Carbonatites from source to surface

Pergamon PII:

Journal of African Earth Sciences, Vol. 25, No. 1, pp. 1-3, 1997 1997 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 0899-5362/97 $17.00 + 0.00

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Carbonatites from source to surface R. E. HARMER Council for Geoscience, Private Bag Xl 12, Pretoria 0001, South Africa

This special issue of the Journal of African Earth Sciences contains papers drawn mostly from presentations made at the IGCP 314 scientific Meeting held under the auspices of the 10 th International Conference of the Geological Society of Africa at Nairobi, Kenya from 9-13 October 1995. This symposium and the associated excursion to Oldoinyo Lengai in northern Tanzania, organised by Abigail Church and Liya Kogarko, were the last official scientific functions of the lUGS-UNESCO supported IGCP 314 project "Alkaline and Carbonatite Magmatism". IGCP 314 was initiated in 1991 and jointly co-ordinated by Liya Kogarko (Vernadsky Institute, Russian Academy of Science), Keith Bell (Carleton University) and J6rg Keller (Freiburg). In the five years of its existence, IGCP 314 was instrumental in bringing together carbonatite researchers at regular intervals and facilitated visits to many carbonatite and alkaline magmatic provinces. It is perhaps fitting that IGCP 314 should close in Africa, a continent richly endowed with alkaline and carbonatitic rocks. The last 10 years have seen a significant resurgence of interest in the petrogenesis and evolution of carbonatite and alkaline silicate magmatism. This has resulted in (and to some extent been engendered by) a combination of the availability of new experimental data at mantle pressures along with substantial amounts of new radiogenic and stable isotopic data gathered in conjunction w i t h conventional petrological and mineralogical observations. Subjects covered in this issue include the presentation of new geological observations, new experimental, chemical and isotopic data along w i t h n e w models for c a r b o n a t i t e petrogenesis and a critique of current carbonatite terminology. Several melting studies on peridotitic starting materials have demonstrated that carbonatite liquids form as near-solidus melts of carbonated peridotite compositions at a range of mantle pressures (Wallace and Green, 1988; Thibault et aL, 1992; Sweeney, 1994). Much is now understood about the relationship between

carbonatite melt composition, composition of the equilibrium peridotite and pressure (Dalton and Wood, 1993; Sweeney, 1994). These studies show that near-solidus melts of carbonated mantle would be magnesian carbonatite liquids with significant amounts of alkalis and that on ascent, if chemical equilibrium was maintained b e t w e e n the melt and enclosing mantle peridotite, these melts would disappear through reaction with mantle peridotite and the evolution of CO2. A large proportion of the recent literature on carbonatites, particularly that relating to the carbonatites of the East African Rift system, is d o m i n a t e d by d e s c r i p t i o n s of c a l c i t e carbonatites. As a consequence this has led to the perception that, since most carbonatites are calcitic, these experimental results have greater application to mantle metasomatism than to the problem of carbonatite petrogenesis. Two papers in the present volume relate directly to this discussion. Harmer and Gittins review the common occurrence of dolomitic carbonatites in the ancient cratonic areas of southern Africa and Canada and show that calcite carbonatites do not dominate in these regions. They present field and p e t r o g r a p h i c a r g u m e n t s w h y these magnesian carbonatites are not the result of replacement of calcitic carbonatites but that they r e p r e s e n t the p r o d u c t s of l o w - p r e s s u r e c r y s t a l l i s a t i o n of magnesian c a r b o n a t i t e magmas. Experimental investigations by Dalton and Wood (1993) have shown that carbonatite melts exist in equilibrium with pargasite-bearing and phlogopite bearing Iherzolite at depths of + 75 km or more, and that, on ascent, reaction between this melt and mantle wall rocks generates clinopyroxene, transforming peridotite to w e h r l i t e . E q u i l i b r a t i o n of m a g n e s i a n ("dolomitic") carbonatite mantle-melts with peridotite at depths shallower than 75 km, shifts the composition of the carbonatite melt to more calcic (" calcitic ") compositions. Using these results, Harmer and Gittins develop petrological schemes whereby both calcitic and dolomitic carbonatites

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C a r b o n a t i t e s f r o m s o u r c e to s u r f a c e

may be derived from primary magnesian mantlederived carbonate melts. Johnson eta/. describe the composition of megacrysts ejected in the Deeti tuff cone which occurs on the flanks of the carbonatite-bearing Kerimasi volcano in northern Tanzania. Interestingly, pargasite, phlogopite and diopsidic clinopyroxene feature prominently amongst the Deeti megacrysts. Preliminary studies show that the ~Sr-~,dvalues of pargasite from Deeti are identical to those measured in the Kerimasi calcitic carbonatites (Church, 1995). Clearly, the Deeti macrocrysts have the mineralogical, chemical and isotopic characteristics of the reaction products of peridotite metasomatised by the parental carbonate melt from which the Kerimasi carbonatite was ultimately derived. Kerimasi lies close to the volcano of OIdoinyo Lengai which is currently active and erupts natrocarbonatite lava. Oldoinyo Lengai has played a significant role in the imagination of most carbonatite petrologists since it was first recognised as an active carbonatitic volcano in 1961. Nyamweru has documented the volcanological development of Oldoinyo Lengai in several reports and her paper in this special issue presents a review of the volcano's activity up to February 1997, and includes observations made during the IGCP 314 field excursion, 1 3-1 7 October 1995. If carbonatites can be derived by the evolution of primitive carbonate mantle melts, then the origin of the alkalic silicate magmatism frequently associated with carbonatites becomes something of an open question. It is perhaps time to challenge the current view that carbonatites are the "secondary" products of the evolution of primary alkalic silicate magmas; indeed, the actual necessity of a direct petrogenetic link between alkaline silicate and carbonatite magmatism should be re-evaluated. Might the relationship be nothing more than spatial? The petrological association of carbonatites and syenites in the Lueshe Complex of Zaire is discussed by Kramm eta/. On the basis of isotopic data these authors argue that a substantial portion of the syenitic components of that complex were generated by the re-mobilisation of fenites produced through interaction between the Lueshe carbonatite and its country rocks. In this situation carbonatite magmatism is interpreted as producing silicate magmas: in contrast to the immiscibility model which regards carbonatites as the products of silicate magmatism. It is worth speculating whether the processes documented at Lueshe might have wider implications. For example, how many of the undersaturated silicate rocks commonly associated with carbonatites could represent melts of deeper crust extensively fenitised by fluids evolved from the carbonatite magma?

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An important consideration in the study of carbonatite evolution is the question of what samples of solid carbonatite, taken from outcrop or from borehole cores, actually represent. The common occurrence of alkali fenitisation of country rocks, as documented at Lueshe, provides evidence of the substantial loss of alkalis and other elements from the crystallising carbonatite: the very low alkali contents in most calcite and dolomite carbonatite samples are clearly not representative of the concentrations in their parental magmas. It is also important to constrain the extent of possible sub-solidus chemical changes to the crystallised carbonatite. Modification of primary carbonatites is dealt with in three papers: Sch~irmann eta/. document the l a t e - s t a g e a l t e r a t i o n of v o l c a n i c and p y r o c l a s t i c c a r b o n a t i t i c d e p o s i t s in t h e Proterozoic Kruidfontein carbonatite of South Africa. The alteration is characterised using petrographic, geochemical and 8~80-513C isotopic data and the mineralisation related to this m o d i f i c a t i o n is d e s c r i b e d . In similar vein Onuonga et al. describe l o w - t e m p e r a t u r e modifications to the carbonatites of the Buru and Kuge C o m p l e x e s in w e s t e r n Kenya. P e t r o g r a p h i c and 5~80-5~3C data i n d i c a t e substantial sub-solidus modification and isotopic r e - e q u i l i b r a t i o n in r e s p o n s e to m e t e o r i c hydrothermal and supergene processes. Such alteration processes are important in enhancing the economic potential of volcanic carbonatite centres. Horstmann and Verwoerd present a compilation of published and new 5180-5~C measurements on South African carbonatites. These data are used to interpret the cause of the "shift" noted in many carbonatite ~180 values to levels higher than that anticipated for primary magmatic carbonates (e.g. as summarised in Deines, 1989). These authors argue that the variation in 5180-513C noted in southern African carbonatites is the result of both magmatic and sub-solidus processes. Interestingly, data on calcite, dolomite and "ankerite" from the Spitskop Complex are interpreted as indicating a replacement origin for the magnesian carbonatites in this complex. Harmer and Gittins, however, showed that the ~s -ENdin these dolomite-ankerite carbonatites are more mantle-like than those for the calcite carbonatites. In addition to their intrinsic petrological interest, carbonatites are important in that they are the principal source of several important metals, most notably the rare earth elements, Nb and Ta. Aspects of the economic mineralogy of carbonatites are addressed by Williams et al. who describe the variations in mineral chemistry

Carbonatites f r o m source to surface

noted in pyrochlore from the Bingo carbonatite in Zaire. Substantial compositional variation is identified in these pyrochlores which is attributed to both primary magmatic zonation and extensive secondary alteration. Perovskite is a common accessory mineral in carbonatites and has economic importance in that it is one of the major hosts of Nb and the rare earths. Mitchell presents new experimental data which define the solubility of the perovskite group minerals in hydrous calcitic carbonatite melts at 1 kbar. These preliminary results show that perovskite and Ioparite are highly soluble in carbonatitic melts of this composition whereas tausonite and lueshite are not stable under these conditions. The ability of a carbonate magma to nucleate phases in which Ti, REE and Nb are major constituents emphasises the need to use terms and concepts such as "incompatible elements" with a degree of circumspection. While REE and Nb are incompatible in basalt systems, they will clearly be compatible in carbonatite magmas c r y s t a l l i s i n g p y r o c h l o r e and p e r o v s k i t e : fractionation or accumulation of these phases will induce order of magnitude changes in concentration levels of these elements. If progress is to be made in understanding carbonatite petrogenesis then it is important that the terminology used to discuss these rocks is precise and unambiguous. "Ferrocarbonatite" dykes are commonly described as late stage features of many carbonatite complexes from which the implication is often drawn that carbonatite complexes evolve to Fe-rich residua. Gittins and Harmer review the ambiguities associated with the current useage of the term "ferrocarbonatite" and demonstrate that almost all published a n a l y s e s of late stage "ferrocarbonatite" dykes are not enriched in Fe, but are actually hematite-calcite carbonatites. They suggest that the term "ferrocarbonatite" be excluded from the lUGS modal classification and propose an alternative to the lUGS chemical classification scheme to limit the field of ferrocarbonatites to those in which Fe-rich carbonates dominate. Research into the origin and evolution of carbonatites is clearly entering an exciting phase. As sophisticated experimental, geochemical and isotopic data continue to accumulate, the need for these to be reconciled with the field and petrographic evidence presented by the rocks t h e m s e l v e s is of g r o w i n g i m p o r t a n c e . Conventional and received wisdoms must be

critically challenged as should existing and developing hypotheses. Perhaps the most important attribute required to advance our understanding of carbonatite petrogenesis as we approach the new millenium is an open mind.

Acknowledgements Successful compilation of a special issue of this type requires the collaboration and cooperation of several groups of people: the authors, the referees and the editorial staff in St. Andrews. The c o n t r i b u t i n g authors are thanked for adhering to the deadlines and for responding to the required modifications in good time. It is important to also acknowledge the efforts of the panel of expert referees for their thorough and constructive reviews of the papers: Torn Andersen, Dan Barker, Thinus Cloete, John Gittins, Chris Harris, J6rg Keller, Mike Le Bas, Roger Mitchell, Dave Reid, Tony Simonetti, Russell S w e e n e y , Ken T a i n t o n , Wilhelm Verwoerd, Frances Wall, and Alan Woolley. Thanks, too, to Bob Thomas who did the editorial handling of the papers on which I was involved. Last but not least, the efforts of the staff of the Editorial Office in St. Andrews in expertly compiling the issue must be acknowledged. In particular, the encouragement and rapid replies to frantic e-mail queries by Christine Hardwick is greatly appreciated.

References Church, A. A. 1995. The petrology of the Kerimasi carbonatite volcano and the carbonatites of OIdoinyo Lengai with a review of other occurrences of extrusive carbonatite. Ph D. Thesis (unpubl.) University College, London. Dalton, J. A. and Wood, B. J. 1993. The compositions of primary carbonate melts and their evolution through wallrock reaction in the mantle. Earth Planetary Science Letters 119, 511-525. Deines, P. 1989. Stable isotope variations in carbonatites. In: Carbonatites: Genesis a n d Evolution (Edited by Bell, K.) pp 301-359. Allen and Unwin, London. Sweeney, R. J. 1994. Carbonatite melt compositions in the earth's mantle. Earth Planetary Science Letters 128, 259-270. Thibault, Y., Edgar, A. D. and Lloyd, F. E. 1992. Experimental investigation of melts from a carbonated phlogopite Iherzolite: implications for metasomatism in the continental lithosphere. Amer i can Mineralogist 77, 784-794. Wallace, M. E, and Green, D. H, 1988. An experimental d e t e r m i n a t i o n of p r i m a r y c a r b o n a t i t e m a g m a composition. Nature 335, 343-346.

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