DMM component

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The DMM component or DM component is a depleted magma component in earth's mantle reservoirs that is significantly involved in the formation of oceanic crustal rocks .

designation

The acronym DMM or just DM is derived from the English Depleted MORB Mantle or Depleted Mantle with the meaning of depleted MORB coat or just depleted coat . MORB is the acronym for Mid Ocean Ridge Basalt , the basalt type of mid-ocean ridges . The name was first introduced by Zindler and Hart in 1986.

Sheath components

The DMM component in the lead isotope diagram

Geochemical studies of oceanic basalt rocks have shown that their parent magmas formed in the upper mantle contain several different end components. The following components could be identified so far:

MORB basalts can usually be explained with a binary mixing process between the DMM and the HIMU components, but oceanic island basalts ( OIB ) already require three or more components.

Characterization of the DMM component

The depleted MORB mantle component can be characterized geochemically as follows:

  • high ε Nd or 143 Nd / 144 Nd isotope ratio : ≥ 0.51326
  • low Δ Sr or 87 Sr / 86 Sr isotope ratio: ≤ 0.70219
  • low lead isotope ratios:
    • 208 Pb / 204 Pb: ≤ 37.280
    • 207 Pb / 204 Pb: ≤ 15.404
    • 206 Pb / 204 Pb: ≤ 17.573
    • 208 Pb / 206 Pb: ≤ 2.121 (very low)
  • low elemental ratios of Ba / Nb, Th / Nb and K / Nb:
    • Ba / Nb: ≤ 2.627
    • Th / Nb: ≤ 0.046

Emergence

The depletion of the silicate-containing earth's mantle goes back very far into the history of the earth. It should have started immediately after accretion of the proto-earth from the chondritic solar nebula and subsequent (in the Hadean ) splitting off of the iron core . The original, primitive coat ( PRIMA ) was subsequently subject to partial melting with simultaneous depletion of incompatible elements such as uranium , thorium and potassium . This can be correlated with the parallel formation of the earth's crust from the Archean . The correlation can be seen very well, for example, from the LREE content of rocks - continental crustal rocks are very strongly enriched in LREE, whereas MORB basalts behave in a complementary manner. The crust formation was consequently at the expense of the earth's mantle, which depleted (depleted) more and more of incompatible elements - at least in its upper area, the area of ​​the DMM component. The extent to which the DMM component reaches in the Earth's mantle remains speculative, the only thing that is certain is that it underlies the oceanic crust in the area of ​​the mid-ocean ridges. In any case, most mantle models assume that it largely fills the upper area of ​​the earth's mantle. Calculations based on radiogenic argon show that the earth's mantle is now 50% depleted. Since the upper mantle only accounts for around 30% of the total mass of the mantle, the lower mantle must also have been affected by depletion, at least in its upper section. This result speaks against a total jacket convection .

Model calculation

A model calculation carried out by Workman and Hart in 2004 to determine the average chemical composition of the DMM component is based on its partial melting with the generation of MORB magmas and abyssal peridotite remaining as residualite . Since the chemistry and isotope ratios of the last two rock groups are known, indirect conclusions can be drawn about the DMM component.

According to the authors, the melting process of the DMM mantle begins to take effect in the border area of ​​the upper garnet mantle facies / spinel mantle facies from a depth of 80 kilometers. The depletion of incompatible elements in the MORB magmas indicates a partial melt formation of 6%. Asimov et al. (2004) come to the same conclusion, who assume that melting occurs with the participation of water .

The model calculation also shows that 33% of the DMM component is required to balance the continental crust. This proportion even increases to 43% if the oceanic crust is also taken into account (see also the argon results already mentioned above with comparable results).

Individual evidence

  1. ^ Zindler, A. & Hart, S .: Chemical geodynamics . In: Ann. Rev. Earth Planet. Sci. tape 14 , 1986, pp. 493-571 .
  2. ↑ C. Allègre, AW Hofmann, K. O'Nions: The Argon constraints on mantle structure . In: Geophysical Research Letters . tape 23 , no. 24 , 1996, ISSN  0094-8276 , pp. 3555-3557 .
  3. ^ Workman, RK and Hart, SR: Major and Trace Element Composition of the Depleted MORB Mantle (DMM) . In: Earth and Planetary Science Letters . 2004.
  4. ↑ Asimov, PD et al .: A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores . In: Geochem., Geophys., Geosyst. tape 5 , 2004, doi : 10.1029 / 2003GC000568 .

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