http://www.sciencedirect.com/science/article/pii/S1674987116000311

Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time.Four types of mantle are characterized from incompatible element distributions in basalts and komatiites:depleted,hydrated,enriched and mantle from which komatiites are derived.Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean,which we ascribe to thermal and convective evolution.Prior to 2.5 Ga,depleted and enriched mantle have indistinguishable thermal histories,whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle.Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes,but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle.During this time,however,high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts.Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti,Al,Ca and Na in basalts derived from enriched mantle sources.After 2.5 Ga,increases in Mg~# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time.A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.     

High field strength element ratios in Archean basalts: a window to evolving sources of mantle plumes?

In terms of high field strength element ratios Nb/Th, Zr/Nb, Nb/Y and Zr/Y, most basalts from non-arc type Archean greenstones are similar to oceanic plateau basalts, suggestive of mantle plume sources. A large number of these basalts have ratios similar to primitive mantle composition. Perhaps the Archean mantle was less fractionated than at present and “primitive mantle” comprised much of the deep mantle and made a significant contribution to mantle plumes. The near absence of Archean greenstone basalts similar to NMORB in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source (DM) was volumetrically insignificant. The element ratios in basalts also indicate the existence of recycled components (HIMU, EM1, EM2) in the mantle by the Late Archean. This suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the Late Archean. High field strength element ratios also indicate an important contribution of continental crust or/and subcontinental lithosphere to some non-arc Archean greenstone basalts. This implies that at least thin continental lithosphere was relatively widespread in the Archean.     

Magmatic processes and mantle heterogeneity beneath the slow-spreading northern Kolbeinsey Ridge segment,North Atlantic

We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition.     

Nature and Evolution of Cenozoic Lithospheric Mantle beneath Shandong Peninsula,Sino-Korean Craton,Eastern China

The Shanwang and Qixia basalts lie within the North China block and were erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene time (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, whereas the Qixia area lies east of the fault zone. The basaltic rocks (alkali olivine basalts, basanites, nephelinites) carry abundant deep-seated xenoliths including spinel lherzolite (dominant), dunite, and pyroxenite, and a megacryst suite including augite, anorthoclase, phlogopite, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. Reconstructed bulk compositions of the lherzolites range from relatively depleted (<3% modal diopside) to fertile (>12% modal diopside). Equilibration temperatures of 850° to 1020°C indicate entrainment of these lherzolites from depths ≤45 km, within the lithosphere; the geotherm may have been higher beneath Shanwang. The Shanwang suite contains less-depleted lherzolites, and more pyroxenites, than the Qixia suite. The chondrite-normalized REE patterns in clinopyroxenes of the Shandong xenoliths vary from LREE depleted, through concave shaped, to LREE enriched; spidergrams for the clinopyroxenes can be divided into depleted, fertile, and metasomatic types. Progressive depletion in Na and Al is accompanied by depletion in moderately incompatible elements such as Y, Yb, and Zr, and an increase in Mg#. Ti and Zr in clinopyroxenes have not been affected by the metasomatic process, and MREE have been little disturbed, whereas the light rare-earth elements, Nb, and Sr have been strongly enriched during metasomatism; this suggests that carbonate-rich fluids/melts were the metasomatic agent. The mantle beneath the Shandong Peninsula sampled by these basalts is dominantly Phanerozoic in character rather than Archean or Proterozoic lithospheric mantle. This mantle probably represents a mixture of older lithospheric mantle and newly accreted material that replaced the Archean lithospheric keel through extension, thermal erosion, and fluid/melt metasomatism. The differences in micro-structures, chemistry, temperature, and fluid/melt activity between Shanwang and Qixia are ascribed to their spatial relationships to the Tanlu fault, which is a major translithospheric suture that hasplayed an important role in the Cenozoic replacement of the pre-existing Archean lithospheric mantle.     

Evidence for heterogenes primary MORB and mantle sources,NW Indian Ocean

Basalts from 5 Deep Sea Drilling Project (DSDP) sites in the northwest Indian Ocean (Somali Basin and Arabian Sea) have general geochemical features consistent with a spreading origin at the ancient Carlsberg Ridge. However, compared to most MORBS from other oceans they have low normative olivine, TiO₂, and Zr contents. There is no evidence that the mantle source of these northwest Indian Ocean basalts was enriched in incompatible elements relative to the Atlantic and Pacific ocean mantles. In detail, incompatible element abundances in these DSDP basalts establish that they evolved from several compositionally distinct parental magmas. In particular, basalts from site 236 in the Somali Basin have relatively high SiO₂ and low Na, P, Ti, and Zr contents. These compositional features along with low normative olivine contents are similar to those proposed for melts derived by two-stage (or dynamic) melting. Published data also indicate there is no enrichment in incompatible elements at the southwest Indian Ocean triple junction, although southwest Indian Ocean basalts have slightly higher ⁸⁷Sr/⁸⁶Sr than normal Atlantic MORB. The data suggest that there are significant subtle geochemical variations in the Indian Ocean mantle sources, but are insufficient to show whether these variations have a systematic temporal or geographic distribution.     

Geochemistry and Petrogenesis of Three Series of Cenozoic Basalts from Southeastern China

Cenozoic basaltic volcanism in southeastern China was related to the lithospheric extension and asthenospheric upwelling at the eastern Eurasian continental margin. The cenozoic basaltic rocks from this region can be grouped into three different series: tholeiitic basalts, alkali basalts, and picritic-nephelinitic basalts. Each basalt series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The mineralogy, petrography, and major and trace-element geochemistry of the tholeiites are similar to oceanic island basalts, implying that the mantle source for these Chinese continental tholeiites was similar to that of the oceanic island basalts—an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements, and their geochemical features can be interpreted as a result of partial melting of an enriched lithospheric mantle, or the mixing products of an asthenospheric magma with a component derived from an enriched lithospheric mantle through thermal erosion at the base of the lithosphere. But the lack of a transitional rock type and continuous variational trends among these basalts suggests that the mixing between asthenospheric magmas and lithospheric magmas probably was not significant in the petrogenesis of the basalts from SE China. Low-degree partial melting of enriched lithospheric mantle alone can account for the observed geochemical data from these basalts.     

Distribution of titanium and the rare earth elements between peridotitic minerals

The concentrations of titanium and rare earth elements (REE) in olivines, orthopyroxenes, clinopyroxenes and spinels from four anhydrous, spinel-bearing peridotite xenoliths have been determined. The distribution of titanium (used as an analogue for the high field strength elements: HFSE) relative to the REE between clinopyroxenes and orthopyroxenes varies as a function of the whole rock composition and modal mineralogy. The distribution coefficients for titanium and the REE in these peridotites do not reflect mineral-melt equilibria. It is believed that subsolidus distribution coefficients for HFSE relative to REE vary with temperature. Ratios of various incompatible elements (e.g., Ti/Eu, Zr/Sm, Hf/Sm and P/Nd) in peridotite minerals differ from those in most primary basalts. However, the abundance ratios of incompatible elements in the bulk peridotite are comparable to those found in modern basalts. Given this and the differing contribution of melt from each phase during melting, near constant ratios of such incompatible elements in primary and primitive basalts and komatiites reflect the buffering of the melt by its residue. These ratios are fixed in the magma during the initial stages of melting because of similar and low distribution coefficients between melt and bulk residue for these element pairs. Differences in the relative abundances of titanium and REE in clinopyroxenes and orthopyroxenes demonstrate that mantle normalized abundance patterns for clinopyroxene are not equivalent to those of the whole rock. Therefore, claims of a widespread HFSE-depleted reservoir in the upper mantle base solely on the relative abundances of incompatible elements in peridotitic clinopyroxenes are invalid.     

High-Field-Strength Elements in Mafic Volcanics from North China Craton: Implications for Archean-Proterozoic Boundary and Source Composition

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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。     

The importance of melt extraction for tracing mantle heterogeneity

Numerous isotope and trace element studies of mantle rocks and oceanic basalts show that the Earth’s mantle is heterogeneous. The isotopic variability in oceanic basalts indicates that most mantle sources consist of complex assemblages of two or more components with isolated long-term chemical evolution, on both global and local scales. The range in isotope and highly incompatible element ratios observed in oceanic basalts is commonly assumed to directly reflect that of their mantle sources. Accordingly, the end-points of isotope arrays are taken to represent the isotopic composition of the different components in the underlying mantle, which is then used to deduce the origin of mantle heterogeneity. Here, a melting model for heterogeneous mantle sources is presented that investigates how and to what extent isotope and trace element signatures are conveyed from source to melt. We model melting of a pyroxenite-bearing peridotite using recent experimental constrains for melting and partitioning of pyroxenite and peridotite. Identification of specific pyroxenite melting signatures allows finger-printing of pyroxenite melts and confirm the importance of lithological heterogeneity in the Earth’s mantle. The model results and the comparison of the calculated and observed trace element-isotope systematics in selected MORB and OIB suites (e.g. from the East Pacific Rise, Iceland, Tristan da Cunha, Gough and St.Helena) further show that factors such as the relative abundance of different source components, their difference in solidus temperature, and especially the extent, style and depth range of melt aggregation fundamentally influence the relationship between key trace element and isotope ratios (e.g. Ba/Th, La/Nb, Sr/Nd, La/Sm, Sm/Yb, ¹⁴³Nd/¹⁴⁴Nd). The reason for this is that any heterogeneity present in the mantle is averaged or, depending on the effectiveness of the melt mixing process, even homogenized during melting and melt extraction. Hence to what degree mantle heterogeneity is reflected in the erupted melts is not only a function of source and melting-induced variability. It also depends on the extent of mixing during melting and melt extraction and thus strongly on the relative incompatibility of the elements considered. The observed trace element variation in erupted melts can be greater or smaller than that of their mantle sources, depending on the incompatibility of the elements investigated. The isotopic variability in erupted melts, on the other hand, is generally smaller than that of their mantle source. Melt mixing during melt extraction consequently has an important influence on the relative extent of variation, and hence the degree of correlation between the isotope and trace element ratios. Overall fewer correlations between trace element and isotope ratios are expected whenever melts are extracted from a restricted depth range, as is the case for ocean island basalts, than for cases where melts are extracted over a larger depth interval (mid ocean ridges and especially ridge centered hotspots like Iceland). While the isotopic composition of the most enriched melts may correspond closely to those of the enriched source component, even the most depleted mid ocean ridge basalts are likely to underestimate the isotopic depletion of the depleted mantle component. These observations imply that using the chemical and isotopic range observed in oceanic basalts as directly representative of that in the corresponding mantle source can be misleading, since this assumption is strictly true only for homogeneous mantle sources. In addition to identifying source or partitioning-related differences in melts from different mantle sources, inferring the true composition, origin, and distribution of heterogeneous components in the Earth’s mantle therefore requires detailed knowledge about the mechanisms of melting and melt mixing during the melt extraction process. Only if these processes and their influence on the isotope-trace element relationship are understood, can the composition and origin of the different source components, and thus mantle heterogeneity, be accurately constrained.     

Late Pliocene lamproites from Bucak,Isparta (southwestern Turkey): Implications for mantle ‘wedge’ evolution during Africa-Anatolian plate convergence

Magmatism in the Kirka–Afyon–Isparta (KAI) region, southwestern Turkey, shows a temporal progression from calc-alkaline to ultrapotassic affinity. Magmatic activity is associated with the geodynamic evolution of the ‘Isparta Angle’ and is typical of a collision-affected convergent plate margin, most magmas being enriched in potassium and other large-ion lithophile elements (LILE) and depleted in high-field strength elements (HFSE) such as Ti, Zr, Nb, Ta, and Hf. However, Late Pliocene ultrapotassic lamproites in the south of ‘Isparta Angle’ show HFSE-rich incompatible element distributions, similar to those of ‘non-orogenic’ intraplate leucite basalts (ILB) and oceanic island basalts (OIB). Their association with HFSE-depleted ‘orogenic’ magmas suggests that ultrapotassic character reflects primarily crustal contamination of their mantle sources, rather than magma-wallrock reaction effects. Their relatively high content of Fe and Ti (for equivalent Mg content), and SiO₂-undersaturated character suggest that they segregated at relatively high pressures (>ca. 2.0 GPa) from fertile sources. In contrast, the older SiO₂-saturated, Afyon (orogenic) magmas which, for equivalent Mg content, show lower contents of Fe and Ti, are better explained as partial melts segregating at ca. 1.0–1.5 GPa from refractory (basalt-depleted) sources, similar to those of basalt-borne xenoliths tapping the lithospheric mantle. The notion of variably fertile contaminated mantle sources is compelling, but needs to be constrained in terms of the dynamic interaction between the lithosphere and asthenosphere and their respective contamination histories. Given the unlikelihood of in situ partial melting of the continental lithosphere mantle, we propose that both orogenic and non-orogenic magmas are generated at different pressures from sources within the convecting asthenosphere, contaminated by both lithospheric mantle and crustal components. This model rests on two testable conjectures: firstly, the interpretation that the continental lithospheric mantle is residual from partial melting at an earlier stage of its history and, secondly, that such material is incorporated into the asthenospheric flow field during and following subduction. The first of these is supported by the ambient compositions of continental basalt-borne xenoliths, while the second is contingent on the prediction that lithospheric mantle may be rheologically transformed during subduction-related hydration prior to its incorporation. The proximity of the Bucak lamproites to the Menderes Massif, a suspected Archean cratonic fragment, highlights the resemblance of these unusual rocks to intra-plate leucite-bearing lamproites elsewhere, whose genesis has been linked to mantle ‘wedge convection’ triggered beneath cratonic and circumcratonic lithosphere domain boundaries.     

Heterogeneous mantle melting and magmatic processes at the East Pacific Rise (2.6–3.1°S): Evidence from mid-ocean ridge basalt geochemistry and Sr–Nd–Pb isotopes

ABSTRACT

We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe₂O₃, and 0.06–0.34 K₂O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), _N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and ⁸⁷Sr/⁸⁶Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.     

Geochemistry of lavas from Mohns Ridge,Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen

 Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)_N<1 to >2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic. Received: 3 May 1995 / Accepted: 6 November 1995     

Mantle Heterogeneity and Crustal Contamination in the Genesis of Low-Ti Continental Flood Basalts from the Paran? Plateau (Brazil): Sr-Nd Isotope and Geochemical Evidence

Continental flood basalts from the Parana plateau are of LowerCretaceous age and are represented by abundant (c. 45 per centby volume) two-pyroxene tholeiites characterized by relativelylow-TiO₂ (< 2 wt. percent) and incompatible (e.g., P, Ba,Sr, La, Ce, Zr) element contents. Low-Ti basalts are distributedthroughout the Parana Basin and predominate in the southernregions, where they represent over 90 per cent by volume ofthe basic activity. Major and trace elements and Sr-Nd isotope ratios were analysedin 43 low-Ti basalts selected so as to cover the entire Paranabasin. In general, low-Ti basalts with initial ^87Sr86Sr ratios (R₀)lower than O7060 may be divided into two groups: (A) those relativelyenriched in incompatible elements (e.g., average K₂O = O.85and P₂O₅ = 0.27 wt. per cent, and Ba = 346, Sr =289, Rb=16;La =18; Zr=132 p.p.m.) and SiO₂ (average 51.1 wt. per cent);and (B) depleted in incompatible elements (e.g., average K₂O= 0.31, P₂O₅ =0.17 wt. per cent, and Ba=178, Sr= 179, Rb= 11,La = 9, Zr = 93 p.p.m.) and SiO₂ (average 49.7 wt. per cent).Low-Ti basalts of Group A are typical of northern Paran? {Ro= O70550–O70596), but a few are also present in centralParan? (Ro = 070577–0–70591), while those of GroupB are exclusive to central Paran– {Ro = 070463–0–70580) Low-Ti basalts with R₀> O7060 are typical of southern Paran?(R₀ = O7O639 –O71137), but are also present in centralParana (Ro = 070620–070890). These low-Ti basalts havechemical similarity (e.g., Ti, P, Sr) with low-Ti basalts depletedin incompatible elements (Group B) from which, however, theydiffer-in possessing significantly higher concentrations ofSiO₂, K₂O, Rb, and Ba. Such chemical diversity, accompaniedby important Ro variations (070463–071137) suggests thatthe low-Ti basalts from southern and part of central Paranamay result from crustal contamination. On the contrary, low-Ti basalts from northern, and part of central, Parana (GroupA) may be considered virtually uncontaminated. Results indicate that crustal contamination by granitic material(s)may be in the range 7–17 per cent. Such contaminationin central Paran? appears compatible with an assimilation-fractionalcrystallization process (AFC), while in southern Parana, othercontamination processes (e.g., mixing of magmasfrom crustaland mantle sources, assimilation of wall rock while magmas flowthrough dykes, etc.) were probably superimposed on AFC. Thedegree of crustal contamination generally decreases from southernto northern Parana. Sr and Nd isotope ratios suggest that mantle source materialfor low-Ti basalts depleted in incompatible elements (GroupB: southern and part of central Parana) had a lower R₀ value(c. O.7046) and a higher ^l43Nd/¹⁴⁴Nd ratio (Nd + c. 0.51274)than that for low-Ti basalts enriched in incompatible elements(Group A: northern and part of central Parana), namely R₀ c.O.7059 and Nd+ c. 0.51242. These Sr-isotopic differences alsoapply to the northern (incompatible-element rich, R₀ c. O.7053)and southern (incompatible-element poor R₀ c. 0.7046) basaltprovinces of Karoo, suggesting that both Parana and Karoo basaltmagmas, differing by about 70 m.y. in age, probably originatedin a similar batch of subcontinental lithospheric mantle inpredrift times (cf. Cox, 1986). The extension of the Dupal Sr-anomaly (i.e. Rio Grande Rise+ Wai vis Ridge + Gough and Tristan da Cunha islands: Sr = 46=53;Hart, 1984) inside the Brazilian continent (Sr = 46–59)suggests that the lithospheric mantle of the Parana (and Karoo)provinces was possibly also the local source of oceanic volcanismup to advanced stages of the opening of the South Atlantic. ^*Reprint requests to E. M. Piccirillo.     

Rift-zone magmatism: Petrology of basaltic rocks transitional from CFB to MORB,southeastern Brazil margin

Compositions of basaltic samples from the southeastern Brazil passive margin (18°–24° S) depict the change from continental to oceanic lithosphere during the opening of the South Atlantic Ocean. Samples studied range from 138 to 105 m.y. old and are from 12 Petrobrás drill cores recovered from the coastline to about 150 km offshore in the Espirito Santo, Campos, and Santos basins. Compositions vary, ranging, for example, from 49–54 wt.% SiO₂, 0.5–3.0 wt.% TiO₂, 0.6–5.0 FeO^*/MgO, and 1-6 La/ Yb_(n), but can be grouped: (i) basalts enriched in incompatible elements, such as K (some K₂O>2.0 wt.%), Rb (>18 ppm), Zr (>120 ppm), and LREE (some FeO^* 16 wt.%; most with SiO₂ 51–54 wt.%), and resembling Serra Geral continental flood basalts (SG-CFB) of southern Brazil; (ii) basalts less enriched, or transitional, in incompatible elements, having K₂O <0.40 wt.% and flat REE patterns, and resemble N. Atlantic diabases and FAMOUS basalts; and (iii) one depleted sample, Ce/Yb_(n)=0.7, where Ce_(n)=4. Expressed in oceanic-basalt terminology and Zr-Nb-Y abundances, enriched samples are P- and T-type MORB (e.g., Zr/Nb 4–25), transitional samples are T-type (Zr/ Nb 8–27), and the depleted sample is N-type MORB (Zr/Nb>30).Trace-element ratios (e.g., Zr/Nb, Zr/Y) link the Brazil margin basalts to a heterogeneous mantle (attributed to metasomatic veining) of variably proportioned mixtures of depleted-mantle (N-MORB) and plume (P-MORB, e.g., Tristan hotspot) materials. The various compositions therefore reflect, in part, different zones of melting during the separation of Gondwanaland, where gradual decompression during rifting enabled concurrent melting of upper, more depleted (non- or sparsely-veined) mantle and enriched (densely-veined) mantle. Within the time represented, melting produced enriched, transitional, and depleted magmas that were emplaced subaerially, hypabyssally, and subaqueously; they mark the transition from CFB before rifting and separation (from deeper, enriched mantle) to N-MORB in the S. Atlantic afterwards (from non- or sparsely-veined upper mantle). While P-type mantle components account for the enriched compositions of some basalts (Zr/Nb<8), continental crust is largely responsible for that of others (e.g., Ti/Zr 40–57; La/Yb_(n) 5–6, and ¹⁸O+12.2 in one sample). Some may be contaminated expressions of otherwise T-type basalts free of crustal components. This study identifies CFB to be from sources similar to those for T- and P-type oceanic rocks, where individual CFB magmas may or may not have acquired crustal signatures.     

The Scourie dyke suite: Petrogenesis and geochemical nature of the Proterozoic sub-continental mantle

The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions.Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial ⁸⁷Sr/⁸⁶Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier.     

Geochemistry and origin of basaltic lavas from Marquesas Archipelago,French Polynesia

The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii.     

Some Key Problems on the Petrogenesis of Seafloor Basalts,Abyssal Peridotites and Geodynamics—A Non-traditional Isotope Approach

This paper discusses some major research to be carried out in the next five years in the newly established Laboratory of Ocean Lithosphere and Mantle Geodynamics. By using our existing sample collections of global mid-ocean ridge basalts, gabbros and abyssal peridotites from the Pacific, Atlantic and Indian oceans, the research includes: ①Using Ti-Zr-Hf stable isotope methods to test the hypothesis that the observed huge Nb-Ta and Zr-Hf fractionations result from mass-dependent fractionation under mantle magmatic conditions; ②Using a MORB sample suite of uniform ratios of incompatible elements and Sr-Nd-Pb isotopes with large major element compositional variation to test the common hypothesis of iron isotope fractionation, i.e, the affinity of heavy Fe with ferric Fe (Fe³⁺), and both heavy Fe and ferric Fe (Fe³⁺) being more incompatible than light Fe and ferrous Fe (Fe²⁺) during magma evolution; while using an incompatible trace element and Sr-Nd-Pb isotope highly variable MORB suite to test the same hypothesis during low-degree mantle melting (i.e, the effect of mantle metasomatism); ③Proposing and testing the hypothesis that the high oxygen fugacity of the Earth’s mantle is a consequence of plate tectonics by subducting partially serpentinized oceanic mantle lithosphere with abundant ferric Fe (e.g. Fe³⁺/SFe>2); ④The recent work by Andersen et al. (Nature, 2015) is a milestone contribution by using U isotope variation in oceanic basalts to hypothesize that the O₂-rich atmosphere since the late Archean (abont 2.4 Ga) mobilized the water soluble U (6+ vs. 4+) from continents, transported to the ocean and subducted with sediments to the upper mantle, which explains the low Th/U in MORB (<2.5) and the high Th/U (>3.5) ocean island basalts (OIB) do not see such U addition effect probably because OIB source materials are all ancient (> abont 2.4 Ga) if there were subducted component. The Cenozoic alkali basalts from eastern China are ideal materials for evaluating the significance of the subducted seafloor materials for the petrogenesis of OIB and enriched MORB by using the U isotope approach, which is expected to revise and improve the Andersen et al hypothesis.     

Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites

Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P₂O₅ concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10^?6 to 10^?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.     

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K₂O, TiO₂, and P₂O₅ contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe²⁺) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe²⁺) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.