The models recognize that ZrSiO4, ZrTiO4,  and TiSiO4, but not ZrO2 or TiO2, are independently variable phase components in zircon. Accordingly, the equilibrium controlling the Zr content of rutile coexisting with zircon is ZrSiO4 = ZrO2 (in rutile) +  SiO2. The equilibrium controlling the Ti content of zircon is either ZrSiO4 + TiO2 = ZrTiO4 + SiO2 or TiO2 + SiO2 = TiSiO4, depending whether Ti substitutes for Si or Zr. The Zr content of rutile thus depends on the activity of SiO2
 $$(a_{\text{SiO}_{2}})$$
 as well as T, and the Ti content of zircon depends on $$a_{\text{SiO}_{2}}$$
  and 
 $$a_{\text{TiO}_{2}}$$
 as well as T. New and published experimental data confirm the predicted increase in the Zr content of rutile with decreasing $$a_{\text{SiO}_{2}},$$
 and unequivocally demonstrate that the Ti content of zircon increases with decreasing $$a_{\text{SiO}_{2}}$$
 . The substitution of Ti in zircon therefore is primarily for Si. Assuming a constant effect of P, unit $$a_{\text{ZrSiO}_{4}},$$
 and that $$a_{\text{ZrO}_{2}}$$
  and $$a_{\text{ZrTiO}_{4}}$$
 are proportional to ppm Zr in rutile and ppm Ti in zircon, [log(ppm Zr-in-rutile) + log
 $$a_{\text{SiO}_{2}}$$
 ] = A1 + B1/T(K) and  [log(ppm Ti-in-zircon) + log
 $$a_{\text{SiO}_{2}}$$
 − log
 $$a_{\text{TiO}_{2}}$$
 ] = A2 + B2/T, where the A and B are constants. The constants were derived from published and new data from experiments with $$a_{\text{SiO}_{2}}$$
 buffered by either quartz or zircon + zirconia, from experiments with $$a_{\text{SiO}_{2}}$$
 defined by the Zr content of rutile, and from well-characterized natural samples. Results are A1 = 7.420 ± 0.105;  B1 = −4,530 ± 111;  A2 = 5.711 ± 0.072;  B2 = −4,800 ± 86 with activity referenced to α-quartz and rutile at P and T of interest. The zircon thermometer may now be applied to rocks without quartz and/or rutile, and the rutile thermometer applied to rocks without quartz, provided that $$a_{\text{SiO}_{2}}$$
  and $$a_{\text{TiO}_{2}}$$
 are estimated. Maximum uncertainties introduced to zircon and rutile thermometry by unconstrained $$a_{\text{SiO}_{2}}$$
  and 
 $$a_{\text{TiO}_{2}}$$
 can be quantitatively assessed and are ≈60 to 70°C at 750°C. A preliminary assessment of the dependence of the two thermometers on P predicts that an uncertainty of ±1 GPa introduces an additional uncertainty at 750°C of ≈50°C for the Ti-in-zircon thermometer and of ≈70 to 80°C for the Zr-in-rutile thermometer.

New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers

Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature:

 $$\log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.$$
 In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T
 −1:

 $$\log ({\text{Zr}}_{{{\text{rutile}}}}) = (7.36 \pm 0.10) - \frac{{4470 \pm 120}}{{T\;(\hbox{K})}}.$$
 The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.

Crystallization thermometers for zircon and rutile

The deep carbon cycle and melting in Earth's interior

The crystal structures of uranyl minerals and inorganic uranyl compounds are important for understanding the genesis of U deposits, the interaction of U mine and mill tailings with the environment, transport of actinides in soils and the vadose zone, the performance of geological repositories for nuclear waste, and for the development of advanced materials with novel applications. Over the past decade, the number of inorganic uranyl compounds (including minerals) with known structures has more than doubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structures that contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological details of their structural units, which are formed by the polymerization of polyhedra containing higher-valence cations. Overarching structural categories correspond to those based upon isolated polyhedra (8), finite clusters (43), chains (57), sheets (204), and frameworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either their graphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement of anions within the sheets.

U6+ minerals and inorganic compounds: insights into an expanded structural hierarchy of crystal structures

Titanium is one of many trace elements to substitute for silicon in the mineral quartz. Here, we describe the temperature dependence of that substitution, in the form of a new geothermometer. To calibrate the “TitaniQ” thermometer, we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt, at temperatures ranging from 600 to 1,000°C, at 1.0 GPa. The Ti contents of quartz (in ppm by weight) from 13 experiments increase exponentially with reciprocal T as described by: 

$$ {\text{Log}}{\left( {X^{{{\text{qtz}}}}_{{{\text{Ti}}}} } \right)} = (5.69 \pm 0.02) - \frac{{(3765 \pm 24)}} {{T(K)}}. $$

Application of this thermometer is straightforward, typically requiring analysis of only one phase (quartz). This can be accomplished either by EPMA for crystallization temperatures above 600°C, or by SIMS for temperatures down to at least 400°. Resulting temperature estimates are very precise (usually better than ±5°C), potentially allowing detailed characterization of thermal histories within individual quartz grains. Although calibrated for quartz crystallized in the presence of rutile, the thermometer can also be applied to rutile-absent systems if TiO2 activity is constrained.

TitaniQ: a titanium-in-quartz geothermometer

Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree. In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content (ppm by weight), pressure (GPa) and reciprocal absolute temperature: $ {\text{log}}({\text{Zr}}^{{{\text{sphene}}}} ,{\text{ppm}}) = 10.52( \pm 0.10) - \frac{{7708( \pm 101)}} {{T(K)}} - 960( \pm 10)\frac{{P({\text{GPa}})}} {{T(K)}} - {\text{log}}(a_{{{\text{TiO}}_{2} }} ) - \log (a_{{{\text{SiO}}_{2} }} ). $The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization.

A thermobarometer for sphene (titanite)

Rutile saturation in hydrous siliceous melts and its bearing on Ti-thermometry of quartz and zircon

It is thought that mixing of Earth's outer core material back into the mantle following core formation may be responsible for the proportion of elements observed in upper mantle rocks, but the mechanism by which this might occur remains unknown. Leslie Hayden and Bruce Watson report experimental results from a study of grain-boundary diffusion of siderophile ('iron-loving') elements through polycrystalline MgO. Siderophile elements are good indicators of a core contribution to the mantle due to their extreme enrichment in the core. They find diffusivities high enough to mark out grain-boundary diffusion as a potentially significant mechanism in the transport of siderophile elements across the core–mantle boundary over geologically significant distances (tens of kilometres) over the age of the Earth. Experimental results from a study of grain-boundary diffusion of siderophile ('iron-loving') elements through polycrystalline MgO are reported. The diffusivities computed are high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. Understanding the geochemical behaviour of the siderophile elements — those tending to form alloys with iron in natural environments — is important in the search for a deep-mantle chemical ‘fingerprint’ in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models1,2. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks3. Such re-mixing has been attributed to an unspecified metal–silicate interaction in the reactive D″ layer just above the core–mantle boundary4. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core–mantle interactions is controversial5. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales6 and that grain boundaries can be primary storage sites for incompatible lithophile elements7, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by ∼1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.

A diffusion mechanism for core–mantle interaction

Single crystals of eight zippeite-group compounds have obtained using mild hydrothermal synthesis techniques. The structure of each has been determined with single-crystal diffraction data collected using Mo K α X-radiation and an APEX CCD-based detector, and refined on the basis of F 2 for all unique data. The structure of zippeite, K 3 (H 2 O) 3 [(UO 2 ) 4 (SO 4 ) 2 O 3 (OH)], is mono-clinic, C 2, a 8.7524(4), b 13.9197(7), c 17.6972(8) A, β104.178(1)°, V 2090.39(17) A 3 , R 1 3.30%, D c 4.794 g/cm 3 . The structure of sodium-zippeite, Na 5 (H 2 O) 12 [(UO 2 ) 8 (SO 4 ) 4 O 5 (OH) 3 ], is monoclinic, P 2 1 / n , a 17.6425(11), b 14.6272(9), c 17.6922(11) A, β104.461(1)°, V 4421.0(5) A 3 , R 1 6.88%, D c 4.517 g/cm 3 . The structure of magnesium-zippeite, Mg(H 2 O) 3.5 [(UO 2 ) 2 (SO 4 )O 2 ], is monoclinic, C 2/ m , a 8.6514(4), b 14.1938(7), c 17.7211(9) A, β 104.131(1)°, V 2110.24(18) A 3 , R 1 2.39%, D c 4.756 g/cm 3 . The structure of zinc-zippeite, Zn(H 2 O) 3.5 [(UO 2 ) 2 (SO 4 )O 2 ], is monoclinic, C 2/ m , a 8.6437(10), b 14.1664(17), c 17.701(2) A, β 104.041(3)°, V 2102.7(4) A 3 , R 1 4.57%, D c 5.032 g/cm 3 . The structure of cobalt-zippeite, Co(H 2 O) 3.5 [(UO 2 ) 2 (SO 4 )O 2 ], is mono-clinic, C 2/ m , a 8.650(4), b 14.252(9), c 17.742(10) A, β 104.092(19)°, V 2122(2) A 3 , R 1 5.55%, D c 4.948 g/cm 3 . The structure of (NH 4 ) 4 (H 2 O)[(UO 2 ) 2 (SO 4 )O 2 ] 2 is monoclinic, C 2/ m , a 8.6987(15), b 14.166(2), c 17.847(3) A, β 104.117(4)°, V 2132.9(3) A 3 , R 1 4.31%, D c 4.442 g/cm 3 . The structure of (NH 4 ) 2 [(UO 2 ) 2 (SO 4 )O 2 ] is orthorhombic, Cmca , a 14.2520(9), b 8.7748(5), c 17.1863(10) A, V 2149.3(2) A 3 , R 1 5.11%, D c 4.353 g/cm 3 . The structure of Mg 2 (H 2 O) 11 [(UO 2 ) 2 (SO 4 )O 2 ] 2 is monoclinic, P 2 1 / c , a 8.6457(4), b 17.2004(8), c 18.4642(9) A, β 102.119(1)°, V 2684.6(2) A 3 , R 1 4.73%, D c 3.917 g/cm 3 . Each structure contains the zippeite-type sheet consisting of chains of edge-sharing uranyl pentagonal bipyramids that are cross-linked by vertex sharing with sulfate tetrahedra, although the compositional details of the sheet are varied. The interlayer configurations are diverse, and are related to the bonding requirements of the sheets.

The crystal chemistry of the zippeite group

http://www2.ess.ucla.edu/~manning/pdfs/hm11.pdf

Leslie A. Hayden

Papers

A thermobarometer for sphene (titanite)

Rutile saturation in hydrous siliceous melts and its bearing on Ti-thermometry of quartz and zircon

A diffusion mechanism for core–mantle interaction

The crystal chemistry of the zippeite group

Rutile solubility in supercritical NaAlSi3O8–H2O fluids