Journal of Petrology - Advance Access

On Silica Activity and Serpentinization: Errata

Frost, B. R., Beard, J. S. — 2008-05-07

The Titanomagnetite-Ilmenite Equilibrium: New Experimental Data and Thermo-oxybarometric Application to the Crystallization of Basic to Intermediate Rocks

Sauerzapf, U., Lattard, D., Burchard, M., Engelmann, R. — 2008-05-07

Although the titanomagnetite–ilmenite thermo-oxybarometer has been widely used to provide information on temperature and oxygen fugacity during magmatic and metamorphic processes, the available formulations yield unsatisfactory results; for example, at high temperature and low to moderate fO₂ (i.e. in conditions relevant to crystallization in basic and intermediate rocks). We present a new version of this thermo-oxybarometer based on numerical fits of a large experimental dataset comprising new results in the Fe–Ti–Al–Mg–O system and those of literature studies. Our new subsolidus experimental results at temperatures in the range 1100–1300°C under low to moderate fO₂ conditions show that the addition of Mg and/or Al in the concentration ranges that are usual in Fe–Ti oxides from basic magmatic rocks can be accommodated by simple projections. We have taken advantage of this fact and performed numerical fits to generate empirical formulations. With the resulting expressions we can retrieve temperature values from X'_usp and X'_ilm (projected mole fractions) of titanomagnetite–ilmenite_ss pairs and fO₂ values from X'_usp and T. The present thermo-oxybarometer model is designed for assemblages of titanomagnetite and hemoilmenite (with the $$R\overline{3}$$ space group), with the usual low Al₂O₃, Cr₂O₃, MgO and MnO contents (less than about 6 wt %), which equilibrated at high temperatures (T ≥800°C) and low to moderate oxygen fugacities (–4 < NNO < +2, where NNO is the nickel–nickel oxide buffer). Tests of our model by using the compositions of titanomagnetite–ilmenite_ss pairs in products of liquidus experiments conducted at known T–fO₂ conditions (literature data and new results) show that the calculated values reproduce the experimental ones within ±70°C, and in most cases within ±50°C. The estimates of the oxygen fugacity are mostly within ±0·4 log units. This is a significant improvement compared with the previous models.

Ultra-refractory Domains in the Oceanic Mantle Lithosphere Sampled as Mantle Xenoliths at Ocean Islands

2008-05-03

Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al₂O₃, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al₂O₃ in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.

The Alkaline-Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source

Marks, M. A. W., Schilling, J., Coulson, I. M., Wenzel, T., Markl, G. — 2008-04-25

The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a_SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a_SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (FMQ = +2–5 to –1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (FMQ = –2) and intermediate a_SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.

Geochemistry of the Volcan de l' Androy Basalt-Rhyolite Complex, Madagascar Cretaceous Igneous Province

Mahoney, J. J., Saunders, A. D., Storey, M., Randriamanantenasoa, A. — 2008-04-19

The 4000 km² Androy massif in southeastern Madagascar is a >2000 m thick sequence of interbedded basalt and rhyolite erupted during a widespread Cretaceous episode of predominantly basaltic volcanism. Two geochemically different groups of basalt, tholeiitic group B1 and mildly alkalic B2, are present, as are two different groups of rhyolite, R1 and R2. Both the basalts and rhyolites appear to have issued from relatively nearby feeders, as compositionally equivalent intrusions are exposed in the vicinity. The R2 rhyolites define a whole-rock Rb–Sr isochron of 84·0 ± 2·4 Ma (2), the same, within error, as an ⁴⁰Ar–³⁹Ar sanidine age reported by earlier workers. Plate reconstructions suggest that the area was near the Marion hotspot at this time. Some involvement of hotspot mantle is allowed, but not required, by Nd–Pb–Sr isotope data for the basalts. The two types of basalt may have formed by different amounts of melting of the same mantle source, which remains rather poorly specified, but group B1 was affected much more than B2 by contamination with continental material, probably Archean crust. The R1 rhyolites are petrogenetically related to the B1 basalts, with which they are interbedded. The R2 rhyolites may be derived from melts of frozen high-_Nd B1 basalt coupled with fractionation and assimilation of relatively small amounts of crust. Alternatively, although these rhyolites were erupted significantly later than the B2 basalts, they may have formed through advanced crystal fractionation of B2-type magma and relatively small amounts of crustal assimilation. Separate magmatic plumbing systems appear to have existed more or less contemporaneously in the Androy area.

Igneous Layering, Fractional Crystallization and Growth of Granitic Plutons: the Dolbel Batholith in SW Niger

Pupier, E., Barbey, P., Toplis, M. J., Bussy, F. — 2008-04-19

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An_≤11), K-feldspar (Or_96–99, Ab_1–4), quartz, edenite (X_Mg = 0·37–0·55), augite (X_Mg = 0·65–0·72) and accessories (apatite, titanite and Fe–Ti-oxides). Whole-rock compositions are metaluminous, sodic (K₂O/Na₂O = 0·49–0·62) and iron-rich [FeO_tot/(FeO_tot + MgO) = 0·65–0·82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite ± plagioclase (C_a/p), edenite + plagioclase + augite + quartz (C_q), and edenite + plagioclase + augite + quartz + K-feldspar (C_k). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal C_a/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ. The subsequent C_q layers correspond to cotectic compositions in the Qz–Ab–Or phase diagram at P_H2O = 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (C_a/p and C_q layers); (3) equilibrium ‘en masse’ eutectic crystallization (C_k layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the C_k layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton construction.