Partitioning of High Field Strength and Rare Earth Elements Between Sector Zones in Diagenetic Titanite

Jon Bouch Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB9 2UE, Scotland

Malcolm Hole Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB9 2UE, Scotland

Nigel Trewin Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB9 2UE, Scotland


Sandstones from the Cock of Arran (Scotland) contain titanite as a pore filling cement phase that crystallised during late diagenesis (van Panhuys-Sigler and Trewin, 1990). A pilot BSE-SEM and WDS-EPMA study revealed the presence of sector zonation within the titanites caused by variations in concentrations of REE, Y, P, Zr, Nb (Hole et al., 1992).

Sector zoning patterns

The pore filling morphology of the majority of the titanites makes it difficult to relate compositional sector zones to specific crystallographic zones. Heavy mineral separates from some samples, however, contain euhedral titanite grains with sector zonation that can be related to crystal morphology. Unlike sector zonation described in some igneous titanites (Paterson and Stephens, 1992), the titanites described here contain approximately equal expression of faces and sectors arising from the crystallographic forms {100} and {111}. Other, unidentified, minor forms are also observable in some sections.

Chemical variations between sectors

The {100} sectors have higher Fe, REE, Y, Zr, Nb, and P and lower Ca, Ti and Si concentrations than the {111} sectors. Calculation of differential partition coefficients (Ddiff = [element]{100} / [element]{111}) shows that P is the most strongly partitioned element between the two sector types (average Ddiff = 55). The REE and other HFSE have Ddiff which range from 2.5 to 10. Al and F are less strongly partitioned between sectors having Ddiff = 1.3 and 1.4 respectively. In the titanites the dominant substitution scheme is;

(Al,Fe)3+ + F- = Ti4+ + O2-. (1)

However, WDS-EPMA analysis for F in these titanites shows that F concentrations are not sufficient to entirely account for all the Al and Fe present and the coupled substitutions;

(P,Nb)5+ + (Al,Fe)3+= 2Ti4+ (2)

(REE,Y)3+ + (Al,Fe)3+ = Ca2+ + Ti4+ (3)

account for much of the remainder. The relative importance of the three schemes varies between sectors and the pronounced partitioning of P between sectors produces particularly large variations in the relative importance of substitution scheme (2).

Rare earth element chemistry

Condrite normalised REE profiles for a number of samples have been constructed using REE abundances measured by ion-microprobe (University of Edinburgh, 1995). There are observable differences between grains, but the profiles obtained are all LREE enriched (average Lan/Ybn = 7) and convex upwards with maxima around Ce-Nd. This pattern may reflect the ability of the titanite structure to accommodate these elements more readily as a result of the close similarity between the ionic radii of Ce, Pr & Nd and that of Ca (1.06Å in sevenfold co-ordination). Within individual grains the main difference in REE chemistry is that different sectors have different absolute REE abundances. However, average Ddiff for the REE show systematic variations, with the HREE being less strongly partitioned (e.g. Yb, Ddiff = 3.16) than the LREE (e.g. Ce, Ddiff = 4.03). While this effect is relatively minor, it is sufficient to cause REE profiles from {100} sectors to have more LREE enriched REE profiles than {111} sectors. In the titanites studied here Pr, with an ionic radius equal to that of Ca, shows the greatest degree of partitioning between sectors (Ddiff = 4.33).


The titanites described here demonstrate that the REE and HFSE can be mobilised in relatively low T (maximum zeolite facies; Hole et al., 1992) aqueous systems. They also show how it is possible to produce pronounced partitioning of trace elements, particularly P, due to disequilibrium crystal growth.


Hole, M.J., Trewin, N.H. & Still, J., J. Geol. Soc., London 149, 689-692 (1992).

Paterson, B. & Stephens, W.E., Contrib. Mineral. Petrol. 109, 373-385 (1992).

van Panhuys-Sigler, M. & Trewin, N.H., Scott. J. Geol. 26, 139-144 (1990).