Showing papers on "Partial melting published in 1996"

A possible role for garnet pyroxenite in the origin of the “garnet signature” in MORB

Abstract: Geochemical data have been interpreted as requiring that a significant fraction of the melting in MORB source regions takes place in the garnet peridotite field, an inference that places the onset of melting at ≥80 km. However, if melting begins at such great depths, most models for melting of the suboceanic mantle predict substantially more melting than that required to produce the 7 ± 1 km thickness of crust at normal ridges. One possible resolution of this conflict is that MORBs are produced by melting of mixed garnet pyroxenite/spinel peridotite sources and that some or all of the “garnet signature” in MORB is contributed by partial melting of garnet pyroxenite layers or veins, rather than from partial melting of garnet peridotite. Pyroxenite layers or veins in peridotite will contribute disproportionately to melt production relative to their abundance, because partial melts of pyroxenite will be extracted from a larger part of the source region than peridotite partial melts (because the solidus of pyroxenite is at lower temperature than that of peridotite and is encountered along an adiabat 15–25 km deeper than the solidus of peridotite), and because melt productivity from pyroxenite during upwelling is expected to be greater than that from peridotite (pyroxenite melt productivity will be particularly high in the region before peridotite begins melting, owing to heating from the enclosing peridotite). For reasonable estimates of pyroxenite and peridotite melt productivities, 15–20% of the melt derived from a source region composed of 5% pyroxenite and 95% peridotite will come from the pyroxenite. Most significantly, garnet persists on the solidus of pyroxenite to much lower pressures than those at which it is present on the solidus of peridotite, so if pyroxenite is present in MORB source regions, it will probably contribute a garnet signature to MORB even if melting only occurs at pressures at which the peridotite is in the spinel stability field. Partial melting of a mixed spinel peridotite/garnet pyroxenite mantle containing a few to several percent pyroxenite can explain quantitatively many of the geochemical features of MORB that have been attributed to the onset of melting in the stability field of garnet lherzolite, provided that the pyroxenite compositions are similar to the average composition of mantle-derived pyroxene-rich rocks worldwide or to reasonable estimates of the composition of subducted oceanic crust. Sm/Yb ratios of average MORB from regions of typical crustal thickness are difficult to reconcile with derivation by melting of spinel peridotite only, but can be explained if MORB sources contain ∼5% garnet pyroxenite. Relative to melting of spinel peridotite alone, participation of model pyroxenite in melting lowers aggregate melt Lu/Hf without changing Sm/Nd ratios appreciably. Lu/Hf-Sm/Nd systematics of most MORB can be accounted for by melting of a spinel peridotite/garnet pyroxenite mantle provided that the source region contains 3–6% pyroxenite with ≥20% modal garnet. However, Lu/Hf-Sm/Nd systematics of some MORB appear to require more complex melting regimes and/or significant isotopic heterogeneity in the source. Another feature of the MORB garnet signature, (^(230)Th)/(^(238)U)>1, can also be produced under these conditions, although the magnitude of (^(230)Th)/(^(238)U) enrichment will depend on the rate of melt production when the pyroxenite first encounters the solidus, which is not well-constrained. Preservation of high (^(230)Th)/(^(238)U) in aggregated melts of mixed spinel peridotite/garnet pyroxenite MORB sources is most likely if the pyroxenites have U concentrations similar to that expected in subducted oceanic crust or to pyroxenite from alpine massifs and xenoliths. The abundances of pyroxenite in a mixed source that are required to explain MORB Sm/Yb, Lu/Hf, and (^(230)Th)/(^(238)U) are all similar. If pyroxenite is an important source of garnet signatures in MORB, then geochemical indicators of pyroxenite in MORB source regions, such as increased trace element and isotopic variability or more radiogenic Pb or Os, should correlate with the strength of the garnet signature. Garnet signatures originating from melts of the garnet pyroxenite components of mixed spinel peridotite/garnet pyroxenite sources would also be expected to be stronger in regions of thin crust.

Constraints from partitioning experiments on the composition of subduction-zone fluids

Abstract: THE generation of calc-alkaline magmas in subduct ion zones is thought to be the most important mechanism for the growth of continental crust since the Proterozoic eon. It is widely assumed that most of these magmas are products of fluid-triggered melting in the mantle wedge above the subducted slab1,2. Fluid transport from the subducted slab into the zone of melting has also been invoked in order to explain many of the trace element and radiogenic isotope characteristics of calk-alkaline magmas3–7. Here I report experimental data on the partitioning of trace elements between fluids, silicate melts and minerals, which suggest that the agent responsible for the transport of trace elements in subduction zones may be an alkali-chloride-rich aqueous fluid. The data show that chemical transport by such a fluid can generate the trace element and isotope enrichment pattern typical for calc-alkaline magmas, including the enrichment of large ionic lithophile elements, lead and uranium, and the characteristic depletion in niobium and tantalum.

Petrogenesis of slab-derived trondhjemite–tonalite-dacite/adakite magmas

Abstract: The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al 2 O 3 at the 70% SiO 2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young ( −1 ) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

Major-element variability in the Hawaiian mantle plume

Abstract: Isotope ratios and concentrations of silica, titanium, aluminium, calcium, sodium and iron are correlated in shield-stage basaltic lavas erupted over the past three million years from eight Hawaiian volcanoes. These correlations indicate the presence of a component beneath Hawaii with a bulk composition which is distinct from typical mantle peridotite. This material is most probably recycled oceanic crust distributed within the Hawaiian plume as segregations of quartz-bearing garnet pyroxenite or eclogite, which yield silica-rich (dacitic) magmas on partial melting.

Genesis of the two main types of peraluminous granitoids

Abstract: Two-mica monzogranites to leucogranites and biotite-rich, cordierite-bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids. They can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation. Except for the rare muscovite-bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas, the majority of peraluminous granitoids are produced by partial melting of crustal rocks. Production of either muscovite-bearing granites or biotite-rich, cordierite-bearing granitoids does not depend only on the nature of the sources, but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced. Biotite-rich, cordierite-bearing granitoids are generated where mantle-derived magmas are injected into or have underplated crustal rocks; two-mica granites are generated where thickened crust is affected by major crustal shears or thrusts. Correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids.

Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii

Abstract: We have investigated the isotopic compositions of Os, Sr, Nd, and Pb in a suite of primitive Mauna Loa lavas from the upper 280 m of the Hawaii Scientific Drilling Project pilot core drilled near Hilo, Hawaii. These lavas were probably erupted from Mauna Loa's northeast rift. Correlations between Os (hosted by olivine) and other isotopes indicate that olivine crystals in these flows are closely related in space and time to the enclosing lava, despite the presence of deformation features and Fe/Mg disequilibrium in some olivines. The temporal isotopic evolution of the lavas matches published data for basalts from Mauna Loa's southwest rift, indicating that the two rifts (as well as the summit) share a common magma feed zone which is distinct from that of Kilauea. The composition of the lowermost HSDP Mauna Loa sample shows some isotopic similarities to modern Kilauea compositions and in this respect compares well with published data on submarine lavas from Mauna Loa's southwest rift. The good correlations among the isotopic tracers of compatible (Os) and incompatible (Sr, Nd, Pb) elements indicate that a depleted upper mantle component is very minor or nonexistent in Mauna Loa lavas. The Os isotope results definitively rule out equilibrium porous flow as a means of melt transport through the lithosphere. The isotopic variations in shield-stage lavas are most consistent with partial melting of two distinct sources within the Hawaiian plume followed by partial mixing and rapid transport of melts through the oceanic lithosphere. The passage of the Pacific lithosphere over a heterogeneous Hawaiian plume can account for the systematic differences in the compositions of volcanoes from the “Kea” and “Loa” trends as well as the geochemical evolution of individual shields on the island of Hawaii.

Melt segregation and magma flow in migmatites: implications for the generation of granite magmas

Abstract: To form a granite pluton, the felsic melt produced by partial melting of the middle and lower continental crust must separate from its source and residuum. This can happen in three ways: (1) simple melt segregation, where only the melt fraction moves; (2) magma mobility, in which all the melt and residuum move together; and (3) magma mobility with melt segregation, in which the melt and residuum move together as a magma, but become separated during flow. The first mechanism applies to metatexite migmatites and the other two to diatexite migmatites, but the primary driving forces for each are deviatoric stresses related to regional-scale deformation. Neither of the first two mechanisms generates parental granite magmas. In the first mechanism segregation is so effective that the resulting magmas are too depleted in FeOT, MgO, Rb, Zr, Th and the REEs, and in the second no segregation occurs. Only the third mechanism produces magmas with compositions comparable with parental granites, and occurs at a large enough scale in the highest grade parts of migmatite terranes, to be considered representative of the segregation processes occurring in the source regions of granites.

Controls on the trace element composition of crustal melts

Abstract: The behaviour of trace elements during partial melting depends primarily on their mode of occurrence. For elements occurring as trace constituents of major phases (e.g. Li, Rb, Cs, Eu, Sr, Ba, Ga, etc.), slow intracrystalline diffusion (D ≍ 10−16 cm2 s−1) at the temperature range of crustal anatexis causes all effective crystal-melt partition coefficients to have a value close to unity and impedes further melt-restite re-equilibration. Usually, therefore, the trace element composition of crustal melts simply depends on the mass balance between the proportion and composition of phases that melt and the proportion and composition of newly formed phases. The behaviour of trace elements occurring as essential structural components in accessory phases (e.g. P, La-Sm, Gd-Lu, Y, Th, U, Zr, Hf, etc.) depends on the solubility, solution kinetics, grain size and the textural position of accessory phases. In common crustal protoliths a significant mass fraction of monazite, zircon, xenotime, Th-orthosilicates, uraninite; etc.—but not apatite—is included within other major and accessory phases. During low melt fraction anatexis the amount of accessory phases available for the melt is not sufficient for saturation, thus producing leucosomes with concentrations of La-Sm, Gd-Lu, Y, Th, U and Zr lower than expected from solubility equations. Low concentrations of these elements may also occur if the melt is prevented from reaching equilibrium with the accessories due to fast segregation. However, the first mechanism seems more feasible as leucosomes that are undersaturated with respect to monazite and zircon are frequently saturated, even oversaturated, with respect to apatite.

Isotopic evolution of Mauna Kea volcano: Results from the initial phase of the Hawaii Scientific Drilling Project

Abstract: We have examined the Sr, Nd, and Pb isotopic compositions of Mauna Kea lavas recovered by the first drilling phase of the Hawaii Scientific Drilling Project. These lavas, which range in age from ∼200 to 400 ka, provide a detailed record of chemical and isotopic changes in basalt composition during the shield/postshield transition and extend our record of Mauna Kea volcanism to a late-shield period roughly equivalent to the last ∼100 ka of Mauna Loa activity. Stratigraphic variations in isotopic composition reveal a gradual shift over time toward a more depleted source composition (e.g., higher 143Nd/144Nd, lower 87Sr/86Sr, and lower 3He/4He). This gradual evolution is in sharp contrast with the abrupt appearance of alkalic lavas at ∼240 ka recorded by the upper 50 m of Mauna Kea lavas from the core. Intercalated tholeiitic and alkalic lavas from the uppermost Mauna Kea section are isotopically indistinguishable. Combined with major element evidence (e.g., decreasing SiO2 and increasing FeO) that the depth of melt segregation increased during the transition from tholeiitic to alkalic volcanism, the isotopic similarity of tholeiitic and alkalic lavas argues against significant lithosphere involvement during melt generation. Instead, the depleted isotopic signatures found in late shield-stage lavas are best explained by increasing the proportion of melt generated from a depleted upper mantle component entrained and heated by the rising central plume. Direct comparison of Mauna Kea and Mauna Loa lavas erupted at equivalent stages in these volcanoes' life cycles reveals persistent chemical and isotopic differences independent of the temporal evolution of each volcano. The oldest lavas recovered from the drillcore are similar to modern Kilauea lavas, but are distinct from Mauna Loa lavas. Mauna Kea lavas have higher 143Nd/144Nd and 206Pb/204Pb and lower 87Sr/86Sr. Higher concentrations of incompatible trace elements in primary magmas, lower SiO2, and higher FeO also indicate that Mauna Kea lavas formed through smaller degrees of partial melting at greater depth than Mauna Loa lavas. These chemical and isotopic differences are consistently found between volcanoes along the western “Loa” and eastern “Kea” trends and reflect large-scale variations in source composition and melting environment. We propose a simple model of a radially zoned plume centered beneath the Loa trend. Loa trend lavas generated from the hot plume axis reflect high degrees of partial melting from a source containing a mixture of enriched plume-source material and entrained lower mantle. Kea trend lavas, in contrast, are generated from the cooler, peripheral portions of the plume, record lower degrees of partial melting, and tap a source containing a greater proportion of depleted upper mantle.

Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships, mineral compositions and trace-element residence

Abstract: Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially, e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite. Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the xenoliths.

Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line

Abstract: Major element, trace element and Sr-Nd-Pb isotopic compositions ofultramafic xenoliths and megacrysts from the continental Cameroon line provide evidence for metasomatism of the uppermost lithospheric mantle by enriched melts during the Mesozoic The megacrysts probably crystallized within the lower continental crust from melts similar to the host magmas. All the xenoliths originated as depleted residues after the extraction of basaltic melts, but some indicate evidence of interaction with enriched partial melts before entrainment. The U—Pb isotopic data on garnet are consistent with cooling through > 900°C at >300 Ma. The Sm—Nd isotope systematics in constituent phases appear to have been in equilibrium on a xenolith scale at the time of entrainment, indicating derivation from mantle that remained at temperatures > 600°C until eruption. Spinel Iherzolites that show simple light rare earth element (LREE) depletions are characterized by isotopic compositions that are comparable with, but slightly more depleted than Atlantic NMORB, suggesting that the unmetasomatized sub-continental lithosphere of the Cameroon line may be isotopically similar to that of sub-oceanic lithosphere. The Nd-depleted mantle model ages of these xenoliths are consistent with late Proterozoic depletion, similar in age to much of the overlying continental crust. In contrast, samples that have LREE-enriched clinopyroxenes (La/Yb —4-7—9-4) contain trace amounts of amphibole, are enriched in U and have more radiogenic Pb and Sr. These xenoliths yield U-Pb and Sm—Nd model ages consistent with Mesozoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Clinopyroxenes record three orders of magnitude enrichment in U and LREE accompanied by progressive K depletion associated with the growth of trace amphibole, with K/ U ratios that range from 12 000 to 1. The ratios of the trace elements thought to have similar bulk D in mantle melting, Ce/ Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopic compositions. Mass balance calculations suggest that the melts responsible for the most recent enrichment of the lithosphere had higher La/Yb and U/Pb than Cameroon line host magmas, and were probably the product of small degrees of partial melting associated with the earliest stages of the breakup of Pangea.

Constraints on mantle melting at mid-ocean ridges from global 238 U– 230 Th disequilibrium data

Abstract: The extent of radioactive disequilibrium between 238U and its daughter product 230Th in mid-ocean-ridge basalts from around the world is negatively correlated with axial ridge depth; local positive correlations with inferred mantle source composition are also observed. The larger 230Th excesses seen in samples from shallow ridges reflect a larger melt contribution from a garnet-bearing source which may be explained by a deepening of the solidus in hotter mantle regions. These observations provide important constraints for modelling melt generation at mid-ocean ridges.

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

Abstract: New mineral and bulk-rock analyses, as well as Nd, Sr and Pb isotope compositions are presented for lavas from Grande Comore, Moheli and Mayotte, thru of the four main islands of the Comores Archipelago in the western Indian Ocean, and these data an used to evaluate the petrogenesis, evolution and mantle source region characteristics of Comorean lavas. The typically silica-undersaturated, alkaline lavas from all three islands can be grouped into two distinct types: La Grille-type (LGT) lavas, which display strong relative depletions in K, and Karthala-type (KT) lavas, which do not. With the exception of the lavas erupted by La Grille volcano on Grande Comore, which exhibit the petrographic and geochemical characteristics expected of primary mantle-derived magmas, all Comorean lavas analysed have experienced compositional modifications after they segregated from their source regions. Much of this variation can be explained quantitatively by fractional crystallization processes dominated by the fractionation ofolivine and clinopyroxene. Semi-quantitative modelling shows that the consistent and fundamental difference in composition between K-depleted LGT lavas and normal KT lavas can be attributed to partial melting processes, provided amphibole is a residual mantle phase after extraction of LGT magmas at low degrees of melting. Low absolute abundances of the heavy rare earth elements in LGT magmas are interpreted to reflect partial melting within the garnet stability field In contrast, KT magmas, which do not show relative K depletions, are considered to be the products of somewhat larger degrees of partial melting of an amphibolefree source at comparatively shallower depths. Whereas the Nd and Sr isotopic compositions of Comorean lavas (which show a significant range: r>'Sr/S6Sr = 0-70319

Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

Abstract: In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 Ma (Early Cretaceous). It occurred about 5 to 10 m.y. after the ∼165 to 159 Ma Josephine ophiolite was thrust beneath older parts of the province during the Nevadan orogeny (thrusting from ∼155 to 148 Ma). The magmatism also corresponds to a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist almost entirely of tonalite and trondhjemite. Poorly-defined concentric zoning is common. Tonalitic rocks are typically of the low-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite. The suite is characterized by low abundances of K2O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate (∼450 ppm) by comparison with Sr-rich arc lavas interpreted to be slab melts (up to 2000 ppm). Initial 87Sr/86Sr, δ18O, and ɛNd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual assemblage of clinopyroxene+garnet±plagioclase±amphibole; residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source. Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low- and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary.

Petrology of lavas from the Puu Oo eruption of Kilauea Volcano: III. The Kupaianaha episode (1986-1992)

Abstract: The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes.

Constraints on the formation of platinum-group element deposits in igneous rocks

Abstract: Mechanisms of concentration of the platinmn-group and other chalcophile elements iu several platinum-group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts. The similarity of the partition coefficieut D (super sulfide melt/silicate melt) of Ir to that of Pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility. In experiments designed to test the possibility that this fractionation was due to the presence of Os-Ir alloy, sulfide and silicate melts were equilibrated with Ir metal at 1,450 degrees C, 8 kbars total pressure, and under sulfur and oxygen fugacity conditions appropriate for nature. The Ir content of the sulfide liquid was found to be > or = 3.7 wt percent and that of the coexisting silicate melt inferred to be > or = 1,000 ppb. These concentrations are far in excess of the Ir contents of natural sulfides or rocks and indicate that Os-Ir alloys are not stable in the presence of sulfide or silicate melts in nature.Using our preferred values of D (super sulfide melt/silicate melt) for the chalcophile elements and their estimated concentrations in the mantle, limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle. The ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits, including the J-M reef (Stillwater Complex), the UG-2 (Bushveld Complex), and the Lac des Isles Complex, are inconsistent with concentration solely by segregation of sulfide liquid. In contrast, the Merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide. Stratigraphic variations in chalcophile element concentrations through the UG-2 and the Main Sulfide zone of the Great Dyke also indicate that metals must have been redistributed on at least the meter scale. Our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism.