Fluid Fluxing Research Articles

U, Th and Ra disequilibria, Sr, Nd and Pb isotope and trace element variations in Sunda arc lavas: predominance of a subducted sediment component

The time scales of magma formation, transfer and storage beneath island arc volcanoes are important because of the insights they provide into the physical processes of magma formation and evolution with possible links to the prediction of eruptive hazards. Here we present new mass spectrometric U, Th and Ra, and Sr, Nd and Pb isotope data, along with major and trace element data, from 19 lavas along the Sunda arc, Indonesia. The lavas range in SiO2 from 49 to 75% and the combination of high SiO2 and low 143Nd/144Nd indicates an important role for shallow-level contamination by ancient crustal materials in Sumatra. Diagrams of 143Nd/144Nd versus Th/Ce and U/Th versus 87Sr/86Sr reveal negative trends which allow contributions from both subducted sediments and fluids to be identified. The fluid component as expressed in elevated U/Th, (238U/230Th) and (226Ra/230Th)o ratios is ubiquitous in all of the lavas including near primary lavas from Galunggung. Sr isotope data indicate that the fluids had variable composition and derived from altered oceanic crust, and the U, Th and Ra isotope data suggest that some fluid components must have been added less than 8,000 years ago. Tholeiitic lavas from Flores have the lowest Th contents in the arc and accordingly the highest (226Ra/230Th)o and (238U/230Th). For these there is evidence that both U and Ra addition probably occurred less than a few thousand years ago. This suggests that the onset of melting was linked to fluid fluxing and cannot have been due to decompression alone. In contrast, the sediment component may have been added as a partial melt and sediment addition can account for the observed along-arc variations in Sr, Nd, Pb and Th isotopes. There is no requirement in our data for an OIB component in the mantle wedge. Increases in SiO2 in the Galunggung lavas are accompanied by decreases in both (238U/230Th) and (226Ra/230Th)o, which are inferred to reflect mixing with melts of pre-existing mafic lavas. Mixing and differentiation must have occurred faster than could be recorded by either the 238U–230Th or 226Ra–230Th systems. There is little evidence in our data for any effect of the Australia–Indonesia collision on the composition of the lavas. Across-arc trends in lava composition indicate an increasing relative contribution from subducted sediment, a correspondingly weaker fluid signal and probable decreases in the degree of partial melting. Residual aluminous clinopyroxene or garnet may become important in controlling REE and U/Th fractionation in the far back-arc.

O–H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O–H isotope signatures of Alpine ultramafic rocks and eclogitic metagabbros of the Erro–Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction-related deformation along serpentinite mylonite shear zones favoured preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and mineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5–8‰) are slightly enriched in 18O compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4–7.6‰). The lowest values occur in high-pressure veins (3.5–5.7‰) and eclogitic metagabbros (3.1–5.3‰). These variations are comparable to variations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic δ18O signatures of the Erro–Tobbio rocks and a lack of oxygen isotope re-equilibration between different shear zones imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generations show a bimodal distribution in δD values: pre-eclogitic lizardite and chrysotile range from –102 to –77‰; high-pressure antigorite in the mylonites and in low strain metaperidotites range from –71 to –57‰ and –83 to –60‰, respectively. Comparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptake in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The 18O-enriched and H-depleted compositions of chrysotile and lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at the early stages of burial in a forearc setting, where slabs undergo large-scale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. The Erro–Tobbio unit thus represents an example of cycling of internally-derived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure metamorphism.

Fluid Fluxing of Cumulates: the J-M Reef and Associated Rocks of the Stillwater Complex, Montana

olivine-bearing rocks in the lower half of OB I. The olivinerich rocks also define pegmatoidal ‘pothole’ structures. (3) Rock composition, texture and grain size vary considerably in olivinebearing units, characterized by segregation between coarser mafic regions and medium-grained felsic regions. (4) Primary plagioclase in olivine-rich rocks has an eroded texture. (5) Modal variations KEY WORDS: layered intrusions; partial melting; platinum-group elements are consistent with progressive incongruent melt reactions, and hydrous minerals are most abundant in olivine-rich rocks. (6) Hydrous melt inclusions (now crystallized to polyphase, hydrous mineral-bearing assemblages) are present in olivine and chromite.

Fluid Fluxing of Cumulates: the J-M Reef and Associated Rocks of the Stillwater Complex, Montana

Fluid Fluxing of Cumulates: the J-M Reef and Associated Rocks of the Stillwater Complex, Montana

Trace element and U-series systematics for 1963-1965 tephras from Irazú Volcano, Costa Rica: implications for magma generation processes and transit times

In order to investigate fluid fluxing and the time-scales of magma genesis in the mantle below Costa Rica, basaltic andesites erupted in 1963–1965 from Irazú volcano were analyzed for major and trace element compositions as well as thorium isotope and 226Ra abundances. The 1963–65 eruption of Irazú produced two groups of tephras termed high-Ra, with ( 226Ra)/( 230Th) = 1.13–1.21 and low-Ra with near equilibrium ( 226Ra)/( 230Th) values. Low-Ra and high-Ra samples have similar overall compositions. For example, barium and HSF element concentrations are about the same in the two magma groups, and all tephras have ( 230Th)/( 232Th) ratios of about 1.20 and ( 238U)/( 230Th) ≅ 0.9. However, low-Ra samples have 6–16 % higher concentrations of many highly incompatible elements compared to high-Ra samples, whereas REE, Sr, and P 2O 5, concentrations are higher by about 3–8% in the high-Ra samples. These compositional differences can be explained by variations in the degree of melting and in the compositions of fluids involved in magma genesis. Low-Ra magmas were generated by dehydration partial melting of garnet-bearing mantle that had been enriched previously by a slab fluid. High-Ra magmas were generated by a similar mechanism, but involved an additional mantle-derived fluid. Trace element compositions of the high-Ra magmas suggest that this fluid was similar to a carbonatite. The temperature and pressures during melting were appropriate for carbonatite-silicate melt miscibility, and the carbonatitic fluid was consumed or partly consumed during melting. The lower concentrations of many highly incompatible trace elements in the high-Ra samples resulted from a higher degree of partial melting of its mantle source caused by fluid-silicate melt miscibility. The approximately equal HFS element and Ba concentrations in the low- and high-Ra samples, and the relatively high REE, Sr, and P 2O 5 concentrations in the high-Ra samples reflects the addition of these elements to the high-Ra magma from the fluid. Ra-Ba-Th geochronometry suggests that the enrichment event occurred between a few centuries and eight millennia before the eruption.

SHRIMP II dating of zircons and monazites: reassessing the timing of high‐grade metamorphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia

ABSTRACT Key insights into the timing of tectonometamorphic events in a complex high‐grade metamorphic terrane can be obtained by combining results from SHRIMP II ion microprobe studies of individual monazite grains with SHRIMP II studies and scanning electron microscope (SEM)‐based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high‐T and low‐P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785–1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM‐based CL imaging. Monazites in a 1806± 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high‐T event prior to 1580 Ma. Most monazites from the region record only a single high‐T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (≤750°C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post‐tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data.The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low‐P, high‐T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.

SHRIMP ionprobe dating of short-lived Proterozoic tectonic cycles in the northern Arunta Inlier, central Australia

Structural and metamorphic evidence from low- P, high- T metamorphic terranes within the northern Arunta Inlier, central Australia, indicates that two discrete tectonic cycles affected the region. Each cycle was associated with compressive deformation, coincident with peak granulite facies metamorphism and granite intrusion. SHRIMP UThPb analysis of zircons from early-, syn- and late-tectonic granitoids allows the age of the two tectonic cycles to be directly measured. The age of the first cycle (D 1) is determined by emplacement of the Mount Stafford Granite, dated at or before 1818±15 Ma, after the first two foliations of the cycle had formed, but before the axial-plane cleavage associated with later upright folding. The regional extent of this tectono-magmatic event is confirmed by an emplacement age of 1818±8 Ma measured on the large, discordant Harverson Granite, 40 km away. The second cycle (D 2) consisted of four distinct deformation stages (D 2a−2d), the ages of which are constrained by 1785-1775 Ma old, early-, syn- and late-tectonic granitoids. The Warimbi Granite (emplaced at or before 1785±22 Ma) and the granitic protolith to the Napperby Gneiss (inferred to be 1780±10 Ma) pre-date, or formed during, the earliest foliation (S 2a). Zircon considered to have grown at peak granulite facies metamorphic conditions, during generation of the dominant S 2b foliation, gives an age of 1774±6 Ma in the Possum Creek Charnockite. Leucogranite in the Napperby Gneiss, which post-dates S 2b and large-scale, upright, NWSE-trending F 2d folds, yields an age of 1775±12 Ma and confirms that the entire tectonic cycle (D 2a−2d) formed in a rapid (∼ 10 Ma) period. Younger zircons, which formed at 1659±6 Ma in the Napperby Gneiss, cannot be related to any local structural event, but are similar in age to a major magmatic/metamorphic episode in the southern Arunta (Black and Shaw, 1992), ∼ 150 km away. They may be the peripheral manifestation of that event. Further zircon growth at ∼ 1600 Ma in the Napperby Gneiss and Possum Creek Charnockite was caused by hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement. These dykes were deformed by amphibolite facies, retrograde shear zones. Ion probe dating of zircon “stratigraphy” from the granitoids has confirmed the discrete successive tectonic cycles identified by earlier structural and metamorphic studies. For the northern Arunta Inlier at least, those cycles (or “orogenies”) are associated with short-lived (∼ 10 Ma) thermal pulses related to granite magmatism, not prolonged thermal perturbations spanning 100 Ma or more (cf. Dirks and Wilson, 1990; Dirks and Hand, 1991). Such short-lived pulses are intimately associated with granite intrusion and may be a general feature of low- P, high- T metamorphism.

Leucogranites of the Himalaya/Karakoram: implications for magmatic evolution within collisional belts and the study of collision-related leucogranite petrogenesis

Two types of leucogranite have been identified in a collisional setting in the Himalaya. Type-a (e.g., Manaslu, High Himalaya) are syn-tectonic crustal melts related to “hot over cold” thrusting and subsequent fluid fluxing. Type-b (e.g., Baltoro, Karakoram) is related to lower crustal melting during/following crustal thickening and thermal re-equilibration, followed by crystal fractionation. For type-b, melting is triggered/promoted in the lower crust by input of heat and material from the sub-continental lithospheric mantle which is contemporaneously melting due to removal of the slab input and downward displacement. Both these granite types can be explained in terms of recent thermal models for continental collision, and so might be expected to be found in other orogenic terranes. These models also show that extension and thinning will occur as a result of the crust becoming gravitationally unstable, due to thickening, some time after collision. Thinning will lead to upwelling and melting of asthenosphere which could then intrude the base of the crust and promote further melting. This could explain the common occurrence of A-type granitoids as final plutonic events in orogenic terranes following intrusion of the collision-related type-a and type-b granites. Existing genetic classification schemes only classify granites in terms of source-type or broad environmental affinity (e.g., collisional). To further subdivide such broad environmental fields it is considered that processes and a general evolutionary model must be used. Source-type is not suitable due to the number of source components that are often involved and the overlap of these components from one environment to another. One or two selected features of a granite are not, however, sufficiently exclusive to identify different petrogenetic histories. We propose that an evaluation of field style, mineralogy, isotope and elemental geochemistry should be used to subdivide the collisional granite field into type-a and type-b (or Manaslu and Baltoro-types) respectively.

Structural controls on metamorphism and syn-tectonic magmatism: the Portuguese Hercynian collision belt

Upper Palaeozoic collisional movements along the Ibero-Armorican arc were accommodated on oblique strike-slip ductile shear zone systems. In north-central Portugal, these shear zones are coincident with long linear belts of high temperature, low pressure metamorphism and S-type granitic plutons. A model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12–15 km; this was achieved by a combination of (i) increased heat flow, (ii) lowering of melting temperatures caused by fluxing of hydrous fluids, and (iii) a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts. Shear zones are proposed as the most important influence in controlling the site of development of such belts; anatexis of metasedimentary crust and emplacement of the resulting S-type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source.

Charnockite and related neosome development in the Eastern Ghats, Orissa, India: petrographic evidence

ABSTRACTCharnockite and homophanous leptynite are two quartz–feldspar neosomes in the late Archaean granulite-migmatite Eastern Ghats terrane of Orissa, India. Three phases of deformation and metamorphism are recognised to have preceded a late phase, or phases in which deformation was not intense. An early granulite facies event (D1–M1) produced basic granulites and khondalite (sillimanite-garnet aluminous paragneiss). A second event (D2–M2) involved some retrogression but temperatures and pressures remained high (possibly c. 750°C—>5 kb). D3–M3 was associated with the development of open folds (F3) with axial planar shear zones in pinched antiforms; temperatures and pressures were also falling (c. 650°C— 4kb), but the shear zones acted as a control on the development of firstly homophanous leptynite, and secondly charnockite. In both cases, neosomes of quartz and potassium feldspar have myrmekitic textures and intergranular occurrence. The palaeosome became strained and garnet–biotite and garnet–biotite–clinopyroxene ± orthopyroxene mineral assemblages were formed in association with the development, respectively, of homophanous leptynite and charnockite. The persistence of straining in the palaeosome and the presence of poikilitic textures indicate that this typical assemblage does not represent one in thermodynamic equilibrium. This has important consequences for geothermobarometry.The structural control on neosome development reflects a regional characteristic where shear zones are the site of peralkaline plutons, fenitisation, carbonatites and epigenetic graphite development. The peculiar features of charnockite are seen as just one of several aspects related to the fluxing of hot CO2-dominated fluids from either the deep crust or mantle during the late Archaean development of the Indian craton.