High Field Strength Cations Research Articles

Structural environment of nickel in silicate glass/melt systems: Part 2. Geochemical implications

The two Ni coordination states put in evidence in Part 1 indicate distinct connections with the silicate glass structure. If low field-strength cations are available for charge compensation, Ni occurs mainly as four-coordinated Ni ( [4]Ni) and belongs to the silicate framework. The higher the field strength of the charge-compensating cation, the lower the relative amount of Ni present in this coordination state. The NiOSi (α) angle keeps a value close to 130° whatever the associated cation. This intertetrahedral angle is smaller than that measured in alkali aluminosilicate glasses, which indicates a smaller repulsion between [4]Ni and Si than between Al and Si. Five-coordinated nickel ( [5]Ni) is present in glasses containing higher field strength cations (Ca, Na, Mg). Bond length-bond strength considerations show that [5]Ni may only bond to oxygens belonging to the tetrahedral network (including bridging oxygens). [5]Ni inherits the dynamic structure of silicate melts and appears to be the consequence of the fast oxygen motion due to the viscous flow, a structural property which is quenched in the glass. Both Ni coordination states are observed in all silicate glasses, and their proportion only depends on glass composition. These structural data provide a rationale for understanding Ni behavior in natural magmas because they are independent of the Ni content of the glass over more than two orders of magnitude (0.1–27 wt% NiO). Crystal Field Stabilization Energy (CFSE) has been calculated for [5]Ni and [4]Ni in glasses; by considering the Ni-site distribution observed in the compositions studied, the average CFSE varies between 45 and 65 kJ · mol −1. There is thus a strong preference of Ni for the octahedral sites of all rock-forming minerals which explains, at least in part, the strong enrichment of Ni in olivines crystallizing from a basaltic melt.

Structural environment of nickel in silicate glass/melt systems: Part 1. Spectroscopic determination of coordination states

Nickel environments in silicate and aluminosilicate glasses have been investigated using X-ray Absorption Near Edge Structure (XANES) spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and optical absorption spectroscopy. Glass compositions included nickel end-members such as Ni-diopside (CaNiSi 2O 6) and Ni-alkali silicates (Na 2NiSi 3O 8 and K 2NiSi 3O 8) as well as glasses with compositions of diopside, anorthite, potassium trisilicate, and sodium trisilicate and disilicate containing 1–3 wt% NiO. Optical spectroscopy, XANES, and EXAFS show that nickel occurs as [4]Ni and [51]Ni in all the glasses investigated, [4]Ni being predominant in potassic glasses as [51]Ni prevails in glasses with higher field strength cations (Na, Ca, Mg). [4]Ni occurs in a distorted tetrahedral site and [5]Ni in a slightly distorted trigonal bipyramid. The geometry of [4]Ni and [5]Ni sites does not vary much among the glasses investigated, as shown by the constant position of [4]Ni and [5]Ni optical absorption bands. XANES parameters are intermediate between those expected for [4]Ni and [5]Ni. EXAFS-derived mean Ni-O distances and number of oxygen neighbors are 1.96–1.99 Å and 3.8–4.7 (±0.5), respectively, depending on glass composition. They also suggest a mixing of [4]Ni and [5]Ni with [4]Ni-O = 1.95 A ̊ and [5]Ni-O = 2.00 A ̊ , respectively. Including the data of the literature, spectroscopic data on Ni are independent on the total Ni content of the glass investigated (0.1–27 wt% NiO). However, the relative proportion of the two Ni coordination states depends upon glass composition. Silicon second neighbors were determined by EXAFS in all glasses at Ni-Si distances of 3.2–3.3 Å, depending on glass composition, but the number of Si neighbors is higher in the potassic glasses. Finally, there is no indication of significant amounts of [6]Ni in silicate glasses, which is consistent with the structural data and models on the sites occupied in silicate glasses by other small divalent cations, such as Mg and divalent transition metal cations.

Fluorine in silicate glasses: A multinuclear nuclear magnetic resonance study

Anhydrous nepheline, jadeite, and albite glasses doped with F as well as hydrous F-containing haplogranitic glasses were investigated using 19F combined rotation and multiple-pulse spectroscopy; 19F → 29Si cross-polarization/magic angle spinning (MAS); and high-power 19F decoupled 29Si, 23Na, and 27Al MAS nuclear magnetic resonance methods. Fluorine preferentially coordinates with Al to form octahedral AlF 6 3− complexes in all glasses studied. In addition, F anions bridging two Al cations, units containing octahedral Al coordinated by both O and F, or tetrahedral Al-F complexes might be present. The presence of Si-F bonds cannot be entirely ruled out but appears unlikely on the basis of the 19F → 29Si CP/MAS spectra. There is no evidence for any significant coordination of F with alkalis in the glasses studied. 23Na spectra are identical for the samples and their F-free equivalents and the spectra do not change upon decoupling of 19F. The speciation of F in the hydrous and anhydrous glasses appears to be very similar. Over the range of F contents studied ( up to 5 wt.% ), there seems to be hardly any dependence of F speciation on the concentration of F in the samples. The spectroscopic results explain the decrease of the viscosity of silicate melts with increasing F content by removal of Al from bridging AlO 4-units due to complexing with F, which causes depolymerization of the melt. The same mechanism can account for the shift of the eutectic point in the haplogranite system to more feldspar-rich compositions with increasing F content, and for the peraluminous composition of most F-rich granites. Liquid immiscibility in F-rich granitic melts might be caused by formation of (Na,K) 3AlF 6 units in the melt with little or no interaction with the silicate component. The presence of F in granitic melts might increase the solubility of high field strength cations by making nonbridging O atoms available which form complexes with these cations.

Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe, Western Australia

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (≈1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al2O3 and CaO < 1%, Na2O≤0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ≠ 91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr2O3 (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ≠ 92–94 ) have comparatively high Cr2O3 (0.2%–0.45%) and Na2O (up to 0.18%) but very low Al2O3 contents (0.5%–0.7%). Diopsides (Mg ≠ 92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr2O3) and have low Al2O3 (0.7%–2.2%) and Na2O (0.5%–1.6%) contents. Many have high K2O contents, typically 0.1%–0.4% but up to 1.3% K2O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe2+)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr2O3, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr2O3) titanate resembling armalcolite but containing significant K2O (1%–2.5%), CaO (0.6%–2.2%), ZrO2 (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly 13C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (≥2 Ga) enrichment in incompatible elements.

Shoshonitic volcanism in the Northern Mariana Arc: 2. Large‐ion lithophile and rare earth element abundances: Evidence for the source of incompatible element enrichments in intraoceanic arcs

The Mariana‐Volcano‐Izu arc system extends 2400 km north to south and is an outstanding example of an intraoceanic magmatic arc. In spite of this, the system is poorly known because most of it is submarine. Volcanism is entirely submarine in the northernmost Mariana arc, between 20°40′ and 24°N. This is the Northern Seamount Province (NSP) and was the focus of a detailed marine geologic and geochemical study, with additional data drawn from adjacent arc segments to the north (Volcano arc) and south (Central Island Province (CIP) of the Mariana arc). Samples from 24 submarine volcanoes and three islands were analyzed for concentrations of K, Rb, Sr, Ba, and the rare earth elements (REE). These data show strong variations along the arc, being relatively depleted in the more mature, tholeiitic and low‐K calc‐alkaline volcanoes of the Volcano arc and the Mariana CIP, containing on average 6100 ppm K, 300 ppm Sr, 200 ppm Ba, and 6 ppm La. All of the NSP is enriched in large ion lithophile (LIL) and light rare earth elements (REE), particularly the northern half (26,000 ppm K, 700 ppm Sr, 900 ppm Ba, 47 ppm La); these lavas have strong shoshonitic affinities. These enrichments do not result from fractional crystallization of CIP‐type melts. The source responsible for these enrichments shares some features in common with Mariana CIP and Volcano arc sources: K/Rb and K/Ba in particular are similar (∼500 and ∼30, respectively). However, Ba/La, Sr/Nd, and (Ce/Yb)n change drastically, with Ba/La and Sr/Nd decreasing to mantle values with increasing LIL and LREE enrichment. The origin of the LIL and LREE enrichments in the NSP shoshonites does not result from variations in the behavior or composition of the subducted lithosphere. Melting occurs exclusively within the mantle wedge, and forward modeling of the REE patterns for all Mariana and Volcano arc lavas indicates that melt generation occurs within the stability field of spinel lherzolite, probably within the upper 40–50 km of the subarc asthenosphere. Lavas from the large volcanoes of the Mariana CIP and Volcano arc result from 10–20% melting of spinel lherzolite, followed by varying amounts of low‐P fractional crystallization. Inferences based on REE forward models that the NSP shoshonites manifest very low (1%) degrees of partial melting of LIL‐ and LREE‐enriched spinel lherzolite are inconsistent with observed similar concentrations in tholeiites and shoshonites of high field strength cations such as TiO2 and Yb. Some of this inconsistency can be explained as resulting from source or melt mixing, with the NSP shoshonites being derived from a LIL‐ and LREE‐enriched source or melt, with Ba/La and La/Yb indistinguishable from that of ocean island basalts (OIB), while Mariana CIP and Volcano arc melts are derived from a depleted mild‐ocean ridge basalt‐like mantle that has been recharged with K, Rb, Sr, and Ba by hydrous fluids. These variations are interpreted as reflecting the evolution of the subarc asthenosphere, with a depletion in time resulting from the continuous extraction of basaltic melts.

Physical and chemical response of zircons to deformation

An investigation of U-Pb isotopic systematics in zircons from mylonitized Henderson Gneiss (Sinha and Glover 1978) revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic Pb at ∼ 460 m.y. To further investigate the relationship between Pb loss, U gain, and grain size reduction associated with increasing strain in the shear zone, we have characterized the chemistry and morphology of zircons in the mylonitic rocks, using both electron microprobe analysis and scanning electron microscopy. SEM photographs of the zircons indicate that strain-correlated fracturing and size reduction of the zircons accompanied Pb loss throughout the mylonite zone. Stresses imposed by the expansion of initially U-rich, α-damaged portions of the crystal resulted in microfracturing of the more brittle crystalline material proximal to the U-rich zones. During mylonitization, fractures propagated preferentially along these zones allowing metamorphic fluids to penetrate the easily-leached, α-damaged portions of the zircons. Removal of ∼75% of the radiogenic Pb from zircons in the least-deformed zone of the mylonites may have occurred via this mechanism. Irregular, porous zircon overgrowths are also evident from the SEM photographs. Overgrowths are strongly enriched in U, Y and P with respect to the relict, Henderson Gneiss-derived cores, and tend to increase in volume from the protomylonite to the blastomylonite. Thus, the development of overgrowths on the zircons accounts for the U gain observed by Sinha and Glover (1978), and indicates that the transport of high field strength cations (e.g., Zr4+, Hf4+, U4+, etc.) occurred during prograde mylonitization at 460 m.y. A retrograde shearing event at ∼ 273 m.y. caused no further disturbance in the U-Pb isotopic systematics of the zircons. Pb retention by zircons during the later episode may have been the result of 1) the participation of H2O-rich, relatively noncorrosive fluids and/or 2) the lack of further fracturing and size reduction in a strain gradient of lower magnitude than the prograde event.