Madeleine Flashcards
(1) Not all hotspots meet criteria for a very deep mantle origin.
Three proposed types:
i. Narrow, primary plumes (Reunion, Hawaii)
ii. Secondary plumes emanating from top of ‘superswells’
i. Shallow tertiary plumes linked to tensile stress in lithosphere
(1) Different plumes
Primary plume – from deepest part of mantle
Secondary plumes – from upper/lower mantle discontinuity
Tertiary plumes – lithosphere
(1) Ocean island volcanism
- Oceanic intraplate volcanism is a product of mantle convection/ recycling processes
- Ocean islands produce isolation that allows for speciation and therefore diversity
- Ecological and evolutionary diversity – geographical isolation (e.g. Galapagos)
- Life systems closely linked to plate tectonics
(1) Magma genesis in ocean islands:
- Ocean island magmas are generated by adiabatic decompression melting (no transfer of heat in or out)
- Isentropic adiabat = DT/DP for subsolidus upwelling, no conductive cooling (small temperature change with pressure).
- Mantle Tp is projected temperature of adiabatic trend at surface
- Solidus temperature increases more steeply with pressure
- Adiabatic gradient is steeper than the change in solidus temperature with pressure, so inevitable melting at low P
- Isentropic melting path has larger DT with pressure due to latent heat of melting
(1) Melting compared to usual
No melting under normal conditions. Need to either increase mantle Tp (thermal anomaly) or decrease pressure (e.g. rifting)
• MORB – extreme rifting
• Mantle pot temp has a large effect on melting by adiabatic decompression
• Adiabatic decompression results from extension of the lithosphere (MORB), or plume upwelling. The extent of melting is controlled by the depth to which decompression can occur (crustal thickness).
• Relatively high melt production rates (e.g. on Hawaii) are consistent with anomalously high mantle potential temperatures (e.g. >1550 ℃)
(1) Experimental constraints on melting
• High-P experiments give insights into conditions of mantle melting
Upper mantle is alumnus lherzolite - fertile, undepleted mantle
Melt extraction leaves residue depleted in clinopyroxene (harzburgite)
(1) The basalt tetrahedron
The basalt tetrahedron can be used to understand and visualize the types of basalt formed in different conditions
Nepheline – a feldspathoid – like feldspar but no water
Silica undersaturated rocks to left, saturated to right
Normative components – on table column to right
Qz tholeiite = Si-oversaturated
Ol tholeiite = Is-saturated
Alkali ol basalt = Si-undersaturated
(1) Conditions of formation
- In the plane Fo-Ne-Qz it is also clear that formation of alkali basalts is favoured by high-pressure melting
- Addition of CO2 shifts eutectic towards more Si-undersaturated alkali compositions
- Addition of CO2 means its easier to produce alkali basalts and towards nepheline
- Addition of H2O shifts eutectic towards more Si-saturated compositions
- Increasing extent of melting also pulls melt composition towards Si-saturation
- Fo-En cotectics as function of pressure (Kushiro 2007)
- Eutectic melt is in eqm with
- Fo + En + Ab
- Once Ab is consumed, melt composition moves up cotectic towards Si- saturated melts
- Increasing extent of melting also pulls melt composition towards Si-saturation
- Alkalis are incompatible so enriched in small degree partial melts
(1) What happens during low-pressure crystallisation of those melts?
- Fo-Ab join is a thermal divide at low pressure so cannot cross this line during crystallisation. End up with alkaline and tholeiitic suites
- Join is a thermal divide that can’t be passed by fractional crystallisation – can’t move between
- Produces either alkaline basalt suites or tholiete basalt suites
(1) Isotope geochemistry of the mantle
- OIBs are abundant and sample oceanic mantle far from crustal contamination – therefore a good way to investigate global mantle geochemistry
- OIBs are generally enriched in incompatible trace elements such as LILE (K, Rb, Cs), LREE (La, Ce), and HFSE (Nb, Zr, Hf)
- OIB magmas (e.g. Hawaii) have more radiogenic Sr and less radiogenic Nd than MORB source. Some even more radiogenic than primitive mantle (EMI, EMII)
- Source must be more enriched mantle reservoir. Significant mantle heterogeneity.
- Depletion in HREE relative to MORB – melting in presence of gt, >60 km
(1) Mantle plume volcanism – Hawaii
• Hawaii represents the ‘classic’ mantle plume case
• Primary melts from Hawaii sample primitive, undegassed deep mantle (high 3He/4He)
• Geoid/ residual depth anomaly:
– Broad 1500km swell (buoyant uplift due to presence of hot plume)
– Localised (200km) trough due to load of volcanic edifice – flexure of elastic lithosphere
• These trends are ‘classic’ case – Hawaii – not all hotspots follow this
• Depression around islands – due to big load causing flexure of the lithosphere
• Broad swell – due to buoyant uplift of hot material under the islands
• Linear age progression away from active volcanism
(1) Stages of volcano: Hawaii
Loihi seamount - alkali and transitional tholeiitic lavas – unusual for OIBs
Less than a million years to end transition
- Pre-shield stage: As the plume impinges, low-degree melts are formed at the margins of the plume. These high pressure, low degree melts are enriched in incompatible elements (including Na, K) and are alkaline.
- Shield-building stage: High degrees of melting over the plume axis result in formation of tholeiitic basalts.
- Post-shield stage: ‘Downstream’ from the plume head, again get lower degrees of melting and hence post-shield alkali basalts.
(2) What is the evidence for magma storage beneath volcanoes?
– Passive degassing at active (but quiescent) volcanoes
– Magmas saturated with multiple low-P phases
– Geophysical evidence (e.g. seismic tomography, imaging of reflectors, surface deformation)
– Large volumes of magma in explosive eruptions
– Ancient magma chambers preserved in eroded crust
– Gravity anomaly across a hot body
– Surface uplift and movement
(2) Thermobarometry
use mineral ± melt ± fluid reactions that are sensitive to pressure and/or temperature
– Gives equilibration P, T
– For example, there is a large volume change for formation of jadeite pyroxene (NaAlSi2O6) from melt (i.e., between reactants and products)
– NaO0.5 (liq) + AlO1.5 (liq) + 2SiO2 (liq) = NaAlSi2O6 (cpx) Large partial molar volume of Al and Na components in liquid Small partial molar volume of NaAlSi2O6 (jadeite component)
– Therefore, this reaction is strongly sensitive to pressure and the jadeite proportion of clinopyroxene can be experimentally calibrated as a function of pressure (and temperature)
– Using crystal geochemistry to understand reactions
(2) What do the pressures mean?
- last conditions of equilibrium between cpx and melt
- suggests locations of prolonged magma stagnation and xlln – therefore main level of magma storage and fractionation is 19-26 km
- No evidence for shallow magma storage region under the edifice
- Crystal variability suggests many smaller reservoirs?
- Crystal compositions and zoning can tell us about the P, T, timing of crystallisation
- Results suggest that last itme the lava was stored and allowed to crystalize for a significant time was 19-26km below- below moho
- No evidence for shallow storage – it is suggested then that there are small plumes that from small sills and reservoirs that are then tapped off and come to the surface
(2) Stokes’ law
Stokes’ law describes the velocity (Vs) of a spherical particle in a free fluid
• BUT crystal settling does not take into account magma convection; irregular particle shapes; particle aggregates; magma yield strength; particle-particle interactions (e.g. McBirney & Noyes 1979)
Stokes law doesn’t take into account convection currents – will interrupt settling
- Doesn’t take into account yield strength – will not move to settle
- Doesn’t take into account crystal aggregates – will have a different drag coefficient and move/settle much differently than a single spherical crystal
(2) How effective is crystal mush compaction?
– Reorganisation of grain packing (initial mush framework) results in relatively small amounts of compaction (a few %)
– Viscosity of coherent mush could be very high (1017 Pa s?) – inhibits further compaction
– Compacting a crystal mush
– Reorganizing crystals to fit/pack together more – decreasing porosity
– Porosity does increase with time (shaking to reorganize) - but very low % changes
(4) What is a glass?
- A glass is an amorphous solid whose atomic configuration is ‘frozen in’, equivalent to a super-cooled liquid/ melt
- Crystals have ordered, repeated (periodic) long-range 3D atomic arrays
- Melts/ glasses lack long-range structure
- But can have short-range structural components, similar to crystalline structures
- Similar building blocks – different patterns
- Oxygen dominates silicate structure
- – Large ions (O2-) and abundant
(4) Glass/ melt structure
- 3D structure (“polymer network”). Basic building blocks are SiO4 tetrahedra, linked by corners
- A polymer is a large molecule formed from repeating sub-units that are covalently bonded, eg. Polythene, PVC, silicone
- Natural variety of silicate minerals is generated by linking by 1, 2, 3 or all 4 corners
(4) NBO vs BO
- Bridging oxygen: oxygen atom that is shared between two network formers (Si, Al)
- Non-bridging oxygen: oxygen atom that is bonded to only one network former
- Parameter “NBO/T” correlates with physical properties (e.g. viscosity) as calculated from melt composition (practical)
- T = tetrahedrally coordinated cation (Si4+, Al3+)
- Simple parameter but correlates with physical properties, e.g. melt viscosity
- Non-bridging oxygens / tetrahedra cations = NBO/T – to a first order tells us about physical properties
(4) Comparison of crystal and glass: single component melt (pure SiO2)
- Pure SiO2 crystal – tetrahedra linked at corners, structured and ordered at short range and long range. Rings have regular size and shape
- Fully polymerised, NBO/T = 0
- Pure SiO2 melt has disorder in ring shape, size, bond lengths, bond angles
- BUT still NBO/T = 0 (identical chemistry)
(4) Multi-component melts
• Addition of network-modifying cations (e.g. Mg2+, Fe2+, Na+, K+) results in breaking of some Si-O-Si bridges
• Network-modifying cations disrupt (modify) the tetrahedral network
• These cations link to (alumino)silicate network by ionic bonding with ‘non-bridging oxygen’ (NBO)
e.g. Si-O-Si + M-O = 2Si-O- + M2+
• Coordination is higher
e.g. octahedral [6-fold]
- ‘Pyroxene’-like crystal has ordered lattice with regular cation sites. Chains of linked tetrahedra
- Two different tetrahedral network-formers (Si4+, Al3+), regularly ordered
- Charge balance Na+, K+ for Al3+
- Pyroxene melt can be disordered in several ways, e.g. random cation arrangement; varied tetrahedral groups; different coordination (Al)
(4) Substituting for network-formers (Si4+)
• Natural silicate melts are aluminosilicates – most common substitute for Si4+ is Al3+
• Al3+ needs charge balance from adjacent cations (typically alkalis ± Ca) to keep overall 4+ charge.
• Any “excess” alkalis can act as network-modifiers
• Two possible structural roles for alkalis and alkaline earths (Ca)
(i) charge balancing cation
(ii) network modifier
- Peralkaline melts – more alkalis than are required to charge-balance Al3+
- But no alkalis are available to act as network-modifiers for peraluminous or metaluminous melts – all required for charge-balance
- In phonolites, ≤20% of alkalis are network-modifier
- In tholeiites, alkalis are never network-modifier
- Ca role varies, Mg is always network-modifier
(4) What are volatiles?
• Elements that would be gaseous at atmospheric pressure, room temperature • H2O, CO2, sulphur (SO2, H2S), fluorine, chlorine, nitrogen, noble gases • Critical influence on magmas through: - melt density - melt viscosity - mineral phase stability - mineral compositions - magma density (vesiculation)
