Mantle Reservoirs

As the amount of available data has multiplied over the past ten years, the number of apparently distinct reservoirs has risen from two (primitive and depleted mantle, respectively) to five (PREMA, Depleted, HIMU = high U/Pb and Two types of EM or enriched mantle reservoirs).

The depleted reservoir produces “normal” (N-type) MORB probably more than 90% of all MORB. The other four reservoirs are sampled by “enriched” or “plume-type” (E- or P- type) MORB and by ocean island volcanism. It is proposed that a single general mechanism, subduction, produces all four types of OIB source reservoirs in spite of their distinctiveness.

Related terms :-

• CC Bulk continental crust.
• PM Primitive mantle (equal to BSE = Bulk Silicate Earth).
• DM Depleted mantle reservoir in a 3-reservoir Earth, the product of extraction of CC from PM; DM should be capable of generating MORB or MORB + OIB.
• EER Early enriched reservoir, either permanently stored in the deep mantle or possibly lost by collisional/accretion erosion.
• EDR Early depleted Reservoir = mantle after removal of EER but prior to extraction of permanent continental crust.
• RM Residual mantle reservoir in a 4-reservoir Earth consisting of PM, EER, EDR, and CC. RM is formed by extraction of CC from EDR.

PREMA or Hawaii-type : recycled P-type MORB, IOB or oceanic plateaus :-

Common characteristics of these are : relatively high Rb/Sr and Th/U ratios, but low Sm/Nd and Lu/Hf ratios. During partial melting under a thickened crust, these element ratios are fractionated because of the presence of residual clinopyroxene and sometimes garnet.

• When such thick oceanic crust subducted, its chances to survive convective mixing and thus to retain its isotopic identity are higher than those of thin, N-type MORB crust. When it is incorporated into new plume the above elements fractioned again.
• In this way , repeated cycling of plume type mantle produces a correlated pattern of mother daughter element ratios, which form the main isotopic “mantle array” of correlated Sr, Nd, Hf and ²⁰⁸Pb*/²⁰⁶Pb* isotopic ratios.
• The repeated enrichment process through partial melting leaves garnet in the residue, causing Lu/Hf ratios to be fractionated, and this accounts for the correlated ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁴³Nd/¹⁴⁴Nd ratios in OIB.

HIMU or St. Helena-type : recycled N-type MORB :-

HIMU basalts have MORB like isotopic compositions of Sr, Nd, Hf and ²⁰⁸Pb*/²⁰⁶Pb* and Ba/La ratios, but much higher ²⁰⁶Pb/²⁰⁴Pb. Thus these sources have lost Pb in preference to U and Th, probably during subduction when island arc volcanism extracted Pb preferentially, acquired by low U/Pb and left a high U/Pb residue in the mantle. It is likely that the original N-type oceanic crust received some uranium during ridge-crest hydrothermal uptake. This would contribute to the low Th/U ratios and the slightly lower than normal Nb/U ratios found in St. Helena basalts.

EM – I or Kerguelen type : Recycled P-type oceanic crust with small additions of old crustal material

The main enrichment trend is the same as that seen in PREMA sources. EM -I sources deviate from these by having relatively low ²⁰⁶Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr but high ²⁰⁷Pb/²⁰⁶Pb ratios. It might be old sedimentary material from a former continental margin or it might be derived directly or indirectly from the lower continental crust. The enrichment patterns of EM I sources are such that some EM Is have isotopic compositions of Sr, Nd, Hf and Pb close to primitive mantle values. However, this is fortuitous because both Nb/U and Ce/Pb ratios of these basalts and their sources have been fractionated and are clearly not primitive.

EM – II or society Island type : recycled N-type MORB with variable but small additions of sediments of relatively young age

The sediments add components high in ⁸⁷Sr and ²⁰⁷Pb, high in Pb and low in Nb and Ta concentrations. This causes the isotopic trends to deviate from the Nd-Sr mantle array and from the so called northern hemisphere correlation of ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb ratios. Low Nb concentrations and Ce/Pb ratios are correlated with high 87Sr /86Sr ratios on Society Island volcanics but sedimentary material is sufficiently small to keep Nb/U ratios between present-day mantle value and primitive mantle.

The overall source of EM – II basalts is dominated by recycled oceanic crust, the bulk composition of which has been modified not only by possible hydrothermal effects noted for the HIMU sources but also by the addition of small amounts of sediments.

REFERENCES :

• mantle reservoirs by Hofmann, A.W.
• https://doi.org/10.1029/2022GC010339
• https://www.researchgate.net/publication/317429076_Hard_Rock_Geochemistry_and_Earth_Mantle_Evolution