Petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the Trenton Formation, Ordovician, Michigan Basin, U.S.A.

Three major types of dolomite occur in the Trenton Formation (Mid-Ordovician) of the Michigan Basin. These are: (1) ‘regional dolomite’ which is confined to the extreme western edge of the basin; (2) ‘cap dolomite’ which occurs in the upper portion of the Trenton and is confined to the basin's southern margin; and (3) ‘fracture-related’ dolomite which occurs in association with both large- and small-scale faults and fractures. These three dolomite types can be distinguished from one another by their major element chemistry, oxygen isotope ratios and rock texture. The regional dolomite is fine-grained, has <0.34 mol% FeCO₃, and mean δ¹⁸O of ?6·8‰_OPBD. The cap dolomite is texturally similar to regional dolomite but contains 3–13·0 mol% FeCO₃ and has a mean δ¹⁸O of ?7·7‰. Fracture-related dolomites are coarse-grained, low in iron, and have the most depleted δ¹⁸O ratios (x?=–9·0%_PDB). Petrographic relationships imply that the regional dolomite, formed prior to the cap dolomite probably during early diagenesis. The cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying Utica Shale and the Trenton Limestone. Fracture-related dolomites post-date the cap dolomite and formed during deeper burial. A temperature of precipitation of approximately 80°C was calculated for fracture-related dolomites using oxygen isotope data. The distribution of the cap dolomite was controlled by the availability of Fe^2? which was in turn controlled by the availability of S^2?. In the centre of the basin Trenton-Utica deposition was continuous. The upper Trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce H_2S from seawater sulphate. The precipitation of iron sulphides (pyrite + iron monosulphide) followed and used up most of the available Fe^2?. As a result only small amounts of ferroan dolomite formed. On the periphery of the basin, subaerial exposure resulted in the oxidation of most of the available organic matter. Sulphate reducing bacteria were therefore limited and produced limited amounts of H₂S. As a result only a minor amount of iron sulphide (iron monosulphide) formed. The remaining Fe^2- was then available for the formation of the ferroan cap dolomite. This model is supported by the following: (1) In the southern margin of the basin, the contact between Trenton cap dolomite and the overlying Utica Shale is sharp and probably unconformable. In the centre of the basin the contact is gradational. (2) In the centre of the basin, the total organic carbon content in the upper Trenton is an order of magnitude higher than in the cap dolomite. (3) The whole-rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre. (4) Iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite. In the cap dolomite, iron sulphide is minor and primarily in the form of iron monosulphide.     

The Stability of Sodalite in a Synthetic Syenite plus Aqueous Chloride Fluid System

Natural feldspathoidal syenites may be approximated by assemblagescontaining some or all of the phases sodalite, nepheline, oneor two alkali feldspars, and aqueous chloride fluid in the systemNaAISi₃O₈-KAISi₃O₈-NaAISiO₄-KAISiO₄-NaCI-KCI-H₂O. The stabilityof sodalite in these assemblages was studied in the range 500–700°C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturatedliquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system,three distinct types of sodalite-bearing syenite can crystallize.Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-twoalkali feldspars rocks and sodalite-analcime-bearing rocks crystallizebelow 1600 bars, between 1600 and 2750 bars and above 2750 barsfluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-twoalkali feldspars-aqueous fluid are different and distinguishablefrom the effects of metasomatism. Closed-system cooling resultsin replacement of K-feldspar by albite, feldspathoids remainingnearly unchanged, while metasomatism generally results in sismultaneousenrichment or impoverishment in sodalite plus K-feldspar.     

Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii

Kilauea Volcano has erupted lava from its summit caldera andfrom two rift zones that extend from the summit towards theeast and south-west. Lavas erupted from the summit of the volcanodiffer from each other principally in their content of olivineand define lines of ‘olivine control’ on magnesiavariation diagrams. Lavas erupted on the rift zones may be similarin composition to the summit lavas or may be differentiatedby processes that involve minerals other than olivine. All ofthe differentiated lavas have less than 6·8 per centMgO and plot off the extension of olivine control lines forthe summit lavas. Prehistoric vents (before A.D. 1750) fromwhich differentiated lavas have been erupted are found on theeast rift zone and in the western Koae fault zone adjacent tothe south-west rift zone; historic vents for differentiatedlavas are confined to the east rift zone. Twenty-one new analysesare presented for several of the east rift differentiates andfor the newly discovered differentiates adjacent to the south-westrift zone. The differentiates have MgO as low as 3·9per cent and SiO₂ as high as 56 per cent; both extremes arefound in the prehistoric lavas adjacent to the south-west rift. Detailed petrochemical studies suggest the following conclusions:

1. Thechemical composition of magma erupted at Kilauea summitvarieswith the date of eruption. Lavas erupted before 1750,duringthe eighteenth and nineteenth centuries, and in the twentiethcentury form groups that can be distinguished chemically. Ona lesser scale, each Kilauea summit eruption in the twentiethcentury has a chemistry that is distinctive with respect tothe chemistry of every other summit eruption.
2. During lateprehistoric time pockets of differentiated magmawere formedwithin the rift zones by separation of the liquidremainingafter partial crystallization of bodies of summitmagma. Thisprocess presumably is still going on within theeast rift zone,but the more recently separated liquids havenot yet been eruptedto the surface. The relative time at whichthese differentiatedmagmas were produced can be estimated fromcalculations basedon their chemical compositions, which showthat the differentiatescould lie on the liquid line of descentfor Kilauea summit magmaof prehistoric composition but noton any liquid line of descentfor younger summit magmas.
3. Lava from some eruptions, notablythe early part of the 1955eruption on the lower east rift,has the composition of theliquid fraction as it is generatedwithin the rift. Lava compositionsof other eruptions, includingthose of the later lavas of 1955,are best explained by mixingof magma supplied from a centralreservoir beneath Kilauea summitwith the differentiated liquidin the rift. Lava from each summiteruption is unique chemically,so it is possible to recognizeits presence or absence as componentsof mixing in such mixedlavas. It appears that summit magmaof composition characteristicof the 1952 and 1961 Halemaumaueruptions contributed to thecomposition of the mixed lavasproduced in the latter part ofthe 1955 eruption. Summit magmaof 1961 composition is alonesufficient to explain the compositionof mixed lavas eruptedin 1960 and 1961. In rift lavas eruptedfrom 1962 to 1965, thecomposition of lava erupted in Halemaumauin 1967, in additionto the 1961 composition, is a componentof mixing, and it isthe dominant summit component in the compositionof the two1965 eruptions. The proportion of summit magma todifferentiatedmagma needed to explain the composition of lavaserupted onthe upper east rift increases from 1961 to 1965;this increaseindicates that the differentiated magma was beingdiluted andused up by repeated flooding of this part of therift zone bymagma supplied from the central reservoir.
4. The fact that componentsof ‘summit composition’appear in rift eruptionsbefore they appear undiluted in Halemaumausuggests that thecentral reservoir is vertically zoned. Rifteruptions are fedfrom lower levels where younger magma is available,and summiteruptions are fed from the relatively older magmaabove. Thechemical distinction between lava of successive summiteruptionsimplies that significant convective mixing of magmadoes nottake place throughout the central reservoir.
5. The unique anduniform composition of lava of each successivesummit eruptionalso suggests that summit eruptions end whenall of the magmaof one composition has been erupted. The magmaerupted fromthe upper levels of the reservoir during one cycleis continuallyreplaced from below by younger magma of differentcomposition.In order for eruption to be renewed in Halemaumau,new magmafrom the mantle must be held in storage at intermediatelevelsbefore it attains an ‘eruptive state’.
6. The hypothesispresented in 2–4 above permits qualitativepredictionsconcerning future lava compositions. The compositionof thenext lava to be erupted in Halemaumau is expected tobe distinctfrom that of the 1967 eruption, and this compositionwill presumablybe identified in rift eruptions occurring between1967 and thetime of its appearance in Halemaumau.
7. Differentiates of prehistoricage also were apparently formedin the same way as those ofhistoric age, but the mixing cannotbe described quantitativelybecause of poor control on the stratigraphyand the compositionsof erupted lavas. One lava in the Koaegroup, that from YellowCone, appears to be a mixture of a picriticmagma (12 per centMgO) with a differentiated liquid with lessthan 2·5per cent MgO and nearly 60 per cent SiO₂.

     

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800{degrees}C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H₂O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H₂O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO₂, Na₂O, and K₂O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al₂O₃, Na₂O, andK₂O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al₂O₃/[Na₂O + K₂O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10^–15m²/s and (0·6–2·4) x 10^–14 m²/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10^–11m²/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H₂O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H₂O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis