Incompatible Trace Element Ratios Research Articles

U–Pb age and Hf isotopes of zircon from basaltic andesite and geochemical fingerprinting of the associated picrites in the Emeishan large igneous province, SW China

The ages of many picrites associated with Permian flood basalts in the Emeishan large igneous province, SW China have been debated for a long time. Some researchers believe that these picrites are co-magmatic with the flood basalts whereas others believe that they are Triassic picritic porphyrites or Cenozoic olivine-rich dikes. We report a U–Pb age of 256.2 ± 1.4 Ma for the youngest zircon grains from the Binchuan volcanic sequence which contains the disputed picrites. This age is within the range of values reported for the intrusive and extrusive rocks of the Emeishan large igneous province. The eHf values (−0.3 to 6.6) of the youngest zircon crystals from the Binchuan volcanic sequence are similar to those (−5 to 10) of co-magmatic zircon crystals from the intrusive rocks of the Emeishan large igneous province. Our new data confirm that the Binchuan volcanic sequence is an integral part of the Emeishan large igneous province. The disputed and undisputed picrites in Sichuan and Yunnan have similar olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. This, together with their spatially close association with the Emeishan flood basalts, indicates that the disputed picrites also belong to the Emeishan large igneous province. The confusing Cenozoic olivine-rich dikes in the region can be distinguished from the picrites of the Emeishan large igneous province by their different olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. The results from this study vindicate the use of the previously-disputed picrites as evidences for Permian mantle plume activity in SW China.

A common parentage for Deccan Continental Flood Basalt and Central Indian Ocean Ridge Basalt? A geochemical and isotopic approach

A comparison of geochemical and Sr–Nd–Pb isotopic compositions for Deccan Continental Flood Basalts (CFBs) and Central Indian Ridge (CIR) Basalts is presented: these data permit assessment of possible parental linkages between the two regions, and comparison of their respective magmatic evolutionary trends in relation to rift-related tectonic events during Gondwana break-up. The present study reveals that Mid-Ocean Ridge Basalt (MORB) from the northern CIR and basalts of Deccan CFB are geochemically dissimilar because of: (1) the Deccan CFB basalts typically show a greater iron-enrichment as compared to the northern CIR MORB, (2) a multi-element spiderdiagram reveals that the Deccan CFBs reveal a more fractionated slope (Ba/YbN>1), as compared to relatively flat northern CIR MORB (Ba/YbN<1), (3) there is greater REE fractionation for Deccan CFB than for the northern CIR MORB (i.e., La/YbN∼2.3 and 1 respectively) and (4) substantial variation of compatible–incompatible trace elements and their ratios among the two basalt groups suggests that partial melting is a dominant process for northern CIR MORB, while fractional crystallization was a more important control to the geochemical variation for Deccan CFB. Further, incompatible trace element ratios (Nb/U and Nb/Pb) and radiogenic isotopic data (Sr–Pb–Nd) indicate that the northern CIR MORBs are similar to depleted mantle [and/or normal (N)-MORB], and often lie on a mixing line between depleted mantle and upper continental crust. By contrast, Deccan CFB compositions lie between the lower continental crust and Ocean island basalt. Accordingly, we conclude that the basaltic suites of the northern CIR MORB and Deccan CFB do not share common parentage, and are therefore genetically unrelated to each other. Instead, we infer that the northern CIR MORB were derived from a depleted mantle source contaminated by upper continental crust, probably during the break up of Gondwanaland; the Deccan CFB are more similar to Ocean island basalt (Reunion-like) composition, and perhaps contaminated by lower continental crust during their evolution.

Temporal geochemical variations in lavas from Kīlauea's Pu‘u ‘Ō‘ō eruption (1983–2010): Cyclic variations from melting of source heterogeneities

Geochemical time series analysis of lavas from Kīlauea's ongoing Pu‘u ‘Ō‘ō eruption chronicle mantle and crustal processes during a single, prolonged (1983 to present) magmatic event, which has shown nearly two‐fold variation in lava effusion rates. Here we present an update of our ongoing monitoring of the geochemical variations of Pu‘u ‘Ō‘ō lavas for the entire eruption through 2010. Oxygen isotope measurements on Pu‘u ‘Ō‘ō lavas show a remarkable range (δ18O values of 4.6–5.6‰), which are interpreted to reflect moderate levels of oxygen isotope exchange with or crustal contamination by hydrothermally altered Kīlauea lavas, probably in the shallow reservoir under the Pu‘u ‘Ō‘ō vent. This process has not measurably affected ratios of radiogenic isotope or incompatible trace elements, which are thought to vary due to mantle‐derived changes in the composition of the parental magma delivered to the volcano. High‐precision Pb and Sr isotopic measurements were performed on lavas erupted at ∼6 month intervals since 1983 to provide insights about melting dynamics and the compositional structure of the Hawaiian plume. The new results show systematic variations of Pb and Sr isotope ratios that continued the long‐term compositional trend for Kīlauea until ∼1990. Afterward, Pb isotope ratios show two cycles with ∼10 year periods, whereas the Sr isotope ratios continued to increase until ∼2003 and then shifted toward slightly less radiogenic values. The short‐term periodicity of Pb isotope ratios may reflect melt extraction from mantle with a fine‐scale pattern of repeating source heterogeneities or strands, which are about 1–3 km in diameter. Over the last 30 years, Pu‘u ‘Ō‘ō lavas show 15% and 25% of the known isotopic variation for Kīlauea and Mauna Kea, respectively. This observation illustrates that the dominant time scale of mantle‐derived compositional variation for Hawaiian lavas is years to decades.

The Deccan tholeiite lavas and dykes of Ghatkopar–Powai area, Mumbai, Panvel flexure zone: Geochemistry, stratigraphic status, and tectonic significance

Mumbai City, situated on the western Indian coast, is well known for exposures of late-stage Deccan pillow basalts and spilites, pyroclastic rocks, rhyolite lavas, and trachyte intrusions. These rock units, and a little-studied sequence of tholeiitic flows and dykes in the eastern part of Mumbai City, constitute the west-dipping limb of a regional tectonic structure called the Panvel flexure. Here we present field, petrographic, major and trace element and Sr–Nd isotopic data on these tholeiitic flows and dykes, best exposed in the Ghatkopar–Powai area. The flows closely resemble the Mahabaleshwar Formation of the thick Western Ghats sequence to the east, in Sr–Nd isotopic ratios and multielement patterns, but have other geochemical characteristics (e.g., incompatible trace element ratios) unlike the Mahabaleshwar or any other Formation. The flows may have originated from a nearby eruptive center, possibly offshore of Mumbai. Two dykes resemble the Ambenali Formation of the Western Ghats in all geochemical characteristics, though they may not represent feeders of the Ambenali Formation lavas. Most dykes are distinct from any of the Western Ghats stratigraphic units. Some show partial (e.g., Sr–Nd isotopic) similarities to the Mahabaleshwar Formation, and these include several dykes with unusual, concave-downward REE patterns suggesting residual amphibole and thus a lithospheric source. The flows and dykes are inferred to have undergone little or no contamination, by lower continental crust. Most dykes are almost vertical, suggesting emplacement after the formation of the Panvel flexure, and indicate considerable east–west lithospheric extension during this late but magmatically vigorous stage of Deccan volcanism.

Sr–Nd–Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) 87Sr/86Sr ratios of 0.70538–0.70642, 143Nd/144Nd of 0.51233–0.51218, 206Pb/204Pb of 17.74–18.25, 207Pb/204Pb of 15.51–15.57, and 208Pb/204Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γOs values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.

Geochemical variability of MORBs along slow to intermediate spreading Carlsberg-Central Indian Ridge, Indian Ocean

We present new major and ICP-MS trace element data from the Carlsberg Ridge MORBs from two different locations (i.e., 3°35′N/64°05′E and 3°41′N/64°09′E). Using this data we reassess the intra-ridge geochemical variations of the Carlsberg Ridge- Central Indian Ridge MORBs. Trace element contents of the Carlsberg Ridge MORBs are similar to the Rodriguez Triple Junction MORBs [e.g., LIL and REE spidergrams and (La/Sm)N ratio etc.]; both closely resemble the average N-MORB. However, the MORBs from the northern- and southern Central Indian Ridge, significantly vary in composition from the average N- and E-MORBs. Most Carlsberg Ridge- Central Indian Ridge MORBs show significantly less fractionation in FeOt, in the MgO–CaO–FeOt diagram, compared to those of the Mid Atlantic- and East Pacific Rise MORBs. Furthermore, the depleted LREE and nearly flat HREE patterns of the Carlsberg Ridge–Central Indian Ridge MORBs (along with their low compatible trace element (Ni, Cr, Sr) variability compared with the increasing incompatible trace element (Y, Zr) contents in the log–log plots, as well as their increasing incompatible trace element ratios) favor partial melting dominated processes for their petrogenetic evolution. Our (Sr, Nd, Pb) isotope data review shows that the Carlsberg Ridge-Central Indian Ridge MORBs are derived from a depleted mantle source that was variously contaminated by continental crust, perhaps during the third stage of Gondwana break up between 155 and 135Ma. Such contamination could also have occurred later during the strike slip movement along a mega fracture (a member of the Davie Transform Faults in the Somali Basin that broke Gondwanaland into the East and West Gondwanas). The 208Pb/204Pb versus 206Pb/204Pb plot of above mentioned MORBs suggests that the depleted mantle source of the Rodriguez Triple Junction MORBs was contaminated by ∼21% lower continental crust, whereas the mantle source of the Central Indian Ridge MORBs was contaminated by upper continental crust, which are ∼19% for the Carlsberg Ridge and Northern Central Indian Ridge MORBs and ∼32% for the Southern Central Indian Ridge MORBs. The contaminated mantle sources were compositionally similar to the Al-depleted Komatiite basalt and significantly enriched in Rb, Ba, La and Ce over the depleted mantle.

Constraints on melting processes and plume-ridge interaction from comprehensive study of the FAMOUS and North Famous segments, Mid-Atlantic Ridge

Detailed major element, trace element and isotopic study of the FAMOUS and North Famous segments within the geochemical gradient south of the Azores platform provides new constraints on controls on chemical variations at the segment scale and the origin of plume geochemical gradients. A comprehensive investigation of 110 samples along the entire length of the FAMOUS segment, coupled with a recent extensive melt inclusion study by Laubier et al. (2012), shows large trace element diversity within a single segment and substantial isotopic variability that largely correlates with trace element variations. Substantial variations are also present along the short (18km) North Famous segment despite the presence of an axial volcanic ridge. These results confirm multiple supply of magmas along the length of these segments, the lack of a centrally supplied magma chamber, and the ability of melting processes to deliver highly diverse melts over short distances and times. With the exception of one group of high Al2O3, low SiO2 magmas (HiAl–LoSi) largely recovered in the original small FAMOUS area, the data can be simply explained by a two-component mixing model coupled with melting variations. The HiAl–LoSi magmas reflect assimilation and mixing in the crust, an interpretation supported by the diverse melt inclusions in these lavas.Since the mantle heterogeneity reflects two-component mixing, the end members can be constrained. Surprisingly, source mixing between the Azores plume and depleted mantle cannot produce the observations. This is evident regionally from the fact that nearly all basalts have highly incompatible trace element ratios (e.g., Th/La, Nb/La) as high or higher than the most plume-influenced MORB near the Azores hotspot, despite being over 300km farther south and much less enriched isotopically. To account for the elevated highly incompatible trace element ratios, a metasomatic component formed by adding deep, low-degree melts of Azores plume material to a depleted mantle is required. The regional gradient south of the Azores then requires different processes along its length. Close to the Azores, plume material mixes with depleted mantle. The pure plume influence is spatially restricted, and enrichment farther to the south is caused by shallow mantle metasomatized by low-degree melts from deep plume flow. North Famous lavas are spatially closer to the Azores and yet are more depleted in trace elements and isotopes than FAMOUS lavas, suggesting delivery of the enriched component to individual segments is influenced by additional factors such as segment size and offset. The extent to which these processes operate in other regions of plume–ridge interaction remains to be investigated.

Petrogenesis of mafic–silicic lavas at Mt. Erciyes, central Anatolia, Turkey

New compositional data are presented for a suite of basaltic andesite to rhyolite flows from the flanks of the Pliocene–Quaternary Mt. Erciyes stratovolcano in Central Anatolia, Turkey. The samples are all part of the most recent (<0.9Ma) New Erciyes eruptive phase. Temperatures estimated from FeTi-oxides vary from ~1000°C in the andesites to ~820°C in the rhyolites, while the dacites preserve a very broad temperature range (~620°C–1020°C), consistent with textural and mineral chemistry evidence for their hybrid nature. Oxygen fugacity is about NNO +1 for most samples. The whole rock compositional data, in conjunction with published data, reveal two distinct trends, one formed by the mafic samples (basalts to basaltic andesites) and one by the silicic samples (andesites–dacites–rhyolites). All Erciyes samples plot above the MORB-OIB array on a Nb/Yb–Th/Yb plot, consistent with involvement of subduction-modified mantle, and their low MREE/HREE ratios imply melting of garnet-free mantle. Ratios of incompatible trace elements demonstrate that the basaltic andesite flows are not related to the basalt flows by closed-system fractional crystallization, but instead are derived from a more enriched mantle source, given their higher La/Sm and lower Zr/Nb. The silicic lavas all have higher Th/Nb than the mafic samples, indicating more extensive crustal assimilation. The silicic lavas show linear compositional trends for major elements and most trace elements for SiO2 contents between 56wt.% and 72wt.% that are attributed to mixing processes, consistent with textural observations and mineral data particularly in the intermediate dacite flows (two distinct plagioclase phenocryst populations — clear An33–53 cores vs. sieve-textured cores of An59–80; wide compositional range for orthopyroxene phenocrysts — Mg# from 63 to 90). We infer that basaltic melts of a heterogeneous, shallow mantle source ponded in the mid- to lower-crust and underwent variable differentiation accompanied by crustal assimilation to form more evolved melt compositions with SiO2>~55wt.%. These melts ascended to shallower crustal levels (4–10km based on amphibole geobarometry) where mixing of diverse melts and entrained crystals gives rise to the linear mixing trends shown by the erupted andesite–dacite–rhyolite lavas.

Chapter 15 Compositional variations of magmas in the Aeolian arc: implications for petrogenesis and geodynamics

Abstract The volcanic rocks of the Aeolian arc exhibit important within-island and along-arc compositional variations that testify to both geochemical heterogeneous mantle sources and different roles and intensities of shallow-level magmatic evolution processes. Calc-alkaline magmas are present on all islands, but dominate in the western arc and at Lipari and Panarea. Shoshonitic rocks are present on the central-eastern islands and are particularly abundant at Vulcano and Stromboli. Mafic and intermediate rocks comprise the bulk of older volcanic sequences for most islands. Rhyolites are restricted to younger activity of the central arc, and become particularly abundant at Lipari and Vulcano. Regional variations of incompatible trace element ratios and Sr-, Nd-, and Pb-isotope signatures in mafic-intermediate rocks document the variable composition of mantle sources, which were contaminated by different types of metasomatic fluids released from an oceanic slab in the western-central sectors and from oceanic slab plus sediments in the east. This metasomatism was superimposed over a heterogeneous mantle wedge, which had a mid-ocean-ridge basalt (MORB-) to ocean-island basalt (OIB)- type character passing from the centre to the margins of the arc. The OIB-type component in the external arc is attributed to asthenospheric mantle inflow from the Africa foreland, around the borders of a narrow slab during rollback.

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia, South America

AbstractGeochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.

Geochemistry and Sm–Nd geochronology of the metasomatised mafic rocks in the Khetri complex, Rajasthan, NW India: Evidence of an Early Cryogenian metasomatic event in the northern Aravalli orogen

The mafic magmatic rocks associated with 1720–1700Ma albitised A-type granites in the northern segment of the Aravalli orogen, NW India show evidence of metasomatism. It is, however, not clear whether the metasomatism of mafic rocks is related to the cooling of these associated granites or whether it took place much later after the emplacement of the granites on a regional scale. For this, we have investigated the mafic magmatic rocks, which occur in close association with these granites. In the Biharipur intrusive, the mafic rocks are intensely commingled with the A-type granites, whereas in the vicinity of the Dosi intrusive, the mafic rocks (clinopyroxenite) do not show any evidence of granite mingling. The commingled and metasomatised Biharipur mafics occur in contact with the albitised granites instead of original granite, indicating that the mafics were metasomatised along with the granites. This is supported by the similarity in REE and spider patterns of the intermixed mafic rocks and the albite granites. On the other hand, the Dosi mafic rocks, free from granite commingling, are scapolitised where the original diopside has been partly transformed to chlorine-rich marialites with a meionite component ranging from Me14.0 to Me16.0. The scapolite, occurring as anastomosing veins, within these rocks is also of similar composition, and the undeformed nature of these veins suggests that the scapolitisation postdates regional metamorphism in the region. Mineralogical, geochemical and Nd isotopic characteristics of the mafic rocks indicate that originally, these were clinopyroxenites, which have been altered to a monomineralic actinolite-bearing rock. The immobile incompatible trace element ratios indicate a continental tholeiite affinity for the mafics, which is in consonance with the A-type nature of the associated granites. During this metasomatic event, the mafic magmatic rocks experienced albitisation and scapolitisation, although the dominance of these processes varied on a local scale depending on the fluid composition.A whole-rock-mineral (clinopyroxene and scapolite) Sm–Nd isochron of the scapolitised clinopyroxenite at Dosi yields an age of 831±15Ma. Synthesis of this age data along with previously published geochronological data indicate an important Early Cryogenian (850–830Ma) metasomatic event in the northern Aravalli orogen, which is also synchronous with the Erinpura granite event in the southern Aravalli orogen.

Petrology, Geochemistry, and Geodynamic Implications of Basaltic Dyke Swarms from the Southern Continental Part of the Cameroon Volcanic Line, Central Africa

Basaltic dyke swarms in the southern continental part of the Cameroon Volcanic Line (Bangangte, Dschang, Manjo areas) are tholeiitic in composition with 46 to 50 wt.% SiO2 and have moderate Mg-numbers (53-59), medium TiO2 contents (1.48-2,05 wt.%), and flat to mildly enriched incompatible trace element patterns. Comparison with trace element patterns of representative Cenozoic basaltic rocks of the Cameroon Volcanic Line (Bana anorogenic complex, Mt. Bambouto, Adamawa Plateau basalts) indicates that these dykes are less enriched in light REE and show different incompatible trace element ratios (La/Yb: 5.7 to 8.6; Zr/Nb: 7.6 to 12.0; Ba/Th: 87.7 to 93.3). The trace element patterns of the dykes and their Sr- and Nd- isotope compositions, however, are similar to those of the pre-Cenozoic volcanic rocks of the Benue Trough in Nigeria. Our data therefore suggest that these dykes represent the magmatic history related to the break-up of Africa and South America and are unrelated to the Tertiary volcanism of the Cameroon Line.

Geochemical constraints on a mixed pyroxenite–peridotite source for East Pacific Rise basalts

The extent to which the lithological heterogeneity of the mantle source controls mid-oceanic ridge basalt (MORB) compositions remains largely debated. The recycling of oceanic crust into the mantle is widely considered to be an important cause of mantle source heterogeneity. We provide data for major and trace elements and Sr–Nd–Pb isotopes of thirteen mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR) to investigate the mantle source heterogeneity introduced by recycled oceanic crust (ROC). Relationships of Sr–Nd–Pb isotopes and incompatible trace element ratios, such as Sm/Yb and Hf/Zr, and ratios of Zn/Fe, Co/Fe and (Pb/Pb⁎)N indicate a geochemically and lithologically heterogeneous mantle source. Mixing between components from depleted mantle (DMM) and enriched ROC-derived pyroxenite is required to explain these relationships. The relatively high Sm/Yb and the good correlation of 87Sr/86Sr vs. Sr/Nd are in concert with variable contributions from a pyroxenite component derived from recycled plagioclase-fractionated E-MORB. The narrow range of MgO contents (7.49wt.% to 8.58wt.%) and the correlations of major element contents with 87Sr/86Sr and Zn/Fe ratios indicate the control of a mixed pyroxenite–peridotite source on the major element compositions of these lavas. To further investigate the major element contribution from the lithologically heterogeneous mantle source (peridotite/pyroxenite), major element compositions were corrected for fractional crystallization both to a given MgO wt.% and a given liquidus temperature. After correction for fractional crystallization, the FeOT and Na2O increase and CaO decreases with increasing melt proportion from the enriched end-member. Based on the observations of this study and the results from partial melting experiments on pyroxenite and peridotite, a melting model, in which magma forms by mixing a deep and enriched melt (pyroxenite melt) with a shallow and depleted DMM melt (peridotite melt), provides a likely mechanism that can produce the major element systematics.

The Petrology and Geochemistry of Lavas from the Western Azores Islands of Flores and Corvo

The islands of Flores and Corvo in the Azores archipelago are the only two of nine subaerial volcanic edifices lying west of the Mid-Atlantic Ridge (MAR). This makes them important for constraining the evolution of this young (5 40 Ma) oceanic plateau. The alkalic basalt suites from Flores and Corvo lie on a single liquid line of descent. Ankaramitic cumulates, with MgO contents up to � 18 wt %, result from clinopyroxene-dominated polybaric crystallization. The parental magmas (MgO � 11wt %) are inferred to be low-degree partial melts (F ¼ 3^5%) of enriched peridotite generated at depths of � 80^90 km. These primary magmas commenced crystallizing at the lithosphere^asthenosphere boundary and this continued in conduits over a pressure range of � 0·6^1·2 GPa. Only lavas with MgO53 wt % fractionated at shallow crustal levels. Nd and Sr isotope data reveal variations in the source of both magmatic systems, suggesting variable contributions from both enriched (E-) and depleted (D-) mid-ocean ridge basalt (MORB)-source mantle components. This is supported by the greater variability of incompatible trace-element ratios within the Flores lavas (e.g. Ba/Nd, La/Sm, Th/Nd), whereas those from Corvo exhibit a good correlation between key trace-element ratios [e.g. (La/Sm)N,Th/Nd] and Sr isotope ratios. Lavas from Flores display a greater variability in Sr and Nd isotope compositions and define a mixing array between an E-MORB source and a common Azores mantle source.The latter signature is restricted to lava suites from the north and east of Flores. We concur with the generally accepted notion that Flores and Corvo are derived from the same mantle plume as is responsible for the eastern Azores islands. However, there is evidence (different Nb/Zr, Ta/Hf and La/Sm, but homogeneous Sr and Nd isotopic composition) that these two islands are dominated by a source component that is not as evident in the eastern archipelago.

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

We investigate the relative influence of mantle upwelling velocity and source heterogeneity on the melting rates recorded by 230Th–238U, 231Pa–235U and 226Ra–230Th disequilibria in post-glacial tholeiites from Iceland’s main rift areas. The measured (230Th/238U) ratios range from 1.085 to 1.247, the (231Pa/235U) ratios from 1.333 to 1.925, and the (226Ra/230Th) ratios from 0.801 to 1.218. A general positive correlation between 230Th excesses and distance from the inferred plume centre is consistent with a model of decreasing mantle upwelling velocity with increasing distance from the plume axis. However, the model is not substantiated by the (231Pa/235U) data as the correlation with distance from the plume centre is weak. On the scale of individual eruption centres, the observed U-series are influenced by variations in melt transport time, source porosity, and local variations in mantle upwelling velocity. Broad correlations between (230Th/238U) and (231Pa/235U) and highly incompatible trace element ratios for samples from the Western Volcanic Zone provide, however, evidence for a significant underlying effect of source heterogeneity on the U-series data. Low 230Th and 231Pa excesses in enriched samples from the Western Volcanic Zone with high U/Th, Nb/U and Nb/La indicate that partial melts from an enriched source component, characterised by high melt productivity but low bulk DU/DTh, influence the U-series systematics of the erupted melts. These results re-affirm the presence of comparatively larger abundances of enriched material in the mantle source beneath the South Western Rift of Iceland, which has been suggested based on relationships between highly incompatible element and Pb isotope ratios in Icelandic basalts. Overall, our results highlight the importance of lithological heterogeneity on the melting behaviour of the upper mantle and the composition of oceanic basalts.

Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling

Abstract A palaeogeographical reconstruction of the South American and African continents back to anomaly C34 (84 Ma) brings together the Rio Grande Rise (RGR) and the central portion of the Walvis Ridge (WR), thus the RGR–WR aseismic ridges may have a common origin. If the construction of the RGR–WR basaltic plateau took place mainly between 89 and 78 Ma, as indicated by the ages of the basalts sampled by DSDP wells, then the basaltic magmas are the result of an ‘on-ridge’ volcanism. Once separated, the normal sea-floor spreading and thermal subsidence of the RGR and WR ridges continued until approximately 47 Ma when an Eocene magmatism took place in the RGR. In the WR, a younger volcanism is observed in the Guyot Province. The available geochemical and isotope data of the WR–RGR basalts do not indicate the participation of the continental crust melting component. Incompatible trace element ratios and isotope signatures of the basalts from the RGR–WR ridges are distinct from the present-day Tristan da Cunha alkaline rocks, and are nearly identical to the high-Ti Paraná Magmatic Province (PMP) tholeiites (133–132 Ma). Both the high-Ti PMP and the WR–RGR basalts are characterized by moderate initial 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb isotope ratios [Enriched Mantle I (EMI) mantle component], suggesting melting from a common source, with significant participation of sub-continental lithospheric mantle (SCLM). A three-dimensional (3D) flexural modelling of the RGR and WR was conducted using ETOPO1 digital topography/bathymetry and EGM2008-derived free-air anomalies as a constraint. The best fit between the observed and calculated free-air anomalies was obtained for an elastic plate with elastic plate thickness ( T e ) of less than 5 km, consistent with an ‘on-ridge’ initial construction of the RGR–WR. The modelling of the crust–mantle interface depths indicates a total crustal thickness of up to 30 km in the RGR–WR. Flexural analysis reinforces the geological evidence that RGR was constructed during two main magmatic episodes, the tholeiitic basalts in the Santonian–Conician times and the alkaline magmatism in the Eocene. Geochemical and geophysical evidence, which rules out the classical deep-mantle plume model in explaining the generation of basalts of these volcanic provinces, is presented. Finally, three models to explain the geochemical and isotope signatures of RGR–WR basalts are reviewed: (1) thermal erosion of SCLM owing to edge-driven convection; (2) melting of fragmented or detached SCLM and lower crust; and (3) thermal erosion at the base of the SCLM with lateral transport of enriched components by mantle flow.

Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province

New ion microprobe (SHRIMP) U–Pb zircon data indicate that voluminous A-type volcanic rocks were extruded c. 1585Ma in the Benagerie Ridge region of the north-central Curnamona Province in southern Australia. Incompatible trace element ratios and whole-rock Sm–Nd isotope compositions (ɛNd (1585)=−4.3 to −2.2) suggest a dominant crustal source region for the felsic rocks. Incompatible trace element ratios in the basalts, enriched light REE/heavy REE ratios (La/Yb=6–7.5), and ɛNd (1585) values between −1.5 and 0.2 suggest that the basalts were derived from a HFSE-enriched magma from within the mantle. These new data show that the Benagerie Volcanic Suite share geochronological, geochemical, and isotopic affinities with the upper Gawler Range Volcanics of the Gawler Craton indicating these two volcanics suites can be considered part of a formerly contiguous Mesoproterozoic silicic large igneous province. We invoke aspects of the two-phase model for the generation of this A-type silicic large igneous province presented by previous workers and attempt to place the initial ‘developmental’ and subsequent ‘mature’ phases of this event into a tectonic framework for the early Mesoproterozoic of the Curnamona Province and the adjacent Gawler Craton. Initiation of the developmental phase likely resulted from lithospheric extension and was accompanied by localised basaltic magmatism. The mature phase, which involved eruption of voluminous felsic volcanic rocks, was a result of widespread crustal melting, potentially induced by the elevated geotherm caused by extension, by ponding of mafic melt in the lower crust, or a combination of these factors. The high geothermal gradient that resulted from extension and magmatism likely primed the crust for the early Mesoproterozoic regional deformation and metamorphism that occurred across the Curnamona Province and Gawler Craton. The far-field tectonic drivers for this silicic large igneous province remain uncertain, although the association between extensive bimodal A-type magmatism, high-temperature metamorphism, and localised compressional deformation, is suggestive of an intracontinental setting, possibly mechanically connected to a far-field subduction zone.

Conditions of melting beneath the Azores

The depth and temperatures of melting beneath Ocean Islands provide important constraints on mantle melting dynamics. The central Azores islands of Faial, Pico, Graciosa and São Jorge are ideally suited for calculating depth and temperatures of melting because they are situated orthogonal to the slow spreading Mid-Atlantic Ridge and thus supposedly on variable lithosphere thicknesses which allow to constrain on the influence of lithosphere thickness on average melting depth. The dataset presented here is used to test the spatial pattern of potential temperature anomalies in the Azores mantle plume. Trace elements indicate comparable degrees of partial melting for all three islands, while Sr–Nd–Pb isotope ratios and incompatible trace element ratios imply only small-scale source heterogeneity that is not related to the major element systematics. Thus, the observed differences in calculated, primary SiO2, FeOT and TiO2 (and Al2O3) are inferred to reflect changes in melting pressures and temperatures that are independent of source composition. Melting temperatures in the Azores are lower and melting starts deeper than at other mantle plumes (e.g., Hawaii). This implies that the smaller size of the Azores compared to other mantle plumes might be the result of a much smaller anomaly with a large variability of depths and temperatures of melting even on the scale of an individual island. This small-scale variability is of importance when comparing pressures and temperatures between different islands or even globally. The Azores lavas have temperatures lower than predicted for a dry mantle solidus indicating the presence of at least 200ppm H2O in the Azores mantle source.

Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics

New bulk rock geochemical data on an extensive suite of samples from the Marion and Prince Edward Islands, located in the Southern Ocean at ~37°45′E, 46°55′S, show highly coherent major and trace element variations. Rock types are dominated by alkali basalt, with lesser hawaiite and minor mugearite and benmoreite. Prince Edward Island has in addition rare trachytes exposed on the plateau, and xenoliths of comenditic rhyolite pumice were found in a basalt flow of the 1980 eruption on Marion Island. Lavas from Prince Edward Island differ from those erupted on Marion Island by having subtly distinct incompatible trace element ratios (e.g. Zr/Nb=9.1 vs 6.7; Ba/Nb=7.8 vs 6.7; Ce/Pb=31 vs 36), and lower 87Sr/86Sr (0.70302) and higher 143Nd/144Nd (0.51300) isotope ratios than found on Marion Island (0.70336–0.70349; 0.51292–0.51295). The two islands are indistinguishable in terms of Pb isotope ratios (206Pb/204Pb=18.61–18.75, 207Pb/204Pb=15.54–15.56).Quantitative modelling suggests that the range in rock types found on each island can be ascribed to extensive low pressure crystal fractionation of phenocryst phases. For example, hawaiite can be related to the more primitive alkali basalts by some 24% fractional crystallisation of olivine, clinopyroxene, plagioclase and minor FeTi-oxides, whereas the more evolved mugearites and benmoreites require, respectively a further 36% and 24% fractional crystallisation to account for their felsic compositions. The trachytes of Prince Edward Island appear to represent the residual 28% magma following extensive crustal fractionation of dominantly clinopyroxene, plagioclase and FeTi-oxide with minor apatite. None of the sampled lavas have compositions characteristic of true primary magmas, but the least evolved, with Mg# ~63, appear to have experienced as much as 18% olivine fractionation from a primary magma with ~15wt.% MgO. The rhyolite pumice is distinct in composition from Bouvet rhyolite or South Sandwich Island pumice and may belong to an earlier, submarine eruption at or near Marion Island.The enriched incompatible element ratios (Zr/Nb=6–9; La/Ybn=10.6; Y/Nb=0.73) and radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd isotope ratios show that the lavas erupted on Marion and Prince Edward Islands resulted from melting an enriched mantle source, consistent with derivation from an upwelling deep mantle plume. Pb isotope ratios (∆8/4Pb~33) confirm the absence of any DUPAL signature in the source region of these magmas.

Melting of a Two-component Source beneath Iceland

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland’s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.