Timothy L. Grove

Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism

Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H2O-saturated with fO2 buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the liquidus of wet high-alumina basalts because H2O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na2O content of the bulk composition as well as to H2O content, with the result that hornblende can form as a near liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na2O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na2O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An90) forms from basaltic melts with high H2O contents but cannot form from dry melts with normal are Na2O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H2O contents. In sum, moderate pressure H2O-saturated phase relations show that magmatic H2O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.

Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts

Abstract Partition coefficients for Th, U, Pb, rare-earth elements (REE), high field strength elements (HFSE), alkaline-earth elements, Sc, Cr, V and K were measured by ion microprobe techniques in two experiments on a natural high-alumina basalt composition from Medicine Lake, California. All elements were measured at natural abundance levels except Th, U and Pb, which were each present in the starting mix at 1-wt% levels. The results show that garnet retains U preferentially over Th ( D gt melt U = 0.0059 , D gt melt Th = 0.0014 ), while clinopyroxene shows the opposite sense of partitioning ( D cpx melt U = 0.0127 , D cpx melt Th = 0.014 ). The experimental Th, U and Pb partition coefficients for garnet-melt and cpx-melt are consistent with garnet-cpx pairs from garnet-bearing ultramafic rocks which exhibit U-Pb isochrons, thus demonstrating equilibrium ( D gt cpx U = 0.30 , D gt cpx Th = 0.072 , D gt cpx Pb = 0.016 ). The partition coefficients for Th and U between clinopyroxene and basaltic melt vary systematically as a function of the tetrahedral Al content of clinopyroxene. Garnet/melt values for Th, U and Pb agree with previous determinations, indicating that mid-ocean ridge basalt (MORB) generation begins in the stability field of garnet lherzolite. However, high 226 Ra 230 Th ratios in MORB require very small porosities near the region where the melts lose chemical equilibrium with the mantle. Partitioning data for HFSE and REE suggest that this region of melt segregation is not in the spinel lherzolite field. This requires either rapid transport of MORB magmas from ϵ 70 km, or some degree of disequilibrium during melt generation and/or transport.

Primary magmas of mid-ocean ridge basalts 2. Applications

Variable initial mantle composition and extent of depletion during dynamic melting processes strongly influence compositions of primary basaltic magmas. The descriptions of the equilibria that pertain to melting in the upper oceanic mantle presented in the companion paper (Kinzler and Grove, this issue) are used to estimate the major element compositions and temperatures of aggregate primary magmas of mid-ocean ridge basalt (MORB) generated in the adiabatically upwelling mantle beneath oceanic spreading centers. Primary MORB magmas with high Na2O abundances that are produced from more fertile mantle compositions or represent initial melts of a depleted spinel-lherzolite have higher SiO2 and Al2O3 and lower MgO, FeO, and CaO abundances, relative to low-Na2O primary magmas. Na2O abundance variation in the mantle source during polybaric, near-fractional melting processes causes melt compositions to vary significantly. The total extents of depletion achieved by the decompression melting process to yield the observed variation in major elements of MORB range from ∼ 6 to 18%; the range of mean pressures of melting is relatively narrow, 8–15 kbar; the total range modeled for the adiabatic, near-fractional melting process is 4 to 25 kbar. Aggregate primary magmas of MORB are not picritic, nor do they resemble sampled primitive MORB (MORB with MgO > 9.0 wt%). Much of the variation in major element composition observed in sampled MORB can be explained by melting a similar depleted MORB-mantle source. The ambient temperature range of the upper mantle beneath the global ridge system required to explain the observed chemical variations is 1475°–1315°C.

Primary magmas of mid‐ocean ridge basalts 1. Experiments and methods

This paper reports experiments carried out between 9 and 16 kbar (0.9–1.6 GPa) using natural, primitive mid-ocean ridge basalt compositions and synthetic analogs of mid-ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. For this low-variance, five-phase assemblage, equations involving pressure, melt NaK # ([Na2O+K2O]/[Na2O+K2O+CaO]; weight ratio), melt Mg # (Mg/[Mg+Fe2+]; total iron as Fe2+), and weight percent TiO2 in the melt predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites at upper mantle pressures. The equations are estimated using a selected set of data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, low-Ca pyroxene, augite, and plagioclase and/or spinel. An experimental test of a liquid compositionally similar to melts produced in a subset of peridotite-basalt sandwich experiments is presented. The composition tested was reported as multiply saturated (with orthopyroxene + augite + spinel + olivine) in the sandwich experiment, but it does not crystallize these phases at the conditions of the experiment. We exclude this liquid and the subset it represents (data from Fujii and Scarfe [1985] and Falloon and Green [1987]) from the data set used to constrain the melting equilibria. With estimates of the melt NaK #, melt Mg #, and weight percent TiO2 of the melt the quantitative descriptions of the melting equilibria can be used to predict the temperatures and major element compositions of melts from lherzolite. Methods are described for estimating these compositional parameters with the nonmodal batch melting equation (for Na2O, CaO, K2O, and TiO2) and a mass balance calculation (for MgO and FeO) from the initial composition and phase proportions of the mantle source, the amount of melt produced and the nature of the melting process, and the stoichiometric coefficients of the mantle melting reaction.

Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation, assimilation and mixing

The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing.A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge basalts (MORBs) from the North Atlantic. Analog models of natural glasses and associated phenocrysts show that several possible parental magmas may undergo low pressure fractional crystallization involving olivine and spinel, followed by plagioclase, and then by augite. The phenocryst phase assemblages in natural deep-sea basalts are closely correlated with the major element compositions of their associated quenched glasses, and the projections of these glasses on the Oliv-Cpx-Qtz pseudoternary correspond to the 1-atmosphere phase boundaries and reaction points defined by laboratory experiments. Comparison of natural phenocrysts with experimental phases indicates that the augites preserved in moderately fractionated MORB from the FAMOUS area may have formed at or near the ocean floor and need not be relics of high pressure processes.

Temperatures and H2O contents of low-MgO high-alumina basalts

Experimental evidence is used to estimate H2O contents in low-MgO high-alumina basalts (HABs) (<6 wt.% MgO) and basaltic andesites (BAs) (<5 wt.% MgO) that occur worldwide in magmatic arcs. Wholerock compositions of low-MgO HABs and BAs, phenocryst assemblages, and mineral chemistry match the compositions of liquids, phase assemblages, and mineral-compositions produced in H2O-saturated melting experiments on HABs at moderate pressure (1–2 kb). Low-MgO HABs and BAs therefore could have existed as H2O-rich multiply-saturated liquids within the crust. Results are presented for melting experiments on two HABs and an andesite at 1 kb pressure, H2O-saturated, with fO2 at the NNO buffer. These data and other experimental results on HABs are used to develop a method to estimate the temperature and H2O content of HAB or BA liquids saturated with olivine, plagioclase, and either high-Ca pyroxene or hornblende. Estimated H2O contents of HAB liquids are variable and range from 1 to 8 wt.%. High-MgO HABs (>8wt.% MgO) could have H2O contents reaching no more than 1–2wt.%. The more common low-MgO HABs could have existed as liquids within the crust with H2O contents of 4 wt.% or higher at temperatures<1100°C. Magmas with these high H2O contents will saturate with and exsolve aqueous fluid upon approaching the surface. They cannot erupt as liquids and must grow crystals at shallow depths, thus accounting for the abundant phenocrysts in low-MgO HABs and BAs.

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

Abstract The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An80−81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: D = 10.99 ( cm 2 /sec) exp (−123.4( kcal/mol )/RT), (T in °K). This value is for diffusion perpendicular to the (03 1) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures. The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids. The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.

Experimental petrology of normal MORB near the Kane Fracture Zone: 22°–25° N, mid-Atlantic ridge

Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in “normal” mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.

Mantle melting as a function of water content beneath back-arc basins

Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.