Michael P. Gorton

The origin of the fractionation of platinum-group elements in terrestrial magmas

Abstract The platinum-group elements (PGEs), when chondrite normalized, have been found to be fractionated in order of descending melting point (Os, Ir, Ru, Rh, Pt, Pd and Au). Mantle-derived material (garnet lherzolite and spinel lherzolite xenoliths and alphine peridotites) have essentially unfractionated PGE patterns. Periotitic komatiites have mildly fractionated patterns ( Pd Ir = 10 ), pyroxenitic komatiites are slightly more fractionated ( Pd Ir = 30 ). Both continental and ocean-floor basalts are highly fractionated ( Pd Ir = 100 ). Data from intrusive rocks show a large range in PGE fractionation from Pd-depleted chromities of ophiolites ( Pd Ir = 0.1 ) to the extreme Pd enrichment in the JM Reef of the Stillwater Complex ( Pd Ir = 865 ). Some possible mechanisms for the origin of this fractionation are: alteration, partial melting and crystal fractionation. Carbonate alteration affects Au and Pt and hydrothermal alteration mobilizes Pd. Solid substitution of Ir (and associated Os and Ru) into olivine and chromite, during crystal fractionation or partial melting is rejected as a mechanism of fractionating the PGEs. It is suggested; that the major factor in PGE fractionation is the differences in solubility of the PGEs in a silicate magma, that Pd, Pt and Rh are more soluble than Os and Ir, which form an alloy and Ru which forms laurite. These differences in PGE solubility could fractionate the PGEs during partial melting or crystal fractionation. During crystal fractionation prior to Fe-Ni-Cu sulphur saturation the low solubility of Os, Ir and Ru leads to the formation of Os-Ir alloys and RuS2 in the magma. These may then be settled out of the magma by whatever phase is crystallizing and the remaining magma becomes fractionated in PGEs.

Correlation Techniques in Tephra Studies

Distinctive tephra layers constitute important time-parallel markers, which if widespread, offer the potential for reliable correlation over long distances. Confident correlations require a multiple criterion approach to tephra characterisation and equivalence of samples should only be considered firmly established if their stratigraphic, palaeontologic, palaeomagnetic, and radiometric age relations are compatible, and the physicochemical properties of their glass shards and phenocrysts agree. The strong susceptibility of tephra to reworking further argues for use of several stratigraphic controls in order to safeguard against gross errors.

Rare-earth element mobility in alteration pipes below massive CuZn-sulfide deposits

Abstract A convincing demonstration of trace-element mobility should satisfy two criteria: (1) the range of trace-element concentrations in the rock before and after alteration must be known with confidence; and (2) the absolute abundances and the ratios of the mobile trace elements should vary between altered and unaltered rocks. These criteria have been applied to the host felsic volcanics and alteration pipes associated with several Canadian massive CuZn-sulfide deposits. Although there is no evidence of rare-earth element (REE) mobility in the pervasive low-grade alteration envelopes adjacent to the deposits, the light and middle REE (LREE and MREE) plus Y have been systematically mobilized relative to the heavy REE (HREE) and other trace elements in the alteration pipes beneath the ore zones. There is evidence of minor LREE and MREE depletion in the most altered rocks at the South Bay Mine (Uchi Lake) and of significant LREE depletion in the alteration pipe beneath the Mattabi Mine (Sturgeon Lake). However, the strongest mobility is found at the Kidd Creek Mine where the LREE were leached from the rocks in the footwall stringer zone and redeposited in a metasomatic chlorite zone directly beneath the massive ore horizon, and the MREE and Y were preferentially lost from the system producing distinctive chondrite-normalized REE patterns with ( Tb Yb )n ratios less than unity. Eu2+ was found to be more mobile than the trivalent REE, especially at Kidd Creek where there is a marked increase in the magnitude of the Eu anomaly in the most altered samples. High-field-strength elements such as U, Th, Hf and Zr have not decoupled and apparently remained relatively immobile during all but the most intense alteration (i.e. Kidd Creek). Preliminary data suggest that the degree of REE mobility increases with the size of the deposit. Thus, REE mobility is a potentially useful method of distinguishing between small and large massive sulfide deposits at an early stage in exploration, however, this hypothesis requires further testing.

Subduction-modified pelagic sediments as the enriched component in back-arc basalts from the Japan Sea: Ocean Drilling Program Sites 797 and 794

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., 87Sr/86Sr=0.70369–0.70503, 204Pb/204Pb=17.65–18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred from mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched. EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not “enriched” enough to be the EM II end-member.

Old Crow tephra: A new late Pleistocene stratigraphic marker across north-central Alaska and western Yukon Territory

Old Crow tephra is the first extensive Pleistocene tephra unit to be documented in the northwestern part of North America. It has a calc-alkaline dacitic composition with abundant pyroxene, plagioclase, and FeTi oxides, and minor hornblende, biotite, apatite, and zircon. Thin, clear, bubble-wall fragments are the dominant type of glass shard. This tephra can be recognized by its glass and phenocryst compositions, as determined by X-ray fluorescence, microprobe, and instrumental neutron activation techniques. It has an age between the limits of 60,000 and 120,000 yr, set by 14C and fission-track measurements, respectively. Old Crow tephra has been recognized in the Koyukuk Basin and Fairbanks region of Alaska, and in the Old Crow Lowlands of the northern Yukon Territory, some 600 km to the east-northeast. The source vent is unknown, but these occurrences, considered in relation to the distant locations of potential Quaternary volcanic sources, demonstrate the widespread distribution of this tephra and underscore its importance as a regional stratigraphic marker.

A comparative study of olivine and clinopyroxene spinifex flows from Alexo, abitibi greenstone Belt, Ontario, Canada

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyroxene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow.The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid.In the olivine spinifex flow the zones can be modelled as frozen liquids in the A2-zone, as initial liquid which has fractionated 30% olivine in the A3-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises.

Variations in trace element partition coefficients in sanidine in the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico: Effects of composition, temperature, and volatiles

Trace element partition coefficients have been measured for one plagioclase and five sanidine mineral separates from the Cerro Toledo Rhyolite, New Mexico. Sanidine partition coefficients vary substantially and systematically within the Cerro Toledo Rhyolite. Partition coefficients for Ca, Sr, Zn, La, and Eu are lowest in the most evolved rhyolites, whereas Sm and HREE partition coefficients are highest. Rubidium partition coefficients remain constant, while those for Ba, Ce, and Th are variable. Variations of the Sr, Zn, La, and Eu partition coefficients are correlated with the Ca contents and partition coefficients of the sanidines. Calcium may have controlled the distribution of these elements in the sanidine by modifying the feldspar structure. The low Ca partition coefficients in sanidines for the most evolved rhyolites may be the consequence of modification of the melt structure, possibly due to increased volatile contents at the top of the magma chamber(s) during evolution of the Cerro Toledo Rhyolite. The Zn and La partition coefficients between sanidine and melt also may have been controlled by this change in melt structure. Modelling using major elements and the constant Rb partition coefficient for sanidine indicates 70% crystallization of magma during Cerro Toledo Rhyolite time by removal of 68% sanidine and 32% quartz. Estimates of the volume (1) of initial parental magma and (2) of the magma that crystallized during this period are 11,670 km3 and 8170 km3, respectively. The average intrusion rate of silicic magma during Cerro Toledo Rhyolite activity was 35 × 10−3 km3/a.

Coexisting K-rich alkaline and shoshonitic magmatism of arc affinities in the Proterozoic: a reassessment of syenitic stocks in the southwestern Grenville Province

Biotite-rich syenitic stocks in the Mont-Laurier area of the southwestern Grenville Province are shown to belong to the first recorded Proterozoic example of an ultrapotassic, K-rich alkaline and shoshonitic rock association with clear arc affinities. The plutons investigated were previously considered mostly syenitic, typical of nepheline syenite alkaline suites, slightly metamorphosed and late-tectonic with respect to the Grenville orogeny. We find that they postdate the regional metamorphism and comprise a felsic to ultramafic range of rock types belonging to two series: (1) a potassic-to-ultrapotassic, silica-undersaturated series of biotite-rich nepheline-bearing syenite, syenite, monzonite, diorite and pyroxenite, and (2) a shoshonitic, critically silicasaturated series of quartz syenite and amphibole-bearing syenite, with rare monzonite and diorite. The ubiquitous biotite, previously regarded as metamorphic, is reinterpreted as igneous and diagnostic of the potassic character. The shoshonitic and potassic series display the strong enrichment in Al, Ca, K and large-ion-lithophile elements relative to the high-field-strength elements (e.g. Ba/Nb≤722, La/YB∼45) and the low contents in Mg that are characteristic of arc-related magmas. The syenitic rocks consistently share the distinctive arc-related geochemical signature of their mafic counterparts. Syenites may thus represent a potential source of paleotectonic information for high grade terranes. Geochemical discriminants (NbN/TaN and HfN/TiN ratios) indicate that the shoshonitic and potassic series are unrelated by closedsystem fractionation processes. Rather, the chemical differences between the two series probably reflect differences in source characteristics and conditions of melting. Similar plutons occur throughout the Central Metasedimentary Belt of the southwestern Grenville Province. They define a 1089 to 1076 Ma, 450-km-long grenvillian potassic alkaline plutonic (PAP) province. The presence of this K-rich alkaline province indicates that the scarcity of K-rich rocks in the Precambrian could be only apparent and a consequence of misidentification of K-rich plutons in metamorphosed Precambrian terranes. These 1.1 Ga ultrapotassic to shoshonitic plutonic rocks are geochemically similar to shoshonites and leucitites of the Sunda arc. This similarity suggests that subduction-type enrichment processes were operating in the Proterozoic in ways similar to those of modern settings.

Age of the Salmon Springs Glaciation in Washington

A tephra bed within the Salmon Springs Drift at its type locality near Sumner, Washington, has a zircon fission-track age of 0.84 ± 0.21 m.y., and the immediately overlying silts are reversely magnetized. The same distinctive tephra at Auburn, Washington, has a glass fission-track age of 0.66 ± 0.04 m.y. and a zircon age of 0.87 ± 0.27 m.y. The bracketing silts are likewise reversely magnetized. Therefore, the Salmon Springs Drift, as defined at its type section, is of middle Quaternary age and is much older than the currently accepted age of about 70,000 yr.

Trace-element geochemistry of Archean volcanic rocks and crystal growth in southwestern Abitibi Belt, Canada

Trace-element geochemistry on 176 controlled samples in three adjoining Abitibi (∼2.7 b.y.) volcanic piles confirms the fundamental concept of Archean volcanic cycles composed of lower tholeiitic and upper calc-alkalic parts. The volcanic piles, each as much as 16 km thick, are either unicyclic or multicyclic. In the latter case, the cycles display secular geochemical trends. All the volcanic rocks studied are indicated to have come from mantle sources. A fundamental feature of Abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources. Abitibi tholeiitic basalts closely resemble modern mid-ocean ridge basalts. Abitibi calc-alkalic andesites and alkalic rocks are very similar to modern oceanic island-arc andesites and to some modern volcanic-arc high-K rocks, respectively. The principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism—a type of “hot-spot tectonics” mechanism involving a layered mantle responsible for the early tholeiitic (depleted mantle source) and later calc-alkalic (undepleted mantle source) parts, respectively, of the volcanic cycles.