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

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

3. For example, at an active Caldera

4. Petrology Field Trip to Bemco Mining District

5. 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 circulations
Gossan 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.

6. Solubility in water The 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 SrSO 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 PbCO Chromates are frequently insoluble. Examples: PbCrO4, BaCrO Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.

7. 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

8. 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.

9. 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.

10. 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

11. Hydrothermal Deposit, Bemco Mine

12. Ch. 10 Origin of Basaltic 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
Seismic evidence -> basalts are generated in the mantle
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

13. Basalts in different Plate Tectonic settings are chemically different
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

14. Two principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Ocean Islands such as Hawaii have both Tholeiitic AND Alkaline Basalts
Table 10-1
Common petrographic differences between tholeiitic and alkaline basalts
Subalkaline: Tholeiitic-type Basalt
Alkaline Basalt
Usually 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
(a third, minor, one is hi-Al, or calc-alk basalt & will be discussed later)
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may occur
Interstitial glass and/or quartz common
Interstitial glass rare, and quartz 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 common
Plagioclase later in sequence, uncommon
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).
A third is hi-Al, or calc-alkaline basalt, usually continental

15. Subalkaline: Tholeiites and Calc-alkaline Basalts
Example: AFM diagram
(alkalis-Fe-Mg)
Silica content variation
in two famous Igneous localities
A (Na2O + K2O) , F( FeO + Fe2O3) and M ( MgO )
Notice: Skaergard has a Ferrobasalt member, Crater Lake does not.
Figure 8-3. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

16. AFM diagram: Tilley: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series F A M
Calc-alkaline T h o l e i t c
MORs and Flood Basalts, and above
Plumes
Figure 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,
Above subduction zones

17. AFM diagram showing “typical” areas for various extents of evolution from primitive magma types. Tholeites go through a Ferro-Basalt stage before continuing
towards Rhyolite.
Recall Skaergard and
Mt. Mazama
AFM diagram showing “typical” areas for various extents of evolution from primitive magma types

18. Ophiolite Suite Some Serpentine is formed
due to hot water (called Hydrothermal)
circulation

19. Samples of mantle material
Ophiolites
Slabs of oceanic crust and upper mantle
Thrust faulted onto edge of continent
Dredge samples from oceanic fracture zones
Nodules and xenoliths in some basalts
Kimberlite xenoliths Plume passes through a subduction zone’s carbon
Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth

20. Lherzolite, Harzburgite and Dunite
Lherzolite is probably fertile unaltered mantle
Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called lherzolite. The molten magma extracted from harzburgite may then erupt on the surface as basalt. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite called dunite

21. Lherzolite is probably fertile unaltered mantle
Dunite and Harzburgite are refractory residuum after basalt has been extracted by partial melting 15
Tholeiitic basalt 10
Partial Melting
Wt.% Al2O3 5
Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.
Lherzolite
Harzburgite
Residuum
Dunite
0.0
0.2
0.4
0.6
0.8
Wt.% TiO2

22. Lherzolite: A type of peridotite with Olivine > Opx + Cpx
Dunite 90
Peridotites
Wehrlite
Harzburgite
Lherzolite 40
Olivine Websterite
Pyroxenites
Orthopyroxenite 10
Websterite 10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2-2 C After IUGS

23. Phase diagram for aluminous 4-phase Lherzolite:
Last was Olivive & Pyroxene, now look at Al mineralss.
Notice the mantle will not melt under normal ocean geotherm!
Al-phase =
CaAl2Si2O8.
Ca++ Plagioclase
shallow (< 50 km)
Spinel Lherzolite Spinel is MgAl2O4
50-80 km
Garnet Lherzolite km
Si[4] ® Si[6] coord.
> 400 km
Si [4] => Si [6]
Note: the mantle will not melt under normal ocean geotherm!
Mg3Al2(SiO4)3
Figure Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

24. How does the mantle melt?
1) Increase 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
No realistic mechanism for the general case
Local hot spots OK
very limited area
Figure Melting by raising the temperature.
solidus
liquidus

25. 2) Lower the pressure: MOR and Rifts
Adiabatic rise of mantle (no conductive heat loss)
Rise to low pressure, lower MP, “decompression melting”
Steeper than solidus
Intersects solidus
D slope = heat of fusion as mantle melts
Adiabatic rise of mantle with no conductive heat loss
Steeper than solidus
Intersects solidus
D slope = heat of fusion as mantle melts
Decompression melting could melt at least 30%
Figure Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

26. Basalt origin 1, at the MOR

27. 3) Add volatiles (especially H2O) lowers Melt Pt changes slope
Remember solid + water = liq(aq) and LeChatelier
Dramatic lowering of melting point of peridotite
Eclogite
Amphibolite
Serpentinite
Eclogite: red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite)
Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

28. Basalt origin 2

29. So, basaltic melts can be created under several circumstances
We saw: Plates separate and mantle rises at mid-ocean ridges
Adiabatic rise ® decompression
Melting
Subduction zones ® dewatering
Third way:
Hot spots ® melting plumes, also basaltic

30. Melting depths vary w\ volcanic province Most within upper few hundred kilometers

31. The 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.

32. E-MORBs
Enriched MORBs (called E-MORBS) have, for example, higher Lanthanum La, Cerium Ce, and also higher Strontium Sr than normal N-MORBs

33. “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-chemistry that [some, the E-MORBs] MORB are generated in the 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 earth
elements (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 signature' 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

34. Plume and MOR interactions
Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N
Yaoling Niu, Ken D. Collerson, Rodey Batiza, J. Immo Wendt, Marcel Regelous
Journal of Geophysical Research: Solid Earth (1978–2012)
Volume 104, Issue B4, pages 7067–7087, 10 April 1999
The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean ridge basalts (E-MORBs) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = – , and 143Nd/144Nd = – ). The 87Sr/86Sr 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 CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two-component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. 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 and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched [E-MORB source] domains, whereas the recycled mantle/peridotitic residues are the most depleted [N-MORB source] matrix. 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 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 lavas may have come from the Hawaiian plume. This “distal plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic Ridge. The so-called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic [enriched] lithologies. The positive Na8-Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.

35. Stable Isotopes of Strontium
The ratio 87Sr/86Sr is a parameter often reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because Strontium has an electron configuration similar to that of calcium, it readily substitutes for Ca in minerals.
87Sr/86Sr is used, for example, to distinguish enriched E-MORBs from depleted source N-MORBs.

36. From a plume
From a plume

37. Can we generate both tholeiitic and alkaline basalts from a chemically uniform mantle?
Variables (other than X)
Temperature
Pressure
Variables (other than X)
Temperature
= % partial melting
Pressure
Fig indicates that, although the chemistry may be the same, the mineralogy varies
Pressure effects on eutectic shift
Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

38. Pressure effects:
Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field
Alkaline basalts are thus favored by greater depth of melting
Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field
Alkaline basalts are thus favored by greater depth of melting
Figure 10-8 Change in the eutectic (first melt) composition with increasing pressure from 1 to 3 GPa projected onto the base of the basalt tetrahedron. After Kushiro (1968), J. Geophys. Res., 73,

39. Crystal Fractionation of magmas as they rise
Tholeiite ® alkaline
by Fractionation at medium to high Pressure
Recall not at low Pressure, due Albite Thermal Divide
Thermal divide, they cannot evolve into one another, separate sources at low Pressure, but fractionation at med to high P does allow evolution of a magma from Tholeiite to Alkaline

40. Initial Conclusions: Tholeiites favored by shallower melting
25% melting at <30 km ® tholeiite
25% melting at 60 km ® alkaline basalt
Tholeiites favored by greater % partial melting
20 % melting at 60 km ® alkaline basalt
incompatibles (alkalis) ® initial melts
30 % melting at 60 km ® tholeiite

41. Initial Conclusions
A chemically homogeneous mantle can yield a variety of basalt types
Alkaline basalts are favored over tholeiites by deeper melting
Fractionation at moderate to high depths can also create alkaline basalts from tholeiites
At low P there is a thermal divide that separates the two series
In spite of this initial success, there is evidence to suggest that such a simple approach is not realistic, and that the mantle is chemically heterogeneous

42. Experiments on melting enriched vs. depleted mantle samples:
Tholeiite easily created
by 10-30% Partial Melting
More silica saturated
at lower P
Figure 10-17a. Results of partial melting experiments on depleted Mantle. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,

43. Experiments on melting enriched vs. depleted mantle (DM) samples:
2. Enriched Mantle
Tholeiites extend to
higher P than for Depleted Mantle
Alkaline basalt field
(purple) at higher P yet
Figure 10-17b. Results of partial melting experiments on fertile Lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic magmas. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73,

44. At a depth of about 670 – 700 km g 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 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct.

45. Mantle model circa 1975
Homogeneous mantle
Large-scale convection (drives plate tectonics?)
Figure 10-16a After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.

46. Newer mantle model Upper depleted mantle = MORB source - Tholeites
Lower undepleted & enriched OIB source - Alkaline
Boundary = 670 km phase transition
Sufficient D density to impede convection so they convect independently
Layered mantle
Upper depleted mantle = MORB source
depleted by MORB extraction > 1 Ga
Lower = undepleted & enriched OIB source
Boundary = 670 km phase transition
Sufficient D density to impede convection so they convect independently
It is interesting to note that this concept of a layered mantle was initiated by the REE concentrations of oceanic basalts
Later support came from isotopes and geophysics
Figure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.