Traces in OIB Show Ancient Carbonates Journeyed from Earth’s Surface to Lower Mantle and Back

Traces of carbonates that once lay on the Earth’s surface in the Archean have been found on Pitcairn Island, a young volcanic island in the South Pacific, according to a recent study reported in the latest issue of the journal Proceedings of the National Academy of Sciences of the United States of America.

According to the study, traces of sedimentary carbonate (its isotopically light magnesium) contained in the young oceanic lavas traveled from the Earth’s surface to the core-mantle boundary (~2900 kilometers deep in the Earth) but ultimately came back to the surface to form the volcanic island, after a period of about two and a half billion years staying down below.

The study was conducted by an international team of researchers from China, Japan, and Germany.

They include members from Nanjing University: Lihui Chen,Xiaojun Wang, Yuan Zhong and Jinhua Shi;

from Japan Agency for Marine-Earth Science and Technology: Takeshi Hanyu, Takashi Miyazaki, Yuka Hirahara, Toshiro Takahashi,Ryoko Senda, Qing Chang, Bogdan S. Vaglarov and Jun-Ichi Kimura;

from Max-Plank Institute for Chemistry: Albrecht W. Hofmann;

from Kochi University: Hiroshi Kawabata; and

from the Institute of Geology and Geophysics, ChineseAcademy of Sciences: Liewen Xie.

Figure 1. Left, the photo of Pitcairn Island, the cliff is where the most EM1-like basalts occur. Subaerial basalts collected from this island were used in this study. Credit: Takeshi Hanyu/Japan Agency for Marine-Earth Science and Technology. Right, the location of the Pitcairn Island in the South Pacific. This volcanic island is thought to be the product of mantle plume—where hot upwelling transports deep mantle materials from the core-mantle boundary to erupt on the ocean floor. Credit: Wang Xiaojun et al./Nanjing University

Ocean islandbasalt

Ocean island basalt is a type of basaltic rocks found on many volcanic islands and seamounts far out in the oceans (see Figure 1 foran example).

The theory of plate tectonics can well explain the occurrence of volcanic activities at plate boundaries—such as mid-ocean ridges and subduction zones, but it cannot be directly related to the basaltic eruptionsin the interior of tectonic plates (far away from plate boundaries).

Scientists believe that these intra-plate volcanic islands are the products of mantle plumes, the hot spots of materials upwelling from deep mantle to erupt on the ocean floor. The famous Hawaiian chains are examples of such islands.

Seismic tomography imaged conduit-like low-velocity anomaly beneath ocean islands, which can extend to the core-mantle boundary (~2900 kilometers deep in the Earth), and this is thought to be the mantle plume. Therefore,it is believed that the raw materials of ocean island basalts are the deep mantle materials transported by upwelling mantle plumes.

Magnesium isotopes

Isotopes are atoms of the same element with different numbers of neutrons. The element magnesium consists of three stable isotopes: magnesium-26,magnesium-25 and the lighter magnesium-24. Since the difference in mass between26 and 24 is larger than that between 25 and 24, geochemists usually use theratio of magnesium-26 to magnesium-24 to reflect the magnesium isotope fractionation.

The fractionation of isotopes of the element magnesiumis inversely proportional to temperature. That is to say, the ratio of magnesium-26to magnesium-24 can be easily changed under the conditions of the Earth’ssurface but not in the deep Earth. Therefore, rocks/materials formed underlow-temperature conditions of the Earth’s surface usually have variable ratios of magnesium-26 to magnesium-24, and they are different from rocks produced through high-temperature processes (e.g., basalts). Basedon such understanding, geochemists utilize magnesium isotopes to gauge the “fingerprint” of some possible surficial materials in the mantle sources of basalts.

Light magnesium going from the surface to the deep and back again

In this new study, the research team focused on the volcanic rocks from the Pitcairn Island in the South Pacific. These rocks have the lowest ratios of lead-206 to lead-204 among oceanic lavas, and they are known as “EM1(enriched mantle type1) compositional endmember.”  The origin of such a mantle reservoir with peculiar isotopic compositions has been under debate for more than 25 years, yet an in-depth analysis of the chemical and isotopic compositions of new fresh basalt samples from the Pitcairn Island yielded previously untouched, fascinating results:

Magnesium isotopes provide the key to the researchers’ discovery. On the Pitcairn Island, the basalts’ ratios of magnesium-26 to magnesium-24 are significantly lower than the undisturbed mantle. Since magnesium is more concentrated in the rocks of the mantle rather than the crust, very little crustal materials, after sinking into the mantle, could modify the magnesium isotopic compositions of the mantle.

An exception, however, is sedimentary magnesium-carbonate (e.g., dolomite), which has abundance of magnesium together with extreme enrichment of magnesium-24 relative to magnesium-26. Therefore, the researchers concluded, the exceptionally low ratios of magnesium-26 to magnesium-24 found in these deep mantle-derived basalts most likely originated from magnesium-carbonates that once lay on the Earth's surface.

Also in Pitcairn EM1 lavas, previous researchers found unusual combinations of isotopes of the element sulfur ,which were common in the Earth's atmosphere until ~2.45 billion years ago but not afterwards. This indicates that the raw materials for these EM1 lavas are ancient crustal materials that subducted before ~2.45 billion years ago.

These two findings, when combined, provide evidence that carbonates, which once lay on the Earth’s surface in the Archean, were recycled into the mantle and re-erupted at a young ocean island.

The researchers, however, found that the low calcium-to-aluminum ratio in the Pitcairn lavas is inconsistent with experiments showing carbonate-bearing source retains a high calcium-to-aluminum ratio on melting. Asolution to this “paradox” is, as suggested by the researchers, the reaction between carbonate and silicate minerals in the late Archean subducted sediments may have exhausted the carbonates, leaving the isotopically light magnesium to incorporate into the silicate minerals, which then entered the lower mantle and ultimately returned to the surface as ingredients of Pitcairn island basalt (see Figure 2). Thus, the low ratio of magnesium-26 to magnesium-24 remains in the basalts as a tell-tale “ghost” of the originally subducted carbonate.

Figure 2. The authors of the paper hold that some residual components of ancient carbonate-bearing sediments once atthe Earth’s surface are now ingredients of a young oceanic island (PitcairnIsland). (1) Carbonate-bearing sediments subduct into the Earth's mantle. (2) Interactions between carbonate and silicate of the sediments exhausted the carbonates, leaving the residual silicate sediments, which then sink into the core-mantle boundary. (3) The long-term (about 2.5 billion years) isolated evolution ofresidual sediments formed EM1 component. (4) The Pitcairn mantle plumetransports the EM1 component into shallow asthenosphere. (5) Partial melting of EM1 mantle sources produces lavas, which then erupt from under the seabed to form islands. The whole circulation process might take about 2.5 billion years, more than half of the Earth's entire history. Credit: Wang Xiaojun etal./Nanjing University

“Some stable isotopes, such as magnesium, are useful for tracing information about the exchange of materials between the Earth’s surface and the interior,” said Professor Chen, of Nanjing University. “ Although we can’t find the identity of some surficial materials in the volcanic rocks,their original chemical or isotopic features are sometimes preserved in those rocks."

This study also has implications for the hot-topic of deep carbon cycle: the magnesium isotopes are the main “carbonate memory” leftin the ocean island basalts, but the identity of carbonate has been destroyed through interactions between carbonate and silicate minerals in subducted sediments. Therefore, findings of this study do not support actual subduction of substantial amounts of carbonates into the lower mantle, at least not duringlate Archean time when the subduction zones were presumably hotter than they were in geologically more recent times.

This study was financially supported by the National Natural Science Foundation of China, the “Program of Introducing Talents of Discipline to Universities” of China, and the “Program A for Outstanding PhD Candidate of Nanjing University.”

More information: http://www.pnas.org/content/early/2018/08/07/1719570115.