high-MgO Basalts Research Articles

The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic-Transitional Basalts From the Loranchet Peninsula

A geochemical study of 28 Ma tholeiitic to transitional basalts from the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine) indicates that three distinct magma types erupted within ∼1 Myr. Low-MgO basalts (∼4–6 wt %) in both sections are overlain by high-MgO basalts (∼7–13 wt %), mostly present in Mont Fontaine. Both high- and low-MgO basalts have nearly identical low 87Sr/86Sr and high 143Nd/144Nd and formed from similar parental magmas that represent mixtures between a depleted mantle component and the Kerguelen plume. The third magma type, predominant in Mont des Ruches, is represented by high-MgO basalts that are isotopically heterogeneous with isotopic ratios that are intermediate between those of the stratigraphically lower basalts and the Kerguelen plume compositions; this third magma type may have formed by mixing of similar material, but with a higher contribution from the Kerguelen plume. The depleted component involved in all three magma types is similar to the source for Southeast Indian Ridge basalts and is present in Kerguelen Archipelago basalts older than 26 Ma, which erupted when the ridge axis was <500 km away from the Kerguelen hotspot. Depleted heterogeneities intrinsic to the plume and entrainment of depleted mantle during plume ascent do not explain the marked cut-off in the presence of a depleted component in the archipelago basalts within a time interval of 1 Myr after 26 Ma. Mixing of depleted asthenosphere with the plume in sublithospheric channels during migration of the Southeast Indian Ridge axis away from the Kerguelen hotspot is proposed as a suitable explanation to account for the temporal distribution of the depleted component in basalts from the Northern Kerguelen Plateau and the >26 Ma Kerguelen Archipelago flood basalts; cessation of plume–ridge interactions may explain the absence of depleted basalts in the youngest sections that erupted further away from the ridge axis.

Source and H 2O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, lesser antilles arc

Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H 2O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe 3+/Fe 2+ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO 2 were imposed (NNO + 1.5 for the 1.5 wt% H 2O series, NNO + 2.3 for the 4.5 wt% H 2O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H 2O) and 1185°C, 16 kbar (4.5 wt% H 2O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H 2O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H 2O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry (∼ 2 wt% H 2O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with ∼ 2 wt% H 2O would require a H 2O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to ∼ 5.5 wt% H 2O.

LIP Reading: Recognizing Oceanic Plateaux in the Geological Record

Basaltic oceanic plateaux are important features in the geological record. Not only do they record ancient mantle plume activity, but they also are believed to be important building blocks in the formation of the continental crust. In this paper we review the salient features of two Cretaceous oceanic plateaux (the Ontong Java and the Caribbean–Colombian): thick sequences of predominantly homogeneous basalt; the occurrence of high-MgO basalt, including komatiites; and an apparent absence of sheeted dyke complexes. In addition, pyroclastic deposits may be scarce. We then explore ways of distinguishing plateaux from basaltic sequences erupted in different tectonomagmatic settings: continental flood basalt provinces; island arcs; back-arc basins; ocean islands and mid-ocean ridges. Using these criteria, potential Archaean and Proterozoic oceanic plateaux are reviewed and identified. Finally, we explore how these remnant oceanic plateaux became incorporated into the continents, by reviewing the proposed accretion mechanisms for the Cretaceous Caribbean–Colombian oceanic plateau, on the basis of evidence from South America and the tonalites of the southern Caribbean island of Aruba.

Re-Os Isotope Systematics of the Voisey's Bay Ni-Cu-CoMagmatic Sulfide System, Labrador, Canada: II. Implications for Parental MagmaChemistry, Ore Genesis, and Metal Redistribution

Re-Os Isotope Systematics of the Voisey's Bay Ni-Cu-CoMagmatic Sulfide System, Labrador, Canada: II. Implications for Parental MagmaChemistry, Ore Genesis, and Metal Redistribution

The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan

The Dir-Utror volcanic series forms a NE–SW trending belt within the northwestern portion of the Kohistan island arc terrane in the western Himalayas of northern Pakistan. The Kohistan arc terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan and Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dominated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocryst in mafic to intermediate rocks, K-feldspar and quartz phenocrysts predominate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolite are the most common metamorphic phases; blue-green amphibole, andesine, muscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespread but less abundant. Phase assemblages and chemistry suggest predominant greenschist facies metamorphism with epidote-amphibolite facies conditions attained locally. Whole rock major element compositions define a calc-alkaline trend: CaO, FeO, MgO, TiO 2, Al 2O 3, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORB-normalized trace element concentrations show enrichment of the low-field strength incompatible elements (Ce, La, Ba, Rb, K) and deep negative Nb, P, and Ti anomalies—patterns typical of subduction related magmas. Mafic volcanic rocks plot in fields for calc-alkaline volcanics on trace element discrimination diagrams, showing that pre-existing oceanic crust is not preserved here. All rocks are LREE-enriched, with La=16–112×chondrite, La/Lu=2.6–9.8×chondrite, and Eu/Eu*=0.5–0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu* ratios, reflecting the dominant role of plagioclase fractionation in their formation. Some andesites have La/Lu ratios which are too high to result from fractionation of the more mafic lavas; chondrite-normalized REE patterns for these andesites cross those of the basaltic andesites, indicating that these lavas cannot be related to a common parent. The high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic volcanics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic arc volcanic and plutonic rocks, and is mafic relative to normal continental margins. Andesites with high La, La/Lu, K 2O, and Rb may be crustal melts; we suggest that garnet-rich high-pressure granulites similar to those exposed in the Jijal complex may be restites formed during partial melting of the crust.

Hafnium and Strontium Isotope Measurements in High-MgO Basalts from Theistareykir, Northern Iceland

We report Hf and Sr isotope compositions from 31 samples from Theistareykir volcano, northern Iceland. The samples are high-MgO olivine tholeiites and picrites with the majority having MgO contents higher than 9 wt.%. The total range of MgO contents is from 5.5 to 22 wt.%. All samples are younger than 10,000 years and were collected from different postglacial lava flows of the Theistareykir volcano in the northern Icelandic rift zone.

The relationship between the hawaiites and basalts of the itcha Volcanic Complex, central British Columbia

Volumetrically subordinate alkaline mafic lava flows form a late capping stage over the earlier felsic lavas that form the shield of the Itcha Volcanic Complex (IVC), of the Anahim Volcanic Belt (AVB) in central British Columbia (B.C.). The mafic capping stage of the IVC is dominated by hawaiites which are the earliest of the mafic lavas, and are succeeded by alkali olivine basalts (AOB) and then by basanites. The alkali olivine basalts can be subdivided into high-, intermediate- and low-MgO AOB groups, all of which share similar HFSE ratios (e.g. Nb/Zr) with the hawaiites. High Al contents and Sr/Zr ratios indicate that hawaiites and Fe-rich evolved AOB were derived from primitive AOB parental magmas by crystal fractionation of a wehrlitic assemblage at pressures on the order of 8 to 10 kbar. High Si and low Fe contents indicate that the majority of the evolved AOB lavas, however, do not represent an intermediate stage in the liquid line of descent to hawaiites, but were most likely produced by gabbroic fractionation from primitive AOB magmas at relatively low pressures. The parental magmas of the majority of these lavas were distinct from those of the observed high-MgO basalts, having higher HFSE contents and being more Si-under-saturated. The high Al, high Sr/Zr signature of high-pressure fractionation of a clinopyroxene-dominated assemblage in the IVC is shared by hawaiites of other alkaline volcanic suites of the Canadian Cordillera, such as the Edziza Volcanic Complex in northern B.C. and appears to be a feature of hawaiites in many localities, including Hawaii and Iceland. Viscosities calculated for both high- and low-pressure crystal fractionation models suggest that aphyric hawaiites are residual liquids escaped from a wehrlitic crystalline network, at elevated pressures, possibly at the base of the crust.

Crustal assimilation during turbulent magma ascent (ATA); new isotopic evidence from the Mull Tertiary lava succession, N. W. Scotland

Assimilation of crustal rocks with concomitant fractional crystallisation (AFC) is a well documented phenomenon in many igneous suites, but geochemical evidence from the Tertiary Mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism. Lavas from the lower half of the Mull Plateau group (MPG) can be divided into two broad sub-types; high (>8%) MgO basalts with elevated Ba and K; and lower MgO (<8%) basaltic-hawaiites with lower Ba and K. The lower crust and most of the upper crust beneath Mull is probably of Lewisian age. The Sr-, Nd-and Pb-isotope compositions of local Lewisian crustal samples yield the following ranges: 87Sr/86Sr=0.71002–0.72348, 143Nd/144Nd=0.51045–0.51058 and 206Pb/204Pb=14.0–14.6. Ten lavas have also been analysed and yield the following ranges: 87Sr/86Sr=0.7028–0.7042, 143Nd/144Nd=0.51214–0.51230 and 206Pb/204Pb=15.1–17.9. However, within this range, it is predominantly the more primitive mafic compositions, with elevated Mg, Ba and K, that show the lowest Nd- and Pb-, and the highest Sr-isotope values. Modelling of these isotopic results, in conjunction with major and trace element data, show that: (1) contamination by Lewisian lower crustal material does occur; (2) that the process involved was not one of assimilation with concomitant fractional crystallisation (AFC). The proposed contamination process is one whereby the hottest (most MgO rich) magmas have assimilated acidic partial melts of Lewisian lower crust during turbulent ascent (ATA) through thin, poorly connected dyke- and sill-like magma chambers. The chemical composition of the contaminated lavas can be modelled successfully through addition of ∼5% acidic Lewisian crust to an uncontaminated lava. In contrast, the more evolved magmas — which probably fractionated at sub-crustal levels — were either not hot enough to molt significant amounts of crust, or did not ascend turbulently because of their higher viscosity, and so are less contaminated with crust.

Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al2O3 from High-MgO Basalt

Klyuchevskoy Volcano, Kamchatka, Russia: The Role of High-Flux Recharged, Tapped, and Fractionated Magma Chamber(s) in the Genesis of High-Al2O3 from High-MgO Basalt

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H2O, which will affect the influence of bulk CaO, MgO, and Al2O3 contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than ∼12 kbar.

Near-liquidus phase relations of an anhydrous high-magnesia basalt from the Aleutian Islands: Implications for arc magma genesis and ascent

The results of anhydrous near-liquidus experiments (1 atm to 20 kbar) with a natural high-MgO basalt (ID-16) from Umnak Island in the central Aleutians are used to constrain models of arc magma genesis. At 1 atm. olivine is the liquidus phase followed at lower temperatures by plagioclase, augite, and finally pigeonite. Olivine remains the liquidus phase at 10 kbar but is joined by orthopyroxene, augite, and plagioclase just 25°C beneath the liquidus. The mineral-in curves for olivine, orthopyroxene, clinopyroxene, plagioclase, and spinel all converge on the liquidus at about 12 kbar. demonstrating that ID-16 melt is saturated with the plagioclase/spinel-peridotite assemblage at this pressure. At 15 kbar. clinopyroxene ( +spinel) replaces olivine as the liquidus phase and is followed by orthopyroxene, plagioclase, olivine, and finally, garnet. Although augite persists on the liquidus to at least 20 kbar, garnet is the second phase to crystallize at this pressure. Our first 12-kbar sub-liquidus run is 49 wt.% crystalline despite being only ∼ 10° C beneath the liquidus and the melt is correspondingly evolved relative to the starting material. As a result, we cannot demonstrate conclusively that the liquidus phases have mantle-like compositions, but we believe that the match is acceptable once allowances are made for the crystallinity of the run product. Two permissible interpretations of our results are: (1) our starting material was generated by small degrees of melting of plagioclase/spinel-peridotite at about 35 km depth, or (2) the melt formed deeper but last equilibrated with the mantle at this depth prior to eruption. We prefer the latter interpretation but note that the required temperature (∼ 1300° C) at 35 km depth is hundreds of degrees hotter than that predicted by steady-state thermal models of subduction zones. Accordingly, we propose that the isotherms beneath Umnak Island have been perturbed upward by the passage of earlier batches of hot magma through a reused conduit, thus enabling ID-16 to ascend from a deeper and hotter level and react with its surroundings without cooling significantly.

Magmatic development and tectonomagmatic models for the Solund-Stavfjord Ophiolite Complex, West Norwegian Caledonides

Geochemical variation and different extrusion rates of the Late-Ordovician Solund-Stavfjord Ophiolite Complex (SSOC) suggest both open and closed magma chambers. Closed chambers existed off axis and within a transform fault and are indicated by dykes and pillow lavas that evolved from high-MgO basalt to ferrobasalt by fractional crystallization of olivine, plagioclase and clinopyroxene. Hybrid magmas with ferrobasaltic composition represent open magmatic systems and are characterized by frequent eruptions of sheet flows. The SSOC is interpreted as representing either a normal or a propagating ridge-transform intersection (RTI). The propagating RTI model is favoured as it explains both the high frequency of ferrobasalt and uplift of ultramafic rocks to above the sea-level which is suggested by detrital ultramafics occurring in an ophiolitic mélange.

Pressure of origin of primary mid-ocean ridge basalts

Summary High-pressure phase equilibrium studies of mid-ocean ridge basalts (MORBs) have shown that some samples are in equilibrium with olivine + orthopyroxene (OPX) + clinopyroxene ± plagioclase at about 10 kbar, whereas others are not in equilibrium with OPX at this pressure. Samples that are OPX-saturated at about 10 kbar are characterized by higher SiO 2 (&gt;49.7 wt%) than most (about &gt;75%) MORB glasses with more than 9.5 wt% MgO, suggesting that only about 25% of these glasses could be derived from primary magmas generated at about 10 kbars. Orthopyroxenes in abyssal peridotites are characterized by Al 2 O 3 contents that range from 2.0 to 6.0 wt% (probably about 2.5–6.5 wt% before subsolidus equilibration), but orthopyroxenes in peridotite partial-melting experiments and liquidus orthopyroxenes for MORBs at about 10 kbar have only 3.5–4.3 wt% Al 2 O 3 . Liquidus orthopyroxenes for high-MgO basalts at 25 kbar and 20 kbar have about 6.6 wt% and 5.5 wt% Al 2 O 3 respectively. It appears that the least depleted (Al 2 O 3 -rich end) of abyssal peridotite orthopyroxenes indicate the dominance of melting and primary MORB genesis at pressures substantially greater than 10 kbar, and probably at 20–25 kbar.

Nickel partitioning between olivine and silicate melt

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) − 0.9 Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated. Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.

Mantle composition derived from the chemistry of ultramafic lavas

ESTIMATES of the composition of the Earth's mantle have been based on both geochemical and geophysical studies (refs 1–3). As might be expected, there are some discrepancies between the estimates particularly for Na2O and TiO2. Such discrepancies in part arise from uncertainties concerning the degrees of melting which many volcanics represent and, possibly, the restricted tectonic settings of the ultramafic nodules studied. High MgO basalts and ultramafic lavas, some of which have been termed komatiites4 occur among the Archaean volcanic rocks. They represent exceptionally high degrees of melting (up to 50% or more) of peridotite source regions. Potentially such volcanics may be used to make a precise estimate of the composition of their mantle source region. High MgO lavas are well preserved in the Archaean Belingwe greenstone belt of southern Rhodesia, where they form part of a sequence deposited unconformably on granitic crust5. We assume that the occurrence of pillow lavas in this sequence containing both skeletal olivine spinifex (Fo92.5 in a rock with 28% MgO, E. G. Nisbet, M. J. Bickle and A. Martin, unpublished) and skeletal olivine microphenocrysts indicates that these particular lavas have a composition close to that of the liquid.