Host Basalt Research Articles

Geology and genesis of the Chomalu polymetallic deposit, NW Iran

The vein-type Chomalu polymetallic epithermal deposit, located in the Tarom-Hashtjin Metallogenic Province (THMP), western Alborz, formed in two separate mineralized zones controlled by fault zones within andesite and olivine basalt host rocks. The deposit occurs adjacent to the Varmarziar quartz monzonite, quartz monzodiorite, and monzosyenite intrusions. Ore mineralization consists of three stages; (i) an early stage dominated by gold-bearing breccia veins mainly consisting of quartz, pyrite, arsenopyrite, chalcopyrite, minor magnetite and bornite assemblages, (ii) a main stage, which is divided in three sub-stages I, II and III. Sub-stage I is dominated by Pb-Zn-Cu-Ag-rich mineral assemblages including galena, Fe-poor sphalerite, minor chalcopyrite, tetrahedrite, and pyrite. Sub-stage II is only represented by galena and Fe-poor sphalerite. Sub-stage III is defined by Fe-Mn-Zn-Ca-Mg carbonate minerals. The late supergene stage (iii) is characterized by malachite, covellite, chalcocite, cerussite, and Fe-oxide and hydroxides. Microthermometry of primary fluid inclusion assemblages indicate homogenization temperatures (Th) of 253° to 305 °C and salinities of 3.5–4.82 wt% NaCl equivalent in the early stage. The main stage indicates homogenization temperatures (Th) of 200° to 280 °C and 180° to 235 °C with salinities of 2.18–4.96 and 0.53–2.90 wt% NaCl equivalent for the sub-stages I and II, respectively. Ore precipitation was possibly caused by simultaneous dilution and cooling of hydrothermal fluids. The epithermal mineralization in the Chomalu deposit may be classified as an E-type intermediate sulfidation epithermal deposit in the extensional back-arc regime.

Sublithosphere Mantle Crystallization and Immiscible Sulfide Melt Segregation in Continental Basalt Magmatism: Evidence from Clinopyroxene Megacrysts in the Cenozoic Basalts of Eastern China

AbstractThis study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to >10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts.

Formation of green-core clinopyroxene in continental basalts through magmatic differentiation and crustal assimilation: Insights from in-situ trace element and Pb isotopic compositions

Green-core clinopyroxene (GCPX) widely occurs in diverse magmas and various tectonic settings. Due to its special textual and compositional zonation, it is usually considered as solid evidence for involvement of exotic felsic melt and some unique geological processes (e.g., magma mixing, mantle metasomatism and recycling of foundered lower continental crust (LCC)). However, this premise is challenged by its more complicated and enigmatic origins than previously thought. To explore potential origins of GCPXs in continental basalts, here we investigate in-situ trace element and Pb isotopic compositions of the GCPXs in the Feixian and Sihetun primitive basalts from the eastern North China Craton (NCC). The results show that the Feixian GCPXs can be subdivided into three types in addition to the Sihetun GCPXs. Regardless of the types of these GCPXs, their surrounding rims are regarded as overgrowths that crystallized from the basaltic host magmas. The wide range of Mg# (67–86), trace element patterns similar to those of clinopyroxene equilibrated with the basaltic host magmas and host lava-like Pb isotopic ratios of the Feixian type-1 GCPXs suggest that they have crystallized from differentiated melts genetically related to the basaltic host magmas. Besides unradiogenic Pb isotopic ratios, the Feixian type-2, type-3 and Sihetun GCPXs are characterized trace element signatures that are identical or similar to those of clinopyroxenes in pyroxene-rich granulite, garnet-bearing granulite and feldspar-rich granulite xenoliths from the NCC, respectively, indicating their derivation from lower crustal granulite wall rocks during the ascending of the basaltic host magmas. Our work suggests that GCPXs can be simply formed through the two common magmatic processes of continental basaltic magmas (i.e., crystallization differentiation and crustal assimilation) and highlights that exotic felsic melt and unique geological processes are not additionally required despite being possible. Before using GCPXs as an indicator of specific magmatic processes, comprehensive data (e.g., trace element and isotopic compositions) should be carefully investigated for their exact origins, especially for those in continental magmas. Additionally, GCPXs may provide valuable information about the structure and composition of the lower crust and a window to unravel crustal assimilation and digestion during magma extraction and their influence on compositions of basaltic magmas.

Classification and generation of the enclaves in Karapınar-Karacadağ volcanic rocks (Central Anatolia)

Karapınar-Karacadağ Volcanic Rocks (KKVR) have very complex magmatic history and outcropped the southwestern part of the Cappadocia Volcanic Province (Central Anatolia). Here we present the petrography and whole-rock chemistry of the enclave-bearing rocks to constrain their source and evolution history. These petrographic observation and geochemical data reveal that the enclaves in the Karapınar-Karacadağ volcanic rocks are magma mixing/mingling enclaves (MME), magma segregation enclaves (MSE), and xenoliths. Here we discriminated these enclaves into eight different types according to their mineral composition and textural features. The magma mixing/mingling enclaves (Type 1, 7, 8) are the mixing products of coeval more felsic and mafic magmas. They show hypocrystalline porphyritic, holocrystalline granular, and intergranular textures, and rich in mafic minerals, and have characteristic petrographic features such as quenched amphibole, bladed biotite, ocelli-quartz, sieved and cellular plagioclases. In andesites they range from basalt to andesite in composition. However, in basalts, they are in basaltic composition. The magma segregation enclaves (Type 2, 4, 5, 6), which are observed in almost all the KKVRs, are cognate xenolith because of plucking from the different parts of the magma chamber. They are holocrystalline and granular in texture. The magma segregation enclaves contained in the andesitic host rocks are hornblende gabbro and pyroxene gabbro in composition, whereas in the basaltic host rocks they are dunite, lherzolite, and basalt in composition. The xenolithic enclaves (Type 3) are observed in the basalts as quartz, plagioclase, biotite, and amphibole xenocrysts. Major oxides and trace element data of the studied rocks indicate that the MSEs are more primitive than their host rocks, and all of the enclaves (MME-MSE) are in accordance with their hosts. According to petrographic observations and geochemical data we propose that fractional crystallization, magma mixing and assimilation processes have a key role in the evolution of the KKVRs and their enclaves.

Indigenous carbon-rich matter associated with unusual aqueous alteration features in Nakhla: Complex formation and preservation history

While indigenous organic matter has been previously reported in the Mars meteorite Nakhla, little is known as to either its form or distribution. A notable feature of Nakhla is the prevalence of secondary phases associated with aqueous alteration. By analogy with the terrestrial environment, our objective was to determine whether Martian secondary minerals could also act to accumulate and preserve such organic matter. Through a multidisciplinary approach, we have characterized the nature of carbonaceous matter and spatially associated phases within the Martian meteorite Nakhla using a wide-ranging suite of analytical instrumentation including optical microscopy, laser Raman spectroscopy, focused ion beam microscopy, secondary ion mass spectrometry, and scanning and transmission electron microscopy. In freshly fractured chips of Nakhla, we found carbonaceous phases intimately associated with secondary aqueous alteration phases, both mineral and amorphous, interpreted to have formed through the low-temperature aqueous dissolution of the host basalt while on Mars. The carbonaceous matter is present both in condensed phases and in a dispersed state spatially associated with secondary alteration phases. In the former, we identified discrete refractory micron to submicron assemblages that appear macromolecular in nature and, in several cases, associated with fluorine and, in one case, significant nitrogen. Textural, chemical, mineralogical and isotopic considerations argue for a non-terrestrial origin of this carbonaceous matter. Additionally, we report the presence of the ferrous hydroxycarbonate mineral chukanovite (Fe2OH2CO3) within some of the secondary mineral aggregates studied. Neither the intimate association of carbonaceous matter with secondary phases nor the identification of chukanovite have been previously reported in any of the Martian meteorites. In this regard, we note such microscale features within alteration phases would most likely be lost in the preparation of conventional polished thin sections and thus explain why they have not been previously reported. In lieu of sample return, the sui generis nature provided by Mars meteorites provide insight to alteration processes on Mars currently denied to robotic exploration and remote sensing. Our results show a variety of habitability-related sample attributes, formed hundreds of millions of years ago near Mars’ surface, have persisted there until very recently and may be more widespread in the surface regolith than previously thought. This may have implications in sample selection criteria for Mars sample return.

Magmatic and hydrothermal evolution of mafic pegmatites and their host basalts, Paraná Large Igneous Province, Brazil

The high-Ti Paranapanema sequence in the Paraná Large Igneous Province consists of a succession of pahoehoe lava flows. Some of these flows host mafic segregation structures, such as horizontal pegmatitic sheet layers and vesicular cylinders. Geochronology, whole-rock and mineral chemistry of two pegmatite occurrences and their host basalts permit characterizing crystal-melt segregation processes that occur during magmatic differentiation after lava flow emplacement. The mineralogy of the pegmatites is similar to that of the host basalts, and it includes plagioclase, pyroxene and titanomagnetite-ilmenite crystals; the only observable difference is the coarser grain size of primary phases and an increase in the volume fraction of clay minerals derived from glass devitrification and vesicle infilling in the pegmatites (~15 vol% clay in the pegmatites vs. ~6 vol% clay in the basalts). The pegmatites from both sites are more fractionated than the enclosing lavas and are enriched in TiO2, FeOtotal, K2O, P2O5 and incompatible trace elements (e.g., Ba, Rb, Nb, Hf, Y, Zr, La, Ce, Yb). Thermodynamic modeling of whole-rock compositional data suggests that the Barracão pegmatites were formed after ~8–27 wt% fractional crystallization of the host basalt, whereas the Capanema pegmatites formed after ~38–43 wt% fractional crystallization of the host basalt. 40Ar/39Ar geochronology indicates slight yet significant age differences between plagioclase phenocrysts from the pegmatites and whole-rock fragments from the enclosing lavas (Capanema: 133.56 ± 0.52 pegmatite vs. 134.30 ± 0.46 Ma enclosing lava; Barracão: 133.23 ± 0.88 pegmatite vs. 134.49 ± 0.31 Ma enclosing lava). These age differences likely result from hydrothermal alteration of the plagioclase phenocrysts (e.g., secondary albite and K-feldspar), and they are not due to resolvable differences in crystallization age. Smectite-celadonite aggregates recovered from the Capanema basalt suggest that hydrothermalism occurred at 123.18 ± 0.61 Ma, approximately ten million years after rock crystallization.

Magnetic and Gravity Surface Geometry Inverse Modeling of the TAG Active Mound

AbstractSeafloor massive sulfide deposits form in remote environments, and the assessment of deposit size and composition through drilling is technically challenging and expensive. To aid the evaluation of the resource potential of seafloor massive sulfide deposits, three‐dimensional inverse modeling of geophysical potential field data (magnetic and gravity) collected near the seafloor can be carried out to further enhance geologic models interpolated from sparse drilling. Here, we present inverse modeling results of magnetic and gravity data collected from the active mound at the Trans‐Atlantic Geotraverse hydrothermal vent field, located at 26°08′N on the Mid‐Atlantic Ridge, using autonomous underwater vehicle and submersible surveying. Both minimum‐structure and surface geometry inverse modeling methods were utilized. Through deposit‐scale magnetic modeling, the outer extent of a chloritized alteration zone within the basalt host rock below the mound was resolved, providing an indication of the angle of the rising hydrothermal fluid and the depth and volume of seawater/hydrothermal mixing zone. The thickness of the massive sulfide mound was determined by modeling the gravity data, enabling the tonnage of the mound to be estimated at 2.17 ± 0.44 Mt through this geophysics‐based, noninvasive approach.

Lithium distribution and isotopic composition in zircon megacrysts as constraints for their mantle origin and volcanic transfer timescales

Lithium in zircon has attracted considerable attention for identifying magma sources and as a diffusion chronometer, despite the exact diffusion mechanisms remaining controversial. Zircon megacrysts sourced from Quaternary intraplate basaltic volcanoes in the Ratanakiri Volcanic Province of northeastern Cambodia were analyzed here for Li isotopic and elemental concentration profiles to constrain diffusion behavior of Li in zircon megacrysts entrained in high temperature melts. This is complemented by high-spatial resolution U-Pb geochronology, O-Hf isotope compositions and trace element abundances to refine the long-term pre-eruptive history of these zircon megacrysts. Ratanakiri zircon megacrysts yield a Tera–Wasserburg regression U-Pb age of 0.98 ± 0.02 Ma (n = 615 spot analyses) that is indistinguishable from eruption ages reported for the host basalts, although zircon was unstable in the enclosing magma as evidenced by marginal corrosion and abundant tubular channels within the crystals. Extreme Li-isotopic variations (within −30 to +96‰ on the δ7LiLSVEC scale) of the zircon megacrysts require kinetic Li fractionation due to diffusive mobilization, which is further supported by offsets in intracrystalline domain boundaries with different Li and Y abundances detected in scanning ion images. Complex abundance and isotopic ratio patterns for Li in zircon megacrysts point to multi-mode Li diffusion that is influenced by variable trace element inventories within individual crystals, which correlate with minor differences in the O-Hf isotopic compositions. The overall tendency of decreasing Li abundances along with increasing δ7Li values toward crystal rims indicates diffusion-driven equilibration between zircon and Li-depleted basaltic melt. Diffusion modeling of Li and δ7Li on domain-boundaries within crystals show pre-eruptive heating timescales of about 18 days, which is consistent with the magma transfer duration estimated from zircon megacryst settling rate calculations, implying the zircon megacrysts should be transported from their source region to the surface by ascending alkali basaltic magma within few weeks. Nearly one to two orders of magnitude longer Li diffusion model timescales were computed for crystal rim domains. This apparent faster diffusion of Li is attributed to H+ loss into a volatile phase during eruption, providing an additional driving force for Li+ diffusion, which might become more pronounced at the crystal boundary where zircon is exposed to strong chemical gradients in volatile elements due to decompression and concomitant degassing. Lithium isotope compositions for the innermost domains of zircon megacrysts are relatively heavy with δ7Li values of +12 to +22‰. These values are considered to be less influenced by Li-loss after entrainment by the host basalts than rim compositions. These values in combination with O (δ18O = +3.5 to +5.3‰) and Hf (εHf(0.98 Ma) = +6.5 to +7.9) isotopic compositions as well as inclusions and internal structures suggest that zircon megacrysts were originally derived from partial melting of a metasomatized lithospheric mantle source altered by fluid released from dissolved carbonates during subduction. Lithium isotopes in Ratanakiri zircon megacrysts provide novel insights into using Li isotopes as a tool to constrain the durations of magma transfer from the mantle to the surface, although Li diffusion mechanisms in zircon need to be further scrutinized.

The Lunar Mare Ring‐Moat Dome Structure (RMDS) Age Conundrum: Contemporaneous With Imbrian‐Aged Host Lava Flows or Emplaced in the Copernican?

AbstractRing‐moat dome structures (RMDSs) are small circular mounds of diameter typically about 200 m and ∼3–4 m in height, surrounded by narrow, shallow moats. They occur in clusters, are widespread in ancient Imbrian‐aged mare basalt host units and show mineralogies comparable to those of their host units. Based on these close associations and similarities, a model has been proposed for the formation of RMDS as the result of late‐stage flow inflation, with second boiling releasing quantities of magmatic volatiles that migrate to the top of the flow as magmatic foams and extrude through cracks in the cooled upper part of the flow to produce the small RMDS domes and surrounding moats. In contrast to this model advocating a contemporaneous emplacement of RMDSs and their host lava flows, a range of observations suggests that the RMDS formed significantly after the emplacement and cooling of their host lava flows, perhaps as recently as in the Copernican Period (∼1.1 Ga to the present). These observations include: (a) stratigraphic embayment of domes into post‐lava flow emplacement impact craters; (b) young crater degradation age estimates for the underlying embayed craters; (c) regolith development models that predict thicknesses in excess of the observed topography of domes and moats; (d) landform diffusional degradation models that predict very young ages for mounds and moats; (e) suggestions of fewer superposed craters on the mounds than on the adjacent host lava flows, and (f) observations of superposed craters that suggest that the mound substrate does not have the properties predicted by the magmatic foam model. Together, these observations are consistent with the RMDS formation occurring during the period after the extrusion and solidification of the host lava flows, up to and including the geologically recent Late Copernican, that is, the last few hundreds of millions of years of lunar history. We present and discuss each of these contradictory data and interpretations and summarize the requirements for magma ascent and eruption models that might account for young RMDS ages. We conclude with a discussion of the tests and future research and exploration that might help resolve the RMDS age and mode of emplacement conundrum.

Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Deciphering a mantle degassing transect related with India-Asia continental convergence from the perspective of volatile origin and outgassing

Mantle degassing transect across different tectonic units within a plate convergent setting has been well documented for oceanic convergent margins by systematic changes in geochemistry (e.g., 3He/4He, δ13C, and CO2/3He) of hydrothermal gases. However, little is known about spatial variations in volatile geochemistry across a continental convergent margin. In this study, we identify a mantle degassing transect in the southeastern Tibetan Plateau using He-CO2 systematics of hydrothermal gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. δ13C-CO2 (−11.8‰ to −3.1‰) and CO2/3He (1.7 × 108 to 7.1 × 1011) values of hydrothermal gases show large variations that are consistent with modification by secondary physico-chemical processes, such as multi-component mixing, hydrothermal degassing, and calcite precipitation. Three levels of He degassing can be recognized based on 3He/4He dataset (0.01–5.87 RA) of the hydrothermal gas samples and their distances to volcanic centers. A magmatic level He degassing (35–74% mantle He) is found near active and/or Quaternary volcanoes fed by mantle-derived magmas. With increasing distance to volcanic centers, the outgassed magmatic volatiles are gradually diluted by crustal components (e.g., radiogenic 4He), defining a transitional level He degassing (13–33% mantle He). The 3He/4He values (8.16–8.48 RA) of olivine phenocrysts indicate a MORB (mid-ocean ridge basalts)-type mantle source for the magmatic and transitional levels of He degassing that are localized in Quaternary volcanic fields. In contrast, a background level He degassing (<12% mantle He) dominates the entire study area, and can be attributed to (i) degassing of sub-continental lithospheric mantle, and/or (ii) higher degrees of crustal contamination than those of the transitional level He degassing near volcanic centers. Combined with olivine 3He/4He data reported in this study and whole-rock 87Sr/86Sr data of host basalts from literature, source components of the mantle-derived magmas are suggested to include the MORB-type convective mantle, subducted Indian slab materials, and less degassed mantle materials, which can account for the possible decoupling between He and Sr isotope systematics. These findings delineate the origin and outgassing of mantle volatiles controlled by the India-Asia continental convergence, and would contribute to a better understanding of the deeply-sourced volatile emissions in these tectonic settings.

Evidence for highly refractory, heat producing element-depleted lower continental crust: Some implications for the formation and evolution of the continents

The composition of the lower continental crust is estimated via the analysis of granulite xenoliths, granulite terrains and geophysical properties. All three proxies generally agree on the lower crust's refractory nature, dominated by mafic granulites. Estimates weighted using seismic velocity reference models yield lower Th and U concentrations and higher K/U ratios than granulite xenolith averages, while terrain granulites are often much less refractory than xenoliths.Here we present new data for lower crustal xenoliths from central Queensland, an understudied part of the xenolith-bearing eastern Australian basalt provinces. The granulite chemistry was estimated using the reconstitution approach in which the modal mineralogy and in situ chemical analyses are combined. High-resolution energy-dispersive X-ray spectroscopy mapping revealed that K is enriched in anastomosing grain boundary networks and fractures in granulite and co-occurring mantle peridotite xenoliths. Laser-ablation inductively-coupled-plasma mass-spectrometry 2-D traverses show that the same networks are also significantly enriched in many highly incompatible elements. There are sharp concentration contrasts with neighbouring phases for elements with very different diffusivities (Li and U), suggesting that the networks formed during entrainment, decompression and heating within the host basalt. We propose that undetected inclusion of such late enrichment skews xenolith chemistry estimates to non-representative, overly fertile compositions. In the case of the studied xenoliths, the carrier basalts are not very strongly enriched in highly incompatible elements, and even when the K-rich networks are included in the reconstitution, the resulting granulite chemistry is very refractory with 0.52 wt% K2O, 0.07 ppm Th and 0.03 ppm U.The locally dominant lithology of lower crustal xenoliths is simple two-pyroxene, plagioclase, ilmenite granulite with few accessory phases. The granulite mineralogy and chemistry were compared with results from thermodynamic models of prograde anatexis of different metabasites, variably hydrated. The comparison shows that the granulites could be restitic calc-alkaline basalts or diorites that experienced episodic melt extraction accumulating up to 50–60% total melt loss at very high temperatures (950–1050 °C), implying that the temperatures recorded by two-pyroxene thermometry (750–830 °C) do not capture the thermal maximum. The corresponding upper crustal section of the northern New England Orogen exposes a range of Devonian to Cretaceous granitoids, some of which have complementary features to the granulites, including the low modal abundance of plagioclase; low relative abundance of Ti, very high Rb/Ba ratios, and high Th/U ratios. Together, the data suggest extensive and protracted melting of the original lower crust upon lithospheric thinning and concomitant magmatic underplating. The required high temperatures favour picritic over basaltic underplates. In such a setting, the gravity-driven delamination of more mafic garnet-rich restites and olivine-rich mafic-ultramafic underplating material is physically plausible. This delamination could help explain the long-established mass balance issue for the formation of continental crust in general.

Characterization of the metasomatizing agent in the upper mantle beneath the northern Pannonian Basin based on Raman imaging, FIB-SEM, and LA-ICP-MS analyses of silicate melt inclusions in spinel peridotite

AbstractSilicate melt inclusions (SMI) containing several daughter minerals, residual glass, and a CO2 bubble were analyzed to constrain the composition and evolution of the metasomatic melt present in the upper mantle beneath the Nógrád-Gömör Volcanic Field (NGVF), northern Hungary to southern Slovakia. The SMI were analyzed with a combination of Raman spectroscopy, FIB-SEM, and LA-ICP-MS to identify phases and obtain their volume proportions and major- and trace-element geochemistry. Slicing through the entire volume of the inclusions and collecting geochemical information at each slice with FIB-SEM allowed us to model the 3D appearance of the phases within the SMI and to use this information to calculate bulk major-element compositions.The partially crystallized SMI are hosted in clinopyroxene in a lherzolite xenolith that shows evidence of a metasomatic event that altered the lherzolites to produce wehrlites. Based on bulk compositions, the SMI trapped the metasomatic melt linked to wehrlite formation in the NGVF. The melt is enriched in Fe and has an OIB-like trace-element pattern, which suggests an intraplate mafic melt similar to the host basalt, but with slightly different chemistry. Pre-entrapment evolution and reaction with the lherzolite wall rock produced an intermediate melt composition. Petrogenetic modeling indicates that the melt was generated as a result of a very small degree of partial melting of a garnet lherzolite source. Following entrapment, a volatile bubble exsolved from the residual melt during ascent to shallow depths as suggested by consistent densities of CO2 in vapor bubbles. Small crystals, including sulfates and mica, that formed at the boundary of the bubble and the glass indicate that the exsolved fluid originally contained S and H2O, in addition to CO2.

Effect of water on the rheology of the lithospheric mantle in young extensional basin systems as shown by xenoliths from the Carpathian-Pannonian region

Incorporation of hydrogen as structural hydroxyl (commonly referred to as water) in nominally anhydrous mantle minerals is known for the ‘hydrolytic weakening’ effect, which decreases the strength and electrical resistivity of the rock. Recent models have provided means of calculating rheological properties from geochemical data of upper mantle xenoliths. In the Carpathian-Pannonian region, upper mantle xenoliths can be found on the surface at five locations, both at marginal and central regions of the basin system. In this study we present a comprehensive overview of water contents in these xenoliths, including previously lacking data from two outcrops (Füzes-tó and Tihany) from the Bakony-Balaton Highland Volcanic Field for the first time. The Tihany xenoliths have significantly higher water contents (< 1–5, 116–353 and 327–1394 ppm in olivine, orthopyroxene and clinopyroxene, respectively) than the Füzes-tó xenoliths (0–2.7, 6.2–114 and 3.1–213 ppm in olivine, orthopyroxene and clinopyroxene, respectively). This can be explained with the older eruption age of their host basalt and greater depth of origin, as they likely represent an asthenospheric layer that became part of the lower lithosphere during thermal relaxation. In contrast, the Füzes-tó xenoliths represent a lot dryer mantle portion and are assumed to have been more affected by decompression-induced water loss resulting from decreased water activity during the extension. In general, the marginal xenolith locations of the Carpathian-Pannonian region, associated to prior supra-subduction environment, contain more water than xenoliths from central locations (with the exception of Tihany locality) which were significantly affected by extension-related lithospheric thinning. To reveal the differences in rheology inferred from the different water contents, we calculated effective viscosities and electrical resistivities for the xenoliths of the Bakony-Balaton Highland and other locationss in the Carpathian-Pannonian region using previously published data for input. Based on the results, the central locations have higher effective viscosities (1.4∙10 20 –2.2∙10 21 Pa s) and electrical resistivities (48–913 Ωm) compared to the marginal locations (9.3∙10 19 –6.8∙10 20 Pa s and 36–182 Ωm, respectively), suggesting that the lithospheric mantle is more rigid in the former than in the latter areas. This may be a common feature for extensional basins, as the extension leads to the ‘drying’ of the upper mantle, whereas subduction zones keep hydrating their overlying mantle wedge. However, for more accurate estimations, other factors such as regional differences in strain rate or the potential presence of melts or fluids in the mantle need to be considered as well. • Water lowers effective viscosity and electrical resistivity in mantle xenoliths • Water contents of xenoliths from different tectonic environments were compared • Central regions of the back-arc basin are drier compared to marginal areas • Rheological differences are present in distinct parts of extension-subduction systems • Decompression related water loss results in strengthening of the lithosphere

Immiscible-melt inclusions in corundum megacrysts: Microanalyses and geological implications

Abstract Controversies on the origin of zircon, corundum, titanomagnetite, and quartz megacrysts in alkali basalts mostly reflect the lack of direct evidence of a “melt reservoir” required for their formation. Various mineral megacrysts are carried up by Cenozoic (mostly younger than 25 Ma) alkali basalts that extend more than 4000 km along eastern China. Here we report unusual inclusions in corundum megacrysts from Changle, and we attribute their origin to the existence of a FeO*-SiO2-Al2O3-ZrO2-rich melt. The inclusions, analyzed using electron microprobe and Raman microscopy, may be divided into two types. Type I inclusions are dominated by glassy materials, may exhibit a dark part in backscattered eletron (BSE) images composed of quartz, corundum, and an amorphous substance (AS-1), and a bright part in BSE images composed of baddeleyite and a second distinct amorphous substance (AS-2). Compared with AS-1, AS-2 has higher concentrations of ZrO2 and FeO* but lower concentrations of Al2O3 and SiO2. We argue that the formation temperature of Type I inclusions is ~1200 °C, and the generation of their bright and dark parts in BSE images may be attributed to the coexistence of immiscible melts. Type II inclusions are composed of zircon, quartz, and an amorphous substance (AS-3). Both types of inclusions might be derived from a similar parent melt, which is FeO*-SiO2-Al2O3-ZrO2-rich. New secondary ion mass spectroscopy (SIMS) in situ U-Pb ages of 18 Ma and 13–14 Ma for zircon inclusions suggest that the corundum megacrysts, occurring in basaltic host rocks distributed along the middle segment of the north and south-trending Tanlu fault zone, formed from precursor residual magmas related to underplating basalts stalled at the crust-mantle boundary, and were brought to the surface by entrainment in later basalts. This study provides new insights into the genesis of the corundum-related megacryst suite.

Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift

AbstractTwo Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

Metasomatized peridotite xenoliths from the cretaceous rift-related Natash volcanics and their bearing on the nature of the lithospheric mantle beneath the southern part of the Eastern Desert of Egypt

Highly carbonated mantle xenoliths have been found in rift-related alkaline basalts at the Wadi Natash area in the southern part of the Eastern Desert of Egypt. Although all olivine and most orthopyroxene was replaced by carbonate and/or quartz, textural and mineral chemical features show that they are plagioclase-free spinel peridotites (lherzolite to harzburgite). Cr and Mg numbers (Cr#, Mg#) of Cr-spinel vary from 0.06 to 0.45 and 0.73 to 0.81, respectively. The correlation between Cr# and Mg# of the Cr-spinel in the studied xenoliths is weakly negative and its TiO2 content is slightly higher than in abyssal peridotite that was not affected by melt injection. The chemistry of ortho- and clinopyroxene suggests enstatite and chromian diopside compositions, respectively, with distinct signatures of a sub-continental mantle source. In particular, the Na2O contents (>1.0 wt%) and AlVI/AlIV ratios (1.2–2.6) of chromian diopside suggest such an origin.Two-pyroxene geothermometry indicates a temperature of about 900 °C, which is slightly lower than that of ordinary spinel peridotite xenoliths from other rift zones. It is evident that the studied peridotite xenoliths had experienced mantle processes (e.g. decompression melting, magma upwelling and metasomatism) at higher pressure than abyssal peridotites. The trace-element chemistry of clinopyroxene, e.g. high LREE/HREE ratios {(Ce/Yb)n = 7}, high LREE contents (>3.6 ppm and up to 30.0 ppm Ce) and high Sr between >85.6 ppm and 466 ppm, indicates metasomatic alteration of the peridotite. Clinopyroxene in one sample has very low Ti/Eu and high LREE/HREE ratios. Clinopyroxene with (Ce/Yb)n higher than 3–4 and Ti/Eu ratio lower than 1500 may have experienced carbonatite or carbonate-rich melt metasomatism prior to their incorporation into the host basalt. The basalt itself is almost devoid of any carbonatization and hence the studied mantle peridotites were carbonatized before the generation of the basaltic magma but following an earlier event of K-metasomatism as indicated by the presence of phlogopite. The studied peridotites from the Wadi Natash area were altered by a carbonate-rich melt during a rifting stage. The results of the present paper indicate that the Natash basalts with their peridotite xenoliths extruded along transversal fractures of the NW-trending Nuqra-Kom Ombo-Kharit continental rift on its western shoulder in the south Eastern Desert of Egypt.

Secular evolution of the lithospheric mantle beneath the northern margin of the North China Craton: Insights from zoned olivine xenocrysts in Early Cretaceous basalts

Abstract Abundant zoned olivine xenocrysts from Early Cretaceous basalts of the Yixian Formation in western Liaoning Province, China, contain critical information about the nature and evolution of the lithospheric mantle of the northern North China Craton. These olivine xenocrysts are large (600–1600 µm), usually rounded and embayed, with well-developed cracks. Their cores have high and uniform forsterite (Fo) contents (88–91), similar to the peridotitic olivine entrained by regional Cenozoic basalts. Their rims have much lower Fo contents (74–82), comparable to phenocrysts (72–81) in the host basalts. These characteristics reveal that the zoned olivine has been disaggregated from mantle xenoliths and thus can be used to trace the underlying lithospheric mantle at the time of basaltic magmatism. The olivine cores have high oxygen isotope compositions (δ18OSMOW = 5.9–7.0‰) relative to the normal mantle value, suggesting that the Early Cretaceous lithospheric mantle was enriched and metasomatized mainly by melts/fluids released from subducted oceanic crust that had experienced low-temperature hydrothermal alteration. Preservation of zoned olivine xenocrysts in the Early Cretaceous basalts indicates that olivine-melt/fluid reaction could have been prevalent in the lithospheric mantle as an important mechanism for the transformation from old refractory (high-Mg) peridotitic mantle to young, fertile (low-Mg), and enriched lithospheric mantle during the early Mesozoic.

Subduction-related melt refertilisation and alkaline metasomatism in the Eastern Transylvanian Basin lithospheric mantle: Evidence from mineral chemistry and noble gases in fluid inclusions

Calc-alkaline and alkaline magmatic activity is generally separated in space and/or in time. The Eastern Transylvanian Basin in Romania is one of the few places where, during Pleistocene, alkaline eruptions occurred contemporaneously with the calc-alkaline activity. Mantle xenoliths entrained in Perşani Mts. alkaline volcanic products have been studied in order to investigate the interaction of metasomatic agents of different magmatic affinities with the mantle wedge. Based on mineral major and trace element and noble gases in fluid inclusions, two main events have been recognized. The first was a pervasive, complete re-fertilization of a previously depleted mantle by a calc-alkaline subduction-related melt, causing the formation of very fertile, amphibole-bearing lithotypes. This is shown by the a) increased amounts of modal clinopyroxene up to 21.9 % with Al2O3 contents up to 8.16 wt%, higher than what is expected for clinopyroxene in Primordial Mantle; b) 4He/40Ar* ratios up to 1.2, within the reported range for mantle production; c) 3He/4He in olivine, opx and cpx of 5.8 ± 0.2 Ra, among the most radiogenic values of European mantle, below the typical MORB mantle value (8 ± 1 Ra), reflecting recycling of crustal material in the local lithosphere. The second event is related to later interaction with an alkaline metasomatic agent similar to the host basalts that caused slight LREE enrichment in pyroxenes and disseminated amphiboles and precipitation of vein amphiboles with a composition similar to amphiboles megacrysts also found in the Perşani Mts. volcanic deposits. This is highlighted by the 4He/40Ar* and 3He/4He values found in some opx and cpx, up to 2.5 and 6.6 Ra, respectively, more typical of magmatic fluids.

Mantle and Recycled Oceanic Crustal Components in Mantle Xenoliths From Northeastern China and their Mantle Sources

AbstractThe subduction of the Paleo‐Pacific plate is widely credited for the destruction of the eastern North China Craton. However, how the Pacific plate subduction has affected the off‐craton lithospheric mantle in northeastern China is less clear, as few studies have focused on the lithospheric mantle‐derived xenoliths in that region. We report petrography, mineral and rock chemistry, and in situ isotopic compositions of Sr in clinopyroxene and O in olivine for 20 peridotite and 11 pyroxenite xenoliths hosted by the Cenozoic volcanic rocks at Jiaohe and Shuangliao localities. The peridotites include lherzolites, harzburgites, and a wehrlite, many are metasomatized. Group 1 garnet pyroxenites have high Al2O3, Ni, and Mg#; positive Eu and Sr anomalies; and olivine δ18O values of 4.8–5.1‰, which we attribute to recycling of oceanic crustal components to the asthenosphere and their reactions with host peridotites. Groups 2 and 3 pyroxenites are garnet‐free and contain peridotite‐like pyroxenes; we argue that they were formed by reactions of lithospheric peridotites with evolved melts derived from recycled oceanic crust. In addition, one pyroxenite contains orthopyroxene surrounded by fine‐grained olivine and clinopyroxene, while another pyroxenite and the wehrlite contain zoned clinopyroxene with gradual increases in Na2O, (La/Yb)N, Sr, and 87Sr/86Sr ratios and decreases in Ti/Eu from core to rim. These characteristics record infiltration of a carbonatite melt shortly before the transport of the xenoliths. The inferred carbonatite melt has 87Sr/86Sr ≥ 0.70415 and mantle‐like olivine δ18O and originated from carbonated asthenosphere. Our results, together with previous studies on the host basalts, suggest that subducted crustal materials were initially transported to the asthenosphere beneath northeastern China to produce carbonate‐bearing domains. Upwelling of the carbonated asthenosphere produced carbonatite melts that migrated upward and metasomatized the overlying lithospheric mantle.