压-超高压变质岩分布区对应,高阻层最厚处在岳西—英山之间;在大别地块内,推测存在燕山期花岗质岩浆活动的通道,它们造成了超高压变质岩的进一步抬升,同时影响了大别地块内存在的壳内相对高导层的分布,壳内相对高导层在层位上相差较大.     

我国东北与华北上地幔某些物理状态特征的比较:来自幔源包体的信息

研究由玄武岩岩浆携带到地表的幔源橄榄岩包体可以获得上地幔的许多信息。本文根据对采自我国东北和华北地区的某些幔源橄榄岩包体样品的研究结果,对这两个样品产地地区上地幔的某些物理状态特征,如温度、压力、流动应力、流动速率及等效粘滞度等作了初步比较。资料表明,分属东北与华北两区样品产地的上地幔自新生代(上新世—更新世)以来显示出裂谷的性质,但是,两区上地幔的古地温和其它一些物理状态参数存在一定的差别。     

江苏响水—内蒙满都拉地壳上地幔电性结构初探——地学断面研究报道之三

本文着重介绍了响水一满都拉断面的深部电性结构特征。在华北地台北缘的构造边界两侧电性差异明显,壳内高导层埋深由21公里跃变到34公里;在南缘的郯庐断裂带及东侧壳内高导层缺失,上地幔出现两个高导层。在华北地台内新生代断陷盆地壳内高导层较为发育,埋藏较浅。上地幔高导层的埋深为60—127公里,总的趋势是西北深,东南浅,在呼包盆地、冀中拗陷和郯庐带下方形成局部上隆区,较好地反映了区域地质以及深部构造运动的特点     

上地幔流变性质的研究进展

本文主要总结了近十多年来橄榄岩流变实验的研究成果,介绍了上地幔流变研究的进展和意义.经过多年的实验研究,橄榄岩流变性质在理论和应用方面都已取得重要突破.人们利用高温高压实验深入研究了熔/流体、铁含量、粒径尺度、温压条件等因素对橄榄石流变性质的影响,得到了表征橄榄石流变特性的幂率本构方程的各参数,并从理论方面探讨了橄榄石等矿物在不同深度处可能存在的变形机制.在应用方面,高温高压流变研究为许多地质现象(如软流圈的成因)的解释和地球动力学(如地幔对流)的模拟提供了实验依据和基础数据,如辉石的粒度效应可以解释上地幔板块边界变形及剪切局域化问题等.尽管对上地幔矿物的流变特性进行了深入地研究,但还有许多问题需要人们进一步厘定,如碳酸盐熔体对橄榄石流变性质的影响,橄榄石变形机制的转化等.     

由包体推导的河北汉诺坝下地壳-上地幔地温线及其地质意义

通过对采自河北汉诺坝玄武岩中的下地壳和上地幔包体的详细研究 ,建立了本区下地壳—上地幔地温线。该地温线高于大洋地温线和古老地盾地温线 ,接近克拉通边缘的地温线 ,符合该区的大地构造环境。由该地温线建立的下地壳—上地幔地质结构剖面表明 ,该区下地壳主要由不同类型的麻粒岩相岩石组成 ,其化学成分以镁铁质为主 ,深度范围为 2 5～ 4 2km。上地幔由超镁铁质的二辉橄榄岩组成 ,在尖晶石二辉橄榄岩和石榴石二辉橄榄岩之间有一过渡层。由地温线确定的壳幔边界位于 4 2km附近 ,与地震资料确定的莫霍面一致 ,但在壳幔边界之上的下地壳底部有下地壳麻粒岩和超镁铁质岩的互层。这一现象可以解释在下地壳底部常见的层状反射层。该区岩石圈底界大约在 95km ,其下的软流层仍由石榴石二辉橄榄岩组成     

唐山地区深部电性研究

大地电磁测深结果表明,唐山地区深部电性中存在壳内高导层和上地幔高导层,其分别埋深为20公里和100公里左右,均显示为南浅北深。资料对比表明唐山、宁河地震都发生在两个高导层埋深变化梯度带上,其震源深度都不超过壳内高导层的埋深。此外,在该区南部发现埋深约45公里高导层,我们认为有可能为上地幔中间高导层     

华北克拉通东部岩石圈地幔碳酸盐熔体交代作用与克拉通破坏

华北克拉通东部在早白垩世经历了显著的破坏作用,不同性质熔/流体活动对岩石圈地幔的改造在克拉通破坏过程中起到了至关重要的作用.碳酸盐熔体作为一种非常重要的地幔交代介质,可以对岩石圈地幔的物理化学性质产生显著影响,并在地幔橄榄岩的单斜辉石中留下独特的地球化学指纹(例如,高Ca/Al、低Ti/Eu比值等),为示踪地幔碳酸盐熔体交代作用提供了依据.文章系统总结了华北克拉通破坏最为显著的东部地区岩石圈地幔经历碳酸盐熔体交代作用的时空变化规律,揭示出三种不同类型的地幔碳酸盐熔体交代作用.第一类碳酸盐熔体交代作用形成的单斜辉石具有异常高的Ca/Al比值(15～70)和富集的Sr同位素组成(~(87)Sr/~(86)Sr=0.706～0.713);第二类碳酸盐熔体交代作用形成的单斜辉石具有相对较高的Ca/Al比值(5～18)和Sr同位素组成(~(87)Sr/~(86)Sr=0.703～0.706);第三类碳酸盐熔体交代作用形成的单斜辉石具有略微偏高的Ca/Al比值(5～9)和低Sr同位素组成(~(87)Sr/~(86)Sr=0.702～0.704).时间上,华北克拉通东部苏鲁造山带及其邻区晚三叠世之前深部石榴石相和浅部尖晶石相岩石圈地幔均经历了第一类碳酸盐熔体交代作用的强烈改造,这可能与携带碳酸盐岩的华南陆块向华北克拉通深俯冲过程中碳酸盐熔体-橄榄岩反应作用有关.第一类碳酸盐熔体交代作用可以显著弱化岩石圈地幔的强度,可能为华北克拉通东部岩石圈地幔的最终破坏提供了关键前提条件.华北克拉通破坏峰期之后(晚白垩世-新生代)火山岩捕获的古老岩石圈地幔残余(全部为尖晶石相方辉橄榄岩)只记录了第二类碳酸盐熔体交代作用的改造,表明那些经历了第一类碳酸盐熔体交代作用强烈改造的古老岩石圈地幔在克拉通破坏过程中并未保留下来,已经被完全改造.相比之下,火山岩中的二辉橄榄岩地幔包体代表了克拉通破坏之后新生、饱满的岩石圈地幔,其中只有部分样品记录了第二类或者第三类碳酸盐熔体交代作用的改造,反映了克拉通破坏之后含有不同比例再循环地壳物质的软流圈来源碳酸盐熔体的改造作用.空间上,胶东及其邻区岩石圈地幔经历碳酸盐熔体交代作用的特征最明显,这与该地区岩石圈地幔减薄和克拉通破坏最显著以及巨量金成矿的特征具有空间一致性.所有这些特征表明碳酸盐熔体交代作用在华北克拉通岩石圈地幔改造、破坏和大规模金成矿过程中可能起到了至关重要的作用.     

高温高压下碳酸盐熔体对地幔岩电导率的影响

为了观测含碳酸盐地幔岩部分熔融过程中电导率的变化,厘清碳酸盐熔体在金伯利岩岩浆形成过程中所起的作用,并探讨Slave克拉通中部Lac de Gras地区约80～120km深处的高导成因,我们利用DS 3600t六面顶压机和Solartron 1260阻抗/增益-相位分析仪在1.0～3.0GPa、673～1873K温压条件下分别测量了含碳酸钠(Na_2CO_3)、碳酸钙(CaCO_3)和大洋中脊玄武岩(MORB)的地幔岩样品的电导率.实验结果表明,地幔岩样品的电导率主要受到温度和组分的影响,而压力对其影响较小.在温度低于1023K时,含Na_2CO_3地幔岩样品的电导率明显高于含同比重CaCO_3和MORB的;温度达到1023K时,含Na_2CO_3地幔岩样品开始熔融;但在之后的200K温度区间内,该部分熔融样品的电导率随温度的增加几乎不发生变化.这一现象或许揭示:地幔深部的碳酸质岩浆在快速上升过程中会同化吸收岩石圈地幔中的斜方辉石(Opx),进而形成金伯利岩岩浆,期间岩浆的电导率几乎不发生变化.含CaCO_3和MORB的地幔岩样品分别在1723K和1423K开始熔融,其部分熔融样品的电导率随温度的增加而快速增加.依据前人的研究结果和我们的实验结果,我们认为可以用含碳酸盐的部分熔融样品来解释Slave克拉通中部Lac de Gras地区约80～120km深处的异常高导现象,并推测熔体中碳酸盐的熔体比例小于2wt.%.     

Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean)

Abyssal peridotites collected along the highly oblique-spreading Lena Trough north of Greenland and Spitsbergen have mineral compositions that are similar to residual abyssal peridotites, except for high sodium concentrations in clinopyroxene (cpx). Most samples are lherzolites with light rare earth element (REE)-depleted cpx trace element patterns, but significantly fractionated middle to heavy REE ratios at relatively high heavy REE concentrations. Such characteristics can only be explained by initial melting of a garnet peridotite followed by low degrees of melting in the stability field of spinel peridotite. The residual garnet signature requires either a high potential temperature of the upwelling mantle, or elevated solidus-lowering water contents. The limited spinel field melting suggests a deep cessation of melt extraction, possibly because of the presence of a thick lithospheric cap. This is consistent with the extremely low effective spreading rate and the vicinity to a passive continental margin, which allow conductive cooling to reach deeper levels than commonly estimated for faster mid-ocean ridges. High sodium concentrations in cpx are neither explainable by melt refertilization, nor by a simple diffusion mechanism. The efficient fractionation of sodium from the light REE requires post-melting metasomatism, which is typically restricted to the subcontinental lithosphere. This might imply that the Lena Trough peridotites represent unroofed subcontinental mantle, from which no melt was extracted during the opening of the Lena Trough. It is more likely that sodic metasomatism occurred after partial melting underneath the Lena Trough, and that such an enrichment process is responsible for elevated sodium concentrations in abyssal peridotites elsewhere. Sodium in cpx of residual peridotites can therefore not serve as an indicator of partial melting or melt refertilization.     

基于电性结构模型的青藏高原东缘上地幔热结构研究

研究青藏高原东缘地区的深部物质结构对于理解青藏高原的隆升及扩张机制具有重要的科学意义.本文将青藏高原东缘实测大地电磁测深剖面反演所得的岩石圈电性结构模型与高温高压岩石物理实验测得的上地幔矿物和熔融体导电性定量关系相结合,通过Hashin-Shtrikman(HS)边界条件建立上地幔电导率与温度、熔融百分比等参数的定量关系,在此基础上计算得到了青藏高原东缘上地幔热结构及熔融百分比分布模型.研究结果表明在青藏高原东缘地区通过大地电磁测深方法所探测到的上地幔低阻体可以解释为由高温作用所产生的局部熔融区域.其中,松潘—甘孜地块上地幔高导体对应的温度介于1300～1500℃之间,熔融百分比可高达10%,支持前人将松潘—甘孜地块内部的低阻体解释为局部熔融的观点.龙门山断裂带以东、四川盆地西缘的上地幔高导体温度介于1200～1400℃之间,熔融百分比介于1%～5%左右,表明扬子克拉通的西缘可能正在经历一定程度的活化作用.龙门山断裂带下方的上地幔高阻体温度介于1100℃附近,基本没有发生局部熔融,具有较冷的刚性块体特征,与该区域频发的地震活动相吻合.四川盆地东部的扬子上地幔温度介于800～900℃之间,没有发生局部熔融,符合古老稳定的克拉通块体的基本特征.     

Peridotite composition from the North Atlantic: regional and tectonic variations and implications for partial melting

Mineral chemical parameters and modal abundances for mantle-derived abyssal peridotite tectonites from 14 locations in the North Atlantic (0–79°N) vary over considerable ranges and are correlated together. The data indicate that there are differences in the bulk chemistry of the peridotites which correspond to the amount of basaltic melt that has been extracted from them. These differences appear to be regional in extent. Peridotites from 34° to 45°N are the most refractory. The more intense depletion could have resulted from greater extents of partial melting, possibly caused by the presence of the Azores hotspot in this region; or it could be a pre-existing depletion, due to an earlier melting event. Basalt chemical data does not rule out either possibility, but supports greater extents of partial melting occurring in this region.The regional variations in mantle peridotite composition correlate with long-wavelength variations of crustal elevation and gravity along the northern Mid-Atlantic Ridge. These correlations support the existence of regional variations of upper mantle thermal structure and composition.Peridotites collected from fracture zones are compared with those collected away from fracture zones in the same region. Based on basalt studies, there may well be a decrease in the amount of melting as certain fracture zones are approached. The peridotite data suggest that this may be true for some fracture zones but not others. In general, peridotites collected from fracture zones are representative of the suboceanic mantle.     

西藏南部蛇绿岩套电导率研究

大地电磁(MT)资料显示,青藏高原地壳及地幔中普遍存在着高导层.作为大陆造山带中古洋盆岩石圈残片,蛇绿岩套的电导率测量可为了解古洋盆地区地壳及地幔的电性结构提供极其有用的信息.本研究中,我们在压力为1 GPa或3 GPa下,用交流阻抗谱法测量了采自西藏南部地区的蚀变辉长岩、玄武岩、角闪橄榄岩及方辉橄榄岩四个样品的阻抗谱,并进一步得出样品的电导率,不同样品电导率与温度之间的关系满足Arrhenius关系式.在实验温度范围内,蛇绿岩套电导率的对数logσ位于-6.0~-0.5 S/m之间,且随着温度的增高,不同样品电导率增大约4~5.5个量级.样品在未脱水的情况下,低温段的活化焓变化范围在0.4~0.6 eV之间,高温段的活化焓变化范围为1.7~2.6 eV之间.同时,我们研究了样品中结构水含量及铁含量对实验电导率的影响,验证了样品电导率与铁含量之间呈正比关系.当对样品结构水含量进行归一化后,相同温度下各样品的电导率随铁含量的增加而增大,而对样品铁含量归一化后,相同温度下各样品的电导率随样品中水含量的增加而增大.将实验电导率与藏南地区大地电磁结果进行了对比,发现本研究中各样品高温段实验电导率结果均落在大地电磁结果范围内.     

Formation time of the big mantle wedge beneath eastern China and a new lithospheric thinning mechanism of the North China craton—Geodynamic effects of deep recycled carbon

High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton.     

Origin of fine‐grained peridotite xenoliths from Iraya volcano of Batan Island,Philippines: deserpentinization or metasomatism at the wedge mantle beneath an incipient arc?

Abstract Peridotite xenoliths from the subarc mantle, which have been rarely documented, are described from Iraya volcano of the Luzon arc, the Philippines, and are discussed in the context of wedge-mantle processes. They are mainly harzburgite, with subordinate dunite, and show various textures from weakly porphyroclastic (C-type) to extremely fine-grained equigranular (F-type). Textural characteristics indicate a transition from the former to the latter by recrystallization. The F-type peridotite has inclusion-rich fine-grained olivine and radially aggregated orthopyroxene, being quite different in texture from ordinary mantle-derived peridotites previously documented. Despite their strong textural contrast, the two types do not show any systematic difference in modal composition. The harzburgite of C-type has ordinary mantle peridotite mineralogy; olivine is mostly Fo91–92 and chromian spinel mostly has Cr#s (= Cr/[Cr + Al] atomic ratios) from 0.3 to 0.6. Olivine is slightly more Fe-rich (Fo89–91) and spinel is more enriched in Cr (the Cr#, 0.4–0.8) and Fe³⁺ in F-type peridotites than in C-type harzburgite. Orthopyroxene in F-type peridotites is relatively low in CaO (<1 wt%), Al₂O₃ (<2 wt%) and Cr₂O₃ (<0.4 wt%). The F-type peridotite was possibly formed from the C-type one by recrystallization including local dissolution and precipitation of orthopyroxene assisted by fluid (or melt) of subduction origin. Textural characteristics, however, indicate a deserpentinization origin from abyssal serpentinite of which protolith was a C-type peridotite. In this scenario the initial abyssal serpentinite was possibly dehydrated due to an initiation of magmatic activity beneath an incipient oceanic arc like Batan Island. The F-type peridotite is characteristic of the upper mantle of island arc, especially of incipient arc.     

Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau

Peridotite inclusions, crystal fragments, and kimberlite breccia at Green Knobs, New Mexico, have been studied to evaluate compositions and processes in the upper mantle below the Colorado Plateau. Most peridotite inclusions are spinel lherzolites and harzburgites, or their partly hydrated equivalents, in the Cr-diopside group. Orthopyroxene-rich websterites and olivine websterites comprise 3% of the peridotites and formed as cumulates. Typical anhydrous or slightly hydrated peridotites contain aluminous, calcic diopside (5–7% Al₂O₃), aluminous orthopyroxene (3–6% Al₂O₃), spinel, and olivine (near Fa₉). Geothermometers based on different mineral pairs yield temperatures from above 1100°C to below 700°C in single rocks. High values, derived from pyroxenes with included exsolution lamellae, may approximate temperatures of primary crystallization. Low values, based on olivine-spinel and olivine-clinopyroxene pairs, approach upper mantle temperatures before eruption. In rare samples, some spinel grains are rimmed by garnet while others are not rimmed; garnet formation was controlled by nucleation kinetics. About one-third of the peridotites were deformed shortly before eruption, with effects ranging from mild cataclasis to the production of ultramylonites.Discrete crystals of garnet, olivine (near Fa₈), and Cr-diopside represent garnet peridotite. Eclogites were not found. The garnet peridotite is more depleted than overlying spinel peridotite, and it is not a likely source for the minettes associated with the kimberlites.The mantle below Green Knobs consists of spinel peridotite from 45 to perhaps 60 km depth immediately underlain by more-depleted garnet peridotite. The position of the spinel-garnet transition may be fixed by kinetics. The kimberlite may have been produced when heat from ascending minette magma released volatiles from otherwise depleted garnet peridotite. Resulting gas-solid mixtures erupted along zones of deformation associated with Colorado Plateau monoclines. Sheared lherzolites formed during renewed movement along these zones.     

Subduction-zone peridotites and their records of crust-mantle interaction

Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth's layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone(SSZ) ophiolites and mantle-wedge-type(MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism,deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism(including silicatic melts, carbonatitic melts and silicate-bearing C-HO fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism,deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.     

Attenuation of compressional waves in peridotite measured as a function of temperature at 200 MPa

A technique has been developed to determine attenuation in rocks at high temperature using a gas-media, high-pressure apparatus. A pulse transmission technique and a spectral ratio method are used to study compressional seismic properties of rocks. Seismic waves are transmitted to and from the sample through buffer rods of mullite. The effect of seismic wave reflections within the sample assembly are cancelled out by taking ratios of the spectra measured at different temperatures. In order to obtain good signal-to-noise ratio for resolving the attenuation at high pressure and temperature, special care is taken in the sample assembly and the ultrasonic coupling between the sample, buffer rods and transducers. A very tight connection of the sample-buffer rod-transducer is essential for obtaining high frequency signals (>300 kHz) at high temperature. A small mass is attached to each outside end of the transducer to drive low frequency signals (<250 kHz) into the sample. Before attenuation measurements, the sample and the buffer rods are tightly compacted in a platinum tube at high pressure and room temperature to ensure pressure seal of the sample assembly. The frequency range of measurement covers 50 to 450 kHz for the sample. Attenuation is very small in the buffer rod compared to the sample for the entire temperature range of the study. Because of the small attenuation, a wide frequency band of 50 kHz to 3.2 MHz can be covered for investigating the attenuation in the buffer rod. The technique has been used to measure attenuation at high confining pressure, and temperatures including sub- and hyper-solidus of upper mantle rocks. Therefore, effects of partial melting on attenuation can be studied.The method is applied to the attenuation measurement in a peridotite as a function of temperature to 1225°C at 200 MPa confining pressure. At high temperature, signal amplitude decays more rapidly at high frequency than at low frequency, from which attenuation (andQ) can be determined using a spectral ratio method. No frequency dependence ofQ is resolved for both the sample and the buffer rod over the entire temperature and frequency ranges of the measurement. The results show thatQ decreases rapidly with increasing temperature even in the temperature range below the solidus of peridotites. Such temperature sensitivity ofQ is probably more useful to probe thermal structure in the upper mantle than that of conductivity at temperatures below the solidus. The results in this study are compared with available seismic velocity, electrical conductivity and solidus data for peridotites, suggesting that there is no discontinuous change in both mechanical and electrical properties of peridotites at the solidus temperature. Even at hypersolidus temperatures, it appears that velocity drops and conductivity increases continuously (not abruptly) with increasing melt fraction. This implies that mechanical and electrical properties of the upper mantle will gradually change at the boundary where the geotherm crosses the solidus.