You recall discussions of Black Smokers, the placement of Sulfide deposits at the mid-ocean

ridges.

The same thing happens at volcanoes when water and magma are in proximity

For example, at an active Caldera

http://vulcan.wr.usgs.gov/Glossary/Caldera/description_caldera.html

Petrology Field Trip to Bemco Mining District

Ores from weathered Sulfide deposits• Mineral deposits

containing sulfide minerals, e.g. copper sulfides, are subjected to weathering, can go into solution and trickle down to the reducing conditions below the water table, where native metals or rich concentrations of ores are precipitated.

e.g. black smokers, hydrothermal circulationsGossan Intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore

deposit or mineral vein. In the classic gossan or iron cap all that remains is iron oxides and quartz often in the

form of boxworks, quartz lined cavities retaining the shape of the dissolved ore minerals.

Solubility in waterThe Solubility Rules

1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH4+) are also

soluble. 2. Salts containing nitrate ion (NO3

-) are generally soluble. 3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble. 4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble.

5. Most sulfate salts are soluble, for example FeSO4 is soluble. Important exceptions to this rule include BaSO4, PbSO4, Ag2SO4 and SrSO4 . 6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble.

7. Most sulfides of transition metals are highly insoluble. Thus, CuS, FeS, FeS2, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble. 8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3. 9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4 10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO4 11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.

Changing insoluble metal sulfides into soluble sulfates

• Oxidizing Zone above the water table • Sulfide minerals, for example ferrous and

copper sulfides, are subject to weathering.

• Sulfide minerals are oxidized near the surface and produce sulfuric acid. For example:

• FeS2 (s) + 7O + H2O →FeSO4 (aq) + H2SO4

Formation of the solvent Ferrous Sulfate

The part played by ferric sulfate Fe2(SO4)3 as a solvent can be seen by the following reactions:

Pyrite FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S Chalcopyrite CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S Chalcocite Cu2S + Fe2(SO4)3 →CuSO4 + 2FeSO4 + CuS Covellite CuS + Fe2(SO4)3 →2FeSO4 + S + CuSO4

Sphalerite ZnS + 4Fe2(SO4)3 + H2O →ZnSO4 + 8FeSO4 + 4H2SO4 Galena PbS + Fe2(SO4)3 + H2O + 3O →PbSO4 + 2FeSO4 + H2SO4

Silver 2Ag + Fe2(SO4)3 → Ag2SO4 + 2FeSO4

· Most of the sulfates are readily soluble, and these cold dilute solutions slowly trickle downwards through the deposit until the proper Eh-pH conditions are met to cause deposition of their metallic content.

Reaction and Trickling Down

• Iron sulfate reacts with sulfides, they go into solution as sulfates, acid rainwater then carries, for example copper, as copper sulfate, down to the water table.

• CuS(s) + Fe2(SO4)3 (aq) →2FeSO4 (aq) + S(s) + CuSO4 (aq)

• The net result is that dissolved copper sulfide trickles down from the oxidizing upper portion of the deposit to that portion at and just below the water table.

Reducing Zone below the water table

• Below the water table, where additional sulfide minerals remain solid and unoxidized (e.g. Pyrite FeS2), any iron sulfide grains present will react with the copper sulfate solution, putting iron into solution and precipitating copper.

• FeS2 (s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s) + 2S(s)

• This process is called Supergene Enrichment

Hydrothermal Deposit, Bemco MineHydrothermal Deposit, Bemco Mine

Ch. 10 Origin of Ch. 10 Origin of Basaltic MagmaBasaltic Magma

Seismic evidence basalts are generated in the mantle …

by partial melting of mantle material

Probably can derive most other magmas from this primary magma by fractional crystallization, assimilation,

etc. Basalt is the most common

magma If we are going to understand the origin of igneous rocks, it’s best to start with the generation of basalt from the mantle

Chapter 9 has one figure which we should look at for today’s topic:Here is one corner of that figure:

Basalts in different Plate Tectonic settings are chemically different

Two principal types of Two principal types of basalt in the ocean basalt in the ocean

basins basins

Table 10-1Table 10-1 Common petrographic differences between tholeiitic and alkaline basaltsCommon petrographic differences between tholeiitic and alkaline basalts

Subalkaline: Tholeiitic-type Basalt Alkaline BasaltUsually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic

Groundmass No olivine Olivine common

Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish)

Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent

No alkali feldsparNo alkali feldspar Interstitial alkali alkali feldspar feldspar or feldspathoid or feldspathoid may occur

Interstitial glass and/or quartz common Interstitial glass rare, and quartz absentquartz absent

Olivine rare, unzoned, and may be partially resorbed Olivine common and zoned

Phenocrysts or show reaction rims of orthopyroxene

Orthopyroxene uncommon Orthopyroxene absent

Early plagioclase Early plagioclase common Plagioclase later in sequence, uncommon

Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims

after Hughes (1982) and McBirney (1993).

Tholeiitic Basalt and Alkaline Basalt

A third is hi-Al, or calc-alkaline basalt, usually continental

Ocean Islands such as Hawaii have both Tholeiitic AND Alkaline Basalts

Subalkaline: Tholeiites and Calc-alkaline Subalkaline: Tholeiites and Calc-alkaline BasaltsBasalts

Example: AFM diagramExample: AFM diagram

(alkalis-Fe-Mg)(alkalis-Fe-Mg)

Figure 8-3. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

A (Na2O + K2O) , F( FeO + Fe2O3) and M ( MgO ) Notice: Skaergard has a Ferrobasalt member, Crater Lake does not.

Silica content variationin two famous Igneous localities

http://petrology.oxfordjournals.org/cgi/content/abstract/45/3/507

F

A M

Calc-alkaline

T

ho leiitic

AFM diagram: AFM diagram: Tilley: Tilley: can further subdivide the subalkaline can further subdivide the subalkaline magma series into a magma series into a tholeiitictholeiitic and a and a calc-alkalinecalc-alkaline series series

Figure 8-14. AFM diagram showing the distinction between selected tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the Columbia River Basalts, and Hawaii (solid circles) plus the calc-alkaline rocks of the Cascade volcanics (open circles). From Irving and Baragar (1971). After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.

MORs and Flood Basalts, and abovePlumes

Above subduction zones

AFM diagram showing “typical” areas for various extents of evolution from primitive magma types. Tholeites go through a Ferro-Basalt stage before continuingtowards Rhyolite.

Recall Skaergard and Mt. Mazama

Ophiolite SuiteSome Serpentine is formeddue to hot water (called Hydrothermal) circulation

Samples of mantle Samples of mantle materialmaterial

OphiolitesOphiolites– Slabs of oceanic crust and upper mantleSlabs of oceanic crust and upper mantle– Thrust faulted onto edge of continentThrust faulted onto edge of continent

Dredge samples from oceanic fracture Dredge samples from oceanic fracture zoneszones

Nodules and Nodules and xenolithsxenoliths in some basalts in some basalts Kimberlite xenoliths Kimberlite xenoliths Plume passes through Plume passes through

a subduction zone’s carbona subduction zone’s carbon– Diamond-bearing pipes blasted up from the mantle Diamond-bearing pipes blasted up from the mantle

carrying numerous xenoliths from depthcarrying numerous xenoliths from depth

Lherzolite, Harzburgite and Lherzolite, Harzburgite and DuniteDunite Lherzolite is probably fertile unaltered Lherzolite is probably fertile unaltered

mantlemantle Harzburgite typically forms by the extraction Harzburgite typically forms by the extraction

of partial melts from the more pyroxene-rich of partial melts from the more pyroxene-rich peridotite called lherzolite. The molten peridotite called lherzolite. The molten magma extracted from harzburgite may magma extracted from harzburgite may then erupt on the surface as basalt. If partial then erupt on the surface as basalt. If partial melting of the harzburgite continues, all of melting of the harzburgite continues, all of the pyroxene may be extracted from it to the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-form magma, leaving behind the pyroxene-poor peridotite called dunitepoor peridotite called dunite

15

10

5

00.0 0.2 0.4 0.6 0.8

Wt.

% A

l 2O3

Wt.% TiO2

DuniteHarzburgite

Lherzolite

Tholeiitic basalt

Partia

l Melt

ing

Residuum

LherzoliteLherzolite is probably fertile unaltered mantle is probably fertile unaltered mantleDuniteDunite and and HarzburgiteHarzburgite are refractory residuum after basalt has been are refractory residuum after basalt has been

extracted by partial meltingextracted by partial melting

Figure 10-1 Figure 10-1 Brown and Mussett, Brown and Mussett, A. E. (1993), A. E. (1993), The Inaccessible The Inaccessible Earth: An Integrated View of Its Earth: An Integrated View of Its Structure and Composition. Structure and Composition. Chapman & Hall/Kluwer.Chapman & Hall/Kluwer.

LherzoliteLherzolite: A type of : A type of peridotiteperidotite with Olivine > Opx with Olivine > Opx

+ Cpx+ CpxOlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

teW

ehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

Figure 2-2 C After IUGSFigure 2-2 C After IUGS

Phase diagram for Phase diagram for aluminous 4-phase aluminous 4-phase

Lherzolite:Lherzolite: Ca++ PlagioclaseCa++ Plagioclase

shallow (< 50 km)shallow (< 50 km) Spinel Spinel Lherzolite

Spinel is MgAl2O4

50-80 km50-80 km Garnet Garnet Lherzolite

80-400 km80-400 km Si[4] Si[4] Si[6] coord. Si[6] coord.

> 400 km> 400 km

Al-phase =Al-phase =

Figure 10-2 Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

Notice the mantle will not melt under normal ocean geotherm!

Si [4] => Si [6]

CaAl2Si2O8.

Mg3Al2(SiO4)3

Last was Olivive & Pyroxene, now look at Al mineralss.

How does the mantle How does the mantle melt?melt?

1) 1) Increase the Increase the temperaturetemperature

Figure 10-3. Figure 10-3. Melting by Melting by raising the temperature.raising the temperature.

No realistic mechanism for the general case because Temps hotter than the Geothermal Gradient are needed. Maybe accumulate radioactive decay heat?Local hot spots OK; very limited area

solidu

s

liquidus

2) 2) Lower the pressure: MOR and RiftsLower the pressure: MOR and Rifts– AdiabaticAdiabatic rise of mantle (no conductive heat loss) rise of mantle (no conductive heat loss)– Rise to low pressure, lower MP, “decompression Rise to low pressure, lower MP, “decompression

melting”melting”

Figure 10-4. Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

Steeper than solidusIntersects solidus slope = heat of fusion as mantle melts

Basalt origin 1, at the MOR

3) 3) Add volatilesAdd volatiles (especially (especially HH22OO) ) lowers lowers

Melt Pt Melt Pt changes slopechanges slope

Figure 10-4 or 10-5 (2Figure 10-4 or 10-5 (2ndnd ed). ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

Eclogite: red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite)

EclogiteAmphibolite Serpentinite

Basalt origin 2Basalt origin 2

So, basaltic melts So, basaltic melts cancan be be created under several created under several

circumstancescircumstances We saw: Plates separate and mantle rises at mid-ocean We saw: Plates separate and mantle rises at mid-ocean ridgesridges

Adiabatic rise Adiabatic rise decompression decompression

MeltingMelting Subduction zones Subduction zones dewatering dewatering

Third way:Third way:

Hot spotsHot spots melting plumes, also basaltic melting plumes, also basaltic

Melting depths vary w\ volcanic provinceMelting depths vary w\ volcanic provinceMost within upper few hundred kilometersMost within upper few hundred kilometers

The melts can mixThe melts can mix

• There is evidence that plume and MOR can mix (following slides)

• Certainly a plume rising through a subduction surface is the favorite model for diamond transport to the surface.

E-MORBsE-MORBs• Enriched MORBs (called E-MORBS) have, for

example, higher Lanthanum La, Cerium Ce, and also higher Strontium Sr than normal N-MORBs

“With increasing depths, the aluminous phase in the upper mantle changes from plagioclase to spinel to garnet The transition from spinel lherzolite (olivine +orthopyroxene + clinopyroxene + spinel) to garnet lherzolite (olivine + orthopyroxene + clinopyroxene+ garnet) could potentially influence the characteristics of some kinds of basalts, particularly mid-ocean ridge basalts (MORB), since this transition is thought to occur at about the same depths at which MORB may originate. There is evidence from trace element and isotope geo-There is evidence from trace element and isotope geo-chemistry that [some, the E-MORBs] MORB are generated in the chemistry that [some, the E-MORBs] MORB are generated in the presence of garnet presence of garnet (Klein and Langmuir 1987; Hirschmann and Stolper 1996). The evidence includes the depletion of heavier rare earth elements relative to lighter rare earthelements (Kay and Gast 1973), depletion in 177Lu/176Hf (Salters and Hart 1989) and elevated 230Th/238U ratios (Beattie 1993a, b; LaTourrette et al. 1993). This is generally referred to as the `garnet garnet signature' in MORBsignature' in MORB. However, if melting started in the garnet lherzolite stability field, simple melting models (e.g. Klein and Langmuir 1987; McKenzie and Bickle 1988; Iwamori et al. 1995) predict a thickness of the oceanic crust much greater than the average crust at 7 +/- 1 km inferred from seismological studies (e.g. White et al. 1992). Several possible solutions have been put forward to resolve this apparent conflict, including: (1) reduced melt productivity of upwelling peridotite (Asimov et al. 1995); (2)variations in depth of melting on the top of the melting zone beneath ridges (Shen and Forsyth 1995); or (3) partial melting of small amounts of garnet-bearing assemblages in veins such as garnet pyroxenites or eclogites (among others: Wood 1979; AlleÁgre et al. 1984; Hirschmann and Stolper 1996).”Klemme and O’Neill (2000) Lu = Lutetium Hf = Hafnium

Plume and MOR interactionsPlume and MOR interactions Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East

Pacific Rise at 11°20′NPacific Rise at 11°20′NYaoling Niu, Ken D. Collerson, Rodey Batiza, J. Immo Wendt, Marcel RegelousYaoling Niu, Ken D. Collerson, Rodey Batiza, J. Immo Wendt, Marcel Regelous

Journal of Geophysical Research: Solid Earth (1978–2012)Journal of Geophysical Research: Solid Earth (1978–2012)

Volume 104, Issue B4, pages 7067–7087, 10 April 1999Volume 104, Issue B4, pages 7067–7087, 10 April 1999

The East Pacific Rise The East Pacific Rise (EPR) at 11°20′N erupts an (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean ridge unusually high proportion of enriched mid-ocean ridge basalts (E-MORBsbasalts (E-MORBs) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR ) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit mantle far from mantle plumes. These basalts exhibit large compositional variations large compositional variations (e.g., [La/Sm]N = (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from These correlations are interpreted to result from melting of a two-component mantle with the enriched component residing as physically distinct domains melting of a two-component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic recycled crustal/eclogitic lithologies are the major source of the enriched [E-MORB source] domainslithologies are the major source of the enriched [E-MORB source] domains, , whereas the recycled whereas the recycled mantle/peridotitic residues are the most depleted [N-MORB source] matrixmantle/peridotitic residues are the most depleted [N-MORB source] matrix. On Pb-Sr isotope plot, the . On Pb-Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas lavas may have come from the Hawaiian plumemay have come from the Hawaiian plume. This “distal plume-ridge interaction” between the EPR and . This “distal plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic RidgeHawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic Ridge. The so-. The so-called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic [enriched] called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic [enriched] lithologies. lithologies. The positive Na8-Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from The positive Na8-Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.mantle compositional (vs. temperature) variation.

Stable Isotopes of StrontiumStable Isotopes of Strontium

The ratio The ratio 8787Sr/Sr/8686Sr is a parameter often Sr is a parameter often reported in geologic investigations; ratios in reported in geologic investigations; ratios in minerals and rocks have values ranging from minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because about 0.7 to greater than 4.0. Because Strontium has an electron configuration Strontium has an electron configuration similar to that of calcium, it readily similar to that of calcium, it readily substitutes for Ca in minerals.substitutes for Ca in minerals.

8787Sr/Sr/8686Sr is used, for example, to distinguish Sr is used, for example, to distinguish enriched E-MORBs from depleted source N-enriched E-MORBs from depleted source N-MORBs.MORBs.

From a plume

From a plume

Can we generate both tholeiitic and alkaline basalts

from a chemically uniform mantle?

Variables (other than X)Variables (other than X)– TemperatureTemperature– PressurePressure

Figure 10-2 Figure 10-2 Phase diagram of aluminous lherzolite Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and with melting interval (gray), sub-solidus reactions, and geothermal gradient. After geothermal gradient. After Wyllie, P. J. (1981). Geol. Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Rundsch. 70, 128-153.

Pressure effects:Pressure effects:

Figure 10-8 Figure 10-8 Change in the eutectic Change in the eutectic (first melt) composition with (first melt) composition with increasing pressure from 1 to 3 GPa increasing pressure from 1 to 3 GPa projected onto the base of the basalt projected onto the base of the basalt tetrahedron. tetrahedron. After Kushiro (1968), After Kushiro (1968), J. J. Geophys. Res.Geophys. Res., 73, 619-634., 73, 619-634.

Increased pressure moves the ternary the ternary eutectic minimum from eutectic minimum from the oversaturated the oversaturated tholeiite field to the tholeiite field to the under-saturated alkaline under-saturated alkaline basalt fieldbasalt field

Alkaline basalts are thus favored by greater depth of melting

Crystal Fractionation of magmas as they Crystal Fractionation of magmas as they riserise

Tholeiite Tholeiite alkaline alkaline

by Fractionation by Fractionation at medium to high Pressureat medium to high Pressure Recall not at low Pressure, due Albite Thermal Recall not at low Pressure, due Albite Thermal

DivideDivide Thermal divide, they cannot evolve into one Thermal divide, they cannot evolve into one

another, separate sources at low Pressure, but another, separate sources at low Pressure, but fractionation at med to high P does allow fractionation at med to high P does allow evolution of a magma from Tholeiite to Alkalineevolution of a magma from Tholeiite to Alkaline

Initial Conclusions:Initial Conclusions:

Tholeiites favored by shallower meltingTholeiites favored by shallower melting– 25% melting at 25% melting at <<30 km 30 km tholeiite tholeiite– 25% melting at 60 km 25% melting at 60 km alkaline basalt alkaline basalt

Tholeiites favored by greater % partial Tholeiites favored by greater % partial meltingmelting– 20 % melting at 60 km 20 % melting at 60 km alkaline basalt alkaline basalt

incompatibles (alkalis) incompatibles (alkalis) initial melts initial melts

– 30 % melting at 60 km 30 % melting at 60 km tholeiite tholeiite

A chemically homogeneous mantle A chemically homogeneous mantle can yield a variety of basalt typescan yield a variety of basalt types

Alkaline basalts are favored over Alkaline basalts are favored over tholeiites by deeper meltingtholeiites by deeper melting

Fractionation at moderate to high Fractionation at moderate to high depths can also create alkaline basalts depths can also create alkaline basalts from tholeiitesfrom tholeiites

At low P there is a thermal divide that At low P there is a thermal divide that separates the two seriesseparates the two series

Initial Conclusions Initial Conclusions

Experiments on melting Experiments on melting enriched vs. depleted mantle enriched vs. depleted mantle

samples:samples:

Tholeiite easily Tholeiite easily createdcreatedby 10-30% Partial by 10-30% Partial

MeltingMelting More silica saturatedMore silica saturated

at lower Pat lower P

1. Depleted Mantle1. Depleted Mantle

Figure 10-17a. Figure 10-17a. Results of partial melting experiments on Results of partial melting experiments on depleted depleted Mantle. Dashed lines are contours representing Mantle. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. these phases are completely consumed in the melt. After After Jaques and Green (1980).Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310.Contrib. Mineral. Petrol., 73, 287-310.

Experiments on melting enriched vs. Experiments on melting enriched vs. depleted mantle (DM) samples:depleted mantle (DM) samples:

Tholeiites extend toTholeiites extend to

higher P than for higher P than for Depleted MantleDepleted Mantle

Alkaline basalt fieldAlkaline basalt field

(purple) at higher P yet(purple) at higher P yet

2. Enriched Mantle2. Enriched Mantle

Figure 10-17b. Figure 10-17b. Results of partial melting experiments on Results of partial melting experiments on fertilefertile Lherzolites. Dashed lines are contours representing percent Lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic the conditions required for the generation of alkaline basaltic magmas. magmas. After Jaques and Green (1980).After Jaques and Green (1980). Contrib. Mineral. Contrib. Mineral. Petrol., 73, 287-310.Petrol., 73, 287-310.

At a depth of about 670 – 700 km Olivine ((Mg,Fe)SiO4 )decomposes into silicate Perovskite (FeSiO3) and Periclase (MgO) + silica SiO2 in an endothermic reaction.

Endothermic systems cool, contract, are less buoyant.

This leads some workers to believe that the 670- 700 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct.

Mantle model circa 1975Mantle model circa 1975

Figure 10-16aFigure 10-16a After Basaltic Volcanism Study Project (1981).After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Lunar and Planetary Institute.

Newer mantle modelNewer mantle model Upper depleted mantle = MORB source - TholeitesUpper depleted mantle = MORB source - Tholeites Lower undepleted & enriched OIB source - AlkalineLower undepleted & enriched OIB source - Alkaline

Figure 10-16bFigure 10-16b After Basaltic Volcanism Study Project (1981).After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Lunar and Planetary Institute.

Boundary = 670 Boundary = 670 km phase km phase transitiontransitionSufficient Sufficient density to density to impede impede convection so convection so they convect they convect independentlyindependently