Basalt Chemistry Research Articles

A correlation between mid-ocean-ridge basalt chemistry and distance to continents.

To fully understand the structure and dynamics of the Earth's convecting mantle, the origins of temperature variations within the mantle need to be resolved. Different hypotheses have been proposed to account for these temperature variations: for example, heat coming from the decay of radioactive elements or heat flowing out of the Earth's core. In addition, theoretical studies suggest that the thermal properties of continental masses can affect mantle convection, but quantitative data that could allow us to test these models are scarce. To address this latter problem, we have examined the chemistry of mid-ocean-ridge basalt--which reflects the temperature of the source mantle--as a function of the distance of the ridge from the closest continental margin. No correlation is observed for oceanic ridges close to subduction zones or hotspots; subduction zones probably inhibit thermal transfer between the mantle beneath continents and ocean, whereas hotspots influence the major-element chemistry of ridge basalts, which makes their interpretation with respect to mantle temperature more difficult. However, we do observe a significant correlation for mid-oceanic basalts from the Atlantic and Indian oceans. From this, we conclude that the location of continental masses relative to active ridges influences the large-scale thermal structure of the mantle and we estimate that the mantle cools by 0.05 to 0.1 degrees C per kilometre from the continental margins.

Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island

The effects of source composition and source evolution during progressive partial melting on the chemistry of mantle-derived mid-ocean ridge basalt (MORB) melts were tested using a comprehensive geochemical and Sr–Nd–Pb isotopic dataset for fresh, magnesian basaltic glasses from the Miocene Macquarie Island ophiolite, SW Pacific. These glasses: (1) exhibit clear parent–daughter relationships; (2) allow simple reconstruction of primary melt compositions; (3) show exceptional compositional diversity (e.g. K2O/TiO2 0·09–0·9; La/Yb 1·5–22; 206Pb/204Pb 18·70–19·52); (4) preserve changes in major element and isotope compositions, which are correlated with the degree of trace element enrichment (e.g. La/Sm). Conventional models for MORB genesis invoke melting of mantle that is heterogeneous on a small scale, followed by binary mixing of variably lithophile element-enriched melt batches. This type of model fails to explain the compositions of the Macquarie Island glasses, principally because incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pb isotope ratios vary non-systematically with the degree of enrichment. We propose that individual melt batches are produced from instantaneous ‘parental’ mantle parageneses, which change continuously as melting and melt extraction proceeds. This concept of a ‘dynamic source’ combines the models of small-scale mantle heterogeneities and fractional melting. A dynamic source is an assemblage of locally equilibrated mantle solids and a related melt fraction. Common MORB magmas that integrate the characteristics of numerous melt batches therefore tend to conceal the chemical and isotopic identity of a dynamic source. This study shows that isotope ratios of poorly mixed MORB melts are a complex function of the dynamic source evolution, and that the range in isotope ratios within a single MORB suite does not necessarily require mixing of diverse components.

Probing the Pacific’s oldest MORB glass: mantle chemistry and melting conditions during the birth of the Pacific Plate

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma. Glass recovered from Jurassic age (170 Ma) Pacific ocean crust [Bartolini and Larson, Geology 29 (2001) 735–738] at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60°C hotter than today. Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma. Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs [Humler et al., Earth Planet. Sci. Lett. 173 (1999) 7–23], which may have been a consequence of the Cretaceous superplume event in the Pacific. Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.

Volcanic and geodynamic evolution of the Bouvet triple junction: Evidence from basalt chemistry

Volcanic and geodynamic evolution of the Bouvet triple junction: Evidence from basalt chemistry

Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source, Mt. Erciyes Stratovolcano, Turkey

The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe2O3, MgO, CaO, low K2O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low 87Sr/86Sr (0.703344-0.703964), and high 143Nd/144Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high 87Sr/86Sr (0.704591-0.70507) and low 143Nd/144Nd (0.51272-0.512394) isotopic ratios. The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %. The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.

Author Index

Anorthite megacrysts from twelve volcanic centers of the Kurile Island Arc were analyzed by the ion microprobe for concentrations of: Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. These 1–2-cm megacrysts are mainly found as early phenocrysts in basalts, but are also present in andesites, rhyolites and hybrid rocks from almost every studied volcanic center, both subaerial and submarine. Electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned, whereas rims often result from overgrowth of sodium-richer plagioclase in equilibrium with its host, silica-richer melt. The cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a `window' into an earlier basaltic stage of magmatic evolution. Petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second (after olivine) liquidus phase in high-Al basalts regardless of varying K2O contents across the arc.Given the narrow compositional range of anorthite megacrysts, the across-arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc. The concentration of K in megacrysts increases linearly by a factor of 11; Ba, 13; Sr, Eu, 4; Be, Ti, P, 3; Li, 2, REE, 5–2; whereas concentrations of Pb, B and halogens and their ratios to HFSE either remain constant or decrease with deepening of the Benioff Zone from 110 to 220 km. We used ion microprobe-derived plagioclase–basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values. The reconstructed concentrations of several key trace elements and their correlation with Benioff Zone depth are consistent with whole-rock basalt data, but all give systematically more primitive compositions. Therefore, anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing. They can be used to assess delicate variations in primary basaltic magma chemistry as a function of Benioff Zone depth. We also report new results for Sr, Nd and Pb isotopes for anorthite-bearing rocks and other rocks from the same volcanoes. Across-arc zoning with respect to trace element concentrations, ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes: trace element transfer by fluids, variation in degree of partial melting and progressive change of the melting source.

Trace and REE content of clinopyroxenes from supra-subduction zone peridotites. Implications for melting and enrichment processes in island arcs

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (<1% modal cpx) and the cpxs have extremely low concentrations in Ti (30–150 ppm) and heavy REE. The light and middle REE, Zr and Sr contents of the cpxs show enrichments compared to cpx from abyssal peridotites. Our cpx data falls in the field of previous studies on modern arc peridotites ([Parkinson, I.J., Pearce, J.A., Thirwall, M.F., Johnson, K.T.M., Ingram, G., 1992. Trace element geochemistry of peridotites from the Izu–Bonin–Mariana forearc, Leg 125. In: Fryer, P., Pearce, J.A., Stokking, L.B. (Eds.), Proc. Ocean Drilling Program. pp. 487–506; Bonatti, unpubl.]) suggesting that these complexes originated above a subduction zone. Dry melting of an upper mantle source similar to a mid-ocean ridge basalt (MORB) source (e.g., [Johnson, K.T.M., Dick, H.J.B., Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. J Geophys. Res. 95, 2661–2678.]) cannot generate the extreme depletions in Ti and heavy rare earth element (HREE) seen in the cpxs from these supra-subduction zone (SSZ) peridotites. We propose that these high degrees of depletion can be achieved during hydrous melting of a MORB-depleted peridotite above a subduction zone. The melting rate of cpx during hydrous melting is expected to be less than in dry melting, while the rate of orthopyroxene (opx) consumption increases. These changes in melting modes allow for larger degrees of total melting and larger depletions in Ti and HREE in the cpx before cpx is exhausted from the residual peridotite. The enrichments in light rare earth element (LREE), MREE, Zr and Sr are best modeled by a constant flux of a slab-derived fluid component to the mantle wedge during melting. Ti and the HREE are not mobile in this fluid and are not enriched in the mantle peridotite. The continuous flux is necessary to sustain hydrous melting in the sub-arc mantle during the complete melting range. The calculated fluid composition matches well previous estimates based on the arc basalt chemistry ([McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett. 102, 358–374; Stolper, E., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett. 121, 293–325.]) and experiments on fluids ([Ayers, J.C., Dittmer, S.K., Graham, D.L., 1997. Partitioning of elements between peridotite and H 2O at 2.0–3.0 GPa and 900–1100°C, and application to models of subduction zone processes. Earth Planet. Sci. Lett. 150, 381–398; Ayers, J.C., 1998. Trace element modeling of aqueous fluid–peridotite interaction in the mantle wedge of subduction zones. Contrib. Mineral. Petrol. 132, 390–404.]). The resulting melts from our model closely resemble melts from an arc environment (boninites) further arguing for the plausibility of this model.

A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

Basement outcrops sampled by submersible and dredge from the inner slope of the Izu–Bonin trench at 32°N and 6200–6700 m water depth have a distinct mid-ocean ridge basalt (MORB) chemistry unlike any other rocks previously sampled in the Izu–Bonin arc. They are low K tholeiites with moderate TiO 2 (0.7–1.8 wt%), extremely low Ba (1.5–7 parts per million), low Ba/La (1.2–3) and are depleted in light rare-earth elements. These samples could represent either an accreted piece of subducting Pacific plate or a trapped remnant of Philippine Sea plate on which the Izu–Bonin arc was built. Although their major and trace element chemistry do not help to distinguish their source, the Sr, Nd and Pb isotopes clearly support a Philippine Sea plate origin. The isotopic signature of the inner trench slope samples matches that of Philippine Sea plate lavas, with 87Sr/ 86Sr=0.70321–0.70373, 143Nd/ 144Nd=0.513057–0.513077 and 206Pb/ 204Pb=18.2–18.5. The samples have elevated 207Pb/ 204Pb (15.3–15.5) and 208Pb/ 204Pb (38.0–38.2) values compared to the Northern Hemisphere reference line (NHRL) and their isotopic signature is distinct from the Mesozoic Pacific MORB being subducted. These are the first samples of trapped Philippine Sea oceanic crust discovered in the Izu–Bonin–Mariana arc. They require that models for the formation of intra-oceanic arc crust account for pre-existing oceanic crust and that estimates of arc magma production rates are lowered accordingly.

Trace elements in anorthite megacrysts from the Kurile Island Arc: a window to across-arc geochemical variations in magma compositions

Anorthite megacrysts from twelve volcanic centers of the Kurile Island Arc were analyzed by the ion microprobe for concentrations of: Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. These 1–2-cm megacrysts are mainly found as early phenocrysts in basalts, but are also present in andesites, rhyolites and hybrid rocks from almost every studied volcanic center, both subaerial and submarine. Electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned, whereas rims often result from overgrowth of sodium-richer plagioclase in equilibrium with its host, silica-richer melt. The cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a `window' into an earlier basaltic stage of magmatic evolution. Petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second (after olivine) liquidus phase in high-Al basalts regardless of varying K 2O contents across the arc. Given the narrow compositional range of anorthite megacrysts, the across-arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc. The concentration of K in megacrysts increases linearly by a factor of 11; Ba, 13; Sr, Eu, 4; Be, Ti, P, 3; Li, 2, REE, 5–2; whereas concentrations of Pb, B and halogens and their ratios to HFSE either remain constant or decrease with deepening of the Benioff Zone from 110 to 220 km. We used ion microprobe-derived plagioclase–basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values. The reconstructed concentrations of several key trace elements and their correlation with Benioff Zone depth are consistent with whole-rock basalt data, but all give systematically more primitive compositions. Therefore, anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing. They can be used to assess delicate variations in primary basaltic magma chemistry as a function of Benioff Zone depth. We also report new results for Sr, Nd and Pb isotopes for anorthite-bearing rocks and other rocks from the same volcanoes. Across-arc zoning with respect to trace element concentrations, ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes: trace element transfer by fluids, variation in degree of partial melting and progressive change of the melting source.

Enigmatic extinct spreading center in the West Philippine backarc basin unveiled

Research Article| December 01, 1999 Enigmatic extinct spreading center in the West Philippine backarc basin unveiled Kantaro Fujioka; Kantaro Fujioka 1Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan Search for other works by this author on: GSW Google Scholar Kyoko Okino; Kyoko Okino 2Hydrographic Department of Japan, Tsukiji, Tokyo 104-0045, Japan Search for other works by this author on: GSW Google Scholar Toshiya Kanamatsu; Toshiya Kanamatsu 1Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan Search for other works by this author on: GSW Google Scholar Yasuhiko Ohara; Yasuhiko Ohara 2Hydrographic Department of Japan, Tsukiji, Tokyo 104-0045, Japan Search for other works by this author on: GSW Google Scholar Osamu Ishizuka; Osamu Ishizuka 3Geological Survey of Japan, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Saturu Haraguchi; Saturu Haraguchi 4Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan Search for other works by this author on: GSW Google Scholar Teruaki Ishii Teruaki Ishii 4Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (12): 1135–1138. https://doi.org/10.1130/0091-7613(1999)027<1135:EESCIT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kantaro Fujioka, Kyoko Okino, Toshiya Kanamatsu, Yasuhiko Ohara, Osamu Ishizuka, Saturu Haraguchi, Teruaki Ishii; Enigmatic extinct spreading center in the West Philippine backarc basin unveiled. Geology 1999;; 27 (12): 1135–1138. doi: https://doi.org/10.1130/0091-7613(1999)027<1135:EESCIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Central Basin fault in the center of the West Philippine Basin was first discovered ∼50 yr ago. It is a 1000-km-long ridge oriented northwest to southeast and is cut by north-south–trending fracture zones. Hypotheses about the origin and development of the Central Basin fault have remained unresolved until recently. Submersible observations and SeaBeam surveys show that the Central Basin fault is a segmented spreading ridge having a morphology similar to that of a slow spreading ridge, with a nontransform offset, a nodal deep, and an inside corner high. The distance from the ridge versus the depth of the sea floor, the obliqueness of sets of small trough and ridge structures, and heat-flow values both of the crestal and off-axis areas of the Central Basin fault suggest that the fault is not a simple spreading center, but rather underwent multiple spreading episodes. The texture and chemistry of basalts obtained from the ridge suggest that the lavas were formed in a backarc basin setting. These data confirm that the Central Basin fault is a slow backarc spreading center that has a more complicated evolutionary history than previously realized. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evolution of the basalts from three back-arc basins of southwest Pacific

Petrography, petrochemistry, and mineral chemistry of basalts from the Woodlark, Manus, and Lau basins from the southwest Pacific, have been studied to understand their magmatic evolution. Basalt from the western Woodlark Basin (Dobu Seamount) indicates mixing of a near-primitive magma with fractionated basaltic melt in shallow magma chambers. Basalts from Manus Basin and Central Lau Spreading Center (Lau Basin) are typical N-MORBs, and they exhibit olivine fractionation under high oxidizing conditions while basalts from Mangatolu Triple Junction (Lau Basin) are enriched in Al2O3, K2O, and Zr indicative of a contribution from a subducting plate.

Tectonostratigraphy of the Glen App area, Southern Uplands, Scotland: anatomy of an Ordovician accretionary complex

The stratigraphy and structure of the Glen App coastal area, northwestern Southern Uplands, are characterized by structural repetition of both chaotic and coherent units of the Middle Ordovician (Caradocian) Tappins Group. The chaotic unit, the Currarie Formation, comprises blocks, including basalt, chert and terrigenous sedimentary rocks, set in folded and in places sheared red to grey siliceous mudstone. The coherent unit is turbiditic, and is divided into the Dalreoch Formation in the north and the Corsewall Formation in the south. The former is characterized by inclined, isoclinal folds and thrusts, whereas the latter shows a homoclinal dip, although both are largely overturned northwards. Small-scale duplexes are observed within the coherent turbidites of the Dalreoch Formation and chaotic units. Larger duplexes, identified on the basis of mapping, are oriented at an oblique angle to the general trend of regional structures. The chemistry of basalt blocks within the Currarie Formation shows a within-plate type, suggesting an origin as seamounts. The tectono-stratigraphy of the area is interpreted as the result of accretion of a substantial seamount at a subduction zone during Mid-Ordovician time. Thrust interleaving of chaotic and coherent units is inferred to have taken place in and above the décollement zone, while thick turbidite beds without strong folding were probably preserved below the décollement and emplaced below a large-scale nappe. Interpretation of the Glen App area as part of an accretionary complex confirms that an ocean basin remained open in part of the Iapetus system, at least until Caradocian times.

K–Ar ages from seamount chains in the back‐arc region of the Izu–Ogasawara arc

The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back‐arc extensional zone (back‐arc knolls zone), and a series of across‐arc seamount chains that cross the extensional zone in an east‐northeast and west‐southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across‐arc seamount chains and extension‐related edifices in the back‐arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc‐alkaline andesitic lavas. The youngest rocks (&lt; 2.8 Ma), characterized by cpx‐ol basalt, occur along the western margin of the back‐arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na2O (&gt; 2.0 wt%), Zr (&gt; 50 p.p.m.) and Y (&gt; 20 p.p.m.) and low CaO (&lt; 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na2O (&lt; 1.8 wt%), Zr (&lt; 50 p.p.m.) and Y (&lt; 20 p.p.m.), but significantly higher CaO (&gt; 12 wt%). The age inferred for the initiation of back‐arc rifting (∼ 2.35–2.9 Ma: Taylor 1992) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low‐Na2O and high‐CaO basalt). This implies that post‐2.8 Ma volcanism in the back‐arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.

Erratum to “Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid”: [Earth Planet. Sci. Lett. 153 (1997) 37–55

Erratum to “Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid”: [Earth Planet. Sci. Lett. 153 (1997) 37–55

Unusual upper mantle beneath Guaniamo, Guyana shield, Venezuela: Evidence from diamond inclusions

The geochemistry of mineral inclusions in diamond is an important source of information about the composition of the continental lithospheric mantle at depths exceeding 120–150 km. At these depths, two main types of geochemical environment support diamond formation; they are ultramafic (or peridotitic) (U-type) and eclogitic (E-type) environments as shown by minerals that occur as inclusions in diamonds. In primary diamond-bearing kimberlite or lamproite rock the ratio of diamonds from these two geochemical environments varies widely between localities. The U-type environment dominates for the majority of diamond occurrences worldwide, whereas the E-type environment dominates for a very limited number of localities in South Africa, North America, and Australia. The present study shows that an uncommonly high percentage (99.4%) of E-type diamonds with extremely variable inclusion assemblages is found in kimberlites in the northwestern part of the Guyana shield at Guaniamo, Venezuela. These variations range from previously unknown silica-undersaturated corundum eclogite to abundant silica-rich coesite eclogite assemblages representing 22% of all E-type diamonds, some of which contain syngenetic ilmenite and magnetite. The compositional variations of garnet and omphacite inclusions are extremely broad. The wide variability of the eclogitic source in which diamonds formed beneath the Guyana shield indicates a broadly basaltic chemistry in this environment, which may represent ancient subducted oceanic crust. Such specific features have not been reported for any other diamond occurrence and reflect an unusual composition of the deep lithospheric mantle in this area.

Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid

The major-element composition of Mid-Ocean Ridge basalts (MORB) shows geochemical variations controlled by the temperature, pressure and source composition produced by convection in the mantle beneath the spreading centers. The long-wavelength signal of the geoid reflects deep mantle processes. In this paper, we characterize the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths to determine the extent to which the major-element composition of the MORB reflects deep mantle circulation. The chemical effects of the low-pressure fractionation have been corrected using a new method based on the FeO t/MgO ratio. The fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the MgO denominator converge to a common point with ∼8% MgO. The separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99% level with the axial depth for both the Mid-Atlantic Ridge (MAR) and the East Pacific Ridge (EPR) and with the geoid only for the MAR. The latter correlation suggests that the major-element composition of the MORB is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle. The lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the K 2O/TiO 2 ratio and the geoid indicates that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle.

Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust1,2,3. Recent studies4,5,6, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8 Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

Mantle Melting and Melt Extraction Processes beneath Ocean Ridges: Evidence from Abyssal Peridotites

This paper presents the results of the first quantitative petrological modelling of abyssal peridotites. The mantle beneath a ridge may be considered as two regions: (1) the melting region between the solidus (Po) at which upwelling mantle begins to melt, and the depth (Pf) at which melting stops because of conductive cooling to the surface; (2) the thermal boundary layer between Pf and the base of crust. In the melting region, decompression near-fractional melting is characterized by the reaction aCpx + bOpx + cSpl = dOl + lMelt, i.e. clinopyroxene, orthopyroxene and spinel melt whereas olivine crystallizes as melting proceeds. In much of the pressure range (Po≤25 kbar), orthopyroxene contributes more than clinopyroxene to the melt (i.e. b>a) during decompression melting, which is unexpected from isobaric melting experiments, but is constrained by the incongruent melting of Opx⇒Ol + SiO2 with decreasing pressure. The melting reaction also explains the so-called local trend of mid-ocean ridge basalt (MORB) chemistry characteristic of slow-spreading ridges. Melts produced over a wide region and depth range in the mantle will ascend and migrate laterally towards the axial zone of crustal accretion. These melts cool and crystallize olivine as they pass through previously depleted residues in the thermal boundary layer. This explains why abyssal peridotites have excess olivine relative to simple melting residues. The greater the ambient extent of mantle melting, the more melt is produced in the mantle. Thus, greater extents of melting lead to more olivine (up to 50% of the rock mass in abyssal peridotites) crystallization at shallow levels. Additional important implications are: (1) neither MORB melts nor the bulk igneous crust is compositionally comparable with partial melts produced in peridotite melting experiments because primary mantle melts crystallize olivine back in the mantle; (2) diffusive porous flow is the primary mode of melt migration even at very shallow levels because excess olivine is observed on thin-section scales in abyssal peridotites; (3) low-pressure melt equilibration during ascent is inevitable because the melting reaction preserved in residual peridotites requires continuous solid–liquid equilibration, and because olivine crystallization in the thermal boundary layer is the natural consequence of melt ascent and cooling; (4) perfect fractional melting is unlikely because melt porosity (a few percent?) in the melting mantle is required by the melting reaction, whole-rock major element data and other observations; (5) compositional variations of both MORB and abyssal periodites are consistent with varying extents (∼10–22%) of mantle melting beneath global ocean ridges.

Mantle Melting and Melt Extraction Processes beneath Ocean Ridges: Evidence from Abyssal Peridotites

This paper presents the results of the first quantitative petrological modelling of abyssal peridotites. The mantle beneath a ridge may be considered as two regions: (1) the melting region between the solidus (P) at which upwelling mantle begins to melt, and the depth (P) at which melting stops because of conductive cooling to the surface; (2) the thermal boundary layer between P and the base of crust. In the melting region, decompression near-fractional melting is characterized by the reaction aCpx + bOpx + cSpl = dOl + 1Melt, i.e. clinopyroxene, orthopyroxene and spinel melt whereas olivine crystallizes as melting proceeds. In much of the pressure range (P a) during decompression melting, which is unexpected from isobaric melting experiments, but is constrained by the incongruent melting of Opx⇒*Ol + SiO with decreasing pressure. The melting reaction also explains the so-called local trend of mid-ocean ridge basalt (MORB) chemistry characteristic of slow-spreading ridges. Melts produced over a wide region and depth range in the mantle will ascend and migrate laterally towards the axial zone of crustal accretion. These melts cool and crystallize olivine as they pass through previously depleted residues in the thermal boundary layer. This explains why abyssal paidotites have excess olivine relative to simple melting residues. The greater the ambient extent of mantle melting, the more melt is produced in the mantle. Thus, greater extents of melting lead to more olivine (up to 50% of the rock mass in abyssal peridotites) crystallization at shallow levels. Additional important implications we: (1) neither MORB melts nor the bulk igneous crust is compositionally comparable with partial melts produced in peridotite melting experiments because primary mantle melts crystallize olivine back in the mantle; (2) diffusive porous flow is the primary mode of melt migration even at very shallow levels because excess olivine is observed on thin-section scales in abyssal peridotites; (3) low-pressure melt equilibration during ascent is inevitable because the melting reaction preserved in residual pendotites requires continuous solid-liquid equilibration, and because olivine crystallization in the thermal boundary layer is the natural consequence of melt ascent and cooling; (4) perfect fractional melting is unlikely because melt porosity (a few percent?) in the melting mantle is required by the melting reaction, whole-rock major element data and other observations; (5) compositional variations of both MORB and abyssalperidotites are consistent with varying extents (∼10-22%) of mantle melting beneath global ocean ridges.

Petrochemistry of Jurassic and Cretaceous tholeiites from Kong Karls Land, Svalbard, and their relation to Mesozoic magmatism in the Arctic

A study of the whole-rock geochemistry and mineral chemistry of high-TiO2 Upper Jurassic and medium-TiO2 Lower Cretaceous basalts from Kong Karls Land, Svalbard, is presented. Geochemical criteria indicate that the basalts are initial rifting tholeiites with weak signs of crustal contamination. The Upper Jurassic basalts appear to be associated with the Olga Rift, part of a trans-Barents rift system which failed to link the proto-Atlantic and proto-Arctic basins. The Lower Cretaceous basalts may be more closely related to initial rifting tholeiites on Franz Josef Land and Spitsbergen generated during the rifting stage of opening of the Canada Basin. During break-up of the Barents Shelf, the sequence of magma types corresponds to the pre-, syn- and post-rifting stages established in other areas of continental break-up. Evidence for a possible hot-spot or plume trail, extending from Siberia to the Yermak Plateau over 250 Ma, is assembled.