Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands

Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO₂ content averages 51.5 wt.%. The flows are high in Al₂O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na₂O (mean 2.7 wt.%), and K₂O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO₂ (about 54 wt.%) and Al₂O₃, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K₂O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K₂O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K₂O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time.     

Calc-alkaline volcanic rocks from NW Sardinia: Evaluation of a fractional crystallization model

The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO₃ content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO₂ by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.     

Volcan ceboruco: A major composite volcano of the mexican volcanic belt

Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO₂=58–61%) to acid dacite (SiO₂=68%) in composition. The rate of increase of K₂O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.     

Eruption of Piip Crater (Kamchatka)

In autumn of 1966 on the northern slope of Kliuchevskoy volcano a chain of new adventive craters broke out at the height of about 2200 m. Eighty-four hours before the beginning of the eruption a swarm of preliminary volcanic earthquakes had appeared. The number of preliminary shocks was 457 with total energy of 4 × 10¹⁷ erg. With the beginning of the lava flow the earthquakes stopped and a continuous volcanic tremor appeared. The total energy of volcanic tremor amounts to 10¹⁶ erg. During the eruption numerous explosive earthquakes with the energy of 10¹⁵–10¹⁶ erg were recorded and besides the microbarograph of the Volcanostation recorded 393 explosions with an energy more than 10¹³ erg and their total energy was equal to 10¹⁷ erg. All together it has been formed 8 explosive craters and the lowest 9th crater was effusive. The slag cone was formed round this effusive crater, the lava effusion of basaltic-andesite composition (52,5% SiO₂) tooke place from the lava boccas at the cone base and from the crater. The lava flow covered a distance of 10 km along the valley of the Sopochnoy river and descended to a height of about 800 m. The lava flow velocity at the outflow reached 800 m/hr, the lava temperature was 1050°C. The effused lava volume amounts to 0.1 km³. The eruption stopped on December 25–26, 1966.     

A comparison of the recent eruptions of Mt. Pelée,Martinique and Soufrière,St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.     

A statistical model for vesuvius and its volcanological implications

The last 300 years of Vesuvius history are reconstructed as a chronological succession of 4 phenomenological states: i) repose, ii) persistent activity, iii) intermediate eruption and iv) final eruption. It turns out that the times of permanence in each state are distributed according to the same exponential law. Vesuvius activity is then described by a Markov chain of these 4 states, with transition probabilities determined from the previous phenomenological analysis. The model reproduces the Vesuvius activity between 1694 and 1872 and possibly also in the 1872–1944 period. It turns out that, at least between 1694 and 1872, the volcano was behaving like a quasistationary system with 4 equilibrium states, perturbed by a stochastic noise responsible for occasional transitions from an equilibrium state to another. Major physical or structural changes of the volcanic system around 1872 and possibly in the whole subsequent period, are clearly shown by the statistical analysis.     

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO₂ contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An₉₅ to An₆₀ in the basalts and An₆₀ to An₅₀ in the andesites, but, within each group, they are not correlated with SiO₂ or Na₂O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO₂ variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.     

Melting of Mg2GeO4 under pressure

Melting point of germanate forsterite, Mg₂GeO₄, was raised by compression at the rate of 30°C/GPa. The triple point, at which three phases of olivine- and spinel-type solids and liquid coexisted, was fixed at 1950°C and 3.5GPa. Wen these results are combined with the thermodynamical data of forsterite, Mg₂SiO₄, it is estimated that the triple point of forsterite lies in a region ranging from 2700° to 3000°C in temperature and from 20 to 30GPa in pressure.     

Voluminous acid volcanism in the Busheveld Complex: A review of the Rooiberg Felsite

The 2.1 b.y. old Rooiberg Felsite roofs and is intruded by the mafic layered rocks and granites of the Bushveld Complex. The felsite unit, which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300,000 km³, is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types. Volcanic rocks of more mafic composition occur towards the base of the sequence. The Rooiberg episode essentially terminated the volcanic activity in the Transvaal basin and heralded the emplacement of the Bushveld Complex. Despite the close spatial and temporal relationships between the Rooiberg Felsite and the Bushveld Complex, the precise nature of the petrogenetic link is obscure. Chemical analyses of felsite have been variously interpreted to suggest cyclic differentiation along a comagmatic trend or to demonstrate anomalous enrichment in SiO₂. Several characteristics delineate the Rooiberg Felsite as a possibly unique occurrence of rhyolitic magmatism, notably the immense volume of the unit, the marked preponderance of lavas over pyroclastic types, and the unusually great thickness and lateral extent of the flows. The thesis that the Rooiberg Felsite represents a shock-produced, meteorite-impact melt cannot be supported on the available evidence.     

Devonian tioga tuff in Northeastern United States

The Tioga Bentonite ranges from a coarse crystal tuff to a tuffaceous shale enclosed in marine strata. It originally was deposited over the entire northeastern United States during the Middle Devonian. Trends in grain size, thickness, and numbers of preserved ash layers indicate a source volcano in west-central or central Virginia.     

Fractional crystallization trends in the system Mg2SiO4-CaAl2Si2O8-FeO-Fe2O3-SiO2 over the range of oxygen partial pressures of 10−11 to 10−0.7 atm

A brief report is made of current laboratory investigations on phase relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid and gas in the system Mg₂SiO₄-CaAl₂Si₂O₈-FeO-Fe₂O₂-SiO₂ over a wide range of oxygen partial pressures. Courses of fractional crystallization under various conditions of oxygen partial pressure are depicted using an anorthite saturation diagram. Starting with a basalt-like composition in the system, fractional crystallization at a moderate oxygen partial pressure (10 atm.) results in an andesite-like residual liquid of composition 55 SiO₂, 14 iron oxide, 6 MgO, 9 CaO, 16 Al₂O₃ at a temperature of 1155°C. With fractional crystallization in a closed system, the end liquid approaches the composition of 45 SiO₂, 38 iron oxide, 6 CaO and 11 Al₂O₃, at a temperature of 1050°C and oxygen partial pressure of about 10^?12 atm. The andesitic final liquid in this system would be expected to further differentiate toward dacitic and rhyolitic compositions if alkalies and water were present in the system. On the basis of these studies, the derivation of liquids of andesitic, dacitic or rhyolitic composition from primary basalts by fractional crystallization seems entirely possible if the oxygen partial pressure is maintained at a moderate or high level.     

The composition and differentiation of the volcanic rocks of Carriacou,grenadines, West Indies

Tertiary volcanic rocks of Carriacou occupy two-thirds of the island. The volcanics include volcaniclastics, lava flows and dome lavas and range in composition from basalts to andesites. Carriacou basalts fall into two petrographic types (a) clinopyroxene-plagioclase-phyric basalts and (b) olivine microphyric basalts; the latter having higher MgO and lower Al₂O₃ than the clinopyroxene basalts. Both types are unusually rich in mafic minerals compared with Lesser Antilles basalts in general, although similar types have been reported from the nearby island of Grenada. The potash to silica ratios are relatively high and confirm the similarity between Carriacou and Grenada basalts and the differences between these basalts and basalts from other islands of the Lesser Antilles. The basaltic andesites and andesites from Carriacou correspond closely in mineralogical and chemical composition with typical andesites found elsewhere in the Lesser Antilles. The geochemistry of the volcanics shows that the olivine microphyric basalts display tholeiitic affinities whereas the clinopyroxeneplagioclase-phyric basalt, basaltic andesites and andesites are calcalkaline. The compositional gradation in both the geochemistry and mineralogy of these volcanics suggests that fractional crystallization played an important role in the derivation of the various magma.     

Ground deformations on Mt. Vesuvius from 1977 to 1981

The results of a ground deformation study (1) of Vesuvius are reported. The study is a part of the geophysical program for the surveillance of this extremely dangerous active volcano. The results obtained in the period 1977–1981 show no significant changes in the dynamic state of the volcano. Corresponding to the southern Aapennines Nov. 23, 1980 earthquake a modification of the zenithal distance between the rim and bottom of the Vesuvius crater was observed which was likely connected with fairly shallow effects.     

Petrology of filicudi,aeolian archipelago

Volcanic rocks on the island of Filicudi are typical of the cale-alkaline association of the Aeolian archipelago, and range in composition from basalt to high-K andesite. All specimens contain abundant phenocrystic plagioclase (cores of An_82–97, rims becoming progressively more sodic with increasing Differentiation Index), clinopyroxene, and magnetite; the most basic samples contain olivine, which is replaced by orthopyroxene at around D.I.=44. SiO₂ generally increases with time, and this in conjunction with the continuity in mineralogy suggests that the lavas are the differentiation products of a single batch of magma. Least-squares mixing calculations are consistent with a shallow fractional model, as is the variation in trace element abundances.     

Boninite as a possible calc-alkalic primary magma

Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO₂ content of ? 55%. The relation of SiO₂ to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high P_H2_O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.     

Seismicity of Vesuvius on its present state of activity

Reference is hereby made to the characteristics of seismic recordings of volcanic origin as obtained at the Vesuvian Observatory up to date. The new seismic equipment recently put into operation and consisting of four three-component seismographs set up on the slopes of the Vesuvius Great Cone is also described. The characteristics of the records obtained by the above said equipment are hereby explained and related to the activity of the volcano.     

Rapakivi texture from the O’Leary Porphyry,Arizona (U.S.A.)

The rhyodactic O’Leary Porphyry which forms the Pleistocene (0.233±0.37 m.y.) volcanic domes of O’Leary Peak and Darton Dome in the San Francisco Volcanic Field (northern Arizona, U.S.A.) contains sanidine phenocrysts with oligoclase mantles (rapakivi texture). Rapakivi texture occurs worldwide in silicic rocks of many ages and has been attributed to various igneous and metamorphic processes. The O’Leary Porphyry contains both mantled and unmantled sanidine (both are Or_63–69 Ab_30–36An₁), oligoclase and quartz phenocrysts, labradorite (An₅₃Ab₄₅Or₂) and kaersutite xenocrysts and andesite xenoliths. The compositional range of oligoclase is the same (An_11–26Ab_70–80Or_r–10) for the rapakivi mantles, the oligoclase phenocrysts, and the oligoclase crystals poikilitic within sanidines. Most mantles are discontinuous. The sanidine appears to have been resorbed prior to mantling. Experimental melting studies on the O’Leary Prophyry show that, for a 15 wgt.% water system, plagioclase crystallized prior to sanidine and quartz crystallized last. The O’Leary Porphyry, although inhomogeneous, plots on a Q-Or-Ab-An diagram well within the plagioclase stability field. Poikilitic plagioclases within sanidines further support crystallization of plagioclase prior to sanidine in the O’Leary Porphyry. Exsolution of a ternary feldspar to form a plagioclase mantle is the most commonly accepted igneous theory of rapakivi texture formation but has been eliminated as the origin of the O’Leary Porphyry rapakivi. Petrologic models by Tuttle and Bowen and by Stewart are rejected for the O’Leary rapakivi because of inconsistencies with the O’Leary occurrences. Two theories are viable for the O’Leary rapakivi texture. First, is a decrease in water vapor pressure which would enlarge the plagioclase stability field possibility causing mantling of metastable sanidines. The second and preferred theory is that of an addition of sodium and calcium by basification (chemical assimilation without melting) of the xenoliths within the O’Leary Porphyry. This would move the bulk composition of the melt into the plagioclase field possibly resulting in crystallization of plagioclase on sanidine crystals. Diffusion of sodium and calcium from the xenoliths to sanidine would result in mantling only those crystals near to the xenoliths. Later, convection would result in distribution throughout the melt of rapakivi, unmantled sanidines, and xenolithic kaersutite as is seen in the porphyry. Basic xenoliths are extremely common in rapakivi-bearing rocks. Those within the O’Leary Porphyry are andesitic and show resorption, and in some areas of O’Leary Peak itself, have been drawn out into schlieren.     

Geology and tectonic setting of the Mule Creek caldera,New Mexico,U.S.A.

The 10-km diameter Mule Creek caldera is the youngest felsic eruptive center in the Mogollon-Datil volcanic field of southwestern New Mexico. The caldera forms a topographic basin surrounded by a raised rim. The caldera wall is well displayed on the south and west sides of the structure where it dips 20–30 degrees toward the center of the basin. Mudflow breccia fills the caldera and is banked up against the caldera wall. Post-caldera porphyritic quartz latite domes and flows crop out along the ring-fracture zone. The caldera is superimposed upon an older volcanic complex of flow-banded rhyolite and porphyritic andesite lava. The Mule Creek caldera probably originated by explosive eruption of about 10 km³ of pumice and ash, in part preserved in the matrix of the mudflow breccia. Periods of explosive volcanism during the deposition of mudflow breccia are documented by tuffaceous beds interbedded with the breccia. A thin rhyolite ash-flow sheet originated in the caldera and overlies the mudflow breccia. The youngest felsic rocks around the caldera are (1) domes and flows of crystal-rich porphyritic quartz latite of variable mineralogy, interpreted as a defluidized magma, and (2) widespread crystal-poor, flow-banded rhyolite, dated at 18.6 m.y., which is not directly related to the caldera sequence. The Mule Creek caldera and other volcanic features farther south represent the only documented overlap of felsic volcanism with early stages of Basin-Range tectonism in the Mogollon-Datil field.     

Volcanic rocks of the Witu Islands,Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO₂ andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO₂ contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.⁸⁷Sr/⁸⁶Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two¹⁴³Nd/¹⁴⁴Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources. Perhaps the most striking feature of the Witu Islands rocks is their compositional diversity. Basalts range from olivine tholeiites similar to marginal-basin basalts from other areas, to quartz tholeiites similar in most respects to those typical of island arcs, and to incompatible-element-enriched tholeiites that are close to silica-undersaturation. Andesites on Unea Island have a strong island-arc signature, but the andesites, dacites, and rhyolite of Garove Island have some features that may be more in common with the silica-oversaturated rocks of oceanic areas. The mineralogy of Witu Islands basalts is characterised by phenocrysts of olivine (Fo_89-54), plagioclase (An_90-55) and Ca-augite. Cr-Al-rich spinels and aluminous magnetites are present as inclusions in some olivine phenocrysts. Groundmass fayalite, alkali feldspar, and dacitic to rhyolitic glasses high in K/Na are found in a few samples. In contrast to basalts from volcanoes above the shallower parts of the New Britain Benioff zone, those of the Witu Islands are characterised by rarity of low-Ca pyroxene, as phenocrysts or in the groundmass. Rocks richer in SiO₂ are characterised by the presence of orthopyroxene phenocrysts and lesser amounts of olivine. Hydrous minerals appear to be absent. Groundmass Fe-Ti oxides define crystallisation temperatures (about 800–1050°C) and oxygen fugacities (fO₂) corresponding to those of the Ni-NiO buffer, but up to two fO₂ log units above it. The suite as a whole is phenocryst-poor compared to most New Britain volcanic rocks. A significant degree of mantle heterogeneity is inferred by the chemical variability of the Witu Islands rocks. However, there are no compelling reasons in support of the interpretation that source heterogeneity is due to the effect of a slab-derived component. The cause of the heterogeneity is unclear, but may be due to mantle differentiation processes related to an anomalous tectonic setting.     

Pumice eruptions of the lesser Antilles

Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue.