Thank you Thomas.

The problem is indeed this. How is possible to obtain credible or reliable data on the geotherm using the heat flux alone? Nearly everything must be hypothesized in complex tectonic settings (i.e., the thickness of the various crustal levels, including the U-Th-K content, the presence of fluids and the type of mantle).

michele

Michele Hi

You can read our papers in the ResearchGate DB:

Pilchin, A.N. and Eppelbaum, L.V., 2006. Iron and its Unique Role in the Earth Evolution. Monogr., Mexican Geoph. Soc., 67 p.

Pilchin, A.N. and Eppelbaum, L.V., 2009. The Early Earth and Formation of the Lithosphere, In: (Eds. Anderson, J.E. and Coates, R.W.), The Lithosphere: Geochemistry, Geology and Geophysics, Nova Science Publishers, N.Y., USA, 1-69.

Pilchin, A.N. and Eppelbaum, L.V., 1997. Determination of magnetized bodies lower edges by using geothermal data. Geophysical Journal International, 128, No.1, 167-174.

Eppelbaum, L.V., Modelevsky, M.M. (Jr.) and Pilchin, A.N., 1996. Thermal investigation in petroleum geology: the experience of implication in the Dead Sea Rift zone, Israel. Journal of Petroleum Geology, 19, No.4, 425-444.

Pilchin, A.N. and Eppelbaum, L.V., 2002. Some peculiarities of thermodynamic conditions in the Earth’s crust and upper mantle. Scientific Israel, No. 1-2, 117-142.

Eppelbaum, L.V. and Pilchin, A.N., 2006. Methodology of Curie discontinuity map development for regions with low thermal characteristics: An example from Israel. Earth and Planetary Sciences Letters, 243, No. 3-4, 536-551.

Best regards

Lev

Thanks Lev,

could you make a resume of the articles you have quoted? There are more than two hundred pages to be read, and I would go directly to the point. I am sure that the focus of your articles are not exactly the same of my question.

michele

Michele,

Thomas replied to first part of your question. 

The mentioned papers are suggested to analysis of thermal field at different depths for searching hydrocarbons and solving different geodynanmic and tectonic problems including Earth's evolution history.

Lev

hi Michele,

somewhat more complex (realistic?) time-dependent geotherms may be approximated using the coupled advection-diffusion equation, see for example (Whittington, Hofmeister & Nabelek 2009 - NATURE; Maino et al., 2015 - GEOLOGY). However, there is no simple way to extrapolate crustal or lithosphere-based geotherms from surface heat flow measurements, as surface heat flow is the resultant of -at least- 2 major components. The first of these components should be heat generated by radiogenic decay (mainly U-Th-K) within the section of interest. This component is generally higher in ancient, felsic crust (1 - 5 uW/m3) and become vanishingly small in peridotites (0.02 - < 0.002 uW/m3). The other component is heat flowing INTO the section of interest or, in other words, the temperature at the base of your model. Therefore, you always need both 'geometric' and compositional constraints, as well as some assumption for the boundary conditions.

hope this help, 

Leonardo

Dear Michele,

Leonardo is right. Heterogeneity of the crust (both vertically and horizontally) may lead to significant differences in deep temperatures. This is well detailed and illustrated in the numerous papers by Jaupart  (next AGU medalist) & Mareschal (e.g. "Heat flow and thermal structure of the lithosphere", Treatise of Geophysics, chapter 6.05, 2007).

If you want some examples/explanations on such differences in the shallow crust, you can have a look in one of my papers on this "heat flow versus temperature problem", in particular in the introduction: https://www.researchgate.net/publication/230821355_Heat_flow_and_deep_temperatures_in_the_Southeast_Basin_of_France_Implications_for_local_rheological_contrasts

paper attached below

In this work, we infer a temperature difference of more than 80°C at 8 km depth, which can be attributed to different heat production rates at depth, but also to anisotropy of thermal conductivity, presence of salty layers, etc...

If you work on the thermal regime of the lithosphere, one of the main unknown is the deep component, the "mantle heat flow", which has been estimated in Archean provinces, by various teams, at ~12-20 mW/m². For younger areas, mantle heat flow may be higher, and for Cenozoic, a transient component may have to be accounted for...

Yes, the distribution of temperature in the Earth's interior in not so easy to estimate ...

Laurent G-F

Article Heat flow and deep temperatures in the Southeast Basin of Fr...

Dear Leonardo,

sorry for the delay in replying to your message.

I am going to check the articles you quote. In any case I have realized that it is a difficult task to infer geotherm from heat flow, but this is usually done in scientific literature, and I do not understand why and how.

Major parameters are the radiogenic element budget and the thickness of upper crust and the total crust. However the second extremely important parameter is the temperature below Moho and the third fundamental parameter is the temperature below the seismic lid (or lithosphere or plate, as you like to call the rigid part of the Earth where heat is transmitted mainly by conduction).

The last parameter is defined as the mantle potential temperature and is commonly used in igneous geochemistry to infer heat anomalies in particular areas (e.g., those where mantle plumes have been proposed).

This is a sort of paradox: in order to estimate the temperature of the mantle I have to fix its temperature to transform heat flow in a geothermal gradient. This is the reason why I continue to do not understand this argument.

michele

Dear Laurent, thanks also to you for your kind message.

I can simply reply my comments written for Leonardo. The results obtained inferring a geothermal gradient from heat flow can be very different assuming different structures, rheology and chemical budget.

I have to read definitively the Jaupart and Mareschal paper in ToG.

Thanks also for your paper, even if I was looking for something deeper, below Moho.

Cheers,

michele

Michele,

if you want to calculate the mantle potential temperature, there is no need to fix the mantle temperature to constrain the geotherm. In this case, you should calculate the mantle potential temperature from surface heat flow data, by subtracting to this component the integrated heat flow of a model lithosphere. As you know, geophysical modeling and geochemistry may help building pretty fine models, whose uncertainty is well below the resolution of thermal models, 

cheers

Leonardo

Dear all,

Well, the "integrated heat flow of a model lithosphere" would be estimated - if i am not wrong - from an averaged lithological composition. If I remember, the "Taylor & McLennan" approach used some averaged thermal regime of the lithosphere to infer distribution of radioactive elements... which in turn would actually define the thermal regime of the lithosphere... This is maybe why these geo-neutrino generated by U-K-Th decay have been used for a couple of years, in order to better constrain crustal and mantle heat production (e.g. Perry et al., EPSL,288, 301-308, 2009).

Laurent

Yes Leonardo,

I do not want to calculate the mantle potential temperature. This is a quite impossible task that only seismologists can approach. Unfortunately nearly all the seismologists assume too simple and unrealistic boundary conditions that render their results far from being realistic (i.e., assuming chemically, texturally and mineralogically homogeneous volatile-free mantle and so on). More or less it is the same for geochemists. All the results are strongly model-dependent and may change also a lot if few parameters are modified.

I do not believe that it is possible to calculate the Tp using surface heat flow. If it could be so easy, one should explain me why Iceland and Hawaii are characterized by no heat flow anomaly.

Laurent

geoneutrinos have a too poor resolution, at the moment. We do not know where and how U and Th are located in the mantle and how much noise is derived from the crust. I think that we have to wait five years more, at minimum, to get reasonable results from geoneutrinos.

There are some emprical formula about it. The temperature at Z depth could be estimated by 15*Z or 20*Z, depending on the regions. 

Hi Yangfan,

I do not think it is so easy to calculate the geotherm. One thing is the theory, another thing is the reality...

Hi Michele,

Heat flow data can generally predict the entire oceanic lithospheric geothermal structure well using either the half-space or plate cooling model. There is none such simple model for the continenal case. However,  the Curie point depths derived from magnetic anomalies can be used to infer the crust-scale geothermals.

In addition, combination of the surface heat flow and Curie points can be used to model both the stable oceanic and continental lithospheric geothermals based on the steady  heat conduction equation. In this case, the thermal conductivities are assumed as temperature-depent. For some complex geological settings (e.g., subduction zones, volcanic areas), the steady heat conduction equation may give rise to some biases.

Fortunately, the geothermal modeling in most regions based on the Curie points is agree with the results from tomographic and MT imagings. If you want some examples on the geothermal structures inferred from the Curie points, you can have a look in any one of our papers as attached below.

In these studies, we estimated the Curie point depths firstly, and then modeling the lithospheric geothermal structures constrained by the surface heat flow data. Our geothermal models are in line with the tomography (western North America) and MT (eastern China) results well. We also compared our model with the half-space cooling model (North Atlantic), and found that the small-scale convections and serpentinization can contribute extra heat.

Best regards

Jian

Article Crustal magmatism and lithospheric geothermal state of weste...

Article 3D geophysical characterization of the Sulu–Dabie orogen and...

Article Thermal evolution of the North Atlantic lithosphere: New con...