Primitive Mid-ocean Ridge Basalt Research Articles

Radiogenic Os in primitive basalts from the northwestern U.S.A.: Implications for petrogenesis

Chemically primitive late Cenozoic tholeiitic basalts from the northwestern U.S.A. have Os-isotopic compositions more radiogenic than observed for most basalts from the ocean basins. This result is inconsistent with the simple petrogenetic model that explains the geographically correlated Sr-, Nd- and Pb-isotopic variation in these basalts as resulting solety from melting of metasomatized lithospheric mantle peridotite of varying age across this area. A magma source composed of a mixture of peridotite and pyroxenite/eclogite also fails because at the high percentage of mafic component required to explain the observed Nd—Os isotope systematics, this mixed source would not produce melts that match the major- and trace-element compositions of the observed olivine tholeiites. A more likely explanation for the observed isotopic compositions involves interaction of sublithospheric primary melts that are similar in composition to mid-ocean ridge basalt (MORB) with high- 187Os/ 188Os materials in the lower crust or lithospheric mantle. Though not uniquely defined, mass-balance calculations suggest that the lithospheric component is mafic-potassic in composition, possibly a small-volume melt of mafic material in the lower crust or upper mantle. If the radiogenic Os is attributable to such interaction, the isotopic compositions of the tholeiites can be satisfied by 2–25% addition of the lithospheric component to a primitive MORB parental magma. These results show that the Os-isotope system in continental basalts can provide a clear distinction between magmas derived by melting isotopically evolved peridotitic lithosphere and those produced from a sublithospheric primary melt contaminated by mafic material in the lithosphere.

The Giant Crater Lava Field: Geology and geochemistry of a compositionally zoned, high‐alumina basalt to basaltic andesite eruption at Medicine Lake Volcano, California

The Giant Crater lava field consists of >4 km3 of basaltic lava, compositionally zoned from first‐erupted calc‐alkaline basaltic andesite to last‐erupted primitive high‐alumina basalt. On the FeO*/MgO (where FeO* is total Fe calculated as FeO) versus SiO2 discrimination diagram commonly used to distinguish tholeiitic from calc‐alkaline series lavas the compositionally zoned eruption crosses from the tholeiitic field to the calc‐alkaline field. The lavas erupted in a brief span of time about 10,500 years ago from several closely spaced vents on the south flank of Medicine Lake volcano in the southern Cascade Range. Six chemical‐stratigraphic groups were mapped. Lower K2O, higher MgO groups always overlie higher K2O, lower MgO groups. Group 6 lavas erupted last and are aphyric, have high contents of MgO and Ni, and contain as little as 0.07% K2O. Group 1 lavas are porphyritic and have as much as 1.10% K2O. Major element contents of primitive group 6 Giant Crater basalt are very similar to a subset of primitive mid‐ocean ridge basalts (MORB). Group 6 lava is more depleted in middle and heavy rare earth elements (REE) and Y than is primitive MORB, but it is enriched in large ion lithophile elements (LILE). These LILE enrichments may be a result of fluid from the subducting slab interacting with the mantle beneath Medicine Lake volcano. The group 6 REE pattern is parallel to the pattern of normal‐type MORB, indicating a similar although perhaps more depleted mantle source. The location of Medicine Lake volcano in an extensional environment behind the volcanic front facilitates the rise of mantle‐derived melts. Modification of the primitive group 6 basalt to more evolved compositions takes place in the upper crust by processes involving fractional crystallization and assimilation. The group 1 calc‐alkaline Giant Crater basaltic andesite produced by these processes is similar to other Cascade basaltic andesites, implying that a similar high‐alumina basalt may be parental.

Basaltic melt evolution of the Hengill Volcanic System, SW Iceland, and evidence for clinopyroxene assimilation in primitive tholeiitic magmas

The thick oceanic crust of Iceland is formed by tholeiitic central volcanoes arranged in en echelon patterns along the 40–50 km wide rift zones. The Hengill central volcano in the southwestern rift zone has produced 25–30 km3 of hyaloclastites and lava during the last 0.11 m.y., with maximum productivity during the isostatic rebound following the degalciations 0.13 and 0.01 m.y. ago. The eruption units cover a compositional spectrum from picrite to rhyolite, but the volume of andesite, dacite, and rhyolite is small. The petrographic relations of pillow rim and hyaloclastite glass indicate that the basaltic melts were saturated with olivine and plagioclase, except for the most primitive ones that were undersaturated with plagioclase. Saturation with clinopyroxene was reached in some of the intermediate and evolved basaltic melts. Corroded and partly resorbed crystals of clinopyroxene and partly disintegrated gabbro nodules with resorbed clinopyroxene indicate that selective assimilation contributed to the evolution of the most primitive melts. The intermediate and evolved basaltic glass compositions fall along the low‐pressure cotectic for mid‐ocean ridge basalt (MORB) compositions saturated with olivine, plagioclase, and clinopyroxene, but the primitive glasses (9–9.7 wt % MgO) fall well inside the low‐pressure olivine + plagioclase primary phase volume. The primitive Hengill glasses have significantly higher CaO and lower Al2O3 than primitive glasses from oceanic spreading centers. Their low pressure undersaturation with respect to clinopyroxene and the absence of clinopyroxene phenocrysts indicate that they are not parental to the intermediate Hengill basalts, since fractionation modelling requires a large proportion of clinopyroxene in the fractionating assemblage. The most primitive melts could be produced by fractionation of olivine and plagioclase combined with 5–30 % assimilation of clinopyroxene, and the intermediate melts could be derived by mainly olivine and plagioclase fractionation, from melts equilibrated with peridotitic residues at pressures of 1–2 GPa. The further evolution of the basaltic spectrum can be explained by fractionation of olivine, plagioclase, and clinopyroxene combined with minor contamination by anatectic crustal melts. The rate of magma supply from the mantle to the crust is controlled by the isostatic conditions and is very low in periods of glacial loading of the crust. This leads to infrequent and small eruptions of dominantly evolved magmas. The dense picritic magmas (9–9.7 wt % MgO in the glass phase) were driven to the surface by magmatic overpressure in the mantle at an early deglaciation stage characterized by the absence of large, trapping magma chambers in the lower crust. The assimilation of clinopyroxene in these melts could proceed by direct contact with the solidified cumulate sequences and gabbro intrusions. Clinopyroxene assimilation in combination with olivine fractionation may also contribute to the chemical evolution of some of the most primitive MORB magmas.

Comments on ?Composition and depth of origin of primary mid-ocean ridge basalts? by D.C. Presnall and J.D. Hoover

Presnall and Hoover (1984), in their evaluation of the nature of primary mid-ocean ridge basalts (MORBs), concluded that earlier data suggesting many of the most primitive MORBs are derived from high-MgO (picritic) magmas are not valid. In reaching this conclusion, Presnall and Hoover (1984) ignore several important considerations regarding 1. analytical uncertainties, 2. TiO2abundances in MORBs, 3. least-squares calculations, 4. complex models of magmatic evolution and 5. enstatite saturation. I briefly discuss these points below.

Composition of the Samail ocean crust

The plutonic rocks of the Samail ophiolite in the southeastern Oman Mountains formed in a spreading-ridge magma chamber that was repeatedly replenished by primitive magma. An average composition of the parent magma for the Samail ocean crust is calculated by mass balance of the igneous stratigraphy. The average parent magma was in equilibrium with most early-formed cumulus minerals from the ophiolite and is very similar to primitive mid-ocean ridge basalt. The magma is basaltic—in contrast to the picritic melts proposed as parents for some ophiolites. Although the average parent magma composition is basaltic, field evidence suggests that individual batches of magma that fed the magma chamber varied in composition because of mantle processes. The primary mantle melts were probably picritic, but owing to reaction during ascent through the upper mantle and mixing or density entrapment within an open-system crustal-level magma chamber, these picritic melts were probably not erupted on the seafloor.

Gabbroic rocks from the Mathematician Ridge failed rift

Plutonic rocks from the ocean crust provide direct evidence on the petrological processes leading to its formation and evolution. The nature of such rocks is important for interpreting the origin of on-land ophiolites. So far, oceanic plutonic rocks have been mainly recovered from slow-spreading ridges and their associated fractures1–13 and from trenches14,15. Here we report the results of petrological studies of a suite of gabbros and related plutonic and volcanic rocks dredged from the Mathematician Ridge (MR), which was a fast-spreading ridge (43 mm yr−1) until it was abandoned ∼3.5 Myr ago16. Textures and mineral chemistry of the rocks together with structural considerations indicate that before abandonment the MR produced fractionated mid-ocean ridge basalt (MORB) and fractionated gabbros. The gradual decrease of spreading rate during abandonment resulted in morphological change, eruption of more primitive MORB and possibly tectonic emplacement of the older ferrogabbros. Alternatively, the latter could have been tectonically emplaced after abandonment.

Composition of basaltic liquids generated from a partially depleted lherzolite at 9 kbar pressure

Two contrasting views exist regarding the nature and origin of mid-ocean ridge basalts (MORB): first, that these olivine-tholeiites with Mg/(Mg = Fe) = 0.70–0.72 are primary and are generated by partial fusion of plagioclase + spinel lherzolite at around 30-km (≈9 kbar pressure) in the mantle1. Others2–8 suggest that even the most primitive MORBs are not primary but are derived by fractionation of olivine±pyroxene from picritic parental magmas that are generated at depths of 45 km or more. Duncan and Green2,3 argued for the existence of a second primary magma, a high-MgO quartz-normative or olivine-poor tholeiite, which is produced at depths ≤25 km beneath the ocean ridges. This problem, concerning the composition and depth of origin of primary MORBS, has been approached here by experimentally melting a plagioclase+spinel lherzolite xenolith from Hawaii at 9 kbar pressure. The experimental melts are very similar to some mid-ocean ridge basalts, which suggests that the latter are probably primary and are generated at shallow depths in the mantle.

Chemical characteristics of island-arc basalts: Implications for mantle sources

Major-element, trace-element and isotopic compositions of approximately 1200 basalts (< 53 wt. % SiO 2) from intra-oceanic island arcs have been compiled to assess the nature and possible sources of primitive island-arc basalts (IAB). The chemical characteristics of IAB are examined with reference to those of mid-ocean ridge basalts (MORB) and intraplate oceanic basalts (IPB). Major-element compositions of primitive [ Mg ( Mg +Fe 2+) > 65 ] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al 2O 3, lower TiO 2 or a lack of Fe enrichment compared to primitive MORB but many do have greater K 2O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher P H 2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB. IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic. Island-arc lavas contain greater concentrations of volatiles and have higher CO 2 H 2 O and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with 3 He 4 He data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust. Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher 87 Sr 86 Sr in IAB compared to oceanic lavas with similar 143 Nd 144 Nd values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB.

A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis

Samples of a primitive mid-ocean ridge basalt (MORB) glass were encapsulated in a mixture of ol (Fo90) and opx (En90) and melted at 10, 15, and 20 kbar. After quenching, the basaltic glass was present as a pool within the ol+opx capsule, but its composition had changed so that it was saturated with ol and opx at the conditions of the experiment. By analyzing the quenched liquid, the location of the ol+opx cotectic in the complex, multicomponent system relevant to MORB genesis was determined.

Basalts as probes of planetary interiors: Constraints on the chemistry and mineralogy of their source regions

Basalt magmas, derived by the partial melting of planetary interiors, have compositions that reflect the pre-accretionary history of the material from which the planet formed, the planets, subsequent evolutionary history, the chemistry and mineralogy of the source regions, and the intensive thermodynamic parameters operating at the source and emplacement sites. Studies of basalt suites from the Earth, its Moon, and the eucrite parent body reveal compositional differences intrinsic to their source regions which are, in turn, a characteristic of the planet and its formational and evolutionary history. Major interplanetary differences are observed in iron, Mg (Mg + Fe 2+) , TiO 2, Al 2O 3, Na 2O, Cr, Ni, and in volatile element abundances. The most primitive mare basalts have Mg#s ∼ 0.6, on the Earth they are 0.70–0.72 for mid-ocean ridge basalts (MORBs) and up to 0.9 for Archean peridotitic komatiites. Eucrites have Mg#s approaching 0.5 (excepting Binda). These differences reflect inherent differences in Mg (Mg + Fe 2+) of their sources. Striking differences in the TiO 2 abundances of the planetary basalts reflect both inter- and intra-planetary variations in source chemistry. Primitive MORBs and primitive oceanic intraplate tholeiites have a factor of 2–3 difference in TiO 2 at comparable Mg# (0.7–1.2 vs 2–3 wt.% respectively). Three major titania groups are recognized in the mare suite; high TiO 2 (8–13 wt.%), low TiO 2 (2–5 wt.%) and very low TiO 2 (<1 wt.%). The eucrites have TiO 2 contents <1 wt.%. The mare basalts and eucrites have pronounced Na 2O depletion relative to all terrestrial basalts. This is a consequence of the preplanetary accretion loss of volatiles from the material that formed the Moon and the eucrite parent bodies. Mare basalts have consistently lower Al 2O 3 contents than the terrestrial basalts. This may be due either to an inherently lower content of Al 2O 3 in the mare sources or it may reflect Al 2O 3 retention in an aluminous phase. The transition metals are fractionated in all three basalt suites. For terrestrial basalts this may reflect core-separation; however, in the case of the Moon and eucrite parent bodies pre-accretionary separation of metal and silicates is a more reasonable explanation. A pronounced Cr anomaly is observed in terrestrial MORBs but not in the mare basalts. This appears to be related to f O 2 differences in the respective mantles. Overall rare earth element abundances are comparable between all three objects. Mare basalts have a pronounced negative Eu anomaly which is inherited from their source region and is record of plagioclase removal from crystallizing magma ocean early in lunar history (4.4–4.6 Ga). Early separation of plagioclase on the Earth appears to have been a relatively unimportant process.

Geochemical characteristics of mid-ocean ridge basalts

Major and trace element (Rb, Sr, Y, Zr, Nb, Ba, Sc, Ni, Co, Cr, V) data are presented on ten mid-ocean ridge basalts (MORB), together with a basalt sample from the offshore slope of the Yap Trench and a basalt from the Tertiary ophiolite complex of eastern Taiwan. Rare earth element (REE) and Sr isotope data are presented on eight of the samples, together with REE data on U.S.G.S. standard rock, BHVO-1. A strong correlation exists between percent TiO 2 (proportional to amount of melting) and Al 2 O 3 /TiO 2 , CaO/TiO 2 ratios of these close to primary MORB. These ratios increase up to a maximum (about 20 and 17 respectively) as TiO 2 decreases, indicating a progressive release of Al and Ca from the mantle source. The limiting values of the ratios are close to chondritic and are reached at about 0.8% TiO 2 . At this high degree of melting, the major Al- and Ca-bearing mineral phases are eliminated from the mantle residue. Model calculations on the major element data indicate that MORB with 0.7% and 1.5% TiO 2 could represent about 25 and 15% melting (respectively). This modelling is consistent with the abundances of the first transition series metals (Sc to Ni), which are also interpretable in terms of degree of partial melting and residual mineralogy. Based on the REE data, the samples can be divided into depleted types (typical of “normal” MORB), and enriched types (the so-called “plume” and “transitional” types). Despite the range of (La/Sm) N in these samples (0.38–1.97) the Ti/Zr ratios remain close to chondritic (about 110) indicating that these two elements have a similar degree of incompatibility at this level of melting. Chondritic normalized patterns for typical “normal” (N)- and “plume” (P)-type MORB are presented for the REE and K, Nb, U, Th, Ba, Rb and Cs and it is suggested that this represents an increasing order of element incompatibility in MORB. The behaviour of Y, Ti, Zr, P and Sr relative to the REE is also discussed and it is shown that for most primitive MORB, Y = Ho, Ti = Eu, Zr = Sm, P = Nd and Sr is between Ce and Nd. In addition the Zr/Nb ratio correlates with the La/Sm ratio. This element correlation can be used to predict the general shape of REE patterns. The Sr isotope data are discussed in terms of already published data from the three oceans. It is suggested that P-type MORB originate from depleted mantle sources which only recently (say ≤300 m.y.) were added to by an incompatible-element-rich phase. Various models proposed to explain both the depletion in N-type, enrichment in the P-type MORB sources and the proposed mantle heterogeneity are discussed. It is concluded that mantle depletion is due to a continuing process rather than an episodic event in the early Archaean or 1.6 b.y. ago.

Petrogenesis of Archaean ultrabasic and basic volcanics: Evidence from rare earth elements

Rare earth element (REE) and major element data are presented on 44 Archaean samples which include spinifex textured ultramagnesian lavas (STPK) spinifex textured basalts (STB) and low MgO tholeiites. The samples come from the Yilgarn and Pilbara Blocks (W. Australia), Barberton (South Africa), Belingwe and Que Que (Rhodesia), Abitibi (Canada) and the 3.7 b.y. Isua Belt of Western Greenland. In addition REE data are given on three near primitive mid-ocean ridge basalts (MORB) and a glassy MORB-type basalt from Taiwan. We suggest that REE patterns, particularly the light REE and Eu, can be affected by metamorphism, but argue that the consistency of pattern from samples both within and between areas enables recognition of primary patterns. La/Sm ratios of 2.7 b.y. STPK are characterised by being lower than those of associated basalts. The 3.5 b.y. STPK Barberton material does not show this feature but instead displays significant heavy REE depletion. The separation of garnet from these liquids is suggested as a possible mechanism for the high CaO/Al2O3 ratios, (Al loss) and the heavy REE and Sc depletion. The REE data on Barberton material is equivocal on the derivation of the so-called basaltic komatiites from the peridotitic komatiites. However, REE analyses on STPK and high magnesian lavas from elsewhere suggests that crystal fractionation is not a viable mechanism to produce one from the other. We suggest instead, that varying amounts of partial melting of different sources is responsible for the spectrum of compositions. The STB appear to be an easily recognised rock type within the Archaean. They are characterised by quench (clinopyroxene) textures and a light REE enriched pattern. It is suggested that these are near primary melts and that their REE patterns mirror their mantle source. We propose a two stage model for the 2.7 b.y. mafic complexes, in which, prior to the generation of ultrabasic magmas, the source underwent a small amount of partial melting which resulted in the removal of a melt enriched in incompatible elements. The depletion process could be achieved either during mantle diapirism or by upward migration of interstitial melts into an Archaean low velocity zone. The spread of La/Sm ratios in STPK and STB is used as an argument that the Archaean mantle was chemically heterogeneous and that the degree of heterogeneity was similar to that observed in modern ocean volcanics. As a result, partial melting of the mantle under different P-T conditions produced a spectrum of magma types. The information presently available on Archaean mafic and silicic magmatism and the incompleteness of geochemical data on present day tectonic environments are two major obstacles in formulating Archaean tectonic models. In addition a comparison of present day and Archaean ultramafic and silicic rocks suggests that plate tectonic models as presently understood may not be suitable analogues for all Archaean tectonic environments.

Geochemical regularities and genetic significance of ophiolitic basalts

Al2O3/TiO2 and CaO/TiO2 ratios of primitive mid-ocean ridge basalts increase along with degree of partial melting (TiO2 content decreases) until ratios seen in chondrites (about 20 and 17, respectively) are reached at ?0.8% TiO2. Low-Ti (<0.6%) basalts from ophiolitic complexes (for example, Troodos and Betts Cove), island arcs, and interarc basins are characterized by much higher Al2O3/TiO2 and CaO/TiO2 ratios (up to 60). Such low-Ti ophiolitic basalts could be formed by remelting of a severely depleted source that had experienced a previous episode of magma extraction of the type in mid-ocean ridges. We suggest that these low-Ti basalts are not formed at mid-ocean ridges but are products of a spreading center close to a subduction zone, for example, in an interarc basin or incipient island-arc site. In this model, remelting of the refractory source is induced by the introduction of water from subducted oceanic crust. Geochemical data on high-Mg volcanic rocks from Papua and the Mariana Trench are presented, and an analogy with low-Ti ophiolitic basalts is suggested. “Basaltic komatiites” from ophiolitic complexes are discussed, and it is suggested that there are critical geochemical differences between these and Archean komatiites.