粤东五华河流域的化学风化与CO2吸收

基于对粤东五华河干流和支流水体的物理、化学组成测试数据,应用质量平衡法和相关分析法探讨湿热山地丘陵地区岩石化学风化过程对大气CO₂的吸收.结果表明:五华河水体的总溶解性固体含量(77.11 mg/L)接近于世界河流的平均值(65 mg/L);离子组成以Ca²⁺、Na⁺和HCO₃^-为主,可溶性Si次之.五华河流域化学径流组成主要源自硅酸盐矿物化学风化过程的贡献,碳酸盐矿物的贡献较少;大气和土壤CO₂是流域内岩石化学风化的主要侵蚀介质.与同一气候带其他河流相比较,五华河流域岩石化学风化过程对大气CO₂的吸收通量(2.14×105mol/(km²·a))较低,这主要是由于流域内缺乏碳酸盐岩所导致.     

Stability of forsterite + CO2 and its bearing on the role of CO2 in the mantle

An experimental determination of the reaction MgCO₃ + MgSiO₃ = Mg₂SiO₄ + CO₂ between 20 and 40 kbars and in the range 1000–1500°C yields an average pressure effect on the equilibrium of 44 bars/°C. This result shows that the assemblage forsterite and carbon dioxide is not stable under most pressure and temperature conditions expected in the upper mantle. Hypotheses requiring the presence of free CO₂ in the low-velocity zone, CO₂ as a drive mechanism for kimberlite emplacement, or action of a free CO₂ phase in ultramafic rocks may need considerable revision.     

Are thermal domes heated by CO2-rich fluids from the mantle?

A large part of the fluid present in the thermal dome at Naxos, Greece, during the main stage of metamorphism was CO₂-rich and of deep-seated origin. Samples of this pervasive fluid were trapped in fluid inclusions, which are particularly abundant in syn-metamorphic quartz segregations. These quartz segregations are considered to represent the main passageways of the escaping fluids. Mass-balance calculations based on δ¹³C values of fluids and rocks indicate that most fluid/rock mass ratios are between 0.06 and 0.8. The heat added to the metamorphic system by the cooling of these fluids may have been the prime cause of metamorphism and updoming.     

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.     

Geothermal gradient of the upper mantle beneath Jeju Island, Korea: Evidence from mantle xenoliths

Abstract Ultramafic xenoliths found in alkali basalts from Jeju Island, Korea are mostly spinel lherzolites accompanied by subordinate amount of spinel harzburgites and pyroxenites. The combination of results from a two-pyroxene geothermometer and Ca-in-olivine geobarometer yields temperature–pressure (T–P) estimates for spinel peridotites that fall in experimentally determined spinel lherzolite field in CaO-Fe-MgO-Al₂O₃-SiO₂-Cr₂O₃ (CFMASCr) system. These T–P data sets have been used to construct the Quaternary Jeju Island geotherm, which defines a locus from about 13 kbar at 880°C to 26 kbar at 1040°C. The geothermal gradient of Jeju Island is greater than that of the conventional conductive models, and may be as a result of a thermal perturbation by the heat input into the lithospheric mantle via the passage and emplacement of magma. Spinel–lherzolite is the main constituent rock-type of the lithospheric mantle beneath Jeju Island. Pyroxenites may be intercalated in peridotites at similar depth and temperature as re-equilibrated veins or lenses.     

Geobarometry of ultramafic xenoliths from Loihi Seamount,Hawaii, on the basis of CO2 inclusions in olivine

Abundant fluid inclusions in olivine of dunite xenoliths (～1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO₂ (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions.Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO₂ liquid and gas, with an aggregate ? = ～ 0.5–0.75 g cm^?3, and represent trapped globules of dense supercritical CO₂ (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO₂ blebs. Spherical sulfide blebs having widely variable volume ratios to CO₂ and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present.Assuming olivine growth at ～ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO₂P-V-T data indicates that the primary inclusions were trapped at ～ 220–470 MPa (2200–4700 bars), or ～ 8–17 km depth in basalt magma of ? = 2.7 g cm^?3. Because the temperature cannot change much during the rise to eruption, the range of CO₂ densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.     

Dunitification of mantle lithosphere below the society archipelago: Evidence for magma-mantle thermal and chemical transfers through xenoliths from the Papenoo Valley, Tahiti

Chemical and thermal transfers between mantle lithosphere and O.I.B. from French Polynesia are described in an inclusion suite from the Papenoo Valley, Tahiti. The methods used consider chemical exchange kinetics in geothermometry and olivine deformation study. The mineralogical study shows the extent of the chemical and thermal exchanges between a lithosphere equilibrated at 1000–1050°C and plume basalts whose liquidus temperature is about 1250°C. The dunitification process along with the spinel and olivine “fertilization” is revealed in samples which preserved features of their former mantle deformation. The deformation study also shows that interaction took place within the mantle itself. The whole of the results implies a significant metasomatism of the mantle lithosphere and alteration of the initial geochemical signature of the deep originating O.I.B. beneath French Polynesia.     

Evidence for crustal assimilation by turbulently convecting,mafic alkaline magmas: Geochemistry of mantle xenolith-bearing lavas from northern Sardinia

Alkali basalt, trachybasalt and basanite magmas, containing abundant xenoliths of upper mantle origin, were erupted during the Plio-Pleistocene (2.4-0.14 Ma) in northern Sardinia. The magmas are enriched in K, Rb, Th and Ba relative to mid-ocean ridge basalts (MORB) and most ocean island basalts (OIB), resulting in high K/Nb, Th/Nb, Ba/Nb and Rb/Nb ratios. The large number of spinel peridotite inclusions in these lavas suggests that these chemical features cannot be explained by combined assimilation and fractional crystallization within the continental crust. However, volcanic rock chemistry can be explained by the assimilation of sialic rocks by turbulently convecting, mafic magmas during their ascent to the surface. Fractionation of Ba and K from the light rare earth elements (LREE) is required to explain the positive correlation of K/La and Ba/La with ⁸⁷Sr/⁸⁶Sr_(i). Consequently, bulk assimilation of crystalline basement rocks by rising, hot basaltic magmas cannot explain the observed chemical trends, and preferential melting of a low melting quartzo-feldspathic crustal component probably occurred, leaving the REE in residual phases such as apatite, zircon, sphene and amphibole. Alternatively, large ion lithophile element (LILE) enrichment may have been related to interaction of rising mafic lavas with metasomatized lithospheric mantle or enriched asthenosphere.     

Origin and source of CO2 in natural gas from the eastern Sichuan Basin

The CO2 contents of natural gas from the eastern Sichuan Basin were mainly less than 2%,and they were generally large when the H2S contents were over 5%.The natural gas in the eastern Sichuan Basin showed high δ13C1 values uncorrelated with H2S contents,and the oil-associated gas generally displayed carbon isotopic reversal between methane and ethane,whereas the coal-derived gas samples displayed positive series.The δ13C values of CO2 in the natural gas could be divided into two types,one with low values(-24‰--12‰)and the other with high values(-8‰-4‰).The natural gas had low R/Ra values and broad CO2/3He values distinct from the mantle-derived gas,and the CO2 was typical of crustal origin.Natural gas with high δ13CCO2 values is distributed in the north region of the eastern Sichuan Basin,and CO2 in H2S-poor and H2S-free reservoirs was mainly derived from the thermal decomposition of carbonate rocks under Permian magmatism and high heat flow,whereas CO2 in the gas reservoirs with high TSR extent was derived mainly from the dedolomitization of carbonate reservoirs under acidic formation water,and the CO2 of TSR origin generally entered into the secondary calcite.The CO2 in natural gas in the south-central region of the eastern Sichuan Basin,which had been rarely affected by the Emeishan mantle plume,displayed low δ13C values and was of organic origin.Though the natural gas in Wolonghe gas field had experienced TSR,the low reaction extent and weak acidity of formation water could hardly have resulted in the dedolomitization,and therefore CO2 in the natural gas was mainly of TSR origin with low δ13C values.     

Origin and evolution of two contrasting thermal groundwaters (CO2-rich and alkaline) in the Jungwon area,South Korea: Hydrochemical and isotopic evidence

In the Jungwon area, South Korea, two contrasting types of deep thermal groundwater (around 20–33 °C) occur together in granite. Compared to shallow groundwater and surface water, thermal groundwaters have significantly lower δ¹⁸O and δD values (> 1‰ lower in δ¹⁸O) and negligible tritium content (mostly < 2 TU), suggesting a relatively high age of these waters (at least pre-thermonuclear period) and relatively long subsurface circulation. However, the hydrochemical evolution yielded two distinct water types. CO₂-rich water (P_CO2 = 0.1 to 2 atm) is characterized by lower pH (5.7–6.4) and higher TDS content (up to 3300 mg/L), whereas alkaline water (P_CO2 = 10^− 4.1–10^− 4.6 atm) has higher pH (9.1–9.5) and lower TDS (< 254 mg/L). Carbon isotope data indicate that the CO₂-rich water is influenced by a local supply of deep CO₂ (potentially, magmatic), which enhanced dissolution of silicate minerals in surrounding rocks and resulted in elevated concentrations of Ca²⁺, Na⁺, Mg²⁺, K⁺, HCO₃⁻ and silica under lower pH conditions. In contrast, the evolution of the alkaline water was characterized by a lesser degree of water–rock (granite) interaction under the negligible inflow of CO₂. The application of chemical thermometers indicates that the alkaline water represents partially equilibrated waters coming from a geothermal reservoir with a temperature of about 40 °C, while the immature characteristics of the CO₂-rich water resulted from the input of CO₂ in Na–HCO₃ waters and subsequent rock leaching.     

Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths

Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H₂O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H₂O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.     

Geochemical features of the geothermal CO2-watercarbonate rock system and analysis on its CO2 sources

Taking Huanglong Ravine and Kangding, Sichuan, and Xiage, Zhongdian, Yunnan, as examples, the authors summarize the hydrogeochemical and carbon stable isotopic features of the geothermal CO₂-water-carbonate rock system and analyze the CO₂ sources of the system. It was found that the hydrogeochemical and carbon stable isotopic features of such a system are different from those of shallow CO₂-water-carbonate rock system, which is strongly influenced by biosphere. The former has higher CO₂ partial pressure, and is rich in heavy carbon stable isotope. In addition, such a geothermal system is also different from that developed in igneous rock. The water in the latter system lacks Ca²⁺, and thus, there are few tufa deposits on ground surface, but it is rich in light carbon stable isotope. Further analysis shows that CO₂ of the geothermal CO₂-water-carbonate rock system is a mixture of metamorphic CO₂ and magmatic CO₂.     

Diffuse CO2 emission rate from Pululahua and the lake-filled Cuicocha calderas,Ecuador

We report herein the first results of two soil CO₂ efflux surveys carried out at Cuicocha lake-filled and Pululahua caldera volcanic systems, Ecuador. A total of 172 and 217 soil CO₂ efflux measurements were taken at the surface environment of Pululahua and Cuicocha calderas respectively, by means of the “accumulation chamber” method during the summer of 2006 to constrain the total CO₂ output from the studied area. Soil CO₂ efflux values ranged from non-detectable up to 48.5 and 141.7 g m^{− 2} d^{− 1} for Cuicocha and Pululahua calderas respectively. In addition, probability graphs were used to distinguish the existence of different geochemical populations. Sequential Gaussian Simulation was used to construct an average map for 100 simulations and to compute the total CO₂ emission at each studied area: 106 and 270 t d^{− 1} (metric tons per day) for Cuicocha (13.3 km²) and Pululahua (27.6 km²) volcanoes respectively.     

Age of the Archean Talga-Talga Subgroup, Pilbara Block, Western Australia, and early evolution of the mantle: new SmNd isotopic evidence

Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal¹⁴³Nd/¹⁴⁴Nd vs.¹⁴⁷Sm/¹⁴⁴Nd isochron plot. The data give an age of 3712 ± 98 Ma and initialε_Nd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the SmNd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, withε_Nd(T) values ranging from at least 0 to +4.0.     

Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.     

Hot-spots,mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts

The Wilson-Morgan hypothesis of hot-spots, characterized by high heat flow, positive gravity anomaly and alkalic volcanism, assumes that such hot-spots are surface expressions of mantle plumes rising by thermal convection. Possible evidence of this mantle upwelling is shown here from textural, structural and chemical aspects of ultramafic xenoliths in alkalic basalts. The xenolith-bearing basalts are constanly associated with Wilson-Morgan hot-spots in the ocean basins and with their continental counterparts in the rift valleys which show extensional tectonics. Most of the xenoliths are considered to be accidental fragments of the lithosphere in the host basalts. One remarkable aspect of xenoliths from all parts of the world is their ubiquitous tectonite fabric. The microstructures of these xenoliths are due to plastic deformation. Some of the xenoliths from Baja California show characteristic deformational features which are also found in the marginal parts of diapirically intruded high-temperature peridotite massifs. A model is proposed for the origin of xenoliths in alkalic basalts by mantle upwelling in which the plastic deformation of the xenoliths reflects this dynamic uprise.     

Phase relations of titan-phlogopite,K2Mg4TiAl2Si6O20(OH)4: a refractory phase in the upper mantle?

Experimental data on the stability of titan-phlogopite [K₂Mg₄TiAl₂Si₆O₂₀(OH)₄] are presented which show it to be stable to substantially higher temperatures than normal phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄]. A qualitative model to explain the role of titan-phlogopite during magma generation is put forward. Breakdown of titan-phlogopite during melting at depth (> 150km) on subducted lithospheric slabs is believed responsible for the concomitant increase of K and Ti observed in magmas erupted during orogenic volcanism. At lower pressures (up to about 10 kbar) beneath mid-oceanic ridges, titan-phlogopite is predicted to behave as a refractory phase during partial melting in the mantle, especially if H₂O-excess conditions pertain, although at higher pressures in this environment it would almost certainly behave as a low-melting component.     

High-pressure phases of Co2GeO4, Ni2GeO4, Mn2GeO4 and MnGeO3: implications for the germanate-silicate modeling scheme and the Earth's mantle

In a diamond-anvil press coupled with YAG laser heating, the spinels of Co₂GeO₄ and Ni₂GeO₄ have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO₃ transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a₀ = 5.084 ± 0.002, b₀ = 5.214 ± 0.002, and c₀ = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO₃ is ?6.6%. Mn₂GeO₄ disproportionates into a mixture of the perovskite phase of MnGeO₃ plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.     

Effects of melt percolation on the Re-Os systematics of continental mantle lithosphere: A case study of spinel peridotite xenoliths from Heilongjiang,NE China

Os isotope ratios of mantle peridotites have been considered to be largely immune to recent melt-rock interaction. However, Os isotope ratios and PGE (Platinum group elements) concentrations of the Yong’an xenoliths have been significantly modified by melt percolation, and are not suitable for determining the formation age of lithosphere mantle in Yong’an. In this study, the Yong’an spinel peridotite xenoliths are divided into two groups: N-Type and E-Type. The N-Type group including cpx (clinopyroxene)-poor lherzolite and harzburgite, shows a large variation of Cr#(sp) (13.2-48) and sulfur contents (from 171 ppm to below detection limit), whereas the E-Type peridotites are mainly refractory harzburgites and are characterized by high Cr#(sp) (35.3-42.2) and overall low sulfur contents (below 51 ppm). Both types show similar major and REE (rare earth element) patterns. Furthermore, the N-Type peridotites display a restricted range of iridium-group PGE (IPGE), Os/Ir and Ru/Ir ratios (Os/Ir = 0.64-1.12, Ru/Ir = 1.52-1.79) and variable palladium-group PGE (PPGE) contents (3.4-14.9 ppb), whereas the E-Type peridotites show a large variation of Os/Ir and Ru/Ir ratios (Os/Ir = 0.33-0.84, Ru/Ir = 0.94-1.6), and a restricted range of PPGE (4.3-6.9 ppb). 187Os/188Os ratios of E-Type peridotites are higher than those of N-Type peridotites at comparable fertility levels. These results suggest that N-Type peridotites may have been overprinted by metasomatism via small melt fractions, in which the percolation of the volatile-rich, small melt fractions only resulted in LILE (large ion lithophile element) enrichment of clinopyroxene, and their whole rock PGE contents and Re-Os isotope values were little changed. Moreover, E-Type peridotites may have been modified by melt-rock reaction involving relatively large melt fractions, which may result in the formation of secondary cpx and olivine and the removal of IPGE-bearing minerals such as Ru-Os-(Ir) alloys or laurite, followed by precipitation of secondary sulfides from melt with radiogenic isotopic signature.     

Low- and high-TiO2 flood basalts of southern Brazil: origin from picritic parentage and a common mantle source

The Serra Geral (Paraná) continental flood-basalt province of southern Brazil has two main basalt types: low-TiO₂ ( 1 wt.%) basalts occupy the southern portion, and high-TiO₂ (> 3 wt.%) basalts are largely in the northern part. Low-Ti basalts are less evolved (Mg# 60) and more radiogenic (e.g., ⁸⁷Sr/⁸⁶Sr 0.708) than high-Ti basalts (Mg# 35; ⁸⁷Sr/⁸⁶Sr 0.705). This is consistent with a model that invokes variable melting of a single mantle source to produce picritic magmas that have relatively lower and higher incompatible element contents. Varying percentages of melting can be related to varying proximity to the early Tristan da Cunha hotspot. The Mg-rich magmas fractionated 60–75% olivine, clinopyroxene, and plagioclase to yield low- or high-Ti flood basalts, assimilating more or less crust in the process. The extent of fractionation and assimilation depended on crustal “warmth” (also tied to location relative to hotspot): (1) above zones of 25% melting, warm crust relatively easily contaminated crystallizing picritic magma that originated by a high degree of melting (i.e., magma with lower incompatible element contents); additionally, high degrees of melting sustained replenishment of magma with low-Ti magma characteristics; (2) above 10% melting zones, cooler crust comparatively restricted assimilation during crystallization (of magma with higher incompatible element contents) and permitted magma evolution to high-Ti derivatives; lesser degrees of melting also limited replenishment magma and thereby allowed greater evolution of existing magma. This model refers all diagnostic geochemical and isotopic features of Serra Geral basalts to percentages of partial melting of an essentially homogeneous mantle material.