FLOW SEPARATION IN UNDISTURBED SOIL USING MULTIPLE ANIONIC TRACERS. PART 2. STEADY-STATE CORE-SCALE RAINFALL AND RETURN FLOWS AND DETERMINATION OF DISPERSION PARAMETERS

A series of experiments designed to study the separation of flow components from two large undisturbed cores under steady-state rainfall (downward) and return (upward) flows under near-saturated conditions is summarized. The experiments were conducted on soil columns collected from Lancaster University and the Slapton Wood catchment, Devon. The use of the relatively conservative tracers, potassium bromide, o-(trifluoromethyl)benzoic acid and 2,6-difluorobenzoic acid and a combination of application rates made it possible to quantify the different sources of water contributing to the discharge hydrographs. There is significant retention of tracer within the cores, despite the application of several pore volumes of water. The use of steady flow conditions allowed the determination of dispersion coefficients, dispersivity and proportion of ‘mobile’ water content parameters of the advection–dispersion equation. It was found that there were significant differences between the dispersivities at different flow-rates under upward and downward flux conditions and that in the undisturbed cores studied here the apparent proportions of mobile pore water ranged between 0.33 and 1.0, with an apparently complex relationship to flux rate. Prediction of transport in undisturbed soil remains problematic and tracer experiments will continue to be needed to provide a fundamental understanding of the complex flow processes involved.     

Identifying Accessory Mineral Saturation during Differentiation in Granitoid Magmas: an Integrated Approach

Numerical reconstructions of processes that may have operatedduring igneous petrogenesis often model the behaviour of importanttrace elements. The geochemistry of these trace elements maybe controlled by accessory mineral saturation and fractionation.Determination of the saturation point of accessory mineralsin granitoid rocks is ambiguous because assumptions about crystalmorphology and melt compositions do not always hold. An integratedapproach to identifying accessory mineral saturation involvingpetrography, whole-rock geochemical trends, saturation calculationsand mineral chemistry changes is demonstrated here for a compositionallyzoned pluton. Within and between whole-rock samples of the BoggyPlain zoned pluton, eastern Australia, the rare earth element(REE)-enriched accessory minerals zircon, apatite and titaniteexhibit compositional variations that are related to saturationin the bulk magma, localized saturation in intercumulus meltpools and fractionation of other mineral phases. Apatite isidentified as having been an early crystallizing phase overnearly the whole duration of magma cooling, with zircon (andallanite) only saturating in more felsic zones. Titanite andmonazite did not saturate in the bulk magma at any stage ofdifferentiation. Although some trace elements (P, Ca, Sc, Nb,Hf, Ta) in zircon exhibit compositional variation progressingfrom mafic to more felsic whole-rock samples, normalized REEpatterns and abundances (except Ce) do not vary with progressivedifferentiation. This is interpreted to be a result of limitationsto both simple ‘xenotime’ and complex xenotime-typecoupled substitutions. Our data indicate that zircon REE characteristicsare not as useful as those of other REE-rich accessory mineralsas a petrogenetic indicator. KEY WORDS: saturation; zircon; apatite; titanite; magma differentiation; trace elements; REE patterns     

Magma Mixing and the Production of Compositional Variation within Granite Suites: Evidence from the Granites of Southeastern Australia

Granite suites are groups of plutons possessing characteristicfeatures that are a result of their derivation from source materialof a specific composition. Variation within suites has beenascribed to a variety of processes. Magma mixing or minglingis a popular hypothesis, generally proposed in terms of blendingbetween a crustal melt and mafic material from the mantle thatcaused that melting. When the compositions of pairs of suitesfrom the Bega Batholith of southeastern Australia are compared,any differences seen at either end of the range in compositionare also seen at the other limit, so that both the most maficand most felsic rocks show similar relative abundances of particularelements. Similar relationships are seen for other granitesin the region. These observations are not consistent with large-scalemagma mixing or mingling and, although those processes may operateon a small scale, they cannot have been responsible for themajor compositional variations. Likewise, assimilation of countryrocks had no significant role in producing variation in thegranites of southeastern Australia. The production of variationby differential separation of melt from residual solid sourcematerial, or restite, must be favoured for many of the granitesuites of this region. KEY WORDS: assimilation; enclaves; granite suites; magma mixing; restite     

High- and Low-Temperature I-type Granites

Abstract: I– and S-type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S-type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I-type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S-types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S-type granites are likely to be closely related in origin, with the former comprising both restite-rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I-type granites into two groups, formed at high and low temperatures. The high-temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low-temperature I–type granites, the compositions extend to lower SiO₂ contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO₂ in the more mafic rocks. While the high-temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low-temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low-temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H₂O.     

Granitoid Compositional Zoning by Side-wall Boundary Layer Differentiation: Evidence from the Palisade Crest Intrusive Suite, Central Sierra Nevada, California

Compositionally zoned plutons are an important feature of theSierra Nevada batholith, California. Two such plutons have beenexamined to determine the mechanism by which crystals separatefrom a magma. The Tinemaha pluton shows continuous compositionalvariation from 58 to 67% SiO₂, whereas the McMurry Meadows plutonis bimodal, with an outer margin of mafic granodiorite (59–60%SiO₂) and an inner core of granite (66–69% SiO₂). Extremedifferentiates also occur as small isolated masses within thesuite and may contain up to 76% SiO₂. Both plutons are uniformin strontium isotopic composition but are different from eachother, with initial ⁸⁷Sr/⁸⁶Sr values of 0?70719 and 0?70651respectively. The Tinemaha pluton is both horizontally and vertically( 1000 m) zoned, with fractionation occurring both inward fromthe contacts and upward. The vertical trends in relative mineralproportions are not consistent with crystal settling. Both thevertical and horizontal variations in the chemical compositionof 50 elements, in mineralogy, and in accessory mineral lightrare-earth element zoning, are all directly relatable to side-wallcrystallization which created a less-dense melt that buoyantlymoved upward along the wall towards the top of the magma chamber.The different rates for diffusive heat exchange and compositionaldiffusion within the magma initiated the double-diffusive gradientin the magma chamber. Compositional variations in the side-wallcrystal accumulation zone occur as boundary layer melts evolve,reflecting changes in the bulk convecting magma. The compositionalgap in the McMurry Meadows pluton is the result of a similarbut more efficient side-wall fractionation process, relatedto a higher proportion of melt to crystals in the initial magmaand a slower rate of side-wall solidification as a result ofthe thermal blanket created by the enclosing Tinemaha pluton.     

Manam Island, Papua New Guinea: Petrology and Geochemistry of a Low-TiO2 Basaltic Island-Arc Volcano

Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO₂ andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO₂contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; ²⁰⁷Pb/²⁰⁴Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, ⁸⁷Sr/⁸⁶Sr, and 100 Mg/(Mg ? Fe^2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin ⁸⁷Sr/⁸⁶Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.     

Geochemical and Isotopic Constraints on the Origin of the Jurassic Dolerites of Tasmania

The Jurassic dolerites of Tasmania represent a small part ofthe widespread Mesozoic magmatism of Gondwana, and possess someof the most extreme isotopic and elemental signatures observedin tholeiitic rocks. This study documents the remarkable uniformityof the magma in major, trace element and isotopic compositionat the time of emplacement. Many of the trace element and isotopiccharacteristics compare more closely with those of continentalcrustal rocks than with typical mantle-derived materials, indicatingthe dominance of a crustal component in the petrogenesis ofthese tholeiites. Difficulties in reproducing the trace elementsignatures of the dolerites using calculations involving crustalassimilation, and the unreasonable isotopic compositions requiredof the contaminant, make such models unlikely. We argue that a more likely explanation involves the introductionof a small quantity of sediment (3 wt.%) into a depleted mantlesource by the process of subduction. It is envisaged that thetrace element and isotopic compositions of the mantle sourcewere overprinted by the crustal signatures owing to the lowabundance of incompatible trace elements originally present.These crustal signatures were subsequently inherited by magmasproduced during partial melting.     

Magma Genesis and Mantle Heterogeneity in the Manus Back-Arc Basin, Papua New Guinea

Geochemical data from back-arc volcanic zones in the Manus Basinare used to define five magma types. Closest to the New Britainarc are medium-K lavas of the island arc association and back-arcbasin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB andmildly enriched T-MORB (transitional MORB) occur along the ManusSpreading Center (MSC) and Extensional Transform Zone (ETZ).The MSC also erupted extreme back-arc basin basalts (XBABB),enriched in light rare earth elements, P, and Zr. Compared withnormal MORB, Manus MORB are even more depleted in high fieldstrength elements and slightly enriched in fluid-mobile elements,indicating slight, prior enrichment of their source with subduction-relatedcomponents. Chemical variations and modeling suggest systematic,coupled relationships between extent of mantle melting, priordepletion of the mantle source, and enrichment in subduction-relatedcomponents. Closest to the arc, the greatest addition of subduction-relatedcomponents has occurred in the mantle with the greatest amountof prior depletion, which has melted the most. Variations inK₂O/H₂O indicate that the subduction-related component is bestdescribed as a phlogopite and/or K-amphibole-bearing hybridizedperidotite. Magmas from the East Manus Rifts are enriched inNa and Zr with radiogenic ⁸⁷Sr/⁸⁶Sr, possibly indicating crustalinteraction in a zone of incipient rifting. The source for XBABBand lavas from the Witu Islands requires a mantle componentsimilar to carbonatite melt. KEY WORDS: Manus back-arc basin, mantle metasomatism, magma generation