High Water Fugacity Research Articles

Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

AbstractInfrared absorption measurements of molecular water in sheared Cambrian quartzites in the footwall to the Moine thrust reveal a decrease in water content from 4080 to 1570 ppm with increasing recrystallization traced toward the overlying thrust at the Stack of Glencoul in northwest Scotland. These results are contrary to the expected correlation between shear strain and water content for quartz deformed by dislocation creep and water-weakening processes. The observed inverse correlation indicates that fluid inclusions and hydrous defects within grains were lost by mobile grain boundary sweeping and grain boundary diffusion. Although reduced water contents might lead to hardening as chemical weakening is diminished, quartz mylonites in the immediate footwall (5 mm) to the thrust are characterized by intense strain localization and contain the least water, and there is little evidence of shear zone widening. Water weakening appears to have been important throughout the quartz mylonites, controlled by the presence of water, not by water concentration. Fluids present within relict inclusions and at grain boundaries may have governed the high water fugacities critical for water weakening.

Petrogenesis of the 2090 Ma Zanhuang ring and sill complexes in North China: A bimodal magmatism related to intra-continental process

It is widely debated as to whether the ∼2100Ma bimodal magmatic events in the Eastern North China craton represent intra-continental rifting or marginal processes. The genetic relationship between the two components of bimodal suites is the key solution to this issue; and this is investigated on a group of newly identified ring complexes, as well as the apparently associated sills in Zanhuang (Southern Taihang Mts.). These ring complexes comprise varied components: pyroxenite, gabbro, anorthosite, K-rich A-type granite and minor Na-rich A-type granite. U-Pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a ∼2090Ma age, which represents crystallization timing and is contemporaneous with A-type granites of the ring complexes, as well as the sills in the Gantaohe Group. The mafic rocks of the ring complexes and sills show similar chemical features: they were characterized by enriched light rare earth elements compared to middle and heavy ones (La/Ybn=1.4−2.8), enriched large lithosphile elements but depleted high field strength elements, and enriched Sr-Nd isotopes (εNdt (t=2090Ma)=−1.9–0.6, Nd TDM=∼2.7Ga). They were most likely originated from the same magma source resided in the sub-continental lithospheric mantle, which formed in the Late Archean. The compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase. The K-rich A-type granite, as well as the dacitic sills, could be originated from this same source with firstly olivine+plagioclase-dominated fractionation in the lower magma chamber to more felsic compositions, and then with significant removal of clinopyroxene±plagioclase in the upper magma chamber, which at the same time produced pyroxenite-gabbro and minor anorthosite cumulates in the ring complexes. Na-rich A-type granite was likely formed locally under relatively high-water fugacity with fractionation of K-feldspar+plagioclase±clinopyroxene from a parental magma similar to the K-rich A-type granite. This bimodal Zanhuang ring and sill complexes, and other comparable bimodal suites widely distributed in the Eastern North China craton, were likely intra-continental, representing a craton-scale extension at ∼2100Ma.

Petrofabric and seismic properties of lithospheric mantle xenoliths from the Calatrava volcanic field (Central Spain)

The microstructural and petrofabric study of peridotite xenoliths from the El Aprisco (Neogene Calatrava Volcanic Field) has provided new information on deformation mechanisms, ambient conditions and seismic properties of the central Iberian subcontinental mantle. Olivine, orthopyroxene, clinopyroxene, amphibole and spinel constitute the mineral assemblage in equilibrium. Their microstructure indicates that they accommodated crystal-plastic deformation under high water fugacity conditions. Crystallographic preferred orientation patterns of key minerals were determined with the EBSD technique. The xenoliths exhibit B, C and A olivine fabrics. B-type fabrics, involving the (010)[001] slip system, may develop in domains where deformation occurs under comparatively lower temperature, higher water-content and faster strain rates. They are interpreted here as the result of deformation in a suprasubduction mantle setting triggered by changing conditions imposed by a cooler subducting slab that incorporated fluids into the system. Xenoliths with olivine C-type fabrics involve activation of the dominant (100)[001] slip system, denote intracrystalline slip at higher temperatures and water-contents. They are here interpreted to sample lithospheric mantle domains where the impact of those new conditions was not so strong. Finally, the A-type fabrics, characteristic of the (010)[100] slip system, are frequent in the mantle under moderate to high temperature. These fabrics are considered here as characteristic of the mantle prior to subduction. The olivine fabrics constrain heterogeneous seismic properties. Propagation orientation of P waves (8.27–8.51km/s) coincides with olivine [100] axis concentrations, whereas the fastest S1 waves (5.13–5.22km/s) propagate parallel to [010] axis minima. The maximum shear wave birefringence (VS1–VS2=0.17–0.37km/s) is close to the direction of the macroscopic lineation. Heterogeneity of calculated seismic properties would concur with preservation of fossil flow perturbations, reworking and metasomatism in the subcontinental lithospheric mantle of central Spain, likely related to changes in the ambient conditions close to paleosubductions.

Phlogopite compositions as an indicator of both the geodynamic context of granitoids and the metallogeny aspect in Memve'ele Archean area, northwestern Congo craton

A barium bearing phlogopite (celsian) has been found for the first time within the charnockitic and tonalitic suites that compose Archean mineral belt in Cameroon. Electron microprobe analyses of these phlogopites are reported and contain moderate contents of BaO (0.42–1.26 wt. %) and up to 5.95 wt. % TiO2. Micas are Mg-rich and their compositions indicate phlogopites rich-meroxenes. Phlogopites from Memve'ele are characterized by a nearly horizontal trend of increasing total aluminium (2.494–2.931 a.p.f.u.) and relatively constant Fe/(Fe + Mg) suggesting contributions of aluminous supracrustal material to the magmas by anatexis or assimilation. Compositions of the barium titanium bearing phlogopite vary systematically according to rock types. It seems that the substitution scheme include Ba + Al + VI (Mg, Fe)2+ + 2 IVSi = K + Si + VITi + 2IVAl was dominant in the Memve'ele area thus, this scheme has made easy incorporation of Ba into phlogopite structure. The binary diagram of aluminium vs. titanium shows that phlogopites from the Memve'ele area have been formed by the same metasomatic mechanism as phlogopites from Canary Island xenoliths and Mezitler andesites but Ba enrichment of phlogopites from the Memve'ele area implies an early Ba-metasomatism contrary to those from Mezitler. The estimated temperature of the studied phlogopites indicated mainly two groups: (1) temperature range from 662 to 688 °C (average 676 °C) for phlogopite grains with High Mg# in the trondhjemite sample and (2) temperatures with interval limits from 757 to 800 °C (average 777.07 °C) for remnant phlogopites; reflecting primary and late crystallization respectively from slightly to highly oxidized magma (−17.30 to −13.87 Kbars). The geothermal gradient with average temperatures are 35.57–53.360 °C/Km and 30.95–46.42 °C/Km corresponding to 14.56–21.84 Km and 14.56–30.58 Km depth of below crust respectively. The crystallizing melt is enriched in Ba emanated from sea water at medium and high temperatures, low and high fugacity and high water fugacity generated Ba-bearing phlogopite grains both in trondhjemite samples (S13W6) and remaining granitoids during partial melting of the mantle. The phlogopite grains with low Mg# in S13W6 sample have more Ba than all remain phlogopites. The presence of high Ba contents in these phlogopite grains can be explained by (1) the admixture of residual and new Ba rich melts or (2) the presence of both low Mg and Mn contents in the octahedral site that generate a large interlayer site which accommodate more Ba or (3) both mechanisms are displayed to crystallize these phlogopite grains. Thus Ba rich phlogopites occur in the fresh granitoids depending only on early magmatic processes or hydrothermal alteration at high temperature. Contents of titanium are only controlled by temperature during phlogopite crystallizations. Moreover, igneous phlogopites are used as metallogenic indicator. The study is useful to the exploration efforts for barium ore and shows that the Ba-rich parent granitoids can produce Ba-rich soils which can be potential economic interest for Cameroon.

Permian hornblende gabbros in the Chinese Altai from a subduction-related hydrous parent magma, not from the Tarim mantle plume

In the Chinese Altai, on the northern side of the Erqis fault, the ~10-m-wide Qiemuerqieke gabbro is composed almost entirely of hornblende and plagioclase. Its relative crystallization sequence is olivine, hornblende, plagioclase, and it shows a narrow compositional range in SiO 2 (48.7–50.2 wt%), MgO (6.33–8.54 wt%), FeO (5.27–6.46 wt%), Na 2 O (3.06–3.71 wt%), and K 2 O (0.26–0.37 wt%). These contents result in a high Mg# value (68–72) and a low K 2 O/Na 2 O ratio of ~0.1. It has ( 87 Sr/ 86 Sr) i ratios of 0.70339–0.70350, e Nd(t) values of 4.8–6.0, and zircon e Hf(t) of 11.4–15.8; these values demonstrate a mantle-derived source, and a whole-rock δ 18 O of ~6.7‰ suggests a mantle wedge origin. The presence of magmatic hornblende suggests a relatively high water fugacity, and the crystallization temperature (715–826 °C) calculated using Ti-in-zircon thermometry is considerably lower than that of a normal mafi c melt but consistent with an origin from a water-bearing magma. The gabbro has a secondary ion mass spectrometry zircon U-Pb age of 276.0 ± 2.1 Ma, which is coeval with the 275 Ma mantle plume in the northern Tarim craton, but the Qiemuerqieke petrological and geochemical data do not indicate an abnormally high mantle temperature or a deep mantle signature, which would commonly characterize a mantle plume source. Our results integrated with published data support a model of juvenile crustal growth by a subduction-related process.

Crystallographic preferred orientation produced by diffusional creep of harzburgite: Effects of chemical interactions among phases during plastic flow

Experimental deformation experiments have been conducted on fine‐grained, two‐phase aggregates of olivine and orthopyroxene to investigate the role of grain and phase boundary sliding on rheology and fabric development. A suite of large‐strain (γ ≥ 1) general shear experiments conducted at T = 1200°C and P = 1.6 GPa on aggregates ranging from 65 to 0 vol % orthopyroxene, to characterize the evolution of fabric, were complemented by small‐strain axial compression experiments at T = 1200°C and P = 0.3 GPa, to better constrain the rheology. Microstructural and rheological data suggest that deformation of these two‐phase aggregates in the diffusion creep regime occurs via interface‐reaction‐controlled diffusion creep that is accompanied by extensive migration of olivine‐orthopyroxene phase boundaries. The resulting rheologies suggest that olivine + orthopyroxene composites are weaker than the olivine end‐member at the conditions tested. Physically, this behavior arises because long‐range, i.e., grain‐scale, diffusion of Si4+ is unnecessary in these pseudobinary two‐phase aggregates. We further demonstrate that interface‐controlled diffusion creep leads to strong crystallographic preferred orientations (CPO) of the component minerals, which develops in the near absence of dislocation activity. The CPO formed in these anhydrous, low‐stress experiments has the olivine a axis aligned perpendicular to the flow direction (“type B” fabric) argued by some to be the unique result of deformation under conditions of high differential stress and high water fugacity. Phase boundary dynamics, thus, are argued as a significant factor in the accumulation of strain in polyphase aggregates.

Garnet-bearing tonalitic porphyry from East Kunlun, Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO 17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (eNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in gener- ating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

The partitioning of water between olivine, orthopyroxene and melt synthesised in the system albite–forsterite–H 2O

Coexisting forsterite, enstatite and silicate glass were synthesised from a water-bearing basalt analogue in programmed cooling experiments at 1.0, 1.5, 2.0 and 2.5 GPa. Hydrous species in each phase were characterised and quantified using infrared absorbance spectroscopy. Al 3+-related hydroxyl defects, with vibrational energies between 3400–3200 cm − 1 , dominate the hydroxyl budget of synthetic forsterite. The vibrational characteristics of these defects indicate that similar absorptions in Fe-bearing systems result from Fe 3+-related OH species. This interpretation resolves a previously recognised, but unexplained, relationship between oxygen fugacity and hydroxyl speciation in experimentally annealed olivines, and suggests that the intensities of bands at 3400–3300 cm − 1 could form the basis of an oxybarometer. OH defect speciation in enstatite is similar to that in aluminous crystals synthesised at high water fugacity and many natural orthopyroxenes. Hydroxyl speciation in orthopyroxene is not, therefore, controlled by water activity. Total water contents of synthetic enstatites from any single experiment vary enormously. This heterogeneity is attributed to variable contributions from a non-intrinsic broad-band hydrous component. Intrinsic hydroxyl concentrations dissolved in enstatite are identical in all crystals from an experiment. The partitioning of water between forsterite, enstatite and coexisting silicate melt is independent of pressure, temperature and total water content over the range of conditions studied, as long as the influence of these parameters on phase compositions is taken into account. D H 2O Fo/melt = 0.0001–0.0003 ( n = 4) and is insensitive to pressure or olivine aluminium contents. D H 2O En/melt is strongly controlled by Al 2O 3 concentration and ranges from 0.003–0.016 ( n = 6). Two samples contain both forsterite and enstatite and yield D H 2O En/Fo = 25 ± 1.

Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and Pressure

AbstractEclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027–1427 °C, confining pressures of 2.5–3.5 GPa, and strain rates of 1 × 10−5 s−1 to 5 × 10−4 s−1. We obtained a power‐law creep for the high temperature and pressure deformation of a ‘dry’ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 103.3 ± 1.0, n = 3.5 ± 0.4, ΔE =403 ± 30 KJ mol−1 and ΔV = 27.2 cm3 mol−1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back‐scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear‐strain conditions, yielding a random LPO similar to that of non‐deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.

Hydroxyl induced eclogite fabric and deformation mechanism

Eclogites from orogens often show strong plastic deformation and high hydroxyl content. We have studied the correlation between crystallographic preferred orientations of garnet and omphacite from natural eclogites with their hydroxyl contents using the electron back-scattered diffraction technique. The results show: 1) Omphacite has typical L-type or SL-type crystrallographic preferred orientations, that is, [001] is distributed in a girdle in the foliation plane with a maximum parallel to lineation; (010) is distributed in a girdle normal to the lineation with a maximum parallel to the foliation plane, suggesting a shear dominant deformation regime. Omphacite fabrics do not vary significantly with hydroxyl content, although the hydrous component may cause lower flow strength. 2) Hydroxyl can influence significantly flow properties of garnet in eclogite. Garnets behave as rigid bodies under low temperature and dry conditions. Grain boundary processes will dominate the deformation and lower the flow strength of garnet under high water fugacity conditions. Garnets show no crystallographic preferred orientation in both cases. These results may have important implications for a better understanding of deformation mechanisms and associated fluid activities during deep subduction and exhumation processes.

Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin, Central Mexican Volcanic Belt

The Sierra Chichinautzin Volcanic Field is a key area for understanding the origin of the controversial Mexican Volcanic Belt (MVB). This is due to its recent volcanic activity, which is located at the front of the Central MVB. Although a direct genetic relationship between the volcanism of the MVB and the subduction of the Cocos plate is accepted by most authors, geological, geophysical and geochemical key features of the MVB depart from those of typical magmatic arcs. This fact has promoted a debate on the nature of the complex mantle beneath the MVB: (1) slab-induced convection in the mantle wedge that causes advection of asthenospheric mantle and (2) anomalously hot mantle related to the eastward channeling of plume-related material. This is a petrological study of the Sierra Chichinautzin volcanism, using mineralogical data from selected representative samples, together with their geochemistry, to refine the existing models of magmatic processes. The integrated petrological analyses have been used to define four main groups of rocks in Sierra Chichinautzin: (1) Ol-mafic rocks, (2) Opx-intermediate rocks, (3) Opx-felsic rocks, and (4) Qtz-intermediate/felsic rocks. Moreover, two different types of mantle-derived primitive mafic magmas have been described based on their mineralogical and geochemical features: (1) OIB-like mafic magmas (presence of olivine+plagioclase phenocrysts; higher incompatible element contents with no HFSE/LILE negative anomalies), and (2) mafic magmas derived from a metasomatized mantle (no plagioclase phenocrysts and more forsteritic olivine; HFSE/LILE negative anomalies). We prove that the diversity of magmas (basalts to dacites) in Sierra Chichinautzin is un-related to fractional crystallization processes from the mafic magmas. Rather, magma mixing phenomena between mafic magmas and two different crustal-derived felsic magma types (Opx-dacite and Qtz+Bt+Amp+Pl+Cpx+Opx-dacite) explain the textural, mineralogical, and geochemical signatures of most of the Sierra Chichinautzin rocks. There is a strong genetic relationship between the mafic and felsic end members of each mixing trend. The relatively anhydrous OIB-like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions, generating Opx-bearing felsic rocks. On the contrary, hydrous mafic magmas derived from a metasomatized mantle, promote basaltic crustal partial melting under high water fugacities, yielding magmas with a complex hydrous phenocryst assemblage. Several important geodynamical implications for the origin of the MVB magmatism can be extracted from this petrological study: (1) the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath Sierra Chichinautzin; (2) present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath Sierra Chichinautzin; and (3) the negative Nb anomaly of most of the Sierra Chichinautzin rocks (andesites and dacites) is due to magma mixing, and therefore is not a signature of the role of fluids from the subducting Cocos plate. These geochemical features, together with the geophysical and tectonic scenario of the Sierra Chichinautzin, indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot OIB-like mantle, and cast further doubts on a significant role of the subducting Cocos plate in the origin of this magmatism.

The uptake and solubility of water in quartz at elevated pressure and temperature

The uptake of water in quartz at 1.5 GPa total pressure, 1173 K and high water fugacity, over times up to 24 h, has been investigated using a newly developed assembly to prevent microcracking. It is found that the uptake is small, and below the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity or the solubility or a combination of both, and is in agreement with the observations of Kronenberg et al. (1986) and Rovetta et al. (1986). It brings into question the interpretation of the early experiments on water weakening by Griggs and Blacic (1964) and the recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985).