Towards a Li barometer for bimineralic eclogites: experiments in CMAS

An eclogite barometer has profound importance in the study of upper mantle processes and potential application to diamond prospecting. Studies on the partitioning of Li between clinopyroxene (cpx) and garnet (grt) in natural samples have shown that this particular element is very sensitive to changes in pressure and could be calibrated as the barometer demanded for bimineralic eclogites. Experiments were performed from 4 to 13 GPa and 1,100–1,400°C in the CMAS (CaO, MgO, Al₂O₃, SiO₂) system with Li added as Li₃PO₄ to quantify this pressure dependence into a barometer expressed in the following equation: P = (0.00255T – ln K _d)/0.2351 where P is in GPa, T is in °C and K _d is defined as the partition coefficient of Li (in ppm) between clinopyroxene and garnet. The experimental pressures are reproduced to ±0.38 GPa (1σ) by this equation. This barometer is strictly applicable only to CMAS. Experiments at 1,300°C, 8–12 GPa showed that Henry’s Law is fulfilled for Li partitioning between cpx and grt in the concentration range of approximately 0.01–1 wt% Li. Direct application of the equation to experiments in natural systems performed at 1,300°C from 4 to 13 GPa consistently overestimates pressures by approximately 2 GPa. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

北大别北东地区榴辉岩温度条件分析

北大别榴辉岩温度条件的精确评价直接涉及到北大别变质单元性质的判定.由于榴辉岩温度分类方案的不同,以及温度估算时温度计选择和绿辉石Fe2+校正方法不同的影响,导致北大别榴辉岩温度评价和分类归属具有较大的不确定性.为此,本研究应用5个不同的温度计和4种不同的绿辉石Fe2+校正方法,评价了北大别北东地区榴辉岩的温度条件.研究显示,当应用Kogh (2000)的Grt-Cpx Fe-Mg温度计和设定绿辉石的铁为Fe3+/Fe2+=1∶1时,温度计算较为合理:P标定=3.5 GPa,温度范围为600～740℃或P标定=5.0GPa,温度范围为670～ 810℃.参照Carswell (1990)的温度分类,北大别榴辉岩应属于中温类型.但结合此次榴辉岩的地质特征、岩相学和矿物成分分析,以及前人的研究资料,北大别榴辉岩与中大别和南大别榴辉岩有明显的差异,其可能是在相对高温的条件下形成的.     

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (～45%) and low-Cr megacrystic (～39%) chemistries, with a significant proportion of eclogitic garnets (～11%), and only subordinate contribution from harzburgitic garnets (～5%) with variable Cr₂O₃ contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ～1150–1250 °C) indicate their derivation from at least ～220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ～45 mW/m² or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ～50–65 kbar diamond window of the Olenek province by the same time.     

桐柏山地区低温榴辉岩变质作用的P-T轨迹

桐柏山地区发育南、北两条低温高压榴辉岩带,中间被桐柏山高级变质杂岩所分隔。本文对两条带内的4个保存完好的榴辉岩体及其退变产物进行了详细的岩石学研究和 P-T 条件估算。结果表明,北带榴辉岩的形成温压条件较高,为530～610℃、1.7～2.0GPa,南带榴辉岩较低,为470～520℃、1.3～1.7GPa,说明两条榴辉岩带不属于同一个高压岩片。实际上,二者分别对应于西大别地区的浒湾榴辉岩带(I 带)和七角山榴辉岩带(V 带)。电子探针分析发现,所有榴辉岩中的石榴石都展示了进变质环带,而退变榴辉岩的成分环带更为复杂,即保留了早期的进变环带,也显示了晚期的低温取代以及后期相对高温条件下的再生长,由此为榴辉宕的变质作用推导出一个“发卡式”演化轨迹。这种轨迹表明,榴辉岩先是在降温条件下缓慢向上回返的,这与目前已建立的多数高压、超高压岩石的快速抬升模式有所差异,而后在其进入到中地壳后又在相对较高的地温梯度下遭受退变质作用的改造。     

Geochemical evolution of high-pressure mafic granulites from the Bacariza formation (Cabo Ortegal complex, NW Spain): an example of a heterogeneous lower crust

 The Cabo Ortegal complex (northwestern Iberian massif) is a klippen formed of several structural units stacked during the Hercynian collision. All these units include ultramafic rocks, metabasites and quartz-feldspathic gneisses affected by different metamorphic conditions. The Bacariza formation is heterogeneous showing a conspicuous layering mainly defined by alternate high-pressure ultrabasic-to-basic granulites, retrogressed garnet amphibolites of intermediate composition and rare acid rocks forming garnet trondhjemitic gneisses. This layering is inherited from a gabbroic protolith showing a composition rich in Fe and Ti. Major and trace elements of these rocks can be correlated to continental tholeiitic series of extensional settings. These high-pressure granulites are situated in normal contact between±serpentinised ultramafic rocks and other high-grade metabasites with lessevolved and more-depleted composition comparable to T-type and N-type MORB. It is suggested that the layered gabbro-type protolith was part of a continuous mafic crust. This crust was initially formed during Early Ordovician in a continental extensional setting and progressively evolved to oceanic spreading. Received: 9 February 1996/Accepted: 10 February 1997     

Eclogite and pyroxenite xenoliths from off-craton kimberlites near the Kaapvaal Craton, South Africa

Mantle xenoliths brought to the surface by kimberlite magmas along the south-western margin of the Kaapvaal craton in South Africa can be subdivided into eclogites sensu stricto, kyanite eclogites and orthopyroxene eclogites, all containing omphacite, and garnet clinopyroxenites and garnet websterites characterised by diopside. Texturally, chemically (major elements) and thermally, we observe an evolution from garnet websterites (T_EG = 742–781 °C) towards garnet clinopyroxenites (T_EG = 715–830 °C) and to eclogites (T_EG = 707–1056 °C, mean value of 913 °C). Pressures calculated for orthopyroxene-bearing samples suggest upper mantle conditions of equilibration (P = 16–33 kb for the garnet websterites, 18 kb for a garnet clinopyroxenite and 23 kb for an opx-bearing eclogite). The overall geochemical similarity between the two groups of xenoliths (omphacite-bearing and diopside-bearing) as well as the similar trace element patterns of clinopyroxenes and garnet suggest a common origin for these rocks. Recently acquired oxygen isotope data on garnet (δ¹⁸O_gnt = 5.25–6.78 ‰ for eclogites, δ¹⁸O_gnt = 5.24–7.03 ‰ for garnet clinopyroxenites) yield values ranging from typical mantle values to other interpreted as resulting from low-temperature alteration or precursors sea-floor basalts and associated rocks. These rocks could then represent former magmatic oceanic rocks that crystallised from a same parental magma as plagioclase free diopside-bearing and plagioclase-bearing crustal rocks. During subduction, these oceanic rock protoliths equilibrated at mantle depth, with the plagioclase-bearing rocks converting to omphacite and garnet-bearing lithologies (eclogites sensu largo), whereas the plagioclase-free diopside-bearing rocks converted to diopside and garnet-bearing lithologies (garnet websterites and garnet clinopyroxenites).     

鲁苏榴辉岩套的岩石——构造组合与地质演化

提出鲁苏榴辉岩套的分布、岩石组合及构造形变特征。依据成岩条件和同位素年龄探讨了其地质演化,认为它们形成于三叠纪早期的碰撞造山作用和白垩纪早期的伸展构造作用。     

Signatures of rift environment in the production of garnet-amphibolites and eclogites from Tso-Morari region, Ladakh, India: A geochemical study

The geochemistry of eclogites and garnet-amphibolites from Tso-Morari region, Ladakh, India has been investigated to characterize their protoliths on the basis of immobile elements, especially trace elements including REE. The eclogites and garnet-amphibolites have coherent compositions, except for the UHP metamorphic minerals being preserved in eclogites. Compositionally, the metabasites range from ‘depleted’ to ‘enriched’, and span from within-plate basalts (WPB) to MORB fields, and match with various enriched or ‘transitional’ MORB types (e.g., on Ti–Zr–Y and Nb–Zr–Y ternary plots). Isotopically they have Sr_i ratio 0.706 which is similar to some of the Ocean Island Basalt (OIB). The rocks under study suggest that the enriched components are probably derived by melting of a mantle source with an enriched OIB-type component rather than due to the crustal contamination. We propose a rift environment for their protoliths and relate to advanced intra-continental rift situation. Furthermore, our geochemical studies envisage an initial phase of plume activity (Cambrian or earlier) resulting in basaltic magma in the eclogitic layers at sub-lithospheric levels, wherein they were subjected to crystallization under ultra-high pressure conditions. At a later stage the reactivation of faults (probably during Permo-Triassic times) acted as channels for the emplacement of the high pressure rocks in the continental crust. Subsequently, the ultra-high pressure rocks got re-equilibrated as amphibolites, with some remaining as relict eclogites, which later got exposed to the surface during various phases of the Himalayan uplift.     

Nature and origin of eclogite xenoliths from kimberlites

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.     

江苏东海毛北榴辉岩的矿物化学及其成因问题

本文利用电子探针,分析了江苏东海毛北榴辉岩中的石榴石和单斜辉石的化学组成。利用矿物地质温度计,计算了毛北榴辉岩的成岩平衡温度。结合本区的大地构造环境,提出在早元古代末期,华北陆块曾发生向南的大型堆覆构造,苏北地区即为推覆体前缘。