GEMOC ARC National Key Centre
4D Lithosphere Mapping: a methodology and philosophy for tracing the architecture and composition of the lithosphere through time.
S.Y. O'Reilly and W. L. Griffin
Aims of our program: to use petrology and geochemistry to constrain remotely-sensed geophysical data and to map the lithosphere in 4-dimensions (space + time). In particular, we map the rock type distribution, the nature and location of the important boundaries (crust-mantle boundary, (CMB), Moho and lithosphere-asthenosphere boundary (LAB)) and the compositional variations in the lithospheric mantle through time, constituting our 4-D Lithosphere Mapping technique.
For more information see the bibliography as well as the following...
O’Reilly, S.Y. and Griffin, W.L. 2006. Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: Geophysical implications. Tectonophysics, 416, 289-309.
O'Reilly, S.Y. and Griffin, W.L., 1996. 4-D Lithosphere Mapping; methodology and examples. Tectonophysics, 262, 1-18
Griffin, W.L., Kaminsky, F.V., Ryan, C.G., O'Reilly, S.Y., Win, T.T. and Ilupin, I.P., 1996. Thermal State and Composition of the Lithospheric Mantle beneath the Daldyn Kimberlite Field, Yakutia. Tectonophysics, 262, 19-33.
O'Reilly, S.Y., Griffin, W.L. and Gaul, O., 1997. Paleogeothermal gradients in Australia: key to 4-D lithosphere mapping. In press, AGSO Centenary Volume
Outline of Lithosphere Mapping
"Holistic" approach to understanding:
- the composition, stratigraphy and thermal state of the lithosphere
- nature and significance of its important boundaries (CMB, Moho, LAB) and how they change with time
Uses xenoliths and xenocrysts (mineral concentrates) from mantle-derived volcanics:
- to establish paleogeotherms
- to identify rock types and processes (mainly from trace elements)
Results in stratigraphic relationships of:
- rock types
- processes (eg metasomatism, heat input)
Combined with geophysics to :
- give 3-D picture of composition, structure and thermal state of the lower crust and upper mantle
- extrapolate vertical lithosphere sections laterally beyond the occurrence of xenoliths
Uses different volcanic events in one region to:
- add time factor (4th dimension)
Figure 1: The yellow layer shows the lithosphere relative to other layers of the Earth
Figure 2: Lithosphere definitions
Most intraplate volcanism is probably derived below the lithosphere in the asthenosphere. The asthenosphere is defined as shown in Figures 1 & 2, to be the convecting lower part of the mantle where the thermal gradient is adiabatic, ie a thermal and rheological definition. Immediately above, is the thermal boundary layer that is the transition from the adiabatic, convecting asthenosphere to the lithosphere where heat loss is generally modelled as conductive - see geotherm, in light blue. The cratonic, Archean LAB probably coincides with the base of this thermal boundary layer as shown to the right while younger or reworked LAB is shallower and closer to the mechanical boundary layer.
Figure 3: Depths of origin of basalts and kimberlites
Basalts sample fragments of lithospheric mantle from depths shallower than 90km, and generally 60km or less. These xenoliths are brought up to the surface so rapidly (probably in less than 30 hours) they are rarely altered and therefore are pristine samples from depth, in contrast to deep sequences that rare tectonically exposed.. They are rarely older than Triassic and most are Cretaceous or younger.
Kimberlites span the geological timescale from Pre-Cambrian to very young and, accordingly, provide mantle samples of the lithosphere throughout Earth's geological evolution - but sample a biased tectonic environment - stable cratonic regions. They also sample greater depths than basalts, up to 250 km. Some xenoliths may have originated as deeply as over 400km and some diamonds encapsulate samples they appear to be from the lower mantle.
Xenoliths and disaggregated garnets and chromites from these xenoliths brought to the surface in these magmas, give us the direct samples on which to base a realistic map of the lithosphere: they give us rock types, geochemical signatures, commonly pressures and temperatures of equilibration (hence empirical geotherms) and samples to measure physical properties such as seismic, thermal conductivity, heat production, electrical properties, magnetics etc.
Thermal flux and heat redistribution in the earth
Heat flow -is perhaps the single most important piece of information we can know about the earth - Heat or thermal energy is the primary driving force for all active geological processes. If there was no internal heat engine, the Earth would have a flat and unchanging surface within a maximum of 100 million years.
This thermal flux and heat redistribution is
- the source of mantle melts/fluids
- the driving force for all tectonic processes
- and it strongly affects physical properties of the mantle eg Vp, density, rheology
Perhaps the most important type of thermal data is that leading to xenolith-derived empirical geotherms that can be constructed using pressure and temperature calculated from xenoliths that contain the key minerals that allow this to be done. Geothermobarometry techniques are critical and require complex evaluation for each new xenolith suite and each new locality. A recent protocol for this is given in Xu et al, 1997). If the earth were losing global heat in a steady-state, there would be no volcanism, and there would be generalised geothermal gradients, simply reflecting heat loss by conduction. It is heat differentials that drive melting and tectonism. Until about 10 years ago, the traditional views were that there were 2 types of geotherms - continental and oceanic (with the then controversial variation by Finnerty and Boyd (1977) of the famous kinked geotherm), and that they could be described by conductive models based on extrapolation of surface heat flow.
Figure 4: Variety of geotherms according to tectonic environment and time
Figure 4 shows a range of geotherms, all derived empirically from xenolith mineral data except the shield and oceanic ones: the southeastern Australian (SEA) geotherm is light blue. Just below, is the average alkali province geotherm and this appears to be a good generalised approximation for regions with intraplate basaltic volcanic activity. The Spitsbergen geotherm lies just higher than that for SEA (being near the flank of an active rift, there is background heat flux). The highest is the geotherm is from the Rio Grande rift.
Kimberlites represent the opposite end of the spectrum - they represent perhaps the least thermal perturbation that can be sustained, and that can still permit volcanism. Thus, geotherms constructed from xenoliths in kimberlites potentially provide information on probably thermally unperturbed lithosphere. In contrast to the geotherms derived from xenoliths in basaltic rocks, they record conductive geotherms with surface heat flux varying from 35 through to approx. 45mW/m2 depending on the particular craton sampled and very close to conductive model geotherms.
Figure 5: Southeastern Australian (SEA) empirical xenolith-derived geotherm
In eastern Australia where xenolith data are available from the dominantly Cenozoic basalts (coinciding with the basaltic provinces down the eastern continental margin) there is a remarkably consistent geotherm independent of the age of the basaltic volcanism. The plotted points shoe the original geotherm constructed using data from one locality in western Victoria (Griffin et al, 1994; O'Reilly and Griffin, 1985)
- This inflected (advective) geotherm is higher than conventional ocean basin geotherms and reflects thermal perturbation associated with volcanic episodes ie advective heat transfer.
- It records the paleothermal state at the time of the particular volcanic activity, and decays towards a conductive geotherm with a relaxation time of 30-50 Ma.
Figure 6: Garnet geotherm, Whitecliffs, NSW
Kimberlites and lamproites rarely contain as many fresh xenoliths as basaltic rocks. Over the past few years, Bill Griffin and coworkers have developed a powerful new tool for constructing geotherms and lithospheric stratigraphies using garnet and chromite heavy mineral concentrates. In many localities, especially of kimberlites, such mineral concentrates are a lot more abundant and more readily available than fresh xenoliths.
This methodology is based on determining Temperature using Ni contents of single garnet grains and Zn contents of single chromites.
These temperatures can then be used in 2 ways to obtain P:
Figure 6 shows the result of the latter method for the Whitecliffs area near the Tasman Line just north of Broken Hill (O'Reilly et al., 1997). Here the lower envelope of all the data, traced by the red dotted line, defines the geotherm (Ryan et al, 1996). This geotherm lies above the calculated 45 mW/m2 conductive geotherm and it lies within the graphite stability field in the mantle. (It would be unlikely to find diamonds associated with that volcanism or to have them preserved after that thermal event.)
Figure 7: North Kimberley geotherm.
Figure 7 shows the same type of garnet geotherm for the North Kimberley region. This geotherm is about equivalent to a 40 mW/m2 conductive geotherm (and lies just within the diamond stability field).
Figure 8: Distribution of geotherm determinations in Australia
Figure 8 shows the distribution of empirical geotherms we have determined using xenoliths and mineral concentrates in Australia. The green shaded areas coincide with the mainly Tertiary volcanism down eastern Australia and show the areas with geotherms derived from xenoliths in basalts. The green stars show regions of garnet geotherm determinations - most of these stars represent numerous localities in a given area (and thousands of garnet and chromite grain analyses).
Lithoosphere Stratigraphy is the core of 4-D Lithosphere Mapping
- it provides a spatial context of rocks and processes, especially depth
- allows integration of the petrological information with geophysical data to extrapolate deep-seated rock types laterally
In regions where volcanism has mechanically sampled the mantle and lower crust, a true stratigraphy of the deep sequences can be constructed. Where pressures (P) and temperatures (T) can be calculated, this is straightforward. If only T and not P can be calculated as is the case for many xenoliths, then the depth can be reasonably inferred if a geotherm has been established, simply by referring the T to that geotherm.
Figure 9: SEA geotherm and seismic section
An example of the use of T histograms for eastern Australia is shown in Figure 9. The green histogram represents temperatures, of mantle-derived spinel lherzolites from eastern Australia: the red histogram, temperatures of granulites. The low T edge of the lherzolite histogram, represents the CMB: ie the incoming with increasing depth of dominant ultramafic rock types (spinel lherzolites). This low T edge can be projected to the geotherm as shown in the diagram, to give depth, and correlated with seismic data to interpret the significance of layers in reflection profiles (also see figure 5)
Figure 10: Depth to Crust-Mantle Boundary (CMB) in eastern Australia
This T histogram method is very powerful and, can be used to estimate the depth to the CMB beneath provinces and hence to map the regional variation in crustal thickness . The CMB ranges from very shallow in western Victoria while the thickest crust as determined from xenolith data is in southeast Queensland. This is corroborated seismically by the results of the Eromanga traverse (Finlayson, 1990; O'Reilly and Griffin, 1990) and reflection seismic data for western Victoria (Finlayson et al., 1993). There is some correlation between depth to the CMB and topographic elevation, but in detail other factors, such as present-day heat flow (thermal isostasy), are important.
Figure 11: Generalised lithospheric cross-section eastern Australia
In eastern Australia, the xenoliths from basalts allow us to determine the location of rock types in stratigraphic order and leads to an underplating model with repetitive intrusions around the CMB and possibly repetitive volcanic episodes. The black lines representing under and overplated basaltic sills and lenses that probably explain the abundant reflectors seen on seismic reflection profiles.
The repetition of this under and over-plating process through time is a significant mechanism of crustal growth at least since the Archean, and has important implications about the average crustal composition - at least the lower crust in Phanerozoic regions such as eastern Australia are probably dominantly mafic - and much of the lower crust is directly of basaltic origin.
Figure 12: Schematic lithospheric cross-section and Vp profiles, eastern Australia
Figure 12 shows details of likely rock mixes are given (with calculated and measured Vp profiles).
At well-characterised locations in eastern Australia, the CMB occurs within a package of seismic reflectors (Figure 9) , and commonly several kilometres shallower than the Moho. The recognition that abundant spinel lherzolites occur at depths as shallow as 25-35 km at all localities where samples are available led to a major re-interpretation of seismic data from eastern Australia. Geophysical models based on the presence of a gradual increase in Vp with depth, and the lack of a typical Moho discontinuity, had led Finlayson et al. (1979) to suggest that the crust beneath eastern Australia was >50 km thick. The xenolith data lead instead to a model involving interlayering of mafic and ultramafic rocks near the CMB, and a relatively thin crust (O'Reilly and Griffin 1985,1995). Finlayson et al. (1993) subsequently confirmed this result for western Victoria using high-resolution seismic reflection data. A similar relationship between mantle stratigraphy and seismic data is shown for central Queensland ( Griffin et al, 1987).
Relationship of CMB with reflection MOHO
This distinction between the crust-mantle boundary and the Moho is not trivial because it concerns our very definitions of the crust and mantle of the earth - the crust comprises rock types formed by secondary processes since the earth's accretion . The Moho is a geophysical discontinuity and does not necessarily coincide with the transition from crustal eclogite or granulites to ultramafic rocks. Therefore depth to Moho cannot be used automatically to give crustal thicknesses.
Lithosphere stratigraphy - the core of 4-D Lithosphere Mapping
- it provides a spatial context of rocks and processes, especially depth
- allows integration of the petrological information with geophysical data to extrapolate deep-seated rock types laterally
Xenolith input to lithosphere stratigraphy provides:
- Distribution of temperature with depth (geotherm) at the time of eruption
- Depth to the crust-mantle boundary
- Nature of the lower crust
- Stratigraphic distribution of rock types to a max of about 90 km
- Stratigraphic distribution of processes in the mantle
Garnet and chromite concentrates provide:
- Thermal structure of the lithosphere to depths up to 250 km
- Detailed rock distribution with depth in the upper mantle
- Depth to the "lithosphere-asthenosphere" boundary
- Depths of processes involving various types of mantle fluids
- Temporal variations (from volcanics of different ages)
Figure 13: Siberian traverse as example of integrated study
Figure 13 shows the rock distribution under Siberia constructed from garnet concentrate information (see Griffin et al., 1996, Tectonophysics for more detail) and using contours of rock type abundance shows that there is abundant depleted harzburgite (shown in blue). Such differences in rock type distribution may influence the interpretation of tomographic images. We see in Anderson's global tomography synthesis at 210 km that cratonic areas have high velocities. In particular, the mantle in the region of this Siberian section is very fast. Cold buoyant Mg-rich depleted mantle is less dense than fertile mantle but has a higher seismic velocity- so tomographic regions of high Vp corresponding with cratons may not just be cold - some of them could represent volumes of Mg-rich residual harzburgite.
What is the Lithosphere/asthenosphere Boundary (LAB) and why is it important?
The cratonic "lithosphere/asthenosphere" boundary recognized from the garnet/chromite geotherm work is located at about 160-200 km in many areas (including the Kaapvaal Craton, South Africa;) and at about 220km beneath the Daldyn-Alakit kimberlite field in Siberia (Griffin et al, 1996). The base of the lithospheric keel beneath some cratonic areas lies within the same depth range.
The Lehman Discontinuity is a geophysically definable boundary which, beneath the Siberian Craton, occurs at a depth of about 210 km (Egorkin et al., 1987) and marks the top of a low velocity region where components of the shear velocity of seismic waves are differently transmitted. This has been interpreted as evidence for large-scale vertical anisotropy above and below this L-discontinuity (eg Jordan 1988) or as a boundary between large mantle regions of intrinsically different composition (and hence of different physical properties) (eg, Kennett and Bowman, 1990). The Siberian Craton is currently the only region where both high-quality geophysical data and a detailed garnet-geotherm mantle stratigraphy are available and here the Lehman Discontinuity and the inferred "lithosphere/asthenosphere" boundary coincide (Griffin et al., 1996).
Wyllie (1988) demonstrates that carbonate/kimberlitic melts derived from deeper levels hit a phase change barrier and must form intrusive bodies at around 200 km, although residual volatiles may percolate upwards. Geophysical models (eg McKenzie and Bickle, 1988) require the change from a convecting to conducting regime to occur at approximately the same level.
The sum of this evidence suggests that at least beneath the Siberian Craton, the region around 200-220 km may be a key to the architecture of the lithosphere, the mechanisms of its chemical evolution and its role in plate tectonic movements. At the present state of knowledge, however, its exact significance is not clear and the answer must lie with further seismic anisotropy and tomography experiments.
The lithosphere-asthenosphere boundary in non-cratonic areas such as eastern Australia, is much shallower, at about 90 to 120km in some regions (eg Muirhead and Drummond, 1990; Cheng et al., 1991). This is also consistent with indirect data for the inferred source depth of many primitive intraplate basaltic magmas (eg O'Reilly and Zhang, 1995).
Fluids are extremely important agents of geochemical change and heat transfer in the lithosphere. Mantle-derived fluids are also the source of the Earth's crust, atmosphere and oceans and ultimately are the agents responsible for economic deposits in the crust. Fluid movement reflects pathways in the mantle and the mechanical properties of xenoliths, at least in the lithospheric mantle. The nature of this movement and the trace elements carried in the fluids leads to an understanding of the mechanisms of fluid transport. Many xenoliths show faceted surfaces that must reflect pre-existing planes of weakness and brittle fracture surfaces within the mantle.
Mantle sulphides are abundant in some mantle sections and along with high pressure fluid inclusions and oxygen fugacity estimates, provide a basis for understanding ionic species that can be carried in fluids through the mantle and into the crust.
If fluids move along cracks, - they are not constrained to be the silicate-bearing or carbonatitic melts which can wet grain boundary surfaces as shown by experiments, as they do not have to move along grain boundaries. Therefore carbon dioxide and water-rich melts can move effectively around the mantle if they move through fractures. Then fluid movement can be fast and a wide range of fluids and trace elements can be transported
Petrophysical Parameters of Xenoliths
Laboratory measurements of physical properties of xenoliths provides parameters to calibrate and constrain geophysical datasets: for example, seismic velocities and densities can be calculated knowing the bulk composition Several key properties can be measured, if necessary at high T and P. These parameters include:
- Seismic (acoustic) velocities (Vp, Vs)
- Thermal (heat production, conductivity at high T)
- Density (gravity interpretation)
Figure 14: Petrophysical results for Australian mantle rock types (VP)
Figure 14 show results for acoustic velocities in three directions for a spinel lherzolite xenolith up to 10 kb confining pressure (O'Reilly et al, 1990). In Eastern Australia, spinel lherzolites (olivine, orthopyroxene, clinopyroxene, spinel) are the dominant mantle rock type between about 30 and 55 km. We have measured the Vp of several samples of these spinel lherzolites with Ian Jackson at ANU. At 10 kb and the ambient T calculated for each sample, the measured Vp varies from 7.7 to 7.9 km/sec
This appears to be anomalously "low" because of:composition (Fe and Ca-rich)
mineralogy (high pyroxene content, some amphibole)
high ambient T These values lower than "standard dunite" (8.0-8.3 km/sec) at same T that is normally used for theoretical models of mantle seismic velocities
The anisotropy is also important. In these samples, the anisotropy is about 5 -6%, but in more foliated samples it is up to 12% -15%
The 4-D Lithosphere Mapping methodology gives us tools to fingerprint the lithosphere throughout time. Our studies are showing that there are significant changes in the composition, thermal characteristics and thickness of the lithosphere form the Archean through the Proterozoic to the Phanerozoic. We are also starting to be able to define the mechanisms of these changes in some regions. Possible mechanisms include:
- Thermal erosion
- Geochemical erosion and Replacement
- Lithosphere upwelling which may well be the same as thermal erosion
Hot, trace-element charged fluids derived from the asthenosphere can displace and cause the erosion of old, buoyant refractory lithosphere keels into thinner, hotter and chemically recharged or metasomatised lithosphere.
Figure 15: Lithosphere sections and thermal erosion
Figure 15 shows lithospheric sections in three cratonic regions and a schematic representation of heating and metasomatism producing transformation of the old refractory lithosphere.
Underneath the diamond field in Northern Siberia (Sakha), the lithosphere is still thick and the asthenospheric fingerprints are confined to depths greater than 200 km. Underneath the Kaapvaal craton in South Africa, the hot, trace-element-enriched asthenospheric fingerprint is at depths as shallow as 150 km. Underneath Tanzania, parts of the lithosphere are severely eroded by asthenospheric melts and fluids and the LAB is quite shallow.
CONCLUSIONS that can be drawn from this type of work:
Volcanics with xenoliths and mineral debris provide
- Drill holes through the lithosphere to different depths
- Different time slices (at the time of eruption)
- record properties measured now
- Definition of the relevant geotherm is critical
- Geotherms allow processes and rock types to be put in a stratigraphic context - enhance the value of geochemical data by giving a vertical spatial reference
- Geochemical and petrological data on the lithosphere can be accessed directly from magmas and xenoliths and extended by seismic data
- Provides a methodology for the integration of petrological, geochemical and geophysical data to build up a picture of the composition and shape of the lithosphere in time and space.
- Direct data from xenoliths provide constraints for geologically realistic geophysical models
- Geophysical data can then be used to extrapolate lithosphere composition and structure over large areas.
- Integration of geophysical data and modelling with all the information from mantle-derived volcanism and xenoliths is the key to understanding the evolution of the Earth and the processes that have driven that evolution throughout time.
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