High-alumina Olivine Tholeiite Research Articles

The Fractional Crystallization of Plagioclase in the Hat Creek Basalt: Observations and Theory

Plagioclase in the Hat Creek high alumina olivine tholeiite decreases in anorthite (An) component with crystallization. Petrography reveals the composition of plagioclase that formed at various stages of crystallization represented by microporphyritic glassy tops of the flow and by segregation veins and their residual glasses. The plagioclase ranges from An79 in the cores of microphenocrysts to An9 rims on crystals in glass-bearing segregation veins as the fraction of crystallization increases from 0 to 0.9. The temperatures of crystallization corresponding to the various fractions of crystallization are estimated on the basis of olivine/liquid and magnetite/ilmenite geothermometry and linear interpolation. The temperatures ranged from 1237°C to 980°C. Supercooling was less than 20°C initially. Most of the plagioclase microphenocrysts have An consistent with the plagioclase geothermometer. Plagioclase in most of the segregation veins differs from that expected. The derived histogram of amount of plagioclase versus its composition reveals that the mode lies at the An-rich end of the spectrum with successively smaller amounts formed in more sodic intervals. The curve for composition versus crystallization extrapolates to pure albite at about 100% crystallized. Except for the anomalous segregation veins, the observations agree with a modified theory for perfect fractional crystallization of plagioclase in dry basaltic magma. The anomalous segregation veins apparently lost or gained residual liquid and some inherited microphenocrysts which were carried in during vein formation. The compositional spectrum of plagioclase in basaltic and gabbroic rocks can be used to substantiate closed system fractional crystallization.

Strontium isotope geochemistry of a low potassium olivine tholeiite and two basalt-pyroxene andesite magma series from the Medicine Lake Highland, California

Three contemporaneous yet mineralogically and chemically distinct lava series which range in composition from olivine basalt through basaltic andesite through two-pyroxene andesite, have been analyzed for their strontium isotopic composition. The Mammoth-Modoc lavas produce a mantle isochron which suggests derivation from a source approximately 243 m.y. old. The Medicine Lake shield-forming extrusives produce a minimum mantle isochron; the age of this source is likely greater than 760 m.y. old. Encompassing the entire time span of eruption for the two basalt-andesite series are a number of low potassium high alumina olivine tholeiite lavas which, when analyzed as a coherent group, produce a zero age. Lavas related to these three magma series have been extruded during the last 105 years and cover a limited geographic area. The isotope data suggest a model with substantial heterogeneity in the subcontinental lithosphere and asthenosphere, at least beneath the Medicine Lake Highland portion of the southern Cascade petrographic province.

A tschermakite-bearing high-alumina olivine tholeiite from the southern Cascades, California

The first occurrence of amphibole-bearing basalt in the southern Cascade volcanic province has been found in the Medicine Lake area of California. Little Mount Hoffman, a late Pleistocene cinder and scoria cone, has erupted high-alumina olivine tholeiite, the petrography of which yields a paragenetic phenocryst sequence of amphibole→olivine→plagioclase. The amphibole is compositionally a tschermakite. Comparison with available experimental data indicates an initial depth of crystallization somewhere between 40 and 100 km, temperature near 1000 °C, and fO2 near the Ni-NiO and QFM buffers.

Melting of Gabbro (Quartz Eclogite) with Excess Water to 35 Kilobars, with Geological Applications

Crystalline, fine-grained, high-alumina olivine tholeiite with excess water was reacted in sealed platinum capsules in piston-cylinder apparatus between 10 and 35 kbar pressure. Runs were planned to determine the curve for the beginning of melting, but combining these with published results at lower pressures permitted delineation of the major features of the phase diagram through the melting interval. Amphibolite melts below 10 kbar; quartz eclogite melts above 25 kbar; between them is a melting interval dominated by the breakdown of amphibole and formation of garnet and jadeitic pyroxene. The results introduce two features for geological applications. The solidus changes slope at about 13.5 kbar; the amphibole maximum-stability curve changes slope in the interval 12.5-15 kbar to such an extent that the amphibole stability field is more restricted at high pressures than anticipated from previous studies. With free water, eclogite is Stable only at depths greater than about 70 km, and amphibolite is Stable in the deep crust. Amphibolite crust thickened in the depth range 40-60 km with aqueous pore fluid melts, forming a liquid enriched in silica and albitic plagioclase; this is a potential source of water-undersaturated liquids for batholiths. The upper boundary of the mantle low-velocity zone could be the boundary between rocks with interstitial amphibole and those with interstitial hydrous silicate liquid. In oceanic crust forming the upper part of a subducted lithosphere slab, it appears that most hydrous minerals dehydrate or melt before they reach 100 km depth. If so, dehydration of subducted oceanic crust does not supply water for andesitic magmatism beyond the arc-trench gap, nor contribute to the chemical variations recorded in andesites across arc complexes (K_2O, K/Rb).

Contrasting Behavior of P, Ti, and Nb in a Differentiated High-Alumina Olivine Tholeiite and a Calc-Alkaline Andesitic Suite

Crystallization differentiation in a low-K 2 O, high-Al 2 O 3 olivine tholeiite (Hat Creek, California) yields segregation veins of basaltic andesite composition, and residual, interstitial glasses of dacite and rhyolite composition. P, Ti, and probably Nb, are progressively enriched in segregation veins and residual dacitic glass by crystallization of olivine, plagioclase, augite, and magnetite. P and Ti are depleted in residual rhyolitic glass by crystallization of magnetite, ilmenite, and apatite. By contrast, in a typical orogenic calc-alkaline suite (from Mount Jefferson, Oregon), P, Ti, and Nb are depleted with increasing K and Si. The most likely minerals capable of producing decreasing P, Ti, and Nb with increasing K are apatite, Fe-Ti oxides, amphibole, biotite, and sphene. There is no direct evidence of these minerals in Mount Jefferson basalt and andesite but they are present in calc-alkaline gabbro and tonalite and may occur at a shallow (crustal) depth beneath Mount Jefferson. Alternatively, mixing of dacite-rhyolite with basalt-andesite may account for the Mount Jefferson trend. By physical and chemical analogy with segregation veins and residual interstitial glasses, basaltic andesite and dacite-rhyolite magmas may segregate from complementary rock at depth, and mix during ascent to the surface.

A review of experimental evidence on the origin of basaltic and nephelinitic magmas

Geological and petrological arguments are used to select those basaltic compositions which have been directly derived from the upper mantle with minimum opportunity for crystal fractionation or for contamination within the crust. The experimental study of the crystallization of these magmas at high pressures, under both anhydrous and hydrous conditions provides evidence of genetic links between magma types. These studies also provide data necessary to evaluate a major constraint on “primary” magmas, i.e. that magmas derived by direct partial melting of the upper mantle must have olivine and enstatite among their liquidus phases at the P, T conditions of magma segregation. The characteristic magma derived from either direct partial melting (∼ 20–25%) of pyrolite or fractional crystallization of olivine-rich tholeiite at depths of 15–35 km is high alumina olivine tholeiite. At 35–70 km partial melting of anhydrous pyrolite or fractional crystallization of olivine-rich tholeiite or tholeiitic picrite produces a series of liquids from olivine-rich tholeiite through olivine basalt and alkali olivine basalt to basanite (∼25% Ol, ∼5% Ne). With availability of water in the pyrolite source or during crystal fractionation, the pyrolite solidus is depressed. This produces changes in the subsolidus mineralogy and in the nature of liquids formed by low degrees of partial melting or by extensive crystal fractionation of less undersaturated magmas. Olivine nephelinites and basanites may be generated by small degrees of melting of water-bearing (0.1–0.2% H 2O) pyrolite at ∼50–70 km and olivine melilite nephelinites, olivine-rich nephelinites and olivine-rich basanites form in a similar way, as hydrous magmas at 70–100 km depth.

Mass spectrographic determinations of lanthanide element contents in basalts

Lanthanide element population patterns in deep sea basalts vary with the degree of silica saturation of the rocks. In the olivine tholeiite series more undersaturated examples show preferential enrichment of the lighter lanthanides. This relationship is not displayed by the high alumina olivine tholeiite.

Genesis of the calc-alkaline igneous rock suite

A high pressure experimental study of the partial melting fields of synthetic high-alumina olivine tholeiite, high-alumina quartz tholeiite, basaltic andesite, andesite, dacite and rhyodacite under dry and wet $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ conditions has been conducted in order to investigate possible origins of the calc-alkaline series from the upper mantle. Detailed analyses of crystallizing phases using the electron microprobe has enabled calculation of the liquid line of descent in these compositions at various pressures. At 27–36 kb garnet and clinopyroxene are the liquidus or near-liquidus phases in dry tholeiite, basaltic andesite and andesite, while quartz is the liquidus phases in dry dacite and rhyodacite. Under wet conditions at 27 kb garnet, not quartz, is the liquidus phase in the dacite. Qualitatively these results show that the low melting fraction of a quartz eclogite at 27–36 kb under dry conditions is of andesitic composition whereas under wet conditions it is rhyodacitic or granodioritic. At these pressures under dry conditions the andesite liquidus lies in a marked low temperature trough between the more basic and more acid compositions. Quantitatively, the calculated compositions of liquid fractionates for varying degrees of melting of the quartz eclogite bulk composition broadly follow the calc-alkaline trend. At 9–10 kb under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ sub-silicic amphibole and pyroxenes are the near-liquidus phases in tholeiite and basaltic andesite compositions. Calcic plagioclase and garnet occur nearer the solidus. The calculated liquid fractionates follow the calc-alkaline trend and demonstrate that the calc-alkaline series may be derived by the partial melting of amphibolite at lower crustal depths under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ , Or by the fractional crystallization of a hydrous basalt magma at similar depths. These experimental results support two complementary hypotheses for the derivation of the calc-alkaline igneous rock suite from the mantle by a two stage igneous process. In the first stage of both hypotheses large piles of basalt are extruded on the earth's surface. Subsequently this pile of basalt may, under dry conditions, transform to quartz eclogite, sink into the mantle and finally undergo partial melting at 100–150 kms depth. This partial melting gives rise to the calc-alkaline magma series leaving a residuum of clinopyroxene and garnet. Alternatively, if wet conditions prevail in the basalt pile and the geotherms remain high, partial melting of the basalt may take place near the base of the pile, at about 10 kb pressure $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ . The liquids so formed constitute the calc-alkaline suite and the residuum consists of amphibole, pyroxenes and possibly minor garnet and calcic plagioclase. Both models may be directly linked to the hypothesis of sea-floor spreading.

The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts

Experimental crystallization of olivine tholeiite (20% olivine) at 9 kb shows that olivine and to a lesser extent, orthopyroxene are the early crystallizing phases, joined at lower temperatures by clinopyroxene and plagioclase. This contrasts with atmospheric crystallization of olivine first, joined at lower temperatures by plagioclase and clinopyroxene. In high-alumina olivine tholeiite (6% olivine) however, clinopyroxene is the liquidus phase at 9 kb, joined at lower temperatures by plagioclase, while from 0–6.8 kb plagioclase is the liquidus phase joined by olivine and clinopyroxene. Alkali basalt and olivine basalt have olivine and clinopyroxene as important near-liquidus phases at 9 kb, compared with olivine and plagioclase at atmospheric pressure. Quartz tholeiite contains olivine, together with plagioclase and clinopyroxene near the liquidus at atmospheric pressure, but at 4.5 kb and 6.8 kb plagioclase and clinopyroxene alone were evident. These results show that derivation of quartz-normative tholeiites from olivine-normative parent magmas is only possible at depths of less than 15 km, while at depths of 15–35 km high-alumina basalts of tholeiitic or alkalic affinities and only slightly enriched in silica are obtained from olivine tholeiite or olivine basalt parents. At depths of 35–60 km fractionation of olivine-rich magmas is largely governed by aluminous pyroxenes and the derivative liquids trend towards undersaturated alkali basalts. The inter-relationships of the three major magma types, quartz tholeiite, high-alumina basalt and alkali basalt, in such volcanic provinces as Japan are explained by magma segregation or fractional crystallization over specific depth ranges. Thus alkali basalts are derived at 35–60 km depth, high-alumina basalts at 15–35 km and quartz tholeiites at less than 15 km.

The nature and composition of rock samples dredged from the Mid-Atlantic Ridge between 22°N and 52°N

Dredge hauls along the Mid-Atlantic Ridge between 22°N and 52°N have brought up various rock types including basalts, serpentinites, gabbros and dolerites. The basalts are of particular interest in that numerous fragments have been obtained with well developed glassy margins. Although the interiors of these fragments are often cryptocrystalline and not readily determinable by optical examination, chemical analysis has demonstrated that the basalts are of tholeiitic character. Amongst them high alumina olivine tholeiite is represented, comparable to the basalt of the Medicine Lake Highlands in California. Glass of this composition clearly demonstrates that magma of high alumina characteristics can exist in a wholly liquid condition. The presence of such glass on freshly fractured fragments dredged from the ocean floor demonstrates that such high alumina basaltic magma cannot be restricted to the continental areas of the earth. Distinct differences exist between the compositions of the dredged basalts and those of basalts exposed on the islands along the Mid-Atlantic Ridge, but there are similarities between the basalts dredged from the Mid-Atlantic Ridge and those obtained by Wiseman (1937) from the Carlsberg Ridge in the Indian Ocean.