Origin of layered continental mantle (Karelian craton, Finland): Geochemical and Re–Os isotope constraints

Studies of mantle xenolith and xenocryst studies have indicated that the subcontinental lithospheric mantle (SCLM) at the Karelian Craton margin (Fennoscandian Shield) is stratified into at least three distinct layers cited A, B, and C. The origin and age of this layering has, however, remained unconstrained. In order to address this question, we have determined Re–Os isotope composition and a comprehensive set of major and trace elements, from xenoliths representing all these three layers. These are the first Re–Os data from the SCLM of the vast East European Craton.

Xenoliths derived from the middle layer B (at  110–180 km depth), which is the main source of harzburgitic garnets and peridotitic diamonds in these kimberlites, are characterised by unradiogenic Os isotopic composition. ¹⁸⁷Os/¹⁸⁸Os shows a good correlation with indices of partial melting implying an age of  3.3. Ga for melt extraction. This age corresponds with the oldest formation ages of the overlying crust, suggesting that layer B represents the unmodified SCLM stabilised during the Paleoarchean. Underlying layer C (at 180–250 km depths) is the main source of Ti-rich pyropes of megacrystic composition but is lacking harzburgitic pyropes. The osmium isotopic composition of layer C xenoliths is more radiogenic compared to layer B, yielding only Proterozoic T_RD ages. Layer C is interpreted to represent a melt metasomatised equivalent to layer B. This metasomatism most likely occurred at ca. 2.0 Ga when the present craton margin formed following continental break-up. Shallow layer A (at  60–110 km depth) has knife-sharp lower contact against layer B indicative of shear zone and episodic construction of SCLM. Layer A peridotites have “ultradepleted” arc mantle-type compositions, and have been metasomatised by radiogenic ¹⁸⁷Os/¹⁸⁸Os, presumably from slab-derived fluids. Since layer A is absent in the core of the craton, its origin can be related to Proterozoic processes at the craton margin. We interpret it to represent the lithosphere of a Proterozoic arc complex (subduction wedge mantle) that became underthrusted beneath the craton margin crust during continental collision  1.9 Ga ago.     

华北克拉通岩石圈地幔置换作用和壳幔生长耦合的Re-Os同位素证据

测定了辽宁复县奥陶纪金伯利岩和河北汉诺坝与山东栖霞第三纪碱性玄武岩中产出的地幔包体的Re Os同位素组成。金伯利岩中地幔包体的Re贫化Os同位素模式年龄 (TRD)为 2 .5～ 2 .8Ga ,从Re Os同位素定年角度证明了华北克拉通确实存在太古宙岩石圈地幔。对汉诺坝二辉橄榄岩包体获得了 (1.9± 0 .18)Ga的Re Os同位素等时线年龄 ,表明现今保存在那里的地幔主要是古元古代时形成的。汉诺坝地区出露有大量新太古代岩石 ,表明曾存在太古宙地幔。由于缺乏太古宙年龄 ,说明由汉诺坝所代表的克拉通中部曾存在的太古宙地幔在古元古代时已被减薄 ,并被 1.9Ga的新生岩石圈地幔置换。该事件与华北克拉通中部广泛的古元古代碰撞造山过程导致的麻粒岩相变质作用的时代相同 ,说明有关的岩石圈置换作用可能主要与拆沉作用有关。栖霞地幔包体具有与现代对流地幔相同的Os同位素组成 ,且Os同位素组成与Re/Os比值没有明显相关性 ,表明年龄很新。结合其它地质地球化学证据 ,说明克拉通东部的太古宙岩石圈地幔的置换作用主要发生在中生代 ,且可能与三叠纪华北和扬子陆块的陆陆碰撞造山导致的岩石圈地幔和下地壳的拆沉作用有关。本研究表明华北克拉通岩石圈地幔置换作用在时空上的分布是十分不均一的。 2 .5～ 2 .8Ga与 1.9Ga不仅?     

Crustal zircons and mantle sulfides: Archean to Triassic events in the lithosphere beneath south-eastern Sicily

Three types of zircon coexist in an unusual lower crustal xenolith from the Valle Guffari diatreme (Hyblean Plateau, Sicily): igneous Type 1 (near-euhedral, weakly zoned; Ce/Ce > 1); partially recrystallised Type 2 (ovoid, structureless; weak Ce anomaly); hydrothermal Type 3 (sugary, spongy-textured, probably related to F-rich aqueous fluids). U–Pb dating by LAM-ICPMS, supported by in situ Hf-isotope analysis, suggests that both Type 1 and Type 2 zircons were originally Archean (ca 2.7 Ga), though many of these grains have experienced severe Pb loss. The U–Pb ages of the hydrothermal zircons cluster around 246 Ma, interpreted as the timing of the hydrothermal event. Their ε_Hf (+ 8.5 to − 1.2) indicates the mixing of old crustal components and material from a juvenile source.

In situ Os-isotope analyses of sulfides hosted in peridotite xenoliths from Valle Guffari show Paleoproterozoic–Archean T_RD minimum ages, corresponding to the age of the oldest zircon grains in the crustal xenolith. Other peaks of T_RD ages suggest that multiple metasomatic events have affected the lithospheric mantle.

These observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the Hyblean Plateau represent the northernmost portion of the African Plate. These two units have coexisted since at least late Archean time, and have remained linked through several episodes of crustal modification, including the Permo-Triassic hydrothermal event, which was probably related to the onset of rifting in the Ionian Basin.     

Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet–peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet–pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m². Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (40 mW/m²) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

Evolution of subcontinental lithospheric mantle beneath eastern China: Re–Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts

The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.     

The age of continental roots

Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.     

Mechanism and timing of lithospheric modification and replacement beneath the eastern North China Craton: Peridotitic xenoliths from the 100 Ma Fuxin basalts and a regional synthesis

Lithospheric thinning beneath the eastern North China Craton is widely recognized, but the mechanism and timing of the thinning are contentious. New data on peridotitic xenoliths from the Cretaceous (∼100 Ma) Fuxin basalts at the northern edge of the craton have been integrated with data from other localities across the craton, to provide an overview of the processes involved. The Fuxin peridotite xenoliths can be subdivided into three types, which can also be recognized in other xenolith suites across the craton. The dominant Type 1, lherzolites with olivine Mg^# ∼90, represents fertile mantle (5-12% partial-melt extraction) that makes up much of the Late Mesozoic-Cenozoic lithosphere beneath the craton. Type 2 consists of magnesian (olivine Mg^# >92) harzburgites, interpreted as shallow relics of the Archean cratonic mantle. Type 3, minor lherzolite xenoliths with olivine Mg^# ∼86 reflect the interaction of the lithosphere with magmas similar to the host basalts. In-situ Re-Os data on sulfides in xenoliths from Hebi (4 Ma, interior of the craton) and Hannuoba (22 Ma, northern edge of the Trans-North China Orogen within the craton) basalts give model ages of 3.1-3.0, 2.5, 2.2-2.1, 1.4 and 0.8 Ga, These correspond to the U-Pb ages of zircons from early Mesozoic (178 Ma) peridotitic xenoliths at the southern margin of the craton, and record events during which the Archean lithospheric mantle was modified. The dominance of fertile peridotite xenoliths in the 100 Ma Fuxin basalts indicates that the mantle replacement beneath the eastern North China Craton at least partly took place before that time. The regional synthesis suggests that Mesozoic-Cenozoic lithospheric thinning and mantle replacement was heterogeneously distributed across the North China Craton in space and time. Lateral spreading of the lithosphere, accompanied by asthenospheric upwelling and melt-peridotite interaction, is the most probable mechanism for the lithospheric thinning beneath the eastern part of the craton. Subsequent cooling of the upwelled asthenosphere caused some re-thickening of the lithosphere; this overall more fertile and hence denser lithosphere resulted in widespread basin formation.     

Os, Nd, and Sr isotopic and chemical compositions of ultramafic xenoliths from Kurose, SW Japan: Implications for contribution of slab-derived material to wedge mantle

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan.

The Kurose xenoliths have less radiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.     

加拿大Slave 克拉通橄榄岩的平衡温压计算

加拿大Slave克拉通Jericho金伯利岩筒携带的橄榄岩包体提供了研究大陆岩石圈地幔物质组成和热结构的窗口。文章总结了地幔岩矿物温压计的研究进展,测量了Jericho金伯利岩携带的9个新鲜橄榄岩包体的矿物主量元素和微量元素,并使用不同的矿物温压计估算了平衡温度和压力。结果表明Nickel 和 Green（1985）的石榴子石—斜方辉石压力计可以较好地估算含石榴子石橄榄岩形成时的压力,Taylor（1998）二辉石温度计和Nimis 和 Taylor（2000）单斜辉石温度计的计算结果一致。具有粗粒变晶结构的尖晶石—石榴子石橄榄岩和石榴子石橄榄岩样品的平衡温度为575~843℃,压力为2.4~3.6 GPa,表明Slave克拉通岩石圈地幔温度较低。而残斑结构尖晶石—石榴子石二辉橄榄岩的平衡温度1109℃,压力为5.0 GPa,来源深度为~156 km,可能被早期金伯利岩浆携带到岩石圈地幔中部冷却,然后再被侏罗纪喷发的Jericho金伯利岩筒带到地表。使用石榴子石—单斜辉石稀土元素温压计获得的平衡温度高于主量元素温度计的结果,表明Slave克拉通岩石圈地幔经历了逐渐冷却的过程。此外,Slave克拉通浅部的尖晶石橄榄岩保留了强烈亏损的早期岩石圈地幔特征,而下部的岩石圈地幔经历了金伯利岩熔体和硅酸盐熔体的交代作用。     

Composition and thermal evolution of cratonic mantle beneath the central Archean Slave Province, NWT, Canada

The composition and thermal evolution of the upper mantle lithosphere beneath the central Archean Slave Province has been studied using mineral chemical and petrographic data from mantle xenoliths entrained in the Torrie kimberlite pipe. Coarse-, granuloblastic-, and porphyroclastic- textured harzburgite, lherzolite, and pyroxenite xenoliths yield equilibration temperatures ranging between 850 and 1350 °C. Thermobarometry of these samples requires a minimum lithospheric thickness of approximately 180 km at the time of kimberlite magmatism. The distribution of pressures and temperatures of equilibration for the xenoliths lie on a calculated 42 mWm⁻² paleogeotherm, ∼10 mWm⁻² lower than the present heat flow measured at Yellowknife, near the SW margin of the Slave Province. The Mg# [Mg/(Mg + Fe)] of olivine in peridotites varies between 0.906 and 0.938 with an average of 0.920. The Torrie xenolith suite shows variable degrees of serpentinization and/or carbonation with the rim compositions of many clinopyroxene grains showing Ca enrichment, but in general, the xenoliths are homogeneous at all scales. The Torrie xenoliths are rich in orthopyroxene similar to low temperature (<1100 °C) peridotites from southern Africa, and Siberia. Estimates of bulk rock composition based on mineral chemical and modal data reveal a negative correlation between Si and Fe, similar to peridotite xenoliths from Udachnaya. The similarity of olivine Mg#s with other cratons combined with the negative correlation of Fe and Si suggest that the lithosphere beneath the Slave craton has experienced a evolution similar to other cratons globally. Received: 22 January 1998 / Accepted: 27 August 1998     

Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton     

The origin of garnet and clinopyroxene in “depleted” Kaapvaal peridotites

A detailed petrographic, major and trace element and isotope (Re–Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P–T range from 60 to 150 km depth. With the exception of one sample (that belongs to the ilmenite–rutile–phlogopite–sulphide suite (IRPS) suite first described by [B. Harte, P.A. Winterburn, J.J. Gurney, Metasomatic and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. (Ed.), Mantle metsasomatism. Academic Press, London 1987, 145–220.]), all samples considered here have high Mg# and show strong depletion in CaO and Al₂O₃. They have bulk rock Re depletion ages (T_RD) >2.5 Ga and are therefore interpreted as residua from large volume melting in the Archaean. A characteristic of Kaapvaal xenoliths, however, is their high SiO₂ concentrations, and hence, modal orthopyroxene contents that are inconsistent with a simple residual origin of these samples. Moreover, trace element signatures show strong overall incompatible element enrichment and REE disequilibrium between garnet and clinopyroxene. Textural and subtle major element disequilibria were also observed. We therefore conclude that garnet and clinopyroxene are not co-genetic and suggest that (most) clinopyroxene in the Archaean Kaapvaal peridotite xenoliths is of metasomatic origin and crystallized relatively recently, possibly from a melt precursory to the kimberlite.

Possible explanations for the origin of garnet are exsolution from a high-temperature, Al- and Ca-rich orthopyroxene (indicating primary melt extraction at shallow levels) or a majorite phase (primary melting at >6 GPa). Mass balance calculations, however, show that not all garnet observed in the samples today is of a simple exsolution origin. The extreme LREE enrichment (sigmoidal REE pattern in all garnet cores) is also inconsistent with exsolution from a residual orthopyroxene. Therefore, extensive metasomatism and probably re-crystallization of the lithosphere after melt-depletion and garnet exsolution is required to obtain the present textural and compositional features of the xenoliths. The metasomatic agent that modified or perhaps even precipitated garnet was a highly fractionated melt or fluid that might have been derived from the asthenosphere or from recycled oceanic crust. Since, to date, partitioning of trace elements between orthopyroxene and garnet/clinopyroxene is poorly constrained, it was impossible to assess if orthopyroxene is in chemical equilibrium with garnet or clinopyroxene. Therefore, further trace element and isotopic studies are required to constrain the timing of garnet introduction/modification and its possible link with the SiO₂ enrichment of the Kaapvaal lithosphere.     

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.     

Re–Os systematics of diamond-bearing eclogites from the Newlands kimberlite

A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re–Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03–1.34 ppb), while Os concentrations show a much more limited range (0.26–0.59 ppb). The three diamond-free eclogites have Re and Os concentrations that are at the extremes of the range of their diamond-bearing counterparts. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1579 to 1.4877, while ¹⁸⁷Re/¹⁸⁸Os varies from 0.54 to 26.2 in the diamond-bearing eclogites. The highly radiogenic Os in the diamond-bearing eclogites (γ_Os=23–1056) is consistent with their high ¹⁸⁷Re/¹⁸⁸Os and requires long-term isolation from the convecting mantle. Re–Os model ages for 9 out of 14 diamond-bearing samples lie between 3.08 and 4.54 Ga, in agreement with FTIR spectra of Newlands diamonds that show nitrogen aggregation states consistent with diamond formation in the Archean. Re–Os isochron systematics for the Newlands samples do not define a precise isochron relationship, but lines drawn between subsets of the data provide ages ranging from 2.9 to 4.1 Ga, all of which are suggestive of formation in the Archean. The Re–Os systematics combined with mineral chemistry and stable isotopic composition of the diamond-bearing eclogites are consistent with a protolith that has interacted with surficial environments. Therefore, the favored model for the origin of the Newlands diamond-bearing eclogites is via subduction. The most likely precursors for the Kaapvaal eclogites include komatiitic ocean ridge products or primitive portions of oceanic plateaus or ocean islands.     

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roots are frequentlyincompatible trace element enriched despite their refractorymajor element compositions. To constrain the trace element budgetof the lithosphere beneath the Canadian craton, trace elementand rare earth element (REE) abundances were determined fora suite of garnet peridotites and garnet pyroxenites from theNikos kimberlite pipe on Somerset Island, Canadian Arctic, theirconstituent garnet and clinopyroxene, and the host kimberlite.These refractory mantle xenoliths are depleted in fusible majorelements, but enriched in incompatible trace elements, suchas large ion lithophile elements (LILE), Th, U and light rareearth elements (LREE). Mass balance calculations based on modalabundances of clinopyroxene and garnet and their respectiveREE contents yield discrepancies between calculated and analyzedREE contents for the Nikos bulk rocks that amount to LREE deficienciesof 70–99%, suggesting the presence of small amounts ofinterstitial kimberlite liquid (0·4–2 wt %) toaccount for the excess LREE abundances. These results indicatethat the peridotites had in fact depleted or flat LREE patternsbefore contamination by their host kimberlite. LREE and Sr enrichmentin clinopyroxene and low Zr and Sr abundances in garnet in low-temperatureperidotites (800–1100°C) compared with high-temperatureperidotites (1200–1400°C) suggest that the shallowlithosphere is geochemically distinct from the deep lithospherebeneath the northern margin of the Canadian craton. The Somersetmantle root appears to be characterized by a depth zonationthat may date from the time of its stabilization in the Archean. KEY WORDS: Canada; mantle; metasomatism; peridotite; trace elements     

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.     

Constraints on the thermal evolution of continental lithosphere from U-Pb accessory mineral thermochronometry of lower crustal xenoliths,southern Africa

U-Pb isotopic thermochronometry of rutile, apatite and titanite from kimberlite-borne lower crustal granulite xenoliths has been used to constrain the thermal evolution of Archean cratonic and Proterozoic off-craton continental lithosphere beneath southern Africa. The relatively low closure temperature of the U-Pb rutile thermochronometer (~400-450 °C) allows its use as a particularly sensitive recorder of the establishment of "cratonic" lithospheric geotherms, as well as subsequent thermal perturbations to the lithosphere. Contrasting lower crustal thermal histories are revealed between intracratonic and craton margin regions. Discordant Proterozoic (1.8 to 1.0 Ga) rutile ages in Archean (2.9 to 2.7 Ga) granulites from within the craton are indicative of isotopic resetting by marginal orogenic thermal perturbations influencing the deep crust of the cratonic nucleus. In Proterozoic (1.1 to 1.0 Ga) granulite xenoliths from the craton-bounding orogenic belts, rutiles define discordia arrays with Neoproterozoic (0.8 to 0.6 Ga) upper intercepts and lower intercepts equivalent to Mesozoic exhumation upon kimberlite entrainment. In combination with coexisting titanite and apatite dates, these results are interpreted as a record of postorogenic cooling at an integrated rate of approximately 1 °C/Ma, and subsequent variable Pb loss in the apatite and rutile systems during a Mesozoic thermal perturbation to the deep lithosphere. Closure of the rutile thermochronometer signals temperatures of 𙠂 °C in the lower crust during attainment of cratonic lithospheric conductive geotherms, and such closure in the examined portions of the "off-craton" Proterozoic domains of southern Africa indicates that their lithospheric thermal profiles were essentially cratonic from the Neoproterozoic through to the Late Jurassic. These results suggest similar lithospheric thickness and potential for diamond stability beneath both Proterozoic and Archean domains of southern Africa. Subsequent partial resetting of U-Pb rutile and apatite systematics in the cratonic margin lower crust records a transient Mesozoic thermal modification of the lithosphere, and modeling of the diffusive Pb loss from lower crustal rutile constrains the temperature and duration of Mesozoic heating to 𙡦 °C for ₞ ka. This result indicates that the thermal perturbation is not simply a kimberlite-related magmatic phenomenon, but is rather a more protracted manifestation of lithospheric heating, likely related to mantle upwelling and rifting of Gondwana during the Late Jurassic to Cretaceous. The manifestation of this thermal pulse in the lower crust is spatially and temporally correlated with anomalously elevated and/or kinked Cretaceous mantle paleogeotherms, and evidence for metasomatic modification in cratonic mantle peridotite suites. It is argued that most of the geographic differences in lithospheric thermal structure inferred from mantle xenolith thermobarometry are likewise due to the heterogeneous propagation of this broad upper mantle thermal anomaly. The differential manifestation of heating between cratonic margin and cratonic interior indicates the importance of advective heat transport along pre-existing lithosphere-scale discontinuities. Within this model, kimberlite magmatism was a similarly complex, space- and time-dependent response to Late Mesozoic lithospheric thermal perturbation.     

东南沿海地区古近纪大陆岩石圈地幔特征及成因

东南沿海地区新生代玄武岩中的橄榄岩包体来自岩石圈地幔 ,上地幔橄榄岩包体的岩石学及地球化学特征都记录了地幔演化的历史。普宁橄榄岩包体斜方辉石含量与太古宙克拉通地幔类似 ,但在矿物学、REE、痕量元素和Sr Nd同位素上又与太古宙岩石圈地幔不同。橄榄岩包体的岩相学、矿物学、REE、痕量元素特征都提供了含H2 O富Si流体交代橄榄岩的证据 ,这种流体可能主要是洋壳物质局部熔融而成。流体交代使橄榄岩富Si,同时富Sr、Pb和强不相容元素等大洋岩石圈物质。这表明普宁大陆岩石圈地幔既保留太古宙岩石圈地幔的特征 ,又具有大洋俯冲地幔的特征 ,它是古老岩石圈地幔向大洋岩石圈地幔转换的一部分 ,这种转换可能是大洋岩石圈与大陆岩石圈地幔相互作用的结果。     

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths.

The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.