The Petragenesis of some Archaean Volcanic Rocks from Southern Africa

Major, rare earth and transition elements, have been determinedon a selection of volcanic rocks from greenstone belts in Rhodesia(2.6 by) and South Africa (3.4 by). In Rhodesia two distinctseries can be recognized: a komatite-tholeiite series whichoccurs early in the greenstone belt evolution and apparentlygrades into a second, calc-alkaline, series at higher structurallevels. Peridotitic komatites reflect higher degrees of partialmelting than any Phanerozoic rocks so far observed and are thereforeused to place limits on the composition of their source. Atlower MgO contents they merge into low K tholeites which havesignificantly higher Ce_N/Yb_N and Rb/Sr ratios (at any MgO content)than those observed in modern ocean floor and island are environments.The calc-alkaline series is characterized by andesites whichexhibit a marked heavy REE depletion, but similar light REEand transition metal contents to the more evolved tholeiticrock types. The continuum of compositions from komatiites totholeiites and calc-alkaline andesites suggests that the bulkof greenstone belt volcanics could have been derived by differentialpartial melting, and polybaric fractionation of an essentiallyhomogeneous peridotite source. Late stage dacitic lavas andintrusions probably reflect melting of a more eclogitic sourceregion. The distinctive trace element geochemistry of Archaeanvolcanics, particularly the high Ni and low Yb values of thecalc-alkaline rocks precludes direct comparison with modernisland are associations. Rather the large decrease in liquidustemperatures (500 °C) with increasing structural heightwithin greenstone belts, coupled with the fact that most ofthe volcanics could have been derived from an essentially homogeneoussource, may suggest that greenstone belts developed in a riftingenvironment. It appears unlikely that the tholeiite/calc-alkalineassociation observed in the Archaean may be taken as an indicationof subduction at that time.     

Chemical and Isotopic Studies of Ultramafic Inclusions from the San Carlos Volcanic Field, Arizona: A Bearing on their Petrogenesis

Compositions of the principal minerals and Pb, Nd, and Sr isotopeanalyses of clinopyroxene (cpx) separates are reported for TypeI spinel peridotite xenoliths from the Peridot Mesa vent ofthe San Carlos Volcanic Field. The principal phases are in chemicalequilibrium within each inclusion. Systematic changes in mineralcomposition accompany lithological changes from fertile lherzolitesto infertile harzburgites. These changes are consistent witha fusion residue origin for the major element component of thexenoliths, as noted previously by Frey & Prinz (1978). ExcessFe is additionally present in some inclusions. Pyroxene equilibrationtemperatures calculated using the Wells (1977) geothermometerfall in the narrow range of 1022?34?C (1 s.d.). Equilibrationpressures poorly limit corresponding depths to anywhere between30 and 65 km within the lithospheric mantle. The geotherm is‘advective’ and elevated by 500?C at the depth ofsampling over a reference conductive shield geotherm. The highheat flow measured at the surface results from a combinationof extension and magmatism, with the temperature perturbationextending into the lithospheric mantle. ¹⁴³Nd/¹⁴⁴Nd ratios (0?51251–0?51367) and ⁸⁷Sr/⁸⁶Sr ratios(0?70190–0?70504) in cpx demonstrate gross isotopic heterogeneitybeneath the Peridot Mesa vent. This largely overlaps the oceanicmantle array, although four inclusions have Nd greater thanmid-ocean ridge basalts (MORB). PM-228J with _Nd = +20 is themost extreme yet reported for a spinel Iherzolite. Pb abundancesin cpx (generally <0?03ppm) are far lower than previouslyreported values. ²⁰⁶Pb/²⁰⁴Pb ratios (17?5–19?1) overlapoceanic basalts and do not correlate with ⁸⁷Sr/⁸⁶Sr ratio. However,some of the inclusions exhibit MORB-like ²⁰⁶Pb/²⁰⁴Pb ratiosbut much higher ⁸⁷Sr/⁸⁶Sr ratios, which suggests a possiblegenetic link of detached lithospheric mantle with certain oceanicislands. Metasomatic trace element enrichment processes are most widespreadin the infertile (Al-poor, Cr-rich) inclusions, as noted byFrey & Prinz (1978). This systematic relationship is a localfeature of the mantle and suggests that some degree of meltingoccurs commensurately with incompatible element addition. Inparticular, anhydrous peridotite above its volatile-presentsolidus that was flushed with C-O-H fluids containing incompatibleelements would melt and form an enriched infertile fusion residue.The ascending magmas responsible for forming Type II peridotiteveins are the most probable source of the volatiles and mayin some cases react to produce chemical gradients in the wall-rock.Prior metasomatism is also evident isotopically in some inclusions.Overall, the lithospheric mantle beneath Peridot Mesa has suffereda multi-stage history of enrichment, depletion and melting atvarious times since it became attached to the crust above.     

The Lead, Neodymium and Strontium Isotopic Structure of Ocean Ridge Basalts

Pb-, Nd- and Sr-isotope compositions and U, Pb, Sm, Nd, Rb andSr concentrations are reported for samples of basaltic glassand altered substrates from spreading centres in the Atlantic,Indian and Pacific Oceans. Correlations are shown to exist between^{208, 207, 206}Pb/²⁰⁴Pb ratios, and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios in basaltic glasses, but they are dominated by samplesfrom the Mid-Atlantic Ridge. Whereas basaltic glasses from EastPacific spreading centres exhibit smaller variability in Pb,Sr and Nd isotope compositions than Atlantic samples, seamountsamples from the E. Pacific have a similar range of Pb-isotopecompositions as Mid-Atlantic Ridge glasses. Contamination ofbasaltic magmas by altered oceanic crust or sediments is notconsidered to be of prime importance in determining the isotopicstructures of MORB glasses. It is proposed that the isotopicheterogeneity in the mantle beneath the Pacific and Atlanticis similar, but magma generation processes associated with fastspreading ridges of the East Pacific more effectively eradicateheterogeneities in the erupted basalts. Alteration of oceanic crust is further investigated with respectto the relative response of the U–Pb, Sm–Nd andRb–Sr systems, and the role of recycled oceanic crustin producing the mantle heterogeneities is evaluated.     

Petrogenetic Significance of Isotope and Trace Element Variations in Volcanic Rocks From the Mid-Atlantic

High precision ⁸⁷Sr/⁸⁶Sr analyses, together with determinationsof Rb, Sr, K₂O, Na₂O and, in a few cases, other elements, arereported for about fifty volcanic rocks (mainly basaltic) fromthe Atlantic Ocean basin. Results for dredged basalts from theReykjanes Ridge and Charlie Gibbs Fracture Zone, and an enstatite-forsteritebasalt from Kolbeinsey islet, support the general observationthat ocean-ridge tholeiites have uniformly low ⁸⁷Sr/⁸⁶Sr ratios(0.70294±4) and lithophile element contents comparedwith the most primitive basalts on ocean islands, includingthe Neovolcanic zones of Iceland, although progressive decreasein these quantities away from Iceland has not been confirmed.In contrast, the ocean island alkali basalts generally havehigher ⁸⁷Sr/⁸⁶Sr ratios (0.70334±5 for the Snaefellsnespeninsula of Iceland, 0.70343±4 for Jan Mayen, 0.70509±4for Tristan da Cunha and 0.70369±3 for Bouvetøya).The chief exception is Ascension Island, where volcanic rocksranging from alkali-olivine basalt to trachyte give a mean valueof 0.70284±4. The constancy of this ratio throughouteruptive sequences on any single island indicates that Sr-isotopecharacteristics are primary features. These variations, which are far outside analytical errors, areconsidered in the light of the geochemistry and isotope systematicsof ocean basalts in general. The implied isotopic (and lithophileelement) heterogeneities of the source regions have to be interpretedaccording to either equilibrium or disequilibrium melting models.The former, which is normally assumed, requires large-scale(domain) isotopic inhomogeneities within the mantle, which musthave existed over thousands of m.y. unless the Rb/Sr ratio ofextracted liquids is lower than that of the bulk source (aswould be the case if phlogopite were a residual phase). In thecase of disequilibrium melting, the inhomogeneities are reducedto the mineral scale, as observed in some studies of ultramaficnodules. It is shown that disequilibrium melting models couldgenerally account for the observed isotopic variations of oceanicrocks, although difficulties are again encountered unless phlogopiteis a stable residual phase. Evaluation of the relative importanceof these melting processes cannot be made at the present time.     

The Source Regions of Ocean Island Basalts

The geochemical modelling of many small-volume continental magmasshows that their source regions must have been depleted by basaltformation, and later enriched by the addition of a metasomaticmelt, formed by melting 03% of the MORB source. The presenceof such magmas throughout western Turkey and the Aegean, whereno plume is present, requires such magmas to be formed at temperaturesconsiderably below the dry solidus. Similar magmas elsewherebring up nodule suites, many of which have the same compositionas the source regions of the host magmas. Pressure and temperatureestimates from garnetbearing suites, and temperature estimatesfrom those without garnet, show that the nodules last equilibratedat pressures and temperatures close to those of the wet solidus.Magmas from the smaller oceanic islands and from some seamountsclosely resemble small-volume continental magmas, and also comefrom sources that have been metasomaticaUy enriched. However,no data sets from any of the oceanic islands that have yet beenmodelled require their source regions to have been depletedbefore being enriched The density of the sources of continentaland oceanic basalts can be obtained from their calculated modes.In the garnet peridotite stability field the sources of oceanisland basalts have densities that are slightly greater thanthat of the MORB source, whereas those of most small-volumecontinental magmas are lighter. Therefore ocean island sourcesalone are easily entrained into the thermal convection beneaththe plates. A numerical experiment shows that material in thehot and cold boundary layers of high Rayleigh number time-dependentconvection tends to remain in the boundary layers for severaloverturns, rather than moving into the interior of the circulation.A simple model that can account for the elemental and isotopiccomposition of ocean island basalts forms their sources by theaddition of metasomatic melt to the undcplcUd MORB source whileit forms the lower part of the mechanical boundary layer beneathcontinents. The isotopic differences between ocean island basaltand MORB are generated before the source becomes entrained inthe cold sinking plumes that fall to the base of the convectinglayer. At the base the material is heated and rises as partof a hot plume. Because the metasomatic melt contains waterand carbonates, the enriched regions start to melt and generatemore melt on decompression than does the MORB source. Such regionscan therefore generate islands and seamounts. Even when theenriched material moves into the interior of the circulationand acquires the mean potential temperature of the mantle, itwill still generate more melt on decompression than will theMORB source, and the isotopic and elemental composition willstill be distinctive. The model can therefore account for theobserved composition of magmas from seamounts that cannot beproduced from either the MORB or the primitive source. ^*Corresponding author     

Partial Melt Distributions from Inversion of Rare Earth Element Concentrations

Inverse theory is used to calculate the melt distribution requiredto produce the rare earth element concentrations in a wide varietyof terrestrial and extra-terrestrial magmas. The concentrationsof the major and minor elements in the source regions are assumedto be the same as those for the bulk Earth, and the peridotitemineralogy calculated from the mineral compositions by leastsquares. Rare earth element partition coefficients are thenused for inversion, assuming the melt generation is by fractionalmelting. The mean composition of the magmas is taken to be anestimate of the average composition of the melt. For n-typcand e-type MORB the results agree well with the adiabatic decompressioncalculations if the potential temperatures are 1300 and 1500?Crespectively. The major and minor element compositions calculatedfrom the melt distribution obtained from the inversion alsoagree well with those observed. The observations are consistentwith a melt fraction that increases monotonically towards thesurface, starting at 80 km and producing 9 km of melt in thecase of n-type MORB, and at 120 km to produce 23 km in thecase of e-type MORB. The inversion calculations show that the melt fractions producedbeneath an intact plate by a plume like that beneath Hawaiiare smaller, and are also in agreement with the adiabatic calculationsif the potential temperature of the plume is 1500?C. Much ofthe melt is produced in the depth and temperature range of thetransition from garnet to spinel peridotite, in agreement withlaboratory experiments and with the full convective models ofthe Hawaiian plume. The inversion calculations show that thesource region for Hawaiian tholeiites changes with time fromprimitive to depleted mantle. This behaviour is likely to resultfrom percolation, and the processes involved can be understoodwith the help of a simple analytic model. The last, post-erosional,magmas produced on Oahu come from a source that has been uniformlyenriched in all rare earth elements by a factor of about two.Magmas associated with island arcs come from two sources. Oneresembles that of n-type MORB, and probably is produced by adiabaticupwelling. The other generates calc-alkaline basalt stronglyenriched in light rare earth elements, but with a smaller constantenrichment between Gd and Lu. This composition is consistentwith the extraction of a melt fraction of 1% from a source containing9% of amphibole. Such a source region can also account for thelow values of Ti and Nb, and perhaps also of Ta, observed inisland arc magmas. Basaltic andesites and andesites from islandarcs show the same amphibole signature, and can be producedfrom the calc-alkaline basalts by fractional crystallizationif amphibole separates with olivine and orthopyroxene. The percolationof a small melt fraction through a mantle wedge that containsconsiderable amounts of amphibole can only transport very incompatibleelements, such as He, U, Th, and Rb, towards the Earth's surface.Sr and Nd are likely to be too compatible to move against thematrix flow, but Pb may do so locally. These results have importantimplications for the isotopic systematics of the upper mantle. The melt distributions obtained from ophiolites are like thosefor island arc tholeiites, though a potential temperature of1400 ?C fits the results better than does one of 1300?C. Archaeantholeiites and basaltic komatiites give melt distributions similarto that of e-type MORB from Iceland, and can be produced byadiabatic decompression if the mantle potential temperatureis 1500cC, with tholeiites having lost more material by fractionalcrystallization. The melt distribution obtained from komatiitesrequires the melt fraction to reach 60% at the surface. Thoughthe calculated compositions agree with those observed, decompressionis unable to generate such large melt fractions. Inversion shows that plateau basalts can be produced from theupper mantle beneath the plates by adiabatic upwelling beneatha mechanical boundary layer 60 km thick. Many of the variedalkali-rich continental magmas are generated by melting an enrichedsource in the stability field of garnet peridotite. The averageenrichment required, by a factor of between two and five, canbe produced by the addition of a small melt fraction. Carbonatitesshow no evidence of amphibole involvement at any stage, a resultthat is consistent with their formation by liquid immiscibility.Inversion of the rare earth element concentrations in shalesgives a melt distribution similar to that from calc-alkalinebasalts from island arcs, with a strong amphibole signature.Generation of the continental crust by separation of calc-alkalinemagma from 40% of the mantle can account for the differencebetween primitive and depleted mantle. Low-K highland basalts from the Moon can be produced directlyfrom the average primitive lunar mantle if the melt fractioninvolved is ?0-5%, and if they were generated in the stabilityfield of plagioclase and spinel peridotite. Intermediate-K highlandbasalts come from a source that has been enriched by a factorof about two, and show no evidence of amphibole involvement.The rare earth concentrations in mare basalts require melt fractionsof up to 7% in the spinel peridotite stability field, and canbe generated by adiabatic upwelling of mantle whose potentialtemperature is 1300?C beneath a mechanical boundary layer thatis 150 km thick. Because lunar gravity is only one-sixth ofthat of the Earth, the thickness of the melting zone and thevolume of melt produced are six times greater for the Moon thanfor the Earth for the same value of Tp. Both low-Ti and high-Timare basalts may have lost as much as 70 and 85% respectivelyof their original material through crystal fractionation. Itis, however, difficult to understand how such an origin canaccount for the high magnesium concentrations. Basaltic achondritesinvolve melt fractions of 10-15%, generated in the spinel orplagioclase stability field.     

Nature and Development of Basalt Magma Sources Beneath Iceland and the Reykjanes Ridge

Twelve new Sr-isotope analyses and seventeen new rare earthelement distribution patterns are reported for basalts fromIceland and the Reykjanes Ridge, together with Rb, Sr, Na₂O,K₂O, TiO₂, and P₂O₅ contents. The samples were chosen to representthe widest range of basalt types known from the Iceland-ReykjanesRidge system. ⁸⁷Sr/⁸⁶Sr ratios range from 0.70291 ?4 to 0.70325?5 for tholeiitesand up to 0.70341 ?7 for alkali basalt. Rare earth elementsalso show a wide range of both total abundance and degree oflight-REE fractionation (chondrite-normalised Ce/Yb ratios of0.30 to 3.36 for tholeiites and up to 7.07 for alkali basalt).As found in previous studies of either Sr-isotope compositionor REE distribution, the ocean floor basalts from the southernportion of the Reykjanes Ridge have lower ⁸⁷Sr/⁸⁶Sr and Ce_N/Yb_Nratios than most of the Icelandic basalts. However, some highlyMg-rich tholeiites from Theistareykir in northern Iceland andKj?lur in central Iceland also have among the lowest valuesfor these parameters and are indistinguishable in this respectfrom the ridge basalts. There is a very strong positive, linear,correlation between ⁸⁷Sr/⁸⁶Sr and Ce_N/Yb_N for all the tholeiitesincluding some up to 16 m.y. old, but this relationship doesnot hold for the alkali basalts which have proportionately farhigher Ce_N/Yb_N ratios. There is also a positive, linear, correlationbetween ⁸⁷Sr/⁸⁶Sr and Sr content, but not between ⁸⁷Sr/⁸⁶Srand 1/Sr. These relationships are found to be incompatible with disequilibriummelting of a single mantle source region, whether by variabledegrees of partial melting with different mineral stabilityconditions, or by removal of successive incremental melts. Itis certain that the data reflect relatively gross chemical heterogeneityin the upper mantle beneath Iceland, but the correlation withSr content apparently rules out simple binary mixing models(mantle-plume hypothesis). It is proposed that the heterogeneities result from establishmentof a lithophile element gradient during a single chemical fractionationevent in the upper mantle at least 100–200 m.y. ago. Itis not possible at present to relate this geochemical gradientto known mantle structure.