DUPAL anomalies

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The dupal anomalies are geochemical anomalies in the lead - isotope ratios of igneous rocks . The anomalies are most clearly formed in basalt rocks of the South Atlantic , central Inds, and central western Pacific . They are a clear indication of large-scale heterogeneities and incomplete mixing processes in the earth's mantle .

designation

The acronym DUPAL is derived from the French geochemists Bernard Dupré and Claude Allègre , the two discoverers of the anomalies in basalts of the Indian Ocean in 1983. It is a contraction of the abbreviation of their family names (Dup + Al). The acronym was coined by Hart a year later.

definition

The two anomalies are defined as follows: 

Δ 7/4 = [(207Pb/204Pb)DS - (207Pb/204Pb)NHRL] x 100 (1)
Δ 8/4 = [(208Pb/204Pb)DS - (208Pb/204Pb)NHRL] x 100 (2)

where Δ 7/4 represents the anomaly of the lead isotope ratio of 207 Pb / 204 Pb and Δ 8/4 the anomaly of 208 Pb / 204 Pb. The DS subscription relates to the data record of the sample being examined, the NHRL subscription is the reference data record with which the sample is compared. The DUPAL anomalies are also accompanied by increased Δ Sr values, which in turn are defined as follows: 

ΔSr = [87Sr/86Sr – 0.7030] × 104 (3)

The subscription NHRL is derived from engl. Northern Hemisphere Reference Line or sometimes Northern Hemisphere Regression Line ( Northern Hemisphere Reference Line or Northern Hemisphere Regression Line ). The NHRL is now defined for the two isotope ratios as follows: 

207Pb/204Pb = 0,1084(206Pb/204Pb) + 13,491 (4)
208Pb/204Pb = 1,2090(206Pb/204Pb) + 15,627 (5)
Δ 8/4 diagram with the position of the DUPAL group in comparison to the NHRL and MORB basalts of the Mid-Atlantic Ridge (MAR) and the East Pacific Ridge (EPR)

The two anomalies Δ 7/4 and Δ 8/4 are an absolute measure of how much the examined igneous rocks differ from the reference line or regression line.

On the NHRL are the records of MORB basalts of the Mid-Atlantic Ridge (MAR - Mid Atlantic Ridge), the East Pacific Rise (EPR - East Pacific Rise) as well as oceanic island basalts (OIB basalts) as Hawaii , Iceland , the Azores , Canary Islands and Cape Verde Islands . Your DUPAL anomalies are, by definition, zero. Data sets above the NHRL are positive, data sets below are negative DUPAL anomalies.

example

After entering the data set in the 207 Pb / 204 Pb - 206 Pb / 204 Pb diagram or in the 208 Pb / 204 Pb - 206 Pb / 204 Pb diagram, the values ​​are read off the respective abscissa, the corresponding NHRL value is subtracted and multiply the result by a factor of 100.

The DUPAL anomalies can also be calculated directly using the formulas listed above. The data set of a seamount basalt glass from the South Atlantic at 26 ° 30 'may serve as an example (data set by M. Regelous): 

206Pb/204Pb: 18,2411
207Pb/204Pb: 15,5476
208Pb/204Pb: 38,1807

Using (5), the 208 Pb / 204 Pb value on the NHRL is first calculated and then inserted in (2):

208 Pb / 204 Pb = 1.209 * 18.2411 + 15.627 = 22.0535 + 15.627 = 37.6805

Δ 8/4 = [38.1807 - 37.6805] * 100 = 0.5002 * 100 = 50.02

The seamount has a positive Δ 8/4 anomaly of + 50.02. The procedure for Δ 7/4 is analogous, resulting in the value + 37.12.

DUPAL group

Oceanic rocks of the so-called DUPAL group have positive DUPAL anomalies which, for example, can assume values ​​of up to + 141 for the ratio Δ 8/4 (like the Ninety East Ridge at equator height in the Indic). Negative DUPAL anomalies are much rarer and less pronounced (see Sankt Helena with - 69).

The positive DUPAL anomalies form a band located at 30 to 40 degrees south latitude. Its maximum extends from the mid-Atlantic ridge into the central Ind and into the western central Pacific. In the South Atlantic, the DUPAL group includes Tristan da Cunha , Gough , the Discovery Seamount , the Rio Grande Ridge , the Whale Ridge and the Paraná Basalts / Etendeka Basalts , in the Indic Amsterdam , Saint-Paul , Crozet and Kerguelen as well as in Pacific Rarotonga . A somewhat more indistinct extension from the South Atlantic towards the Oceanographer Fracture Zone and the Azores can also be made out.

The DUPAL anomalies of the Western Pacific were separated from Castillo in 1989 and renamed the SOPITA anomaly ( South Pacific Isotope and Thermal Anomaly ).

In addition to these classic DUPAL occurrences, DUPAL signatures have meanwhile also been found in igneous rocks of the Philippine Plate and eastern China , in Taiwan ophiolites , in the Sea of ​​Japan , in eastern Siberia ( Miocene basalts in Eastern Sayan ) and in the Arctic .

Mixing operations

The records of the rocks dupal group are all located above the NHRL along which depleted asthenosphere (Engl. -Magmen Depleted Mantle or DM - sometimes Depleted Mantle Magma or DMM ) come to rest the ocean ridges. The isotope ratios of the DUPAL rocks have therefore moved from the DM component in the direction of enriched magmas (English. Enriched Mantle or EM ). As a first approximation, this can be explained with a binary mixing process. Data sets from the Indian Ocean suggest that an additional component - the so-called Saint Helena component ( SHC ) - was present, which is below the NHRL and is characterized by quite high 206 Pb / 204 Pb ratios. Recently, other mixed components have also been brought into play, such as the sediment components EM I (pelagic sediments plus mantle above the subduction zone) and EM II (continental sediments), HIMU ( high mu - recycled oceanic crust) and PREMA / FOZO ( prevalent mantle / focal zone - non-enriched sub- mantle component).

geophysics

Geoid anomaly

There is a very good correlation between the DUPAL anomaly pattern and an equatorial bulging of the geoid , which, according to Busse, corresponds to a buoyancy zone of the deeper mantle . The DUPAL anomaly could thus possibly bear the characteristic signature of the deeper mantle.

Seismics

In the area of ​​the DUPAL anomalies, the lower mantle shows a reduced speed of the seismic P waves ( Low Velocity Region - LVR ), those in the South Atlantic up to - 25 m / s and in the equatorial West Pacific up to - 30 m / s can be. Since seismic waves propagate more slowly in less dense media, it is consequently much warmer in these areas, which indicates a possible surge. The low velocity region of the lower mantle also shows a very good correlation with marine hot spots , most of which occur in this area (exceptions are the hot spots of Bermuda , Sankt-Helena and Tubuai ).

Emergence

The origin of the DUPAL anomalies is still controversial. There are two general explanations:

The buoyancy model has several variants that relate to the place of origin of the diapire. Some authors see him at the core-mantle boundary (Engl. Core mantle boundary layer or CMBL ), others on the 660-km discontinuity between Upper and Lower mantle. A simultaneous side by side of both buoyancy systems is also considered possible.

The sinking model also has several variants. For example, the crust contamination model assumes continental rifting in connection with the decay of Gondwana , which began in the South Atlantic in the Lower Cretaceous 132 million years ago . The resulting delamination of the mafic lower crust was absorbed by the upper mantle and changed its isotope ratios in the direction of the DUPAL anomalies. For example, the Parana-Etendeka basalts that formed when South America and Africa broke up document very high DUPAL values.

Some researchers also consider an extraterrestrial entry from meteorites that would change the isotope ratio to be entirely possible.

Web links

Individual evidence

  1. ↑ B. Dupré, CJ Allègre,: Pb-Sr isotopic variations in Indian Ocean basalts and mixing phenomena . In: Nature . tape 286 , 1983, pp. 17-22 .
  2. ^ SR Hart: The DUPAL anomaly: a large-scale isotopic anomaly in the southern hemisphere . In: Nature . tape 309 , 1984, pp. 753-756 .
  3. ↑ a b M. Regelous: Shallow origin for South Atlantic Anomaly Dupal from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26 ° S . In: Lithos . tape 112 , 2008, p. 57-72 .
  4. ^ R. Hickey-Vargas: Isotope characteristics of submarine lavas from the Philippine Sea: implications for the origin of arc and basin magmas of the Philippine tectonic plate . In: Earth and Planetary Science Letters . tape 107, 2 , 1991, pp. 290-304 .
  5. ↑ SL. Chung, SS. Sun: A new genetic model for the East Taiwan Ophiolite and its implications for Dupal domains in the Northern Hemisphere . In: Earth and Planetary Science Letters . tape 109 , 1992, pp. 133-145 .
  6. ↑ M. Tatsumoto, Y. Nakamura: DUPAL anomaly in the Sea of ​​Japan: Pb, Nd and Sr isotopic variations at the eastern Eurasian continental margin . In: Geochimica et Cosmochimica Acta . tape 55, 12 , 1991, pp. 3697-3708 .
  7. ↑ E. Demonterova: Lithospheric origin of the DUPAL anomaly: A case study of a suite of Miocene basalts across the Siberian craton boundary . In: EGU General Assembly (Ed.): Geophysical Research Abstracts . tape 11 , 2009.
  8. ^ FH Busse: Quadrupole convection in the lower mantle . In: Geophys. Res. Lett. tape 10 , 1983, p. 285-288 .
  9. ↑ a b c P. Castillo: The Dupal anomaly as a trace of the upwelling lower mantle . In: Nature . tape 336 , 1988, pp. 667-670 .
  10. ↑ S. Escrig: osmium isotopic constraints on the nature of the Dupal anomaly from Indian mid-ocean-ridge basalts . In: Nature . tape 431 , 2004, p. 59-63 .