Showing papers on "Basalt published in 2013"

The Paraná-Etendeka Province

Abstract: Stratigraphic data and 40 Ar- 39 Ar ages for the Early Cretaceous Parana-Etendeka flood basalts indicate that the main magmatic episode lasted for several my (129-134 Ma) and was linked to the northward opening of the South Atlantic Ocean, but with some earlier magmatism (135-138 Ma) found inland far from the eventual oceanic rift The regional distribution of distinct high-Ti/Y (Urubici, Pitanga, Paranapanema, Ribeira) and low-Ti/Y (Gramado, Esmeralda) magma types in the lavas and associated dyke swarms implies that magma generation occurred over a wide area and involved different mantle sources Low MgO contents (3-7 wt%) indicate extensive fractional crystallisation, and upper crustal assimilation was important in the evolution of the Gramado magmas However, Parana basalts that are considered to be uncontaminated by crust have trace element and isotope characteristics (eg Nb/La 1000 km 3 ) that can be correlated across the Atlantic Ocean accompanied the final magmatic phase in the southeast Parana and the Etendeka The flood basalts post-date most estimates for the Jurassic-Cretaceous boundary, ruling out any link to a faunal extinction

Along‐Strike Variation in the Aleutian Island Arc: Genesis of High Mg# Andesite and Implications for Continental Crust

Abstract: Based on a compilation of whole rock geochemistry for approximately 1100 lava samples and 200 plutonic rock samples from the Aleutian island arc, we characterize along-strike variation, including data for the western part of the arc which has recently become available. We concentrate on the observation that western Aleutian, high Mg# andesite compositions bracket the composition of the continental crust. Isotope data show that this is not due to recycling of terrigenous sediments. Thus, the western Aleutians can provide insight into genesis of juvenile continental crust. The composition of primitive magmas (molar Mg# > 0.6) varies systematically along the strike of the arc. Concentrations of SiO 2 , Na 2 O and perhaps K 2 O increase from east to west, while MgO, FeO, CaO decrease. Thus, primitive magmas in the central and eastern Aleutians (east of 174°W) are mainly basalts, while those in the western Aleutians are mainly andesites. Along-strike variation in Aleutian magma compositions may be related to a westward decrease in sediment input, and/or to the westward decrease in down-dip subduction velocity. 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb /204 Pb and 87 Sr/ 86 Sr all decrease from east to west, whereas 143 Nd/ 144 Nd increases from east to west. These data, together with analyses of sediment from DSDP Site 183, indicate that the proportion of recycled sediment in Aleutian magmas decreases from east to west. Some proposed trace element signatures of sediment recycling in arc magmas do not vary systematically along the strike of the Aleutians, and do not correlate with radiogenic isotope variations. Thus, for example, Th/Nb and fractionation-corrected K concentration in Aleutian lavas are not related to the flux of subducting sediment. Th/La is strongly correlated with Ba/La, rendering it doubtful that Ba/La is a proxy for an aqueous fluid component derived from subducted basalt. Ce/Pb > 4 is common in Aleutian lavas west of 174°W, in lavas with MORB-like Pb, Sr and Nd isotope ratios, and is also found behind the main arc trend in the central Aleutians. Thus, Ce/Pb in Aleutian lavas with MORB-like isotope ratios is not always low, and may be affected by a component derived from partial melting of subducted basalt in eclogite facies. Enriched, primitive andesites, with high Sr/Y, steep REE patterns, and low Yb and Y, are an important lava type in the Aleutians west of 174°W. High Sr/Y and Dy/Yb, indicative of abundant garnet in the source of melting, are correlated with major element systematics. Lavas with a garnet signature have high SiO 2 , Na 2 O and K 2 O.

Emplacement of continental flood basalt lava flows

Abstract: We propose that continental flood basalt (CFB) lavas were predominantly emplaced as inflated compound pahoehoe flow fields via prolonged, episodic eruptions Our most detailed observations come from the ∼14,7 Ma Roza flow field of the Columbia River Basalt (CRB) Group The Roza flow field seems to be typical of many flood basalt lavas Individual flows show a wide range of pahoehoe surface features and a three-part internal structure in vesicularity and other textural parameters This three-fold division into an upper crust, core, and basal crust appears to be diagnostic of the inflation process and is ubiquitous in basaltic lava flows over a remarkable range of sizes The pahoehoe surface features and indications of inflation are inconsistent with rapid emplacement of these lava flows Instead, we interpret the observations to imply that the Roza, and other CFB flows, were emplaced over an extended period of time From the thickness of the upper crust, which we suggest formed while the flow was actively inflating, and an empirical expression for the rate of crust growth of Hawaiian inflated sheet flows, we estimate that individual Roza flows were emplaced over 5 to 50 months and that the Roza flow field was constructed over a period of 6 to 14 years However, even with this longer eruption duration, the average lava effusion rate of ∼4000 m 3 /s is similar to that of the highest-effusion-rate eruption in recorded history (the 1783-4 Laki eruption in Iceland) Our observations of lava characteristics in other CRB flows and in the Deccan Traps suggest that this emplacement style is typical of many, if not most, CFB flows Initial estimates of the volatile release from the Roza eruption indicate that prodigious amounts of S, Cl, and F were injected into the upper troposphere and lowermost stratosphere; thus this single flood basalt eruption could have had a significant effect on the global atmosphere If other flood basalt eruptions produced similar amounts of volatiles, volatile release might provide a link between flood basalt eruptions and mass extinctions

Origin and age of the earliest Martian crust from meteorite NWA 7533

Abstract: Chemical analysis of the meteorite NWA 7533 indicates that it may be a Martian regolith breccia and, if so, that the crust of Mars may have formed in the first 100 million years of the planet’s history. This paper identifies the NWA 7533 meteorite from Northwest Africa as the first sample of Martian highlands rock in the meteorite collection. Munir Humayun and co-authors show that NWA 7533 has a composition indicative of a highlands breccia. It also contains zircons more than 4.4 billion years old, implying that early crustal differentiation on Mars occurred in the first 100 million years of its history, coeval with earliest crust formation on the Moon and the Earth. The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation1,2, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 70343) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory4, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon5 and the Earth6. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically2,7.

Large igneous provinces and silicic large igneous provinces: Progress in our understanding over the last 25 years

Abstract: Large Igneous Provinces are exceptional intraplate igneous events throughout Earth’s history. Their significance and potential global impact is related to the total volume of magma intruded and released during these geologically brief events (peak eruptions are often within 1-5 Myrs duration) where millions to tens of millions of cubic kilometers of magma are produced. In some cases, at least 1% of the Earth’s surface has been directly covered in volcanic rock, being equivalent to the size of small continents with comparable crustal thicknesses. Large Igneous Provinces are thus important, albeit episodic episodes of new crust addition. However, most magmatism is basaltic so that contributions to crustal growth will not always be picked up in zircon geochronology studies that better trace major episodes of extension-related silicic magmatism and the silicic Large Igneous Provinces. Much headway has been made on our understanding of these anomalous igneous events over the last 25 years, driving many new ideas and models. This includes their: 1) global spatial and temporal distribution, with a long-term average of one event approximately every 20 Myrs, but a clear clustering of events at times of supercontinent break-up – Large Igneous Provinces are thus an integral part of the Wilson cycle and are becoming an increasingly important tool in reconnecting dispersed continental fragments; 2) compositional diversity that in part reflects their crustal setting of ocean basins, and continental interiors and margins where in the latter setting, LIP magmatism can be silicicdominant; 3) mineral and energy resources with major PGE and precious metal resources being hosted in these provinces, as well as magmatism impacting on the hydrocarbon potential of volcanic basins and rifted margins through enhancing source rock maturation, providing fluid migration pathways, and trap formation; 4) biospheric, hydrospheric and atmospheric impacts, with Large Igneous Provinces now widely regarded as a key trigger mechanism for mass extinctions, although the exact kill mechanism(s) are still being resolved; 5) role in mantle geodynamics and thermal evolution of the Earth, by potentially recording the transport of material from the lower mantle or core-mantle boundary to the Earth's surface and being a fundamental component in whole mantle convection models; and 6) recognition on the inner planets where the lack of plate tectonics and erosional processes and planetary antiquity means that the very earliest record of LIP events during planetary evolution may be better preserved than on Earth.

Chemical Diversity of Abyssal Volcanic Glass Erupted Along Pacific, Atlantic, and Indian Ocean Sea-Floor Spreading Centers

Abstract: The abundant glassy rinds and chilled contacts of submarine extrusions and intrusions provide the most reliable indicators of primary magmatic compositions of deep-sea igneous rocks. They are homogeneous at the precision of electron microprobe analyses, so that even small sub-samples are reliable indicators of the composition of larger eruptive units. Analyses of such sea-floor glasses by electron microprobe techniques for Si, Al, Fe, Mg, Ca, Na, K, Ti, P reveal an extremely diverse compositional spread, ranging form highly differentiated basalt to varieties extremely depleated in large ion lithophile elements. A preliminary look at glasses from active spreading centers suggests that the FETI basalt group (high FeO and TiO2, low MgO) is more common at rapidly spreading ridges (East Pacific rise system) than at slower spreading ridges (mid-Atlantic ridge system). The data presented here appear for the most part inadequate for meaningful assessment of chemical gradients along spreading centers. There is strong evidence, though, that some segments of spreadings centers have erupted similar lava compositions over tens of millions of years. These similar chemical groups are symmetrically distributed on each side of spreading segments as is expected from sea-floor spreading models. There is very little overlap in the compositions of the basaltic chemical groups at the level precision of the electron probe analyses. However, there are a few Atlantic chemical groups that occur both in the North and South Atlantic. Even these intra-ocean matches of chemical groups from one locality to another are, however remarkably few. This survey shows the great predominance of various basalt types at spreading centers, but other rock types, including soda rhyolite from the Galapagos rise, occur rarely.

Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust

Abstract: Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials1,2. However, the residence time of these subducted materials in the mantle is uncertain and model-dependent3, and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust3,4. Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago5-7. Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur--probably derived from hydrothermally altered oceanic crust--was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative D33S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions8,9. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

Abstract: The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoro-hegy and the Fuzes-to scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoro-hegy and Fuzes-to alkaline basaltic magmas have a complex ascent history, and that most of their minerals (∼30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoro-hegy and Fuzes-to alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.

Non-chondritic sulphur isotope composition of the terrestrial mantle.

Abstract: Earth’s mantle is shown to display heterogeneous sulphur isotope ratios, with a depleted end-member that is not chondritic as has been thought; the mantle’s inferred composition can be accounted for by fractionation during core–mantle differentiation. The differentiation of early Earth's subsurface material into core and mantle layers should be reflected in the residual mantle composition, as most of the iron-loving elements, presumably including sulphur, would have been scavenged by the liquid core. However, previous analyses of Earth's mantle have identified stable sulphur isotope ratios resembling those seen in chondritic meteorites, perhaps the result of a 'late veneer' meteoritic origin for mantle material. But here Jabrane Labidi et al. provide evidence that the mantle displays heterogeneous sulphur isotope ratios directly correlated to strontium and neodymium isotope ratios. The authors conclude that these results can be reconciled by fractionation during core-mantle differentiation. Core–mantle differentiation is the largest event experienced by a growing planet during its early history. Terrestrial core segregation imprinted the residual mantle composition by scavenging siderophile (iron-loving) elements such as tungsten, cobalt and sulphur. Cosmochemical constraints suggest that about 97% of Earth’s sulphur should at present reside in the core1, which implies that the residual silicate mantle should exhibit fractionated 34S/32S ratios according to the relevant metal–silicate partition coefficients2, together with fractionated siderophile element abundances. However, Earth’s mantle has long been thought to be both homogeneous and chondritic for 34S/32S, similar to Canyon Diablo troilite3,4,5,6, as it is for most siderophile elements. This belief was consistent with a mantle sulphur budget dominated by late-accreted chondritic components. Here we show that the mantle, as sampled by mid-ocean ridge basalts from the south Atlantic ridge, displays heterogeneous 34S/32S ratios, directly correlated to the strontium and neodymium isotope ratios 87Sr/86Sr and 143Nd/144Nd. These isotope trends are compatible with binary mixing between a low-34S/32S ambient mantle and a high-34S/32S recycled component that we infer to be subducted sediments. The depleted end-member is characterized by a significantly negative δ34S of −1.28 ± 0.33‰ that cannot reach a chondritic value even when surface sulphur (from continents, altered oceanic crust, sediments and oceans) is added. Such a non-chondritic 34S/32S ratio for the silicate Earth could be accounted for by a core–mantle differentiation record in which the core has a 34S/32S ratio slightly higher than that of chondrites (δ34S = +0.07‰). Despite evidence for late-veneer addition of siderophile elements (and therefore sulphur) after core formation, our results imply that the mantle sulphur budget retains fingerprints of core–mantle differentiation.

Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt

Abstract: Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown By drilling into 35-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ(13)C- and δ(34)S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ(13)C signatures locally attenuated by heterotrophic metabolism

Plume/Lithosphere Interaction in the Generation of Continental and Oceanic Flood Basalts: Chemical and Isotopic Constraints

Abstract: The plume initiation model for flood basalt genesis predicts that melt generation will occur almost entirely within the plume head, beneath the lithosphere. However, isotopic and trace element differences between continental and oceanic flood basalts (CFBs and OFBs, respectively) require the incorporation of a lithospheric component in the former. Debate persists as to whether the continental signatures present in many CFBs derive from contamination of (plume-derived) melts with small volumes of highly incompatible-element-enriched lithospheric components (e.g., through crustal assimilation) or substantial melt generation within the lithospheric mantle. The chemical and isotopic compositions of OFBs and CFBs are used to constrain the mantle sources and melting environment responsible for flood basalt generation and the extent of crustal assimilation. Major and trace element trends in CFBs reflect smaller extents of partial melting than in OFBs and the presence of garnet in CFB source regions. These observations are consistent with the plume initiation model, because thick continental lithosphere will inhibit ascent and melt generation in sublithospheric plumes. However, melt generation from refractory, Fe-poor lithospheric mantle is also indicated for several CFBs, including the Siberian Traps. Depth of melt generation typically decreases with time for a given province, as indicated by the removal of a garnet signature in trace element ratios (e.g., decreasing Sm/Yb). These chemical and temporal trends suggest early melt generation within hydrous but otherwise refractory lithospheric mantle, followed by mechanical erosion of the lithosphere that allows additional ascent and melt generation within the underlying plume.

Chemical Systematics and Hydrous Melting of the Mantle in Back‐Arc Basins

Abstract: This paper explores the chemical systematics of back-arc basins and the physical processes that give rise to them, making use of published data from the Scotia, Mariana, Lau, and Manus Basins. A new low-pressure fractionation model is used to back-correct data with greater than 5.5 wt.% MgO. Even after hydrous correction, back-arc basin basalts (BABB) have low TiO 2 and FeO contents relative to basalts from other ridges. The low TiO 2 both absolutely and relative to Na 2 O requires a source depletion followed by a Na enrichment. This signature is critical to evaluate the range of mantle temperature at back-arc basins, which is about 100°C. In addition to a subduction component and wedge depletion, BABB reflect a prevalent enriched component akin to enriched ocean ridge basalts worldwide, despite the absence of mantle plumes. Data from the Mariana Basin suggest this component arises from very recent addition of low-degree (low-F) melts, which may be an important general agent of mantle heterogeneity. Important aspects of the back-arc data are the linear relationships among all major element parameters with each other and with water. Previous models involving isothermal, isobaric melting with increasing water contents do not account for these relationships. A constraint on permissible physical models is that both trace element and major element data show no inherited effects from garnet in back-arc basins, which constrains generation and transport of melts from great depth to the base of the melting regime. Quantitative modeling of the effects of water on mantle melting shows that previous conclusions based on the MELTS thermodynamic approach are not consistent with experimental data for the mantle. Our new models can account for the back-arc systematics by mixing between dry, pooled fractional melts, formed similarly to open ocean ridges, with hydrous melts generated from sources enriched in H 2 O, Na 2 O, and K 2 O that have equilibrated at low pressure. Thus, successful physical models must be able to produce melts by these two different mechanisms. The effects of H 2 O in back-arc basin ridges and open ocean ridges contrast markedly. In the open ocean, increased water is associated with lower mean extents of melting, increased TiO 2 contents, and an increased garnet effect. In back-arc basins, increased water is associated with increased extents of melting, lower TiO 2 , and no garnet influence. These differences can be accounted for by contrasts in the melting regimes and tectonic setting of the two environments. In the open ocean, deep, low-degree, hydrous melts are produced in the "wings" of the melting regime and combine with higher degree drier melts produced at a range of shallower pressures. At back-arcs, geometrically and thermally, there is no room for "wings" on the arc side of back-arc spreading centers. On the arc side of the spreading center, where water is added, shallow hydrous melting is important, and melt must get to the surface in the context of descending mantle flow. On the back side, dry melting under relatively anhydrous conditions occurs, similar to open ocean ridges. Mixing between melts from the dry side and the wet side should then lead to the characteristic spectra of parental BABB compositions. Both the geometry of melting and the fact of continual rifting of young lithosphere may contribute to the very different water signatures in the open ocean and back-arc settings.

Stratigraphy and Age of Karoo Basalts of Lesotho and Implications for Correlations Within the Karoo Igneous Province

Abstract: The Lesotho remnant contains the type succession for Karoo low-Ti basalts of central southern Africa. The 40 Ar/ 39 Ar dating indicates that the sequence was emplaced within a very short period at about 180 Ma and consists of a monotonous pile of compound basalt lava flows which lacks significant palaeosols and persistent sedimentary intercalations. We have used geochemistry to establish a stratigraphic subdivision of the lava pile. Thin units of basalt flows, the Moshesh's Ford, Golden Gate, Sani, Roma, Letele, and Wonderkop units, with diverse geochemical character and restricted geographical distribution, are present at the base of the succession. These are overlain by extensive units of compositionally more uniform basalt, the Mafika Lisiu, Maloti, Senqu and Mothae units, which build the bulk of the sequence. A single palaeomagnetic polarity reversal occurs within the lower third of the basalt succession and is consistently located within the Mafika Lisiu unit. This and the persistent and relatively uniform thickness of the stratigraphic units suggest that the pile was constructed in a uniform manner by eruption of basalt onto a generally planar surface from a widespread plexus of dykes. The stratigraphic sequence in Lesotho closely resembles that in the thinner sequence of low-Ti basalts of the Springbok Flats remnant, some 400 km to the north. A thin unit of high-Ti basalt within the upper part of the Springbok Flats sequence can be correlated with the thick high-Ti basalt suite along the rift-related Lebombo structure on the eastern margin of the Karoo province. This is the first established correlation between these two important outcrops of Karoo volcanic rocks and demonstrates that the low-Ti basalts of Lesotho and the cratonic interior are the approximate time equivalents of the lower part of the Lebombo sequence. This conclusion has important implications for models for the origin of the Karoo flood basalt province.

Crystallization of oxidized, moderately hydrous arc basalt at mid- to lower-crustal pressures: implications for andesite genesis

Abstract: This study focuses on the production of convergent margin calc-alkaline andesites by crystallization–differentiation of basaltic magmas in the lower to middle crust. Previous experimental studies show that dry, reduced, subalkaline basalts differentiate to tholeiitic (high Fe/Mg) daughter liquids, but the influences of H2O and oxidation on differentiation are less well established. Accordingly, we performed crystallization experiments at controlled oxidized fO2 (Re–ReO2 ≈ ΔNi–NiO + 2) on a relatively magnesian basalt (8.7 wt% MgO) typical of mafic magmas erupted in the Cascades near Mount Rainier, Washington. The basalt was synthesized with 2 wt% H2O and run at 900, 700, and 400 MPa and 1,200 to 950 °C. A broadly clinopyroxenitic crystallization interval dominates near the liquidus at 900 and 700 MPa, consisting of augite + olivine + orthopyroxene + Cr-spinel (in decreasing abundance). With decreasing temperature, plagioclase crystallizes, Fe–Ti-oxide replaces spinel, olivine dissolves, and finally amphibole appears, producing gabbroic and then amphibole gabbroic crystallization stages. Enhanced plagioclase stability at lower pressure narrows the clinopyroxenitic interval and brings the gabbroic interval toward the liquidus. Liquids at 900 MPa track along Miyashiro’s (Am J Sci 274(4):321–355, 1974) tholeiitic versus calc-alkaline boundary, whereas those at 700 and 400 MPa become calc-alkaline at silica contents ≥56 wt%. This difference is chiefly due to higher temperature appearance of magnetite (versus spinel) at lower pressures. Although the evolved liquids are similar in many respects to common calc-alkaline andesites, the 900 and 700 MPa liquids differ in having low CaO concentrations due to early and abundant crystallization of augite, with the result that those liquids become peraluminous (ASI: molar Al/(Na + K + 2Ca) > 1) at ≥61 wt% SiO2, similar to liquids reported in other studies of the high-pressure crystallization of hydrous basalts (Muntener and Ulmer in Geophys Res Lett 33(21):L21308, 2006). The lower-pressure liquids (400 MPa) have this same trait, but to a lesser extent due to more abundant near-liquidus plagioclase crystallization. A compilation of >6,500 analyses of igneous rocks from the Cascades and the Sierra Nevada batholith, representative of convergent margin (arc) magmas, shows that ASI increases continuously and linearly with SiO2 from basalts to rhyolites or granites and that arc magmas are not commonly peraluminous until SiO2 exceeds 69 wt%. These relations are consistent with plagioclase accompanying mafic silicates over nearly all the range of crystallization (or remelting). The scarcity of natural peraluminous andesites shows that progressive crystallization–differentiation of primitive basalts in the deep crust, producing early clinopyroxenitic cumulates and evolved liquids, does not dominate the creation of intermediate arc magmas or of the continental crust. Instead, mid- to upper-crustal differentiation and/or open-system processes are critical to the production of intermediate arc magmas. Primary among the open-system processes may be extraction of highly evolved (granitic, rhyolitic) liquids at advanced degrees of basalt solidification (or incipient partial melting of predecessor gabbroic intrusions) and mixing of such liquids into replenishing basalts. Furthermore, if the andesitic-composition continents derived from basaltic sources, the arc ASI–SiO2 relation shows that the mafic component returned to the mantle was gabbroic in composition, not pyroxenitic.

The Caribbean‐Colombian Cretaceous Igneous Province: The Internal Anatomy of an Oceanic Plateau

Abstract: The Late Cretaceous Caribbean—Colombian igneous province is one of the world's best-exposed examples of a plume-derived oceanic plateau. The buoyancy of the plateau (resulting from residual heat and thick crust) kept it from being totally subducted as it moved eastward with the Farallon Plate from its site of generation in the eastern Pacific and encountered a destructive plate margin. In effect, the plateau makes up much of the Caribbean Plate; it is well exposed around its margins, but more so in accreted terranes in western Colombia (including the well-known Gorgona komatiites and Bolivar mafic/ultramafic cumulates). Compositionally, the lavas of the plateau form three groups: (a) basalts, picrites, and komatiites with light-rare-earth-element (LREE)-depleted chondrite-normalised patterns; (b) basalts with LREE-enriched patterns; and (c) basalts with essentially flat REE patterns (the most dominant type) similar to many of the basalts from the Ontong Java Plateau. These three types demonstrate the heterogeneous nature of the mantle plume source region. The picrites and the komatiites seem to lie nearer the base of the plateau than the more homogeneous basalts; thus, the more MgO-rich melts may have been erupted before large magma chambers had a chance to develop. A reconstructed crustal cross section through the plateau consists of dunitic and pyroxenitic cumulates near the base which are overlain by layered olivine-rich gabbros and more isotropic gabbros. The lowermost eruptive sequence comprises compositionally heterogeneous picrites/komatiites overlain by more homogeneous pillow basalts. Spectacular hornblende-plagioclase veins cut the Bolivar assemblage and these may represent local partial melts of the plateau's base as it was thrusted onto the continent. Subduction-related batholiths and extrusive rocks found around the margin of the province are of two distinct ages; one suite represents pre-plateau collision-related volcanism whereas the other suite, slightly younger than the plateau, may be associated with obduction.

Petrological and Nd-Sr-Os isotopic constraints on the origin of high-Mg adakitic rocks from the North China Craton: Tectonic implications

Abstract: The Mesozoic high-Mg dioritic rocks in the North China Craton have been suggested to be part of adakitic rocks. The origin of the high-Mg diorites has been attributed to equilibration of partial melts from delaminated mafic crust (eclogite) with mantle peridotite. Here we present petrological and Os isotopic data against the delamination model, and propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting for their origin. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively, and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Our proposed model is probably applicable to the general mode of origin and tectonic settings of high-Mg adakitic magmas.