However, it can be a residue of komatiite melt extraction at >7 Melting experiments on fertile peridotite KR4003, a ‘pyrolitic’ GPa from a mantle enriched in SiO2 relative to pyrolite. composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25°C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase

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Melting of Garnet Peridotite and the Origin of Komatiite and Depleted Lithosphere

▪ Abstract The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic 187Os/188Os, indicating long-term i...

THE Re-Os ISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

http://people.earth.yale.edu/sites/default/files/herzberg10.pdf

Thermal history of the Earth and its petrological expression

Several methods have been developed to assess the thermal state of the mantle below oceanic ridges, islands, and plateaus, on the basis of the petrology and geochemistry of erupted lavas. One leads to the conclusion that mantle potential temperature (i.e., TP) of ambient mantle below oceanic ridges is 1430°C, the same as Hawaii. Another has ridges with a large range in ambient mantle potential temperature (i.e., TP = 1300–1570°C), comparable in some cases to hot spots (Klein and Langmuir, 1987; Langmuir et al., 1992). A third has uniformly low temperatures for ambient mantle below ridges, ∼1300°C, with localized 250°C anomalies associated with mantle plumes. All methods involve assumptions and uncertainties that we critically evaluate. A new evaluation is made of parental magma compositions that would crystallize olivines with the maximum forsterite contents observed in lava flows. These are generally in good agreement with primary magma compositions calculated using the mass balance method of Herzberg and O'Hara (2002), and differences reflect the well-known effects of fractional crystallization. Results of primary magma compositions we obtain for mid-ocean ridge basalts and various oceanic islands and plateaus generally favor the third type of model but with ambient mantle potential temperatures in the range 1280–1400°C and thermal anomalies that can be 200–300°C above this background. Our results are consistent with the plume model.

/pdf/temperatures-in-ambient-mantle-and-plumes-constraints-from-t2dh8hln70.pdf

Temperatures in Ambient Mantle and Plumes: Constraints from Basalts, Picrites, and Komatiites

Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span. Beginning at 3.8 Ga (Isua, West Greenland), they are part of Archean cratons and range in age from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum at about 2.5 Ga (iron-formations in the Hamersley Basin of Western Australia) and they disappear from the geologic record at about 1.8 Ga, only to reappear between 0.8 and 0.6 Ga.

The stratigraphic sequences in which BIFs occur are highly variable. Most Archean iron-formations are part of greenstone belts that have been deformed, metamorphosed, and dismembered. This makes reconstruction of the basinal setting of such BIFs very difficult. The general lack of metamorphism and deformation of extensive BIFs of the Hamersley Range of Western Australia and the Transvaal Supergroup of South Africa allow for much better evaluations of original basinal settings. Most Archean iron-formations show fine laminations and/or microbanding. Such microbanding is especially well developed in the Brockman Iron Formation of Western Australia, where it has been interpreted as chemical varves, or annual layers of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8 Ga (e.g., those of the Lake Superior region, U.S.A., Labrador Trough, Canada, and the Nabberu Basin of Western Australia) commonly exhibit granular textures and lack microbanding.

The mineralogy of the least metamorphosed BIFs consists of combinations of the following minerals: chert, magnetite, hematite, carbonates (most commonly siderite and members of the dolomite-ankerite series), greenalite, stilpnomelane, and riebeckite, and locally pyrite. Minnesotaite is a common, very low-grade metamorphic reaction product. The Eh-pH stability fields of the above minerals (and/or their precursors) indicate anoxic conditions for the original depositional environment.

The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in age, is very similar. They are rich in total Fe (ranging from about 20 to 40 wt%) and SiO2 (ranging from 43 to 56 wt%). CaO and MgO contents range from 1.75 to 9.0 and from 1.20 to 6.7 wt%, respectively. Al2O3 contents are very low, ranging from 0.09 to 1.8 wt%. These chemical values show that they are clean chemical sediments devoid of detrital input. Only the Neoproterozoic iron-formations (of 0.8 to 0.6 Ga in age) have very different mineralogical and chemical make-ups. They consist mainly of chert and hematite, with minor carbonates.

The rare-earth element profiles of almost all BIFs,with generally pronounced positive Eu anomalies, indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water.

The prograde metamorphism of iron-formations produces sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-containing assemblages. Such metamorphic reactions are isochemical except for decarbonation and dehydration.

The common fine lamination (and/or microbanding) as well as the lack of detrital components in most BIFs suggest that such are the result of deposition, below wave base, in the deeper parts of ocean basins. Those with granular textures are regarded as the result of deposition in shallow water, platformal areas. Carbon isotope data suggest that for a long period of time (from Archean to Early Proterozoic) the ocean basins were stratified with respect to δ13C (in carbonates) as well as organic carbon content. In Middle Proterozoic time (when granular BIFs appear) this stratification diminishes and is lost.

The Neoproterozoic BIFs occur in stratigraphic sequences with glaciomarine deposits. These BIFs are the result of anoxic conditions that resulted from the stagnation in the oceans beneath a near-global ice cover, referred to as “Snowball Earth.”

All of the most “primary” mineral assemblages appear to be the result of chemical precipitation under anoxic conditions. There are, as yet, no data to support that BIF precipitation was linked directly to microbial activity. The relative abundance of BIF throughout the Precambrian record is correlated with a possible curve for the evolution of the O2 content in the Precambrian atmosphere.

Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins

Archaean felsic metavolcanic rocks in the Superior Province of the Canadian Shield may be divided into three major groups on the basis of trace-element abundances and ratios. (1) FI felsic metavolc...

Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada

Magmatic Fe-Ni-Cu ± platinum-group element (PGE) sulfide deposits form when mantle-derived mafic and ultramafic magmas become saturated in sulfide and segregate immiscible sulfide liquid, commonly following interaction with crustal rocks. Although the metal contents of primary magmas influence ore compositions, they do not control ore genesis because the metals partition strongly into the sulfide liquid and because most magmas capable of segregating sulfide liquid contain sufficient abundances of ore metals. More important controls are the temperature, viscosity, volatile content, and mode of emplacement of the magma, which control the dynamics of magma emplacement and the degree of interaction with crust. By this measure, high-temperature, low-viscosity komatiites and tholeiitic picrites are most capable of forming Ni-Cu-(PGE) deposits, whereas lower-temperature, volatile-rich alkali picrites and basalts have less potential. In most deposits, ore formation is linked directly to incorporation of S-rich country rocks and only indirectly to contamination by granitic crust. However, the geochemical signature of contamination is easily recognized and is a useful exploration guide because it identifies magmas that had the capacity to incorporate crustal material. Several aspects of the ore-forming process remain poorly understood, including the control of mantle melting processes on the PGE contents of mafic-ultramafic magmas, the mechanisms by which sulfur is transferred from wall rocks to ores (bulk assimilation, incongruent melting, and/or devolatilization), the distances and processes by which dense sulfide melts are transported from where they form to where they become concentrated (as finely-dispersed droplets, as segregated layers, or by deformation-driven injection of massive sulfide accumulations), and the dynamic processes that increase the metal contents of the ores.

https://hal.archives-ouvertes.fr/hal-00016864/document

Mantle-Derived Magmas and Magmatic Ni-Cu-(PGE) Deposits

REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

Volcanogenic massive Cu-Zn-(Pb) sulfide (VMS) deposits occur primarily in subaqueous rift-related environments (e.g., oceanic, fore-arc, arc, back-arc, continental margin, or continental), are hosted primarily by bimodal, mafic-felsic volcanic successions, and are typically associated with felsic volcanic rocks with specific geochemical characteristics. FI alkalic dacites and rhyodacites, despite being abundant in the rock record, are typically barren. Some FII calc-alkalic rhyodacites and rhyolites host VMS deposits, but most are barren. FIII tholeiitic and FIV depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host VMS deposits, regardless of age, and FIII rhyolites appear to host the largest deposits.

Most petrogenetic models proposed for the formation of FII and FIII-FIV felsic volcanic rocks link felsic magma genesis to fractionation processes in high-level magma chambers now represented by associated subvolcanic intrusions, where the magma is also interpreted to have supplied the heat and/or metals required to generate and sustain the VMS-forming convective hydrothermal system. However, the relatively constant compositions of FII and FIII-FIV felsic volcanic rocks within individual areas, the high eruptive temperatures (at or above liquidus) of FIII rhyolites, and the bimodality of VMS-hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of FII and FIII-FIV magmas. This, coupled with detailed geological, geochemical, and geochronological studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later, most voluminous phases often cut ore-associated, hydrothermally altered rocks.

A reassessment of the physical conditions responsible for producing the geochemistry of ore-associated FII and FIII-FIV felsic volcanic rocks and a review of the compositions of felsic volcanic rocks associated with VMS deposits that range in age from Mesoarchean to Cenozoic provide important constraints on models for VMS-associated felsic volcanic rocks and their relationship to mineralization. The compositions of felsic volcanic rocks may be explained by low to moderate degrees of partial melting of mafic sources at a range of depths within rift environments where the mineralogy and composition of the source regions, modes, and degrees of partial melting, pressure and temperature of melting, and, to a lesser extent, subsequent fractionation processes, account for the compositional variations from FI through FII to FIII-FIV. Long-lived, enhanced heat flow and structural permeability of rift environments that allows partial melting to form some FII rhyolites at midcrustal levels (10–15 km) and FIII-FIV rhyolites at shallow crustal levels (<10 km), both within the zone of brittle fracture permeability, are essential to sustain the high-temperature convective hydrothermal systems that are required to form large VMS deposits and camps. Rift environments contain long-lived, thermal, magmatic, and structural corridors that focus magma ascent, heat flow, high-temperature convective hydrothermal systems, and emplacement of subvolcanic intrusions that are favorable environments for the formation of VMS deposits and FII and FIII-FIV felsic volcanic rocks.

C. M. Lesher

Papers

Temperatures in Ambient Mantle and Plumes: Constraints from Basalts, Picrites, and Komatiites

Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada

Mantle-Derived Magmas and Magmatic Ni-Cu-(PGE) Deposits

REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

TRACE ELEMENT GEOCHEMISTRY AND PETROGENESIS OF FELSIC VOLCANIC ROCKS ASSOCIATED WITH VOLCANOGENIC MASSIVE Cu-Zn-Pb SULFIDE DEPOSITS