T.T. Win

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

Abstract The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO 2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P - T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m 2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.

Trace elements in garnets and chromites: Diamond formation in the Siberian lithosphere

Proton-microprobe analyses of trace elements in garnet and chromite inclusions in diamonds (DI) from the Mir, Udachnaya, Aikhal and Sytykanskaya kimberlites in Yakutia, CIS, provide new insights into the processes that form diamond. Equivalent data on garnet and chromite concentrates from these pipes yield information on the thermal state and chemical stratification of the Siberian lithosphere. Peridotite-suite diamonds from Yakutia have formed over a temperature interval of ca. 600°C, as measured by Ni and Zn thermometry on garnet and chromite inclusions in diamonds. Individual diamonds contain inclusions recording temperature intervals of >400°C; ranges of >100°C are common. Diamond formation followed a severe depletion event(s), and a separate enrichment in Sr. Comparison of temperatures on DI garnet and spinel with temperatures derived from diamondiferous harzburgites, exposed inclusions in boart and concentrate minerals suggests that the diamond-containing part of the lithosphere has cooled significantly since the Siberian diamonds crystallized. The peridotite-suite diamonds probably formed mainly in response to one or more relatively short-lived thermal events, related to magmatic intrusion. The northern part of the Daldyn-Alakit district may have had a typical cratonic geotherm at the time of diamond formation, and during kimberlite intrusion. The southern part of the district, and the Malo-Botuobiya kimberlite field, probably had a relatively low geotherm (ca. 35 mW/m2). The vertical distribution of garnet and chromite types indicates that the mantle above 120 km depth is dominated by lherzolites, whereas the deeper parts of the lithosphere are a mixture of lherzolites and more depleted harzburgites and dunites.

Quantitative, high sensitivity, high resolution, nuclear microprobe imaging of fluids, melts and minerals

Samples of fluids and melts trapped and preserved as inclusions in growing minerals or healed fractures provide unique windows on a range of geological processes from mantle melting and metasomatism through to economic ore formation and remobilization. Recent advances in nuclear microprobe (NMP) technology at the CSIRO provide powerful tools for the study of these inclusions and associated mineral assemblages. These tools include a new NMP designed for high resolution and high sensitivity, PIXE analytical methods for quantitative imaging and analysis, and simultaneous PIGE imaging. The quantitative imaging and analysis methods are based on the dynamic analysis approach, which generates a fast matrix transform for projection of list-mode PIXE data onto pure elemental images. Recent advances provide rapid pixel-by-pixel correction for matrix and absorption effects in different (mineral) compositions across the image area to yield true quantitative images. These methods are combined in a software package called GeoPIXE II.

Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia

Abstract The proton microprobe has been used to study the distribution of trace elements in garnet and chromite concentrates from the Udachnaya kimberlite and three smaller, low-grade kimberlites from the Daldyn kimberlite field. Garnet thermobarometry and classical P-T estimates for megacrystalline peridotite xenoliths both suggest a Paleozoic geotherm beneath the Daldyn area that is close to a 35 mW/m 2 conductive model. Finer-grained xenoliths with T C scatter above this geotherm; high-temperature sheared xenoliths lie near or above a 40 mW/m 2 model geotherm at 1150–1400°C. The higher- T results are interpreted as the result of short-term heating, caused by magmatic intrusion and perturbation of a relatively cool conductive geotherm, especially near the base of the lithosphere. The stratigraphic distribution [inferred from nickel temperature ( T Ni )] of garnets with different major-element chemistry indicates that the lithosphere is strongly layered in terms of rock type; depleted lherzolites predominate to depths of ca. 150 km, harzburgites comprise up to 60% of the volume between 150 and 180 km, and these are underlain by a mixture of depleted and metasomatically enriched lherzolites. Zinc temperatures ( T Zn ) indicate that chromite-bearing peridotites are essentially absent at depths > 190 km. High- T lherzolite garnets carry a distinctive trace-element fingerprint showing enrichment in Zr, Ti, Y and Ga, interpreted as due to the infiltration of asthenosphere-derived melts. This melt-related metasomatic signature becomes the dominant one at ca. 220–230 km depth, and this is interpreted as the base of the lithosphere. This depth also corresponds approximately to the Lehman Discontinuity at the top of a pronounced low-velocity zone, observed in deep seismic sounding experiments across this part of the Siberian Platform. The techniques used here provide a means of mapping the lithosphere in terms of thermal structure, lithology and fluid-related processes; both lateral (3-D) and temporal (4-D) variations may be mapped using readily available garnet and chromite concentrates from the widespread kimberlite intrusions across the Siberian Platform.

Quantitative PIXE trace element imaging of minerals using the new CSIRO-GEMOC Nuclear Microprobe

Abstract The design of the new CSIRO–GEMOC Nuclear Microprobe for high sensitivity imaging of mineral samples, combined with a new GeoPIXE software environment, incorporating the dynamic analysis (DA) method for projecting quantitative PIXE trace element images, yields quantitative PIXE images at 1.8 μm spatial resolution with detection limits in silicates for extracted concentrations down to ∼0.2 ppm (Br). Detection limits (99% confidence level) down to 39 ppb (Ge) have been achieved in diamond. Advances in PIXE technique include the correction of DA images for the effects on PIXE yields of spatial variation of sample composition, interactive software tools for DA projection and concentration extraction, and their integration with simultaneous PIGE imaging for light elements in a new efficient PC environment. Application areas include the analysis and imaging of melt inclusions, sulfides from ancient and modern settings, and fluid inclusions in minerals.

Imaging fluid inclusion content using the new CSIRO–GEMOC nuclear microprobe

Abstract The analysis of fluid inclusions in minerals holds the key to understanding ore formation processes. PIXE analysis of fluid inclusions using the nuclear microprobe provides a direct non-destructive method to determine the composition of these trapped fluids. However, some results have been controversial. In order to demonstrate the reliability of the PIXE approach, and eliminate spurious sources of important elements, the internal contents of fluid inclusions have been imaged using the new CSIRO–GEMOC nuclear microprobe (NMP) in a method that uses identification and quantitative integration of the inclusion signal from PIXE images. Examples of this approach show clearly the elements that reside within the fluid inclusions, and allow discrimination against solid phases outside and in close proximity to the inclusions.

Distribution and characteristics of diamonds from Myanmar

Abstract Diamonds occur in headless placers at several locations within Myanmar. Twenty-six stones from the Momeik area of northern Myanmar and 111 stones from the Theindaw area of southern Myanmar have been studied to characterise their morphology, crystal forms, colour, degree of resorption, surface features, internal structures, mineral inclusions, and nitrogen content and aggregation state. Most stones grew originally as octahedra, but now show very high degrees of resorption, and highly polished surfaces, reflecting transport in a magma. Etch features are abundant, and breakage and abrasion are common, due to alluvial transport. Brown radiation spots are common, suggesting that these diamonds have a long history in surface environments. Cathodoluminescence (CL) images of plates and whole stones commonly display marked oscillatory zoning of yellow and blue bands, outlining octahedral growth zones. Many other stones show uniform yellow CL. Syngenetic mineral inclusions identified thus far are mainly of peridotitic paragenesis and include olivine, chromite and native iron. Infrared spectroscopy studies show that ∼10% of the diamonds have very low-N contents (Type II diamonds). More N-rich diamonds show high degrees of aggregation (Type IaAB). Both types are consistent with derivation from the upper mantle, rather than from crustal metamorphic sources. The primary source of these diamonds is believed to be an alkaline igneous rock (lamproitic rather than kimberlitic) but they may have reached their present locations via a secondary collector such as a sedimentary rock.

Micro-PIXE analysis of trace element concentrations of natural rubies from different locations in Myanmar

Abstract The trace element concentrations found in geological samples can shed light on the formation process. In the case of gemstones, which might be of artificial or natural origin, there is also considerable interest in the development of methods that provide identification of the origin of a sample. For rubies, trace element concentrations present in natural samples were shown previously to be significant indicators of the region of origin [S.M. Tang et al., Appl. Spectr. 42 (1988) 44, and 43 (1989) 219]. Here we report the results of micro-PIXE analyses of trace element (Ti, V, Cr, Fe, Cu and Ga) concentrations of a large set ( n = 130) of natural rough rubies from nine locations in Myanmar (Burma). The resulting concentrations are subjected to statistical analysis. Six of the nine groups form clusters when the data base is evaluated using tree clustering and principal component analysis.

Mapping the Earth's mantle in 4D using the proton microprobe

Abstract The CSIRO proton microprobe is used to study the trace element chemistry of garnet and chromite grains recovered from kimberlites and other volcanic rocks, both to develop new diamond exploration methods and to further understanding of the makeup and evolution of the earths upper mantle. Analysis of the partitioning of trace and major elements between garnet and chromite and their coexisting phases in mantle rocks has led to the development of two single-mineral thermometers and a barometer. Trace Ni in Cr-pyrope garnet is used to determine the equilibration temperature (TNi) of each garnet grain. This is the temperature of the garnet in its source rock before it was entrained in the erupting magma. Similarly, trace Zn in chromite yields an estimate (TZn) of its equilibration temperature. To relate these temperature to depth in the lithosphere a measure of pressure (PCr) has been developed that estimates pressure directly from Cr-pyrope garnet composition and TNi. This breakthrough enables the information on rock composition and metasomatic processes held in the trace and major element chemistry of each garnet to be located in P and T and thus placed in its stratigraphic context. Y, Ga and Cr provide information on mantle depletion by partial melting. Zr, Y and Ti provide clues to metasomatic processes such as infiltration of asthenospheric melts. Together the result is both an improved diamond exploration tool and a method of mapping the 3D structure, lithology and metasomatic processes in the lithosphere. With the added knowledge of the date of each intrusion, these methods permit the construction of 4D maps of the lithosphere, charting variation in mantle composition both laterally, with depth and through time.

Diamond exploration and mantle structure imaging using PIXE microanalysis

The compositions of garnets, sampled throughout the earths upper mantle and carried to the surface in volcanic eruptions, show PIXE trace element signatures that reveal details of the chemistry, thermal structure, and history of fluid alteration, of their source rocks. Trace Ni concentration (5–150 ppm) is strongly temperature dependent, providing a useful geothermometer (TNi). Cr concentration can be used to estimate pressure (PCr) in Cr-saturated rocks. Trace Zn in chromite also provides a measure of temperature (TZn), and shows the extent of Cr-saturation (chromite present). For those garnets from a particular volcanic pipe originating in Cr-saturated rocks in the upper-mantle, TNi, PCr maps out the paleogeotherm (the Garnet Geotherm), that is the relationship between T and P (and depth) prevailing at the time of eruption. Ti and trace Zr, Y, and Ga show the effects of metasomatic fluids and deep mantle-derived melts, and show clearly the depth extent of depleted (Y < 10 ppm) rocks which helps to define the effective base of the lithosphere, the earths solid outer shell. The Garnet Geotherm and the base of the lithosphere then define the range of depths within the diamond stability field traversed by the erupting magma, and the histogram of TNi provides a direct measure of the sampling in this range and hence the diamond potential of the pipe. The result is both a powerful diamond exploration tool and a method to map the composition and evolution of the earths mantle.