Ocean island basalt (OIB) is a
volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
, usually
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
ic in composition, erupted in oceans away from
tectonic plate boundaries. Although ocean island basaltic
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...
is mainly erupted as basalt
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...
, the basaltic magma is sometimes modified by
igneous differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of ...
to produce a range of other volcanic rock types, for example,
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
in
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its ...
, and
phonolite
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...
and
trachyte
Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
at the intraplate volcano
Fernando de Noronha
Fernando de Noronha () is an archipelago in the Atlantic Ocean, part of the State of Pernambuco, Brazil, and located off the Brazilian coast. It consists of 21 islands and islets, extending over an area of . Only the eponymous main island is in ...
. Unlike
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...
basalts (MORB
s), which erupt at spreading centers (
divergent plate boundaries), and
volcanic arc lavas, which erupt at
subduction zones (
convergent plate boundaries), ocean island basalts are the result of
intraplate volcanism
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of ...
. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and
Samoa
Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands ( Savai'i and Upolu); two smaller, inhabited islands ( Manono and Apolima); ...
, which is located near a subduction zone.
In the ocean basins, ocean island basalts form
seamount
A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...
s, and in some cases, enough material is erupted that the rock protrudes from the ocean and forms an island, like at
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
, Samoa, and Iceland. Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or
guyot
In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .[hotspots
Hotspot, Hot Spot or Hot spot may refer to:
Places
* Hot Spot, Kentucky, a community in the United States
Arts, entertainment, and media Fictional entities
* Hot Spot (comics), a name for the DC Comics character Isaiah Crockett
* Hot Spot (Tra ...]
, which are thought to be the surface expressions of melting of thermally buoyant, rising conduits of hot rock in the
Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly so ...
, called
mantle plume
A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...
s. Mantle plume conduits may drift slowly, but Earth's tectonic plates drift more rapidly relative to mantle plumes. As a result, the relative motion of Earth's tectonic plates over mantle plumes produces age-progressive chains of volcanic islands and seamounts with the youngest, active volcanoes located above the axis of the mantle plume while older, inactive volcanoes are located progressively farther away from the plume conduit (''see Figure 1'').
Hotspot chains can record tens of millions of years of continuous volcanic history; for example, the oldest seamounts in the
Hawaiian–Emperor seamount chain
The Hawaiian–Emperor seamount chain is a mostly undersea mountain range in the Pacific Ocean that reaches above sea level in Hawaii. It is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, ...
are over 80 million years old.
Not all ocean island basalts are the product of mantle plumes. There are thousands of seamounts that are not clearly associated with upwelling mantle plumes, and there are chains of seamounts that are not age progressive. Seamounts that are not clearly linked to a mantle plume indicate that regional mantle composition and tectonic activity may also play important roles in producing intraplate volcanism.
Isotope geochemistry
The
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...
of ocean island basalts is useful for studying the chemical and physical structure of Earth's mantle. Some mantle plumes that feed hotspot volcanism lavas are thought to originate as deep as the
core–mantle boundary
The core–mantle boundary (CMB) of Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2,891 km (1,796 miles) depth beneath Earth's surface. The boundary is observed ...
(~2900 km deep). The composition of the ocean island basalts at hotspots provides a window into the composition of mantle domains in the plume conduit that melted to yield the basalts, thus providing clues as to how and when different reservoirs in the mantle formed.
Early conceptual models for the geochemical structure of the mantle argued that the mantle was split into two reservoirs: the upper mantle and the lower mantle. The upper mantle was thought to be geochemically depleted due to melt extraction which formed Earth's continents. The lower mantle was thought to be homogenous and “
primitive”. (Primitive, in this case, refers to silicate material that represents the building blocks of the planet that has not been modified by melt extraction, or mixed with subducted materials, since Earth's accretion and core formation.) Seismic
tomography
Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, ...
showed subducted slabs passing through the upper mantle and entering the lower mantle, which indicates that the lower mantle cannot be isolated. Additionally, the isotopic heterogeneity observed in plume-derived ocean island basalts argues against a homogenous lower mantle. Heavy,
radiogenic isotopes are a particularly useful tool for studying the composition of mantle sources because isotopic ratios are not sensitive to mantle melting. This means that the heavy radiogenic isotopic ratio of a melt, which upwells and becomes a volcanic rock on the surface of the Earth, reflects the isotopic ratio of the mantle source at the time of melting. The best studied heavy radiogenic isotope systems in ocean island basalts are
87Sr/
86Sr,
143Nd/
144Nd,
206Pb/
204Pb,
207Pb/
204Pb,
208Pb/
204Pb,
176Hf/
177Hf and, more recently,
187Os/
188Os. In each of these systems, a radioactive parent isotope with a long half-life (i.e., longer than 704 million years) decays to a “radiogenic” daughter isotope. Changes in the parent/daughter ratio by, for example, mantle melting, result in changes in the radiogenic isotopic ratios. Thus, these radiogenic isotopic systems are sensitive to the timing, and degree, of parent/daughter the changed (or fractionated) parent daughter ratio, which then informs the process(es) responsible for generating observed radiogenic isotopic heterogeneity in ocean island basalts. In mantle geochemistry, any composition with relatively low
87Sr/
86Sr, and high
143Nd/
144Nd and
176Hf/
177Hf, is a referred to as “geochemically depleted”. High
87Sr/
86Sr, and low
143Nd/
144Nd and
176Hf/
177Hf, is referred to as “geochemically enriched”. Relatively low isotopic ratios of Pb in mantle-derived rocks are described as ''unradiogenic''; relatively high ratios are described as ''radiogenic''.
These isotopic systems have provided evidence for a heterogenous lower mantle. There are several distinct “mantle domains” or endmembers that appear in the ocean island basalt record. When plotted in multi-isotope space, ocean island basalts tend to form arrays trending from a central composition out to an endmember with an extreme composition. The depleted mantle, or DM, is one endmember, and is defined by low
87Sr/
86Sr,
206Pb/
204Pb,
207Pb/
204Pb,
208Pb/
204Pb, and high
143Nd/
144Nd and
176Hf/
177Hf. The DM is therefore geochemically depleted, and relatively unradiogenic. Mid-ocean ridges passively sample the upper mantle and MORBs are typically geochemically depleted, and therefore it is widely accepted that the upper mantle is composed mostly of depleted mantle. Thus, the term depleted MORB mantle (DMM) is often used to describe the upper mantle that sources mid-ocean ridge volcanism. Ocean island basalts also sample geochemically depleted mantle domains. In fact, most ocean island basalts are geochemically depleted, and <10% of ocean island basalts have lavas that extend to geochemically enriched (i.e.,
143Nd/
144Nd lower than the Earth's building blocks) compositions.
There are two geochemically enriched domains, named enriched mantle 1 (EM1), and enriched mantle 2 (EM2). Though broadly similar, there are some important distinctions between EM1 and EM2. EM1 has unradiogenic
206Pb/
204Pb, moderately high
87Sr/
86Sr, and extends to lower
143Nd/
144Nd and
176Hf/
177Hf than EM2.
Pitcairn
The Pitcairn Islands (; Pitkern: '), officially the Pitcairn, Henderson, Ducie and Oeno Islands, is a group of four volcanic islands in the southern Pacific Ocean that form the sole British Overseas Territory in the Pacific Ocean. The four i ...
,
Kerguelen
The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a group of islands in the sub-Antarctic constituting one of the two exposed parts of the Kerguelen Plateau, a large ...
-
Heard, and
Tristan
Tristan ( Latin/Brythonic: ''Drustanus''; cy, Trystan), also known as Tristram or Tristain and similar names, is the hero of the legend of Tristan and Iseult. In the legend, he is tasked with escorting the Irish princess Iseult to wed ...
-
Gough
Gough ( ) is a surname. The surname probably derives from the Welsh (English: "red"), given as a nickname to someone with red hair or a red complexion or as a reduced form of the Irish McGough which itself is an Anglicized form of Gaelic , a patr ...
are the type localities of EM1. EM2 is defined by higher
87Sr/
86Sr than EM1, and higher
143Nd/
144Nd and
176Hf/
177Hf at a given
87Sr/
86Sr value, and intermediate
206Pb/
204Pb.
Samoa
Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands ( Savai'i and Upolu); two smaller, inhabited islands ( Manono and Apolima); ...
and
Society
A society is a group of individuals involved in persistent social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same political authority and dominant cultural expectations. Soc ...
are the archetypal EM2 localities.
Another distinct mantle domain is the HIMU mantle. In isotope geochemistry, the Greek letter µ (or mu) is used to describe the
238U/
204Pb, such that ‘high µ’ (abbreviated HIMU) describes a high
238U/
204Pb ratio. Over time, as
238U decays to
206Pb, HIMU Earth materials develop particularly radiogenic (high)
206Pb/
204Pb. If an Earth material has elevated
238U/
204Pb (HIMU), then it will also have elevated
235U/
204Pb, and therefore will produce radiogenic Pb compositions for both the
206Pb/
204Pb and
207Pb/
204Pb isotopic systems (
238U decays
206Pb,
235U decays to
207Pb). Similarly, Earth materials with high U/Pb also tend to have high Th/Pb, and thus evolve to have high
208Pb/
204Pb (
232Th decays to
208Pb). Ocean island basalts with highly radiogenic
206Pb/
204Pb,
207Pb/
204Pb,
208Pb/
204Pb are the products of HIMU mantle domains.
St. Helena, and several islands in the
Cook
Cook or The Cook may refer to:
Food preparation
* Cooking, the preparation of food
* Cook (domestic worker), a household staff member who prepares food
* Cook (professional), an individual who prepares food for consumption in the food industry
* ...
-
Austral volcanic lineament (e.g.,
Mangaia
Mangaia (traditionally known as A'ua'u Enua, which means ''terraced'') is the most southerly of the Cook Islands and the second largest, after Rarotonga. It is a roughly circular island, with an area of , from Rarotonga. Originally heavily popula ...
) are the type localities for HIMU ocean island basalts.
The final mantle domain discussed here is the common composition that ocean island basalts trend toward in radiogenic isotopic multi-space. This is also most prevalent mantle source in ocean island basalts, and has intermediate to geochemically depleted
87Sr/
86Sr,
143Nd/
144Nd, and
176Hf/
177Hf, as well as intermediate
206Pb/
204Pb,
207Pb/
204Pb,
208Pb/
204Pb. This central mantle domain has several names, each with slightly different implications. PREMA, or “Prevalent Mantle” was the first term coined by Zindler and Hart (1986) to describe the most common composition sampled by ocean island basalts. Hart et al. (1992) later named the location of the intersection of ocean island basalt compositions in radiogenic isotopic multi-space as the “Focus Zone”, or FOZO. Farley et al. (1992) in the same year described a high
3He/
4He (a primitive geochemical signature) component in plumes as the “Primitive Helium Mantle”, or PHEM. Finally, Hanan and Graham (1996) used the term “C” (for common component) to describe a common mixing component in mantle derived rocks.
The presence of a particular mantle domain in ocean island basalts from two hotspots, signaled by a particular radiogenic isotopic composition, does not necessarily indicate that mantle plumes with similar isotopic compositions are sourced from the same physical reservoir in the deep mantle. Instead, mantle domains with similar radiogenic isotopic compositions sampled at different hotspot localities are thought to share similar geologic histories. For example, the EM2 hotspots of Samoa and Society are both thought to have a mantle source that contains recycled upper continental crust, an idea that is supported by stable isotope observations, including δ
18O and δ
7Li. The isotopic similarities do not imply that Samoa and Society have the same physical mantle source, as evidenced by their slightly distinct arrays in radiogenic isotopic multi-space. Thus, hotspots that are categorized as “EM1”, “EM2”, “HIMU”, or “FOZO”, may each sample physically distinct, but compositionally similar, portions of the mantle. Furthermore, some hotspot chains host lavas with wide range of isotopic compositions so that the plume source seems to either sample multiple domains which can be sampled at different times in the volcanic evolution of a hotspot.
Isotopic systems help to deconvolve the geologic processes that contributed to, and in some cases the timing of, the formation of these mantle domains. Some important examples include the presence of crustal fingerprints in enriched mantle sources that indicate that material from Earth's continents and oceans can be subducted into the mantle and brought back up to the surface in buoyantly rising mantle plumes. Sulfur isotopic analyses have shown
mass-independent-fractionation (MIF) in the sulfur isotopes in some plume-derived lavas. MIF of sulfur isotopes is a phenomenon that occurred in Earth's atmosphere only before the
Great Oxidation Event
The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...
~2.3 Ga. The presence of recycled material with MIF signatures indicates that some of the recycled material brought is older than 2.3 Ga, formed prior to the Great Oxidation Event and has resurfaced via mantle plume volcanism.
Noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
isotopic systems, such as
3He/
4He,
20Ne/
22Ne, and
129Xe/
130Xe, have been used to demonstrate that parts of the lower mantle are relatively less degassed and have not been homogenized despite billions of years of mantle convective mixing. Some large, hot mantle plumes have anomalously high
3He/
4He. Since
4He is being constantly produced within the Earth via alpha decay (of
235,238U,
232Th, and
147Sm), but
3He is not being generated in appreciable quantities in the deep Earth, the ratio of
3He to
4He is decreasing in the interior of the Earth over time. The early
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
began with high
3He/
4He and therefore the Earth first accreted with high
3He/
4He. Thus, in plume-derived lavas, high
3He/
4He is an “ancient” geochemical signature that indicates the existence of a well-preserved
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
reservoir in the deep mantle. The timing of the formation of this reservoir is constrained by observed anomalies of
129Xe/
130Xe in ocean islands basalts, because
129Xe was only produced by decay of
129I during the first ~100 My of Earth's history.
Together, high
3He/
4He and
129Xe/
130Xe indicate a relatively less degassed, primitive noble gas domain that has been relatively well preserved since the early
Hadean
The Hadean ( ) is a geologic eon of Earth history preceding the Archean. On Earth, the Hadean began with the planet's formation about 4.54 billion years ago (although the start of the Hadean is defined as the age of the oldest solid materia ...
.
Mantle sources
There are various sources identified for ocean island basalt magma in Earth's mantle. These mantle sources are inferred from differences in
radiogenic isotope ratios that magmas inherit from their source rock. Sources have been defined from a combined analysis of
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is e ...
(Sr),
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarn ...
(Nd) and
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
(Pb) isotopes.
The sources as defined by radiogenic isotopes are:
Footnotes
References
; Notes
; Sources
*
*
{{Basalt
Basalt
Hotspot volcanism