Large Volumes Of Magma Research Articles

Black smokers fuelled by freezing magma

If ocean floor hot spring activity is to continue long enough to produce a large massive sulphide deposit, heat must be stripped by the circulating water from a store within the ocean crust. We argue here that this heat store can only be a magma chamber, and that production of a large sulphide deposit requires very rapid crystallization of large volumes of magma (∼1 km3 per 100 yr for several hundred years). The effects of such rapid solidification should be apparent in the gabbroic components of ophiolite complexes, as well as in plutons in other environments where large high temperature massive sulphides are formed.

Development of heterogeneities in the subcontinental Mantle: Evidence from the Ferrar Group, Antarctica

Tholeiitic lava flows (Kirkpatrick Basalts) and dolerite sills and dikes (Ferrar Dolerites) of the Jurassic Ferrar Group from Antarctica and dolerite sills from Tasmania, Australia are characterised by initial strontium isotope ratios ranging from 0.7089 to 0.7153. The mean and standard deviation of 85 analyses is 0.7115±0.0012. Some of the scatter in the initial 87Sr/86Sr ratios can be attributed to sample inhomogeneity, analytical uncertainties and sample alteration. The published major element data show well-defined trends that are consistent with an evolution by fractional crystallization. Recognition of a parental magma is difficult due to the fractionated nature of the rocks. Trace element analyses, particularly the rare earth elements (REE) support a differentiation model. Compared to mid-ocean ridge basalts, Ferrar Group rocks are enriched in light REE. Kirkpatrick Basalts from the central Transantarctic Mountains show significant correlations between initial 87Sr/86Sr ratios and major elements only for SiO2 and CaO. The general lack of strong correlation is the basis for rejecting the possibility of wholesale contamination by sialic material as a possible cause of the high 87Sr/86Sr ratios. Selective contamination of the basaltic magmas is a possibility and cannot be completely discounted. It would probably involve a fluid phase in order to transport and mix the light REE, Rb, 87Sr, and other elements. By analogy with selective contamination of ocean ridge basalts by sea water it is difficult to envisage a similar process acting on magma emplaced in a non-marine environment. Because of the elevated values of the initial 87Sr/86Sr ratios, their similar average value over 2,500 km and the large volume of magma involved (4× 105 km3) a mantle origin for the high Sr ratios is preferred. Models to account for the enrichment of Rb and light REE in the Antarctic mantle during or prior to the Jurassic include: 1. addition of continental material from a Palezoic Mesozoic subduction zone; 2. metasomatism of volatile elements from the lower mantle; and 3. evolution of a mantle with a high Rb/Sr ratio.

Investigation of volcanic gas analyses and magma outgassing from Erta' Ale lava lake, Afar, Ethiopia

The analyses of 18 volcanic gas samples collected over a two-hour period at 1075°C from Erta' Ale lava lake in December 1971 and of 18 samples taken over a half-hour period at 1125–1135°C in 1974 display moderately to intensely variable compositions. These variations result from imposed modifications caused by (1) atmospheric contamination and oxidation, (2) condensation and re-evaporation of water during collection, (3) analytical errors, and (4) chemical reactions between the erupted gases and a steel lead-in tube. Detailed examinations of the analyses indicate the erupted gases were at chemical equilibrium before collection. This condition was partially destroyed by the imposed modifications. High-temperature reaction equilibria were more completely preserved in the 1974 samples. Numerical procedures based on thermodynamic calculations have been used to restore each analysis to a composition representative of the erupted gases. These procedures have also been used to restore the anhydrous mean compositions reported for two series of collections taken at the lava lake in January 1973. The corrected analyses for 1971 and 1973 have similar compositions (69.6–71.3% H 2O, 1.6–2.4% H 2, 17.8–19.4% CO 2, 0.8–1.6% CO, 4.9–8.8% SO 2, 0.2–0.5% S 2, and 0.4–1.0% H 2S); those for 1974 were somewhat different (77.1% H 2O, 1.6% H 2, 11.7% CO 2, 0.5% CO, 7.4% SO 2, 0.3% S 2, 0.9% H 2S and 0.4% HCl). The O 2 and S 2 fugacities of all restored analyses are similar when compared at the same temperatures. O 2 fugacities are close to those of the quartz-magnetite-fayalite buffer. The restored analyses show no evidence of significant short-term (minutes, hours) variations in the compositions of the gases released from the lava lake, and evidence of long-term variations is limited. The restored analyses indicate the O 2 and S 2 potentials of the lava lake remained nearly constant from 1971 to 1974. However, there is a relative decrease in CO 2 between the 1973 and 1974 corrected compositions. This change is qualitatively compatible with magma outgassing controlled by solubility differences and could reflect a longer term trend. The chemistry of the Erta' Ale gases is consistent with models invoking the generation of basalts resembling mid-ocean ridge tholeiites along axial zones of crustal spreading in the Afar Triangle. The CO 2-rich vesicle gases of submarine basalts and the relatively H 2O-rich volcanic gases of Erta'Ale appear to be end-member compositions of an outgassing trend controlled largely by the effect of pressure on volatile solubilities in tholeiitic basalts from spreading zones. By analogy with submarine basalts H 2O and sulfur in the Erta' Ale gases are mainly contributed by magma outgassing down to shallow depths (50–250 m); CO 2 contributions probably come from larger volumes of magma down to depths of at least 1000 m.

Trace element indicators of the genesis of the Rhum layered intrusion, Inner Hebrides

Synopsis Almost constant concentration of nickel in cumulus olivine crystals taken from the bases and tops of many of the macrorhythmic units of Rhum indicates that a large volume of magma, if predominantly liquid, was needed to produce each unit. Some units represent less than 2 per cent by mass of a liquid parent magma. Rare earth element concentrations in cumulus plagioclases indicate that the parent magma was enriched in the light relative to the heavy rare earths. The estimated rare earth abundance pattern of the parent magma suggests that the melt was produced from a small amount of partial melting of upper mantle material.