Age constraint on Burmese amber based on U–Pb dating of zircons
Introduction
Amber from northern Myanmar, called Burmese amber or merely “burmite”, is the only Cretaceous amber deposit in the world that is exploited commercially, as well as the first to have been studied scientifically. The history of its use has been reviewed by Zherikhin and Ross (2000), Grimaldi et al. (2002), Cruickshank and Ko (2003), and Ross et al. (2010). Briefly, burmite had been used primarily in carvings for at least two millennia by Chinese people, for which the material is ideally suited (Grimaldi, 1996). The deposits in the Kachin state, northern Myanmar, are productive (an estimated 83 tons were exported between 1898 and 1940), and some amber pieces are very large (the largest is 15 kg, in the Natural History Museum, London). Moreover, colours vary from a transparent yellow to a highly desirable deep red (Fig. 1, Fig. 2, Fig. 3), the amber resists fracturing and is relatively hard (1.2 times harder than Baltic amber), and it receives a glassy polish. Burmite mining lapsed from just before the independence of Burma from Britain in 1947, and did not resume until the late 1990s. The greatest value of burmite, however, is scientific.
Amber in general preserves biological inclusions with microscopic fidelity, so as a mode of fossilization it is unparallelled for phylogenetic and palaeontological studies of Cenozoic and late Mesozoic terrestrial life forms (Grimaldi and Engel, 2005). Amber from the Cretaceous is further significant since it coincides with the radiation of the angiosperms and major tectonic shifts in continental positions, and precedes the famous end-Cretaceous impact event. Of the seven major deposits of amber from the Cretaceous Period (Table 1), Burmese amber contains probably the most diverse palaeobiota. For example, approximately 228 families of organisms (primarily arthropods) have been reported from burmite, compared to a range of 68–125 families recorded thus far in the other six major amber deposits. Only the much larger, commercially exploited deposits from the Miocene of the Dominican Republic and Mexico, and the Eocene Baltic amber have yielded more families and species. Interestingly, burmite contains an exceptional diversity and abundance of the most diverse order of insects, the Coleoptera (16% of all studied inclusions, representing more than 40 families, vs. 2–8% and around a dozen families in the other Cretaceous ambers).
Among the more significant records of organisms in burmite is the only Mesozoic fossil of the phylum Onycophora (“velvet worms”) (Grimaldi et al., 2002), as well as the oldest definitive Mesozoic records of mosquitoes, family Culicidae (Borkent and Grimaldi, 2004), and the insect orders Embiodea (Engel and Grimaldi, 2006), Strepsiptera (Grimaldi et al., 2005a, b), and Zoraptera (Engel and Grimaldi, 2002) (Fig. 4). Oddly, burmite also preserves the youngest records of several archaic insect groups, notably Postopsyllidium of the hemipteran family Protopsyllidiidae (previously known from the Permian–Jurassic) (Grimaldi, 2003), and Parapolycentropus, of the scorpionfly family Pseudopolycentropodidae (Triassic–Barremian) (Grimaldi et al., 2005a) (Fig. 4). Parapolycentropus is remarkable for the loss of the hind wings, specialized antennae, and long, styletiform proboscis, convergently resembling a mosquito. Burmite also preserves early, primitive species in groups that are highly social today, notably Formicidae (ants) and Isoptera (termites) (Engel and Grimaldi, 2005; Engel et al., 2007). One of these presumably social insects is Haidomyrmex (Dlussky, 1996), arguably the most peculiar ant known (Fig. 4).
Despite its scientific significance, precise dating of Burmese amber has been elusive. For the first 80 years of its scientific study, burmite was widely considered to be Eocene–Miocene in age, although Cockerell (1917) insightfully considered a Cretaceous age based on the insect inclusions. When Alexandr Rasnitsyn of the Palaeontological Institute in Moscow examined the burmite collection in the Natural History Museum, London in 1995, he noticed the presence of some Cretaceous insect groups in this amber, notably Serphitidae and the extinct subfamily of ants Sphecomyrminae (Zherikhin and Ross, 2000). This and other evidence established a Cretaceous age for the material, corroborated by expanded studies of myriad arthropod taxa in the NHML and AMNH collections (Grimaldi et al., 2002). Based on 21 insect taxa found within various stages of the late Mesozoic as well as in Burmese amber, a Cenomanian age was hypothesized by Grimaldi et al. (2002). Cruickshank and Ko (2003) reviewed the geology of the burmite deposits, based on published and original observations, and reported an ammonite specimen taken 2 m above an amber bed at the principal mine at Noije Bum, identified as Mortoniceras, which has a stratigraphic range of Middle–Upper Albian (Wright et al., 1996). Cruickshank and Ko (2003) cited unpublished reports by E.H. Davies (Branta Biostratigraphy Ltd.) of the fossil pollen, spores, and dinoflagellates, which further indicated an age of the sediments and thus the amber as “most likely Albian to early Cenomanian”. The late Albian age proposed by Cruickshank and Ko (2003) is widely cited in original and review papers on burmite (e.g., Ross et al., 2010, and references therein).
Section snippets
Amber/geological samples
Cruickshank and Ko (2003) described and mapped the locality of the amber mines that are the primary sources of the collections in the Natural History Museum, London and the American Museum of Natural History, New York. Chhibber (1934) listed 13 amber outcrops in the valley, some of which are outside the mine area described by Cruickshank and Ko (2003). Briefly, the mines are located in the Hukawng Valley (Hukawng Basin) of Kachin State, northern Myanmar, and specifically on Noije Bum, a hill
Petrography of amber-bearing matrix
The rock matrix containing amber is a greyish to bluish-green volcanoclastic to mudstone, consistent with the description by Cruickshank and Ko (2003) that the amber is located in the finer facies of sedimentary rocks they encountered at Noije Bum. On close inspection, the rocks consist of sub-millimetre black, yellow, grey and light green clasts. Thus it is understandable why Chhibber (1934) described the rocks as appearing blue in colour, while Cruickshank and Ko (2003) said that they are
Methods and results
The selected zircons were then cast in a transparent epoxy mount together with the standard zircon Plešovice. Cathodoluminescence (CL) imaging of zircon grains employed a scanning electron microscope (LEO1450VP with MiniCL instrument) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Backscattered electron (BSE) images and mineral compositions of the rock matrices were obtained using a JXA-8100 Electron Microprobe Analyzer (EMPA) at IGGCAS with a voltage of 15 kV,
Discussion and conclusions
As the zircons are from sedimentary rocks, the depositional age cannot be older than the youngest ages of Group-II zircons (i.e., 98.79 ± 0.62 Ma). There are two possible interpretations: either the depositional age is essentially the same as the Group-II zircons, implying that the volcaniclastics were airfall which immediately became sediments, or the zircons did not become sediment until some unspecified time later. Petrographic observations do not help in resolving between the two possibilities.
Acknowledgements
We are grateful to L.C. Chen and M.S. Du for the samples, and much appreciate the constructive reviews of A. Ross, E. Peñalver and A. Arillo, and the editorial handling by D.J. Batten. This investigation is financially supported by the National Basic Research Program of China (2009CB421008), the NCET in China (NCET-07-0771) and the Fundamental Research Funds for the Central Universities (2001YXL048).
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