Magmatismo cenozoico dell’area mediterranea Laurea Magistrale in Geologia di Esplorazione Dipartimento di Scienze della Terra Università degli Studi.

Magmatismo cenozoico dell’area mediterranea Laurea Magistrale in Geologia di Esplorazione Dipartimento di Scienze della Terra Università degli Studi di Roma La Sapienza Michele Lustrino Tel: Stanza 116)

Part Five. Basic geochemical definitions of mantle sources. Principles of isotope geochemistry applied to igneous petrology: Rb-Sr, Sm-Nd and U-Th-Pb systematics. Mantle end-members and geochemical jargon used in igneous petrology.

PETROLOGICAL MODELLING
Petrological modelling of igneous rocks is based on a multidisciplinar approach. Geochemistry is the fundamental tool to infer mantle source characteristics and the main physical state of the variables that can change the chemical composition from its production to its solidification. Of course also Petrography (including Mineralogy) has a fundamental role in petrological modelling. Particular emphasis is given also to Geophysics and rheological characteristics of mantle rocks (e.g., tomographic reconstructions of upper and lower mantle).

Geochemical study of igneous rocks follows three main lines of investigation: Major element chemistry Trace element chemistry Isotope geochemistry

Major element composition of igneous rocks can furnish basic information on the gross type of mantle source involved in petrogenetic processes. E.g., from a major element point of view a mantle source can be defined as: Fertile (i.e., a source never tapped by partial melting) Depleted (i.e., a residual source after extraction of partial melts).

Major element composition of igneous rocks can furnish basic information on the gross type of mantle source involved in petrogenetic processes. Question: What type of chemical difference do you expect in terms of major element composition between a Fertile and a Depleted mantle source? Depleted sources have lower content of “basaltic components” (i.e., elements that are relatively enriched in basaltic melts compared to peridotitic sources).

Depleted sources have lower content of “basaltic components” (i.e., elements that are relatively enriched in basaltic melts compared to peridotitic sources).

Second Question: What type of mineralogical difference do you expect in terms of mineralogy between a fertile and a depleted mantle source? Fertile mantle sources have typically lherzolitic composition with modal Cpx 10-15% and modal Opx 15-25%. The remaining part is essentially Olivine (50-70%) and Sp/Gt (5%). Depleted mantle sources have lower pyroxene/olivine and cpx/opx ratios, ranging to near pyroxene-free lithologies (e.g., dunites) to cpx-poor (<5%) lithologies (harzburgites).

From a trace element point of view, a mantle source can be defined either Depleted or Enriched. In the first case a Depleted Mantle (DM) Source is characterized by relatively low content of lithophile trace elements (i.e., elements that “like” entering the melt). The lithophile elements are those that are incompatible in the structure of the typical mineral assemblage of the mantle (i.e., olivine, pyroxenes, spinel and garnet). As a general rule, a mantle sources with refractory major element composition (e.g., low Ti, Al, Ca, alkalies) should have low content of lithophile (incompatible) trace elements.

From a trace element point of view, a mantle source can be defined either Depleted or Enriched. However it is not uncommon to see mantle sources (e.g., mantle xenoliths, abyssal and tectonic peridotites) with depleted major element composition and enriched trace element composition. This may look very strange and puzzling… In many cases the more depleted (from a major element p.o.v.) the more enriched (from a trace element p.o.v.).

Why the more major element-depleted the more trace element-enriched? Major element-depleted mantle sources are also characterized by high olivine/pyroxenes ratio. Olivine-rich lithologies permit easier percolation of what are called METASOMATIC AGENTS. This because in ol-rich lithologies the connectivity of the melt phase is strongly favoured. Experimental studies have shown that the melt phase in a partially molten peridotite is disconnected at the grain boundaries surrounded by the pyroxene grains.

From a trace element point of view, a mantle source can be defined either Depleted or Enriched. The enrichment can be exclusively “invisible” (i.e., cryptic enrichment) when it can be observed only on the basis of analytical investigation. In this case the metasomatic agent is a very thin film around peridotite crystals. Expected WR ?

From a trace element point of view, a mantle source can be defined either Depleted or Enriched. The enrichment can be also “visible” (i.e., modal enrichment) when it is possible to observe new “exotic” phases, typically volatile-bearing (e.g., phlogopite, pargasite, dolomite, magnesite and others) or glassy pockets. Phlogopite Phlogopite Phlogopite Plane-polarized and crossed polar view of Finero Peridotite (Celodurite) Courtesy of L. Melluso

The relationship among the different possible metasomatic agents and between the metasomatic agents and the chemical composition of the partial melts is far from being fully understood. A typical example regards the relationships between hydrous phases (mostly phlogopite and pargasite) and the glassy pockets or veinlets occasionally found in mantle xenoliths. Both these features (minerals and glasses) show chemical compositions that cannot be produced by partial melting of typical mantle minerals (ol-opx-cpx-sp/gt).

For the amphibole/phlogopite and glass presence an Egg-and-Chicken type dilemma exist: Are the hydrous phases the crystallization product of metasomatic melts (represented by the glass) or is the glass the break-down product of decompressing hydrous phases?

One aspect is clear for sure:
PETROLOGICAL MODELLING Up to now there is ambiguous evidences for these hypotheses. Probably both are correct (i.e., in some cases hydrous phases break-down producing liquid with “exotic” compositions, whereas in other cases hydrous phases crystallize from metasomatic melts in colder regions). One aspect is clear for sure: The percolation of metasomatic agents (either fluids or melts) can strongly change the chemical (and mineralogical) original composition of mantle volumes.

Please pay attention on another fundamental concept: Also simple rising of basaltic melt may have “metasomatic effects” on the volume of mantle through whish it passes. We have seen how basaltic melts produced beneath oceanic ridges may strongly modify both major and trace element content of abyssal peridotites. In this case the chemical modifications are not related to any “exotic” composition but simply to “normal” basaltic melts.

The concept of Mantle Metasomatism is rather vague and is systematically used to explain puzzling and or unexpected results/data. “Metasomatic modifications of the mantle sources” can explain almost everything in igneous petrology… Seriously speaking, metasomatism may have profound effects in changing the chemical and mineralogical composition of the mantle. Metasomatic modifications are related to percolation of fluids/melts reacting with ambient mantle.

Mantle Origin Crustal Origin The metasomatic agents can have a:
PETROLOGICAL MODELLING The metasomatic agents can have a: Mantle Origin Crustal Origin Metasomatic Agents of mantle origin can be divided into two different types: “Normal” partial melts derived by partial melting of “normal” peridotites (we have seen these effects when dealing with the composition of abyssal peridotites); “Exotic” partial melts/fluids derived from very deep and undegassed (i.e., never tapped by basaltic extraction) sources.

PETROLOGICAL MODELLING The metasomatic agents can have a: Mantle Origin Crustal Origin Metasomatic Agents of crustal origin are related to the recycling of crustal lithologies in the mantle. Such a recycling can develop as consequence of: Subduction of oceanic lithosphere, partially including also the thin sediment cover; Detachment and delamination of gravitationally unstable thick lithospheric keel (including lower crust) in tectonically active areas. The first mechanism is by far the most effective process of crustal lithology recycling.

PETROLOGICAL MODELLING The metasomatic agents can have a: Mantle Origin Crustal Origin It is not easy to clearly identify the origin of an “exotic” component in the mantle. I.e., the anomalous K-rich composition of some mafic lavas can be related either to recycling of K-rich crustal lithologies in subduction settings or to metasomatic effects related to rising of small degrees partial melts from deep (i.e., fertile, never tapped by basaltic extraction) mantle sources. We will treat carefully this argument later on, especially when dealing with Italian K-rich lavas.

CHEMICAL GEODYNAMICS Chemical Geodynamics is an integrating study of the chemical and physical structure and evolution of the Solid Earth. The marriage between geochemistry and geophysics is important because geophysics can furnish three-dimensional picture of the present state of the Earth, while geochemistry can furnish an historical or time-averaged view of the Earth but in an a-dimensional way. The two approaches are complementary to each other. Zindler and Hart (1986) Annu. Rev. Earth Sci., 14,

Asthenosphere and Transition Zone Mantle; Lower Mantle;
CHEMICAL GEODYNAMICS The basic assumption of chemical geodynamics is that, from a geochemical point of view, the Earth can be divided into several regions or “Reservoirs”: Crust; Lithospheric Mantle; Asthenosphere and Transition Zone Mantle; Lower Mantle; Outer Liquid Core.

CHEMICAL GEODYNAMICS Crust; Lithospheric Mantle;
The basic assumption of chemical geodynamics is that, from a geochemical point of view, the Earth can be divided into several regions or “Reservoirs”: Crust; Lithospheric Mantle; These first two reservoirs are rigid. This means that these are not subject to vigorous stirring and heterogeneities can form (in case of isotopic ratios).

Asthenosphere and Transition Zone Mantle; Lower Mantle;
CHEMICAL GEODYNAMICS The basic assumption of chemical geodynamics is that, from a geochemical point of view, the Earth can be divided into several regions or “Reservoirs”: Asthenosphere and Transition Zone Mantle; Lower Mantle; Outer Liquid Core. These three reservoirs are probably more homogeneous because vigorously stirred.

CHEMICAL GEODYNAMICS On the basis of these assumptions, strong chemical and isotopic heterogeneities can develop exclusively in the Lithosphere (Crust + Lithospheric Mantle). The rest of the Earth would be “not heterogeneous”, i.e., probably it is “homogeneous”, i.e. “normal”. On these grounds the asthenospheric mantle is classically expected to produce partial melts with “normal” basaltic compositions, whereas “anomalous” compositions (e.g., K-rich magmas) are expected to derive from lithospheric sources.

CHEMICAL GEODYNAMICS A powerful tool to study the chemical structure, hystory and evolution of Earth’s Reservoirs is Isotope Geochemistry. Among the dozens of isotope parent-daughter lines three/four are the most commonly used in igneous petrology: Rb-Sr Systematic Sm-Nd Systematic U-Th-Pb Systematic U-Th-He Systematic

This is NOT a course of Geochemistry.
CHEMICAL GEODYNAMICS ATTENTION: This is NOT a course of Geochemistry. For a full treatment of these arguments you are invited to follow more specific courses.

CHEMICAL GEODYNAMICS Rb-Sr Systematic
87Rb is radioactive, and decays to the stable isotope 87Sr by b- decay. In a classical four phase mantle assemblage (ol, cpx, opx, pl/sp/gt) both Rb and Sr are incompatible elements but Rb is more incompatible than Sr. Given an hypothetical Rb/Sr ratio = 1 in a primitive mantle (i.e., mantle that has never involved in partial melting): What will be the Rb/Sr in the residual source? What will be the Rb/Sr in the partial melt?

What does this mean? CHEMICAL GEODYNAMICS Rb-Sr Systematic
A residual mantle source evolves with low to very low Rb/Sr ratios. A solidified partial melt evolves with high to very high Rb/Sr ratios. A (residual) source with low Rb/Sr has also low 87Rb. This means that there will be few nuclides that can decay in 87Sr. An (enriched) source with high Rb/Sr has high 87Rb and, consequentially, high time-integrated 87Sr.

In every type of solid source 87Rb diminishes continuously and 87Sr increases continously. After a given time interval, the residual source will have lower 87Rb and lower 87Sr than the solidified partial melt derived from that source. It is common to compare the 87Sr content (i.e., the daughter isotope after 87Rb decay) with the stable 86Sr isotope. Note that the 86Sr isotope is stable (i.e., its concentration in a solid source does not vary with time).

What does this mean? CHEMICAL GEODYNAMICS Rb-Sr Systematic
The basic principle of isotope geochemistry is that two isotopes of a single element DO NOT FRACTIONATE (i.e., the ratio between two isotopes does not vary with typical processes acting in the mantle and in the crust). What does this mean? The 87Sr/86Sr ratio does not change during partial melting and fractional crystallization processes. (This may be not correct if the mantle sources are composed of “exotic” Rb-bearing phases like mica).

So What? Question: CHEMICAL GEODYNAMICS Rb-Sr Systematic 87Sr/86Sr
Let we assume that a primitive mantle has a 87Sr/86Sr isotopic ratio = Question: What is the 87Sr/86Sr isotopic ratio of the residual source and the solidified partial melt immediately after (i.e., 1 year after) the partial melting? 87Sr/86Sr 87Sr/86Sr So What? Residual Source Partial Melt 1.0000 1.0000

In absolute terms (e.g., elemental concentration in ppm) the residual source has lower Sr than partial melt, but isotopically the two components have the same isotopic ratio. An important difference lies on the much lower Rb/Sr ratio of the residual source compared to the partial melt. This means that the residual source has much lower 87Rb and the partial melt has much higher 87Rb than the primitive mantle.

The Half-Life of 87Rb is 4.8*1010 yr.
CHEMICAL GEODYNAMICS Rb-Sr Systematic A source with high 87Rb will produce a lot of 87Sr, whereas a source with low 87Rb will be characterized by low 87Sr. Of course the amount of 87Sr produced by the decay of 87Rb depends on the velocity of decay of the parent isotope. This amount is function of the so-called Isotope Half-Life. This is the time necessary to reduce to an half the amount of a given parent isotope. Each radioactive nuclide has a typical Half-Life. The Half-Life of 87Rb is 4.8*1010 yr.

Question: CHEMICAL GEODYNAMICS Higher Higher Rb-Sr Systematic
Come back to the primitive mantle source with a 87Sr/86Sr isotopic ratio = After a geologically relevant period (e.g., Myr) the residual source and the solidified partial melt will assume different isotopic compositions in terms of 87Sr/86Sr ratios. Question: After 10 Myr a residual source will have 87Sr/86Sr isotopic ratio higher or lower than the original value (1.0000)? Higher Higher And the solidified partial melt?

The difference is that the solidified partial melt is much more enriched in 87Sr/86Sr than the residual source. 87Sr/86Sr Time Partial Melt (high Rb/Sr) Partial melting event Residual Source (low Rb/Sr) Primitive mantle source

Question: CHEMICAL GEODYNAMICS
A partial melt produced from a depleted mantle source would have relatively high or relatively low 87Sr/86Sr? Partial Melt (high Rb/Sr) Partial melting event 87Sr/86Sr Residual Source (low Rb/Sr) Primitive mantle source Time

CHEMICAL GEODYNAMICS 87Sr/86Sr = 0.70445 YES Rb-Sr Systematic
The present-day estimated composition of Bulk Silicate Earth (BSE) in terms of Sr isotopes is: 87Sr/86Sr = Sr isotopic values of igneous rocks higher than BSE indicate an origin from Enriched Sources. Values lower than BSE indicate Depleted Sources. Average 87Sr/86Sr of North Atlantic MORBs? <0.7030 Are North Atlantic MORB sources depleted? YES But depleted in what?...

CHEMICAL GEODYNAMICS 87Sr/86Sr = 0.70445 YES Rb-Sr Systematic
The present-day estimated composition of Bulk Silicate Earth (BSE) in terms of Sr isotopes is: 87Sr/86Sr = Sr isotopic values of igneous rocks higher than BSE indicate an origin from Enriched Sources. Values lower than BSE indicate Depleted Sources. Average 87Sr/86Sr of Continental Upper Crust? >0.7200 Is Upper Crust an enriched source? YES But enriched in what?...

CHEMICAL GEODYNAMICS The 87Sr/86Sr of the Earth is continuously grown since its formation. What is the 87Sr/86Sr value of the Earth when it formed? An answer comes from the BABI value. Basaltic Achondrite Best Initial 87Sr/86Sr =

CHEMICAL GEODYNAMICS Sm-Nd Systematic
Samarium and Neodymium belong to the Rare Earth Element (REE) group. In this case 147Sm decays in 143Nd with a Half-Life of 1.06 * 1011 yr. As seen for Rb-Sr systematic, also for the Sm-Nd systematic the daughter isotope is compared to a stable isotope. In this case the radiogenic 143Nd is compared to the stable 144Nd (143Nd/144Nd).

* Sm less incompatible than Nd *
CHEMICAL GEODYNAMICS Sm-Nd Systematic As seen for Rb-Sr systematic, also Sm-Nd are very incompatible in typical mantle assemblages. In the Rb-Sr isotope system the Parent Isotope (Rb) is more incompatible than the Daughter Isotope (Sr). However, in the Sm-Nd isotope system the Parent Isotope (Sm) is less incompatible than the Daughter Isotope (Nd). Remember the REE incompatibility order: La>Ce>Pr>Nd>Sm>Eu>Gd>Tb>Dy>Ho>Er>Tm>Yb>Lu. * Sm less incompatible than Nd *

CHEMICAL GEODYNAMICS Sm-Nd Systematic Question: A residual mantle source would have a Sm/Nd higher or lower than a primitive mantle source? 10 15% 20% 25% 30% 5% Cpx DM WR Higher * Sm less incompatible than Nd *

CHEMICAL GEODYNAMICS Sm-Nd Systematic Question: A residual mantle source would have a Sm/Nd higher or lower than a partial melt derived from that source? Primitive Mantle Residual Mantle Partial Melt Higher * Sm less incompatible than Nd *

Did you understand why? CHEMICAL GEODYNAMICS Sm-Nd Systematic
What does this mean? A time-integrated residual mantle source will always have a higher 147Sm/144Nd and, consequentially, a higher 143Nd/144Nd. The average composition of the Solid Earth in terms of 143Nd/144Nd is given by the ChUR (Chondritic Uniform Reservoir) Estimate = As seen for the 87Sr/86Sr isotope systematic, values higher than the ChUR estimate are considered depleted, whereas values lower are enriched. Did you understand why?

Sr-Nd and Sm-Nd Isotope Systematics
CHEMICAL GEODYNAMICS Sr-Nd and Sm-Nd Isotope Systematics 87Sr/86Sr 143Nd/144Nd Depleted Quadrant ChUR ( ) Enriched Quadrant BSE ( )

Few basic assumptions:
CHEMICAL GEODYNAMICS These two isotopic systematics are commonly used together to infer physical composition and chemical evolution of the sources of igneous rocks. Few basic assumptions: Bulk silicate Earth has BSE and ChUR Sr and Nd isotopic composition; Partial melting produces a shift in the composition of the mantle sources towards the depleted quadrant; Partial melt is characterized by aged higher 87Sr/86Sr and lower 143Nd/144Nd isotopic ratios (enriched quadrant).

CHEMICAL GEODYNAMICS WHY? Question:
Compared to the Primitive Mantle estimate, the MORB source is depleted or enriched? WHY? What is the probable Sr-Nd isotopic composition of a MORB source? 87Sr/86Sr 143Nd/144Nd ChUR ( ) BSE ( ) DR If there is a depleted reservoir there must be an enriched reservoir, right? ER Where is the ER possibly located?

CHEMICAL GEODYNAMICS WHY? Question:
Compared to the Primitive Mantle estimate, the MORB source is depleted or enriched? WHY? What is the probable Sr-Nd isotopic composition of a MORB source? If there is a depleted reservoir there must be an enriched reservoir, right? Where might the ER be located?

(Depleted MORB Mantle)
CHEMICAL GEODYNAMICS The isotopic composition of depleted mantle sources is RELATIVELY homogeneous. Here we define the first FUNDAMENTAL end-member of mantle compositions that is: DMM (Depleted MORB Mantle) We will see that this mantle end-member is more or less present in every type of igneous rock, but, obviously, it is the predominant component of melts produced beneath oceanic ridges.

Atlantic MORBs are very different from Pacific and Indian MORBs.
CHEMICAL GEODYNAMICS Typical 87Sr/86Sr ratios of DMM range from to (i.e., lower than BSE estimate). Typical 143Nd/144Nd ratios of DMM range from to (i.e., higher than ChUR estimate). Note that not all the MORBs have the same Sr-Nd isotopic composition… Atlantic MORBs are very different from Pacific and Indian MORBs. Differences exist also at a smaller scale (e.g., N-Atlantic MORBs are different from S-Atlantic MORBs).

CHEMICAL GEODYNAMICS 87Sr/86Sr and 143Nd/144Nd ratios are commonly expressed also in a slightly different form, i.e., in terms of epsilon (e) notation. For Rb-Sr isotopic system the eSr value is: eSr = (87Sr/86Sr)sample (t) (87Sr/86Sr)BSE (t) 1 * 10,000 For Sm-Nd isotopic system the eNd value is: eNd = (143Nd/144Nd)sample (t) (143Nd/144Nd)ChUR (t) 1 * 10,000

CHEMICAL GEODYNAMICS Obviously, eSr values >0 imply 87Sr/86Sr values >BSE ( ) and vice versa. eNd values >0 imply 143Nd/144Nd values >ChUR ( ), and vice versa. eSr = (87Sr/86Sr)sample (t) (87Sr/86Sr)BSE (t) 1 * 10,000 eNd = (143Nd/144Nd)sample (t) (143Nd/144Nd)ChUR (t) 1 * 10,000 The epsilon notation is much more commonly used for Nd isotopes rather than Sr.

The key factor is the REE incompatibility order!
CHEMICAL GEODYNAMICS Obviously, eSr values >0 imply 87Sr/86Sr values >BSE ( ) and vice versa. eNd values >0 imply 143Nd/144Nd values >ChUR ( ), and vice versa. Depleted Reservoir Remember: The key factor is the REE incompatibility order! Partial 3.5 Ga Enriched Reservoir

Nearly all the MORBs plot in the Depleted Quadrant
CHEMICAL GEODYNAMICS Nearly all the MORBs plot in the Depleted Quadrant ChUR BSE

CHEMICAL GEODYNAMICS Question:
Let we assume that a depleted reservoir is interested in another partial melting process 1 Ga after the first melt extraction. Question: What is the Sr-Nd isotopic composition of melt derived from this source? 87Sr/86Sr 143Nd/144Nd ChUR ( ) BSE ( ) DR ER This means that it is possible to hypothesize a depleted source for magmas plotting in the Depleted Quadrant.

CHEMICAL GEODYNAMICS In other words:
Values of eNd >0 and eSr <0 in an igneous rock indicate a mantle source that has suffered melt extraction for a long time (i.e., this rock derives from a Depleted Mantle). 87Sr/86Sr 143Nd/144Nd ChUR ( ) BSE ( ) DR ER When did the depletion event occur? Short time before magma production? Or much earlier?

CHEMICAL GEODYNAMICS YES, of course Question:
Can Enriched Regions interact with Depleted Reservoirs? YES, of course What is the commonest way to allow interaction between these two regions? DR ChUR ( ) 143Nd/144Nd Recycling of crustal lithologies essentially (but not exclusively!) via subduction processes. ER BSE ( ) 87Sr/86Sr

Why? CHEMICAL GEODYNAMICS DR ER
Recycling crustal lithologies has the typical effect of raising 87Sr/86Sr and lowering 143Nd/144Nd of the contaminated mantle source. The change of isotopic composition of the depleted reservoir is generally substantial. Why? DR have very low Sr and Nd compared to ER. In a mixing process the two most important factors are: Sr-Nd elemental content and Sr-Nd isotopic difference. DR ChUR ( ) 143Nd/144Nd ER BSE ( ) 87Sr/86Sr

CRUST-MANTLE INTERACTION
CHEMICAL GEODYNAMICS CRUST-MANTLE INTERACTION If we plot the concentration of any two elements in different samples of this mixture against each other, they must lie on a straight line between the two end members. Element B Element A 1 10% 2 30% 2 50% 2 2 90% 2

CHEMICAL GEODYNAMICS CRUST-MANTLE INTERACTION However, if we plot ratios of elements, or isotope ratios, they commonly do not lie on a straight line. Rather they will define a curve whose equation is: Ax + Bxy + Cy + D = 0 where x and y are the variables of the abscissa and ordinate, respectively.

CHEMICAL GEODYNAMICS CRUST-MANTLE INTERACTION Ax + Bxy + Cy + D = 0 Let we designate the two end members as 1 and 2 and have call ratios x1 and y1, and x2 and y2 respectively: Mantle = 1 Crust = 2 87Sr/86SrMantle = X1 143Nd/144NdMantle = Y1 87Sr/86SrCrust = X2 143Nd/144NdCrust = Y2 1 2 143Nd/144Nd (Y) 87Sr/86Sr (X) Y1 Y2 X1 X2

CHEMICAL GEODYNAMICS CRUST-MANTLE INTERACTION Ax + Bxy + Cy + D = 0 If end members are designated 1 and 2 and have ratios x1 and y1, and x2 and y2 respectively, then A = a2b1y2 – a1b2y1 B = a1b2 – a2b1 C = a2b2x1 –a1b2x2 D = a1b2x2y2 – a2b1x1y1 where ai is the denominator of yi and bi is the denominator of xi. The curvature of the mixing line will depend on the ratio r: r = a1b2/a2b1

CHEMICAL GEODYNAMICS r = a1b2/a2b1 a1 = 144NdMantle a2 = 144NdCrust
The greater the value of r, the greater the curvature. Only in the special case were r = 1 is the line straight. 143Nd/144Nd 87Sr/86Sr 1 Remember that ai is the denominator of yi and bi is the denominator of xi: a1 = 144NdMantle a2 = 144NdCrust b1 = 86SrMantle b2 = 86SrCrust 2 r = a1b2/a2b1

CHEMICAL GEODYNAMICS The greater the value of r, the greater the curvature. Only in the special case were r = 1 is the line straight. 1 Since the amounts of 144Nd and 86Sr are proportional to total Nd and Sr respectively, r is approximated by: Nd1Sr2/Nd2Sr1 i.e.: 143Nd/144Nd 2 87Sr/86Sr NdMantleSrCrust/NdCrustSrMantle

CHEMICAL GEODYNAMICS A mantle end-member (1) with much Nd and low Sr will interact with a crust with a lower Nd/Sr (i.e., higher Sr) (2) following the line: 1 A mantle with low Nd and much Sr will interact with a crust with higher Nd/Sr (i.e., lower Sr) following the line: 143Nd/144Nd 2 87Sr/86Sr NdMantleSrCrust/NdCrustSrMantle

EM-I (Enriched Mantle Type One) EM-II (Enriched Mantle Type Two)
CHEMICAL GEODYNAMICS Mantle sources that have interacted with crust-derived lithologies are therefore Enriched in terms of Sr-Nd isotopic ratios, at least compared to typical DMM sources. The second set of Mantle component identifiable with Sr-Nd isotopes is represented by Enriched Mantle (EM) components. These are generally divided into two main types: EM-I (Enriched Mantle Type One) EM-II (Enriched Mantle Type Two)

CHEMICAL GEODYNAMICS On a Sr-Nd isotopic diagram the approximate location of EM-I and EM-II end-members are: PRIMA MORB Field EM-II EM-I

From a general Sr-Nd point of view:
CHEMICAL GEODYNAMICS EM-I has very low (unradiogenic) 143Nd/144Nd (~0.5121) and mildly radiogenic 87Sr/86Sr (~0.705) EM-II has low 143Nd/144Nd (~0.5124) and strongly radiogenic 87Sr/86Sr (~0.708). These differences require very different explanations. From a general Sr-Nd point of view: EM-I rocks are similar to Lower Continental Crust; EM-II rocks are similar to Upper Continental Crust.

DMM EM-I EM-II CHEMICAL GEODYNAMICS To resume:
On the basis of Sr-Nd isotope systematics it is possible to distinguish at least three very different geochemical end-members in oceanic basalts: DMM Very Low 87Sr/86Sr and very high 143Nd/144Nd EM-I Mildly radiogenic 87Sr/86Sr and low 143Nd/144Nd EM-II High 87Sr/86Sr and low 143Nd/144Nd

But what does prosaically this mean?
CHEMICAL GEODYNAMICS But what does prosaically this mean? DMM MORB Field OIB Field Nearly all the Sr-Nd isotopic composition of the oceanic basalts (MORBs and OIBs) can be defined in terms of only three mantle end-members. EM-II EM-I

CHEMICAL GEODYNAMICS However, it is very common to classify oceanic basalts in terms of another mantle end-member, the so-called HIMU-end member. MORB Field DMM EM-I EM-II HIMU

CHEMICAL GEODYNAMICS However, it is very common to classify oceanic basalts in terms of another mantle end-member, the so-called HIMU-end member. DMM MORB Field HIMU The HIMU end-member is fully defined with Pb isotopes. From a Sr-Nd point of view it is not extremely different to DMM. EM-II EM-I

To resume (for the second time):
CHEMICAL GEODYNAMICS To resume (for the second time): The Sr-Nd isotopic composition of oceanic basalts (MORBs + OIBs) can be expressed in terms of % of FOUR MANTLE END-MEMBERS: DMM HIMU EM-I EM-II End-member particularly common along oceanic ridges (but not exclusive of this setting). End-members particularly common among Oceanic Island Basalts (OIBs) but found also in other settings.

CHEMICAL GEODYNAMICS There are some key Type-Localities where these end-members are particularly rich. DMM? Particularly evident in North Atlantic MORBs; HIMU? Particularly evident in St. Helena Island and French Polynesia (e.g., Rurutu, Rarotonga) basalts; EM-I? Particularly evident in Pitcairn basalts; EM-II? Particularly evident in Pacific Ocean Islands (e.g., Society, Marquesas) basalts.

Typical sampling locality to study Chemical Geodynamics…
Other MORBs and OIBs are characterized by various amounts of these main four end-members. Typical sampling locality to study Chemical Geodynamics…

CHEMICAL GEODYNAMICS Pacific Ocean Islands are divided into :
Melanesia (Black Islands) Micronesia (Small Islands) Polynesia (Many Islands) There are islands

CHEMICAL GEODYNAMICS Pitcairn Volcano

Typical Melanesia Oceanic Island…
CHEMICAL GEODYNAMICS Typical Melanesia Oceanic Island…

CHEMICAL GEODYNAMICS Come back to Earth…

CHEMICAL GEODYNAMICS Did you ask yourself why all these geochemical end-members have been identified in oceanic areas (i.e., far away from continental plates)? In order to avoid the potential risk of crustal contamination of mantle melts. In this way the isotopic message of the basalts more likely reflects the composition of their sources.

CHEMICAL GEODYNAMICS Did you ask yourself why all these geochemical end-members have been identified in oceanic areas (i.e., far away from continental plates)? Basalts emplaced on continental crust (average thickness km up to km) can acquire some crustal isotopic composition en route to the surface, therefore masking the original isotopic composition of the mantle.

The U-Th-Pb system. CHEMICAL GEODYNAMICS
Before to continue the chemical geodynamic approach of mantle petrology we need to consider at least another isotope system: The U-Th-Pb system.

CHEMICAL GEODYNAMICS 238U decays to 206Pb 235U decays to 207Pb
U-Th-Pb Systematics The U-Th-Pb system is somewhat of a special case since there are 3 decay schemes producing isotopes of Pb. In particular, two U isotopes decay to two Pb isotopes, and since the two parents and two daughters are chemically identical, we get two decay systems for the price and one and together they provide a particularly powerful tool 238U decays to 206Pb 235U decays to 207Pb 232Th decays to 208Pb

CHEMICAL GEODYNAMICS 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb
U-Th-Pb Systematics The three Pb daughter radiogenic isotopes are commonly normalized to the stable 204Pb isotope. Typical views of Pb isotopic ratios are therefore: 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb The first two leads are called “Uranogenic Pb” because they are derived from the decay of U isotopes. The third isotope (208Pb) is called the “Thorogenic Pb” because derives from Th.

CHEMICAL GEODYNAMICS U-Th-Pb Systematics
As seen for Rb-Sr and Sm-Nd isotopes, Pb isotopic ratios of an igneous rock are a function of the amount of parent isotope (235U, 238U and 230Th) and the age of the sample (i.e., the time-integrated isotopic growth of the daughter isotopes). The isotopic ratio 238U/204Pb is called m. Rocks with high 238U/204Pb are called High-m (HIMU). Igneous rocks with high 206Pb/204Pb (up to 21-22) imply derivation from sources with high 238U/204Pb (because 206Pb derives from 238U).

There are also low-206Pb/204Pb (down to 16-17) basalts. These are necessarily related to partial melting of mantle sources with low 238U/204Pb (because 206Pb derives from 238U). This type of rocks has been in some cases called LoMU (Low-m) rocks. EM-I and EM-I-like basalts belong to LoMU types. We will no use the LoMU definition hereafter.

The abundances of the two uranogenic lead isotopes (206Pb and 207Pb) depend on the amount of 238U and 235U transformation. Because the half-life of 235U is much shorter than that of 238U, 235U decays more rapidly. As a result, on a plot 207Pb/204Pb vs. 206Pb/204Pb, Pb isotopic evolution follows curved paths. The exact path that is followed depends upon μ.

Three such evolution curves are shown here. All systems that begin with a common initial isotopic composition at time t0 along a straight line at some later time t. This line is the Pb-Pb isochron. 1 Ga ago 2 Ga ago 3 Ga ago Present-Day Pb isotopic composition of the Earth t0 (4.55 Ga ago) [Initial Pb isotopic composition of the Earth]

The Geochron is the present-day Isochron.
CHEMICAL GEODYNAMICS U-Th-Pb Systematics The Total composition of the Entire Earth must lie Along the Geochron. 1 Ga ago 2 Ga ago 3 Ga ago The Geochron is the present-day Isochron. Present-Day Pb isotopic composition of the Earth t0 (4.55 Ga ago) [Initial Pb isotopic composition of the Earth]

It should be clear that the U/Pb or 238U/204Pb ratio of a volcanic rock is not necessarily the same as that of its source. This because U has a different compatibility from Pb. It follows that, starting from a primitive mantle source with a given m0, partial melt and the residual peridotite likely have different m values (m1 and m2). However, the sum of m1 + m2 must be equal to m0.

Reservoirs within the Earth, such as the crust and mantle and individual rock units, have evolved with different m values for 4.55 Ga, so their isotope ratios need not fall on the Geochron. The mean of all such reservoirs should, however, fall on the Geochron. CLEAR? Nun sò tant’ convint’...

If a system has experienced a decrease in U/Pb at some point in the past, its Pb isotopic composition will lie to the left of the Geochron. If its U/Pb ratio increased, its present Pb isotopic composition will lie to the right of the Geochron. This because U is more incompatible than Pb.

Incompatible element-depleted reservoirs should plot to the left of the geochron, incompatible element enriched reservoirs should plot to the right of the Geochron. We would therefore predict that continental crust should lie to the right of the Geochron and the residual mantle (e.g., DMM) to the left. DMM Crust

Upper continental crustal rocks do plot mainly to the right of the Geochron, as expected, but surprisingly, mantle-derived rocks also plot mostly to the right of the Geochron! This indicates the U/Pb ratio in the mantle has increased, not decreased as expected. DMM Crust

Pb paradox CHEMICAL GEODYNAMICS U-Th-Pb Systematics
Upper continental crustal rocks do plot mainly to the right of the Geochron, as expected, but surprisingly, mantle-derived rocks also plot mostly to the right of the Geochron! This phenomenon is known as the Pb paradox DMM Crust Mmmhh…

Lower Crust CHEMICAL GEODYNAMICS U-Th-Pb Systematics
Probably there is a solution to this paradox… Effectively there is a physical reservoir on the Earth that evolves on the left of the Geochron… This reservoir is the Lower Crust DMM Crust Mmmhh…

CHEMICAL GEODYNAMICS UPV CiMACI rocks MORB 207Pb/204Pb 206Pb/204Pb
4.55 Ga 14 15 16 17 18 19 20 21 22 14.50 14.75 15.00 15.25 15.50 15.75 16.00 Continental crust (Ref. in Kramers & Tolstikhin, 1997) Upper Crust Zartman & Haines (1988) Asmerom & Jacobsen (1993) Kramers & Tolstikhin (1997) Hemming & McLennan (2000) Millet et al. (2004) UPV (Anorogenic rocks of Sardinia) Geochron Lower Crust xenoliths (Ref. in Murphy et al., 2001) Zartman & Haines, 1998 Kramers & Tolstikhin, 1997 (young lower crust) Liew et al., 1992 (model B) Liew et al., 1992 (model A) Kramers & Tolstikhin, 1997 (total lower crust) Kramers & Tolstikhin, 1997 (old lower crust) Rudnick & Goldstein, 1990 Davies et al., 1984 Newsom et al., 1986 CiMACI rocks MORB Ba/Nb

Now try to define the Pb isotopic composition of the previously identified mantle end-members: Remember that on Sr-Nd isotopic grounds the DMM and HIMU end-members were not so easy to distinguish.

Now try to define the Pb isotopic composition of the previously identified mantle end-members: Also in this case, with the four end-members it is possible to define the entire isotopic variation of OIB. However, on the basis of Pb isotopes, the two end-members plot in completely different spaces.

? ? ? ? CHEMICAL GEODYNAMICS U-Th-Pb Systematics
Now try to define the Pb isotopic composition of the previously identified mantle end-members: I have some doubts about the position of the DMM end-member in terms of Pb isotopes… ? ? ? ?

Now try to define the Pb isotopic composition of the previously identified mantle end-members: A partially different isotope topology of at least two end-members is possible. EM-I DMM

Now try to define the Pb isotopic composition of the previously identified mantle end-members: Geochron EM-I DMM

? ? ? ? ? ? ? ? CHEMICAL GEODYNAMICS U-Th-Pb Systematics
Now try to define the Pb isotopic composition of the previously identified mantle end-members: ? ? ? ? ? ? ? ?

Now try to define the Pb isotopic composition of the previously identified mantle end-members:

DMM HIMU EM-I EM-II CHEMICAL GEODYNAMICS To resume:
On the basis of Sr-Nd-Pb isotope systematics it is possible to reduce the isotopic variability of the oceanic basalts (MORBs and OIBs) in terms of four fundamental end-members: DMM HIMU EM-I EM-II

Who knows? CHEMICAL GEODYNAMICS To summarise:
For each of these end-members several isotopic compositions or isotopic ranges have been proposed in literature. What is the correct value for each of these end-members? Who knows?

The typical sequence of rationale is:
CHEMICAL GEODYNAMICS Why do these end-members are considered so important in modern igneous petrology? The typical sequence of rationale is: The HIMU, EM-I and EM-II end-members have been identified in OIBs. The OIBs are related to mantle plumes. The identification of one of these end-members in other areas (e.g., continental basalts) must be considered as isotopic evidence for the existence of a mantle plume (deep thermal anomaly typically coming from the Core-Mantle Boundary Layer).

CHEMICAL GEODYNAMICS This is the ground over which most if not all the studies aiming to identify the existence of mantle plumes with isotopic tools are built. But are the isotopic composition of the OIB end-members really compatible with a deep mantle source? i.e., can these be used as a proof for the existence of a derivation from primitive (undegassed) source?

HIMU-EMI-EMII really mean?
CHEMICAL GEODYNAMICS In other words: What do HIMU-EMI-EMII really mean? More: what do other hypothetical mantle components (EM-III, FOZO, C, PREMA, LVC, CMR, EAR) really mean? All these acronyms are HYPOTHETICAL end-member compositions resident somewhere in the upper and lower mantle, as well as at the core-mantle boundary.

PREMA = Prevalent Mantle; LVC = Low Velocity Component;
CHEMICAL GEODYNAMICS FOZO = Focus Zone; C = Common Component; PREMA = Prevalent Mantle; LVC = Low Velocity Component; CMR = Common Mantle Reservoir; EAR = European Asthenospheric Reservoir. All these are components identified on isotopic grounds. These are the areas where large amounts of subduction-unrelated (i.e., anorogenic) basalts plot.

CHEMICAL GEODYNAMICS HIMU DMM CiMACI Rocks PREMA A EAR CMR LVZ

CHEMICAL GEODYNAMICS CiMACI Rocks EM-II HIMU LVZ CMR EAR PREMA A DMM
206/204 >22 LVZ CMR EAR PREMA A EM-I DMM

CHEMICAL GEODYNAMICS A few comments:
OIB is a generic term that should be avoided; under this term at least three types of basalts (HIMU-EMI-EMII) with different isotopic features and trace element ratios are grouped; HIMU-, EMI-, EMII-like isotopic compositions are not direct evidence of the presence of mantle plumes or hot-spots; Most of the end-member compositions (e.g., EMI, EMII) do not exist as discrete reservoirs in the Earth’s mantle; rather they represent heterogeneous regions that inherited their features during long periods of metasomatic processes and recycling of crustal materials;

d) A single model to explain all the isotopic systematics and incompatible trace element contents for each of the end-members do not exist; e) In the mixing process, the pre-metasomatic mantle reacts with low-degrees crustal melts rather than bulk crustal material; the result of this process is probably similar to the SUMA Model; f) The HIMU end-member (and, similarly also FOZO and PREMA) is found in many oceanic and continental localities. This must imply the existence of a relatively uniform and geochemically homogeneous source, probably in the convecting asthenospheric mantle;

g) the DMM previously considered uniform is now considered very heterogeneous (e.g., 206Pb/204Pb ratios ranging from <17 to >20) and therefore to talk of a general DMM end-member has lost its meaning without specifying which kind of Morb type is under consideration (e.g., N. Atlantic, SW Indian, and so on); h) virtually no portion of the upper mantle has primitive composition and most has interacted with recycled crustal material.

There are four most important variables underpinning the existence of the mantle end-members: The degree of fertility of the pre-metasomatic mantle; The composition of the crustal material; The degree and modality of interaction between crustal and mantle lithologies; The possibility storing and isolating these heterogeneities permitting radiogenic growth of isotopic systematics.

One of the greatest errors when dealing with isotope geochemistry applied to igneous petrology is to associate geochemical figures (e.g., isotopic or trace element ratios) to physical reservoirs (e.g., pieces of mantle). I mean that it is not so obvious how to associate a given geochemical number to a given and unique melt source.

CHEMICAL GEODYNAMICS In the next lessons I will show you the most important geochronological, major and trace element as well as Sr-Nd-Pb isotopic compositions of the most important igneous districts of the Mediterranean Area. We will discuss these data in terms of geodynamic evolution of the entire area.

Credits: Hofmann (1997) Nature, 385, 219-229.
Hofmann (2004) In: Treatise on Geochemistry Vol. 2, Lustrino and Wilson (2007) Earth Sci. Rev., 81, 1-65. W.M. White (2003) Geochemistry Zindler and Hart (1986) Annu. Rev. Earth Sci., 14, Anyone can use this material for scientific and non-scientific purposes. Please quote and acknowledge the source of data

Magmatismo cenozoico dell’area mediterranea Laurea Magistrale in Geologia di Esplorazione Dipartimento di Scienze della Terra Università degli Studi.

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Magmatismo cenozoico dell’area mediterranea Laurea Magistrale in Geologia di Esplorazione Dipartimento di Scienze della Terra Università degli Studi.

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Presentation on theme: "Magmatismo cenozoico dell’area mediterranea Laurea Magistrale in Geologia di Esplorazione Dipartimento di Scienze della Terra Università degli Studi."— Presentation transcript:

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