Explosive Alkaline Volcanism on the SW Indian Ridge

doi:10.21203/rs.3.rs-1761254/v1

Download PDF

Article

Explosive Alkaline Volcanism on the SW Indian Ridge

https://doi.org/10.21203/rs.3.rs-1761254/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

We report explosive alkaline volcanism at a segment of the ultraslow spreading (14 mm/yr) Southwest Indian Ridge (SWIR). While gentle effusive tholeiitic (MORB) volcanism typifies the global ridge system, little evidence for violent alkaline volcanism producing debris cones, lava bombs and scoria as at ocean islands has been reported. However, morphologic evidence in the form of craters exists 1, and alkali basalts have been found on the SWIR and Lena Trough e.g., 2,3,4. Ours are the first definitive samples of pyroclastic debris sampled from a cratered terrain on an ocean ridge. Critically, they are also the result of shallow mixing of an alkaline magma with entrained MORB crystal mush from beneath the flank of a rift valley. This suggests a major paradigm shift for ocean ridge volcanism. The alkali basalt represents an early volatile-rich low-viscosity melt fraction erupted from near the base of the melting column violently to the seafloor — the product of deep low-degree melting of local enriched heterogeneities in an otherwise depleted mantle. The MORB mush represents a later volatile-poor viscous melt erupted effusively from beneath the axial valley onto the seafloor — the product of high-degree melting due to penetrative flow and melt-rock reaction through the remaining ascending depleted mantle to the base of the lithosphere. Mixing of small volumes of the former with the latter in the melt storage region beneath the ridge then explains the local major element uniformity of MORB and its notorious isotopic and trace element diversity. We suggest this may apply globally as at magmatically more robust ridges the role of alkali basalt is likely masked by the far more voluminous MORB.

Location

Our samples were dredged by RV Thompson Cruise TN365 on the northeastern Marion Rise in March 2019 on the eastern third of the ~255-km long amagmatic supersegment between the Discovery and Indomed FZ’s. There two orthogonal to spreading magmatic ridge segments are linked by an intervening 67-km long oblique-amagmatic segment oriented at ~15° to the spreading direction (Fig. S1). The entire region is an area of extreme low melt productivity, where only scattered volcanics lie on serpentinized mantle between widely spaced magmatic segments. The explosive lavas were recovered in Dr 30 at 40°08.6’S, 45°28.2’E high on the southern rift valley wall of the 70-km long 45°30’E ridge segment (Fig. 1). The dredge was from 2,195 to ~1,850-m depth up the north side of a 1-km wide cratered debris cone, with a secondary ~200-m wide cone at its center. Additional candidate craters occur on the rift valley walls and adjoining rift mountains (Fig. 1). While some are locally cut by normal faults, others are undisturbed indicating likely off-axis volcanism. Possible craters in the rift valley are obscured by fissure eruptions along the axial neovolcanic zone. The physiography indicates two overlapping volcanic phases – explosive and effusive, one occurring largely in the rift valley, the other manifesting over a broader zone into the rift mountains. Lavas were also recovered from the 45°30’E segment at 44°09.54’S, 45°14.73’E on the south wall of the inside-corner high at its intersection with the oblique amagmatic segment in Dr 31 (Fig. S1). Some 37-km to the southwest at 40°25.98’S, 44°59.82’E Dr. 27 recovered additional basalts from the north wall of the 20-km long 45°E magmatic segment (Fig. S1).

Samples

Dr 30 recovered fresh pillow basalt, glassy volcanic scoria and lava bombs ranging from 6 to 15 cm in size (Fig.’s 2a-c, S3a-b), and a large amount of siliceous sinter, some with thin hydrogenous manganese coverings (Fig. 2d). The lavas have 15 to 25 vol.% 2-3 mm plagioclase and ~1% <2 mm olivine phenocrysts and 22-61 vol.% vesicles measured by cross-sectional computed tomographic imaging and microscopy (Fig. 2a-b). Some of the glassy scoria are partly covered with very-thin silica sinter (Fig. 2c). SEM imaging and EDS show the sinters formed beneath a thin sediment layer with numerous silica balls pseudomorphing foraminifera, and abundant round holes left by their dissolution (Fig. S2).

Dredge 31 contained large chunks of ultraphyric diabase or basalt with 68% ~10-mm plagioclase and 2% 1-mm olivine phenocrysts. One vuggy weathered and altered pillow basalt with zeolites filling 1 to 3 mm coarse vesicles may represent a flow top where vesicles accumulated and grew. No glass is preserved.

Dr 27 dredged light to moderately weathered aphyric to plagioclase phyric pillow basalts, and moderate to heavily weathered avesicular ultraphyric basalts and diabase. The pillow basalts have 0-10% vesicles, (ave. 5%, n=25), and 0-10% plagioclase phenocrysts, several with 1-2% olivine phenocrysts. The contrast in vesicularity between Dr 27 and 30 glasses is striking, with much higher volatiles indicated in the latter (Fig. S3c-d).

Rock and Mineral Chemistry

Analytical methods, and isotopic, major and trace element data for the sinters, lavas, glasses, and minerals are in the supplemental materials (Tables S1 to S8). Dr 30 glasses range from olivine tholeiite to nepheline normative alkali basalt (0 to 3.67 wt.% Ne) similar to ocean island basalts with elevated La and K₂O (0.93 to 1.28 wt.%), La/Yb, K/Ti, ⁸⁷Sr/⁸⁶Sr, and low ¹⁴³Nd/¹⁴⁴Nd (Fig.’s 4, S4c, S5, S6; Table S2). However, K/Ti vs Mg# and isotopic compositions are distinctly different from Marion and Crozet Island basalts (Fig. 4b, d). Though straddling the MacDonald-Katsura line between Hawaiian tholeiites and alkali basalts ⁵ (Fig. 4a), the majority lie in the alkali field. ³He/⁴He ratios in the Dr 30 and Dr 27 glasses (6.6 to 8.2 R_A) (Table S7) are typical for Marion Island lavas and normal MORB ⁶ (Fig. S7), but well below those of Hawaiian and Icelandic plume basalts as well as Crozet Island Lavas ⁷. The Dr 30 heavy isotopes and K₂O-TiO₂ are also distinct from Marion or Crozet (Fig. 4b, d). This is consistent with a distance >1,000 km from Marion Island, and the intervening Discovery and Indomed FZ’s, which would block sub-axial mantle flow from either direction ⁸. The Dr 31 and 27 basalts are alkali-rich quartz normative tholeiites (1.12 to 8.45 wt.% qtz) lying below the MacDonald-Katsura line in the silica-alkalis plot (Fig. 4a), with flat to light REE depleted trace-element patterns (Fig.’s 4c, S5). The fractional crystallization trend between Mg# and other oxides defined by Dr 31 and 27 volcanic rocks is clearly different from that of Dr 30 glasses and lavas (Fig. S4).

The silicic sinters from Dr 30 are nearly pure amorphous SiO₂ with little Fe oxides/hydroxides, except for parts entraining pelagic sediment. Oxygen isotopes indicate they precipitated from 26°C to 45°C (Table S8), but differ from other low-temperature precipitates at spreading centers (Juan de Fuca, Lau Basin and SWIR etc.) due to their low iron contents. They likely formed by venting of hydrothermal fluids due to shallow seawater circulation within the cooling lavas.

Nearly all the Dr. 30 basalt plagioclase and olivine phenocrysts are primitive xenocrysts (An_{80.4 - 85.9}, An_82.6 ave.; Fo_81.5-89.4, Fo_87.3 ave.) and in disequilibrium with the enclosing basalt (Fig. 3c; Table S4). Plagioclase crystals are rounded (Fig 3a) to sub-equant, indicating variable resorption by the host magma; though in several samples there is a second population of subhedral to euhedral plagioclase laths, which indicates one or more stages of magma mixing occurred during ascent to the seafloor ^9-11.

Dr 30 olivine melt inclusions and plagioclase xenocrysts have ⁸⁷Sr/⁸⁶Sr compositions intermediate to the Dr 30 and Dr 27 glasses, with the inclusions lying close to the Dr 27 glasses and the plagioclase phenocrysts close to the Dr 30 glasses (Fig. 3b; Table S3). Moreover, they range from LREE enriched, like the enclosing alkali basalt, to flat N-MORB-type patterns (Fig.’s 4c). This indicates that the alkali basalt incorporated a large population of xenocrysts but little interstitial melt from a tholeiitic crystal mush, preceding and possibly triggering eruption, with partial requilibration of the xenocrysts and their inclusions. This is consistent with the Dr 30 spinel Cr/(Cr+Al), which ranges from 39.1 to 56.6 with TiO₂ from 0.34 to 1.62 wt.% (0.99 ± 0.29 ave.) (Table S4). These are typical MORB spinels, where high-Al coexisting with high-Cr spinel likely reflects high pressure crystallization followed by shallow resorption and crystallization of new Cr-rich spinel during magma mixing ^{12, 13}.

Volatiles

Volatiles in our basalts are highly variable, with low CO₂ (85 ppm ave.) unlike volatile-rich MORB from the Juan de Fuca Ridge or the Mid Atlantic ridge (MAR) at 14° and 34°N (Fig. 5). Dr 30 olivine melt inclusions, however, have significantly higher CO₂ (550 ppm ave.; Table S6), than in Dr 30 glasses or Dr 27 and 31 olivine melt inclusions. As CO₂does not correlate with the other volatiles, this would reflect preferential degassing during ascent (Fig. S8a, c). Due to explosive degassing, then, the Dr 30 volatiles do not tell us that much about primary melt concentrations, and may account for the low CO₂ in the glass compared to the DR 27 glasses, and DR 27 and 31 melt inclusions where there is no evidence for explosive degassing.

Any assimilation of seawater (18,980 ppm Cl) during magma fountaining and fall back, or interaction with hydrothermally altered rocks, will drastically affect melt Cl concentrations. Highly vesiculated basalt fragments in the Dr 30 scoria provides such evidence (Fig. S3b). Chlorine is much higher in the Dr 30 (554 ppm ave.) than in DR 27 glass and melt inclusions (Fig. S8), and both glasses and melt inclusions have very low F/Cl indicating assimilation of seawater during eruption, which could contribute to high H₂O contents in the Dr 30 glass and melt inclusions (5,254 and 5,248 ppm ave.). Successively higher F/Cl in the DR 27 glasses, and DR 27 and DR 31 melt inclusions, then, indicate decreasing interaction with seawater (Table S6). The lack of correlation between volatiles in the Dr 27 glass and inclusions indicates that their concentrations were controlled by stochastic mixing of variably fractionated tholeiitic melts, and unsurprisingly S, H₂O, and F in the Dr 30 and Dr 27 olivine melt inclusions are quite similar.

H₂O, S, Cl and F due show good correlations with each other in the Dr 30 glasses (Fig. S8a, b), and some weak correlations in the Dr 30 Ol-melt inclusions. Both the glasses and inclusions have high H₂O (0.5 wt.% ave.) ranging up to 0.8 wt.% in inclusions (Table S5) compared to typical MORB (Fig. 5) (< 0.2% ¹⁴). Thus, the alkali basalt was saturated only in CO₂ during ascent, while the remaining volatiles were largely enriched by crystal fractionation consistent with only a small amount of melt in the tholeiitic crystal mush. The scatter in the olivine melt inclusions indicates their concentrations were modified during entrainment, as were the trace element and isotopic compositions of the xenocrysts.

Including their vesicularity, the Dr 30 lavas and scoria had total CO₂ at the depth of eruption ranging from 0.79 wt.% to 4.10 wt.%, assuming the vesicles contained only CO₂. However, the explosive threshold for CO₂ is ~7.8 wt.% if the generally accepted 75 vol% CO₂ is needed to cause explosion ¹⁵. Thus, the volatiles in the melt reservoir approximated by the olivine melt inclusion contents are nominally insufficient for the volcanic explosion at the dredge depth ^{16, 17}. Subsurface accumulation of exsolved volatiles, primarily CO₂, as in strombolian or vulcanian eruptions ¹⁶, seem to be required to explain the bombs and scoria in Dr 30.

Evidence for true explosive alkaline volcanism at ocean ridges has been scarce and ambiguous. What we describe is distinct from the hyaloclastite MORB sands and gravels of largely phreatic and phreatomagmatic origin previously reported ^18,19−22. Rather it has the physical characteristics of explosive eruptions at ocean islands and continental volcanic fields producing scoria, lava bombs, and major craters from a hundred meters to many kilometers across. Hekinian et al. ¹ reported ‘alkaline’ explosive volcanism on the MAR at 34°50’N down to ~ 2,000 m water depth, beautifully documented in images of craters, hyaloclastites, and scoria deposits. Poorly vesicular rift valley MORB sheet flows and pillow lavas, however, are abundant in the rift valley, while the ‘alkali basalts’ are localized on a median ridge and a nearby off-axis seamount. Of the 73 glasses in the PetDB database for the ridge segment, however, only 8 contain a small amount of nepheline (0.04 to 1.69%, indicating minor silica undersaturation, while 8 are quartz saturated (0.24 to 1.75%). The remaining 57 normal olivine normative MORB (Fig. 4a). With only one sample lying just in the field for alkali basalt (Fig. S9), Hekinian’s suite would be collectively regarded as MORBs distinct from ocean island basalts by most petrologists. The Dr 30 samples, on the other hand, plot well into the alkaline field (Fig.’s 4, S9) and show unambiguously that explosive alkaline melts do form beneath ocean ridges.

The isotopic, major, and trace element compositions of xenocrysts and melt inclusions show the Dr 30 alkali basalts entrained crystals from a tholeiitic melt, presumably a near-axis crystal mush zone. This evidence of direct mixing of an alkaline melt and tholeiitic crystal cumulate supports a new model for the origin and variable composition of MORB (Fig. 6), explaining a major conundrum of mantle geochemistry – the lack of good correlations between MORB major elements and their trace element and isotopic compositions which are effectively decoupled ²³. This is seen in both slow and ultraslow spreading ridges. For example, some 40 dredge hauls from the FAMOUS region of the MAR define an incredibly tight range in normative olivine-plagioclase-pyroxene space ²⁴ reflecting a wide range in crystal fractionation but a single liquid line of descent, and thus an apparent very uniform major element mantle source — despite their large trace element heterogeneity that would seem to require different mantle sources ²⁵. Similar situations characterized SWIR ridge segments ^23,26, where the basalt major element compositions vary systematically along the ridge with ridge depth and proximity to mantle hotspots, with little correlation with a trace element and isotopic compositions ^27,28.

MORB melts, though very restricted in major element composition compared to subaerial lavas, are divided into three groups based on their highly variable trace element and isotopic chemistry: N-MORB (depleted), T-MORB (transitional), and E-MORB (enriched) reflecting different mantle sources. E-MORB dominates near mantle hotspots and ocean islands like Iceland or the Azores, while N-MORB dominates far from hotspots with intervening compositional gradients. This was attributed to mixing enriched plume derived mantle with depleted mantle beneath the ridge ^29,30,31. A fundamental problem with this is solid-state mixing of enriched mantle flowing from a mantle plume with depleted mantle upwelling beneath the ridge seems physically unlikely for steady-state laminar flows. It would require some form of sub-axial turbulent flow entraining layers of different composition beneath the ridge — unlikely given mantle viscosities and the time scales involved.

An alternate is a laterally heterogenous mantle consisting of patches and veins of eclogite, and pyroxenite embedded in locally metasomatized depleted mantle, that increase in proportion near mantle hotspots. This latter hypothesis is consistent with the large variability of incompatible trace elements and heavy isotopes in MORB near hotspots. Small-scale geochemical heterogeneities in the upwelling mantle should melt early and inhibit melting of the host depleted mantle generating a spectrum of early melts ^{26,32,33−38}. Isotopically and trace element heterogeneous mantle at Zabargad Island emplaced during rifting of the Red Sea is a likely analog ^39–41 for such a source. Mixing early melts with those generated later from the depleted mantle host would explain the chemical variability of MORB, and why within a single ridge segment, sometimes in a single dredge, E-, T-, and N MORB are found.

The problem with this second hypothesis is that MORB formation is increasingly recognized as the result of penetrative melt flow and melt-rock reaction through an upwelling mantle with melt mass increasing as it ascends. This is consistent with modelling the trace element composition of abyssal peridotites ⁴², with magmas separating from the mantle near the base of the lithosphere. Prima facies evidence of this is the major element composition of the inferred parent MORB corrected for fractionation is quite uniform, varying systematically with ridge depth and proximity to mantle hotspots and with the composition of the underlying mantle ^27,28,43,44. The segment-scale major-element uniformity effectively requires zone refining ⁴⁵ where melts move through large masses of ascending mantle focused to a ridge segment. This, however, should eliminate any MORB isotopic and trace element variability due to small enriched heterogeneities in the mantle.

How you preserve the trace element variability at a single ridge segment during melt migration through an upwelling mantle is a longstanding and generally ignored conundrum. This is resolved if the early melting component is removed as alkali basalt and transported independently to crustal levels where it then mixes with MORB at crustal levels in the rift valley.

Alkali basalts are generally believed to form as deep low-degree melts of an enriched parent material ⁴⁶. Enriched in volatiles, alkalis, and incompatible trace elements, these are low viscosity melts that often erupt violently on ocean islands and continents. Numerous mantle xenoliths, many weighing up to ten or twenty kilos or more, testify to the rapid and violent mode of transport of such lavas, whose composition is thus largely unaffected by melt-rock interaction with the mantle. Small volumes of such melts mixing into shallow mantle and crustal magma reservoirs would have an enormous effect on the incompatible trace element and isotopic composition of MORB, with little impact on its major element composition. Thus, the discovery of our explosive alkali basalts is of great importance.

Alkali basalts are distinct from E-MORB and have not been previously associated with the gentle effusive volcanism characteristic of ocean ridges. The two are distinguished using their position in an alkalis-silica diagram relative to the famous MacDonald-Katsura line separating Hawaiian alkali and tholeiitic basalts as in Fig. 4a. This line is the trace of the critical plane of silica saturation in the nepheline-olivine -clinopyroxene-quartz basalt tetrahedron ⁴⁷ (Fig. S10). Basalts lying above the MacDonald-Katsura line fractionate to silica undersaturated compositions, while those below it follows a silica saturated tholeiitic trend out to quartz tholeiites. The precise position of this line, however, depends on the unique composition of the primary basalt as determined by the underlying mantle composition and melting process in a specific tectonic province. Critically, however, the array defined by the compositions of the Dr 30 glasses in the alkalis-silica plot can only be explained by magma mixing, as silicate melts cannot fractionate across the critical plane of silica saturation.

Our evidence shows that alkaline magmas formed at the earliest stages of mantle melting, representing only a few percent melting or less, can escape and undergo rapid transport to the crust violently erupting independent of effusive MORB. This reflects their low viscosity, density, and high volatile content. Based on crater distribution into the rift mountains, these melts, though small in volume, erupt over a broader zone than ordinary MORB, and thus most likely found in rift mountains where not obscured by effusive MORB volcanism. As a result, MORB mixing with alkali basalt would be a stochastic process producing a spectrum of variably enriched lava in the rift valley. The apparent near absence of alkali basalt along ocean ridges likely reflects over-sampling of axial volcanic zones where their signal is muted, and extreme under-sampling of the adjacent rift mountains where they erupt independently.

Acknowledgements

H.Z. was supported by National Key Basic Research Program of China (No. 2012CB417300) and the China NSF grant 91428207. HJBD received funding from the US NSF grants 2114652 and 1657983. S.Q. was also supported by NSF grant 41902044. We thank J.H.W, L.Z. and T.B. for assistance in the sample analysis. The authors express appreciation for the hard work of the RV Thomas G. Washington Crew, and to the entire scientific party of cruise TN365, with particular thanks to our co-chief scientist Juergen Koepke.

Author Contributions

Funding for Cruise TN365 was financially supported by the US National Scientific Foundation grants 2114652 and 1657983 to HJBD and M. Tominaga, and National Key Basic Research Program of China (No. 2012CB417300) to HZ, respectively, who served as Co-Chief Scientists along with Juergen Koepke of the Leibniz University of Hanover of the RV Thomas G. Thompson. All analytical work and the first draft of the manuscript were completed by HZ and SQ, and subsequently HJBD modified and rewrote the manuscript in discussion with HZ and SQ.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Hekinian, R. et al. Deep sea explosive activity on the Mid-Atlantic Ridge near 34 degrees 50 ' N: Magma composition, vesicularity and volatile content. Journal of Volcanology and Geothermal Research 98, 49–77, doi:Doi 10.1016/S0377-0273(99)00190-0 (2000).
Standish, J. J., Dick, H. J. B., Michael, P. J., Melson, W. G. & O'Hearn, T. MORB generation beneath the ultraslow spreading Southwest Indian Ridge (9–25 degrees E): Major element chemistry and the importance of process versus source. Geochemistry Geophysics Geosystems 9, doi:Artn Q05004 10.1029/2008gc001959 (2008).
Nauret, F., Snow, J. E., Hellebrand, E. & Weis, D. Geochemical composition of K-rich lavas from the Lena Trough (Arctic Ocean). Journal of Petrology, doi:10.1093/petrology/egr024 (2011).
Snow, J. E. et al. Oblique nonvolcanic seafloor spreading in Lena Trough, Arctic Ocean. Geochemistry Geophysics Geosystems 12, 1–11 (2011).
Bryan, W. B. Mineralogical studies of submarine basalts. Carnegie Institution Washington Yearbook 71, 396–403 (1972).
Parman, S. W. Helium isotopic evidence for episodic mantle melting and crustal growth. Nature 446, 900–903, doi:10.1038/nature05691 (2007).
Jackson, M. G., Konter, J. G. & Becker, T. W. Primordial helium entrained by the hottest mantle plumes. Nature 542, 340-+, doi:10.1038/nature21023 (2017).
Georgen, J. E., Lin, J. & Dick, H. J. B. Evidence from gravity anomalies for interactions of the Marion and Bouvet hotspots with the Southwest Indian Ridge: effect of transform offsets. Earth and Planetary Science Letters 187, 283–300 (2001).
Davidson, J. P., Morgan, D. J., Charlier, B. L. A., Harlou, R. & Hora, J. M. Microsampling and isotopic analysis of igneous rocks: Implications for the study of magmatic systems. Annual Review of Earth and Planetary Sciences 35, 273–311, doi:DOI 10.1146/annurev.earth.35.031306.140211 (2007).
Qian, S. P. et al. Petrogenesis of Early Cretaceous basaltic lavas from the North China Craton: Implications for cratonic destruction. J Geophys Res-Sol Ea 122, 1900–1918, doi:10.1002/2016jb013548 (2017).
Ramos, F. C. & Reid, M. R. Distinguishing melting of heterogeneous mantle sources from crustal contamination: Insights from Sr isotopes at the phenocryst scale, Pisgah Crater, California. Journal of Petrology 46, 999–1012, doi:10.1093/petrology/egi008 (2005).
Dick, H. J. B. & Bryan, W. B. Variation of basalt phenocryst mineralogy and rock compositions in DSDP Hole 396B. Initial Reports of the Deep Sea Drilling Project XLVI, 215–225 (1978).
Dick, H. J. B. & Bullen, T. Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas. Contributions to Mineralogy and Petrology 86, 54–76 (1984).
Ligi, M., Bonatti, E., Cipriani, A. & Ottolini, L. Water Rich Basalts at Mid-Ocean-Ridge Cold Spots. Nature 434, 66–69 (2005).
Navon, O. & Lyakhovsky, V. Vesiculation processes in silicic magmas. Geological Society of London, Special Publications 145, 27–50, doi::10.1144/GSL.SP.1996.145.01.03 (1998).
Head, J. W. & Wilson, L. Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits. Journal of Volcanology and Geothermal Research 121, 155–193, doi:Pii S0377-0273(02)00425-0 Doi 10.1016/S0377-0273(02)00425-0 (2003).
Helo, C., Longpre, M. A., Shimizu, N., Clague, D. A. & Stix, J. Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas. Nature Geoscience 4, 260–263, doi:10.1038/Ngeo1104 (2011).
Dick, H. J., Honnorez, J. & Kirst, P.,Origin of the Abyssal Basaltic Sand, Sandstone, and Gravel from DSDP Hole 396B, Leg 46, in Initial Reports of the Deep Sea Drilling Program Vol. 46 (eds L. Dmitriev, J. Heirtzler, & et al.) 331–337 (US Government Printing Office, 1978). doi:10.2973/dsdp.proc.46.121
Clague, D. A., Paduan, J. B. & Davis, A. S. Widespread strombolian eruptions of mid-ocean ridge basalt. Journal of Volcanology and Geothermal Research 180, 171–188, doi:doi:10.1016/j.jvolgeores.2008.08.007 (2009).
Davis, A. S. & Clague, D. A. Volcaniclastic deposits from the North Arch volcanic field, Hawaii: explosive fragmentation of alkalic lava at abyssal depths. Bulletin of Volcanology 68, 294–307, doi:10.1007/s00445-005-0008-7 (2006).
Pontbriand, C. W. et al. Effusive and explosive volcanism on the ultraslow-spreading Gakkel Ridge, 85 degrees E. Geochemistry Geophysics Geosystems 13, doi:Artn Q10005 10.1029/2012gc004187 (2012).
Sohn, R. A. et al. Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean. Nature 453, 1236–1238, doi:10.1038/nature07075 (2008).
Dick, H. J. B. Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. in Magmatism in the Ocean Basins, Geological Society Special Publication No. 42 (eds A.D. Saunders & M.J. Norry) 71–105 (1989).
Bryan, W. B. Regional variation and petrogenesis of basalt glasses from the FAMOUS Area, Mid-Atlantic Ridge. Journal of Petrology 20, 293–325 (1979).
Langmuir, C. H., Bender, J. F., Bence, A. E., Hanson, G. N. & Taylor, S. R. Petrogenesis of basalts from the FAMOUS Area: Mid-Atlantic Ridge. Earth and Planetary Science Letters 36, 133–156 (1977).
le Roex, A. P. et al. Geochemistry, mineralogy and petrogenesis of lavas erupted along the Southwest Indian Ridge between the Bouvet Triple Junction and 11 degrees east. Journal of Petrology 24, 267–318 (1983).
Dick, H. J. B., Fisher, R. L. & Bryan, W. B. Mineralogic variability of the uppermost mantle along mid-ocean ridges. Earth and Planetary Science Letters 69, 88–106 (1984).
Klein, E. M. & Langmuir, C. H. Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. Journal of Geophysical Research 92, 8089–8115 (1987).
Hart, S. R., Schilling, J. G. & Powell, J. L. Basalts from Iceland and Along Reykjanes Ridge - Sr Isotope Geochemistry. Nature-Phys Sci 246, 104–107, doi:DOI 10.1038/physci246104a0 (1973).
Schilling, J.-G. Azores mantle blob: Rare earth evidence. Earth and Planetary Science Letters 25, 103–115 (1975).
White, W. M., Schilling, J.-G. & Hart, S. R. Evidence for the Azores mantle plume from strontium isotope geochemistry of the Central North Atlantic. Nature 263, 659–663 (1976).
Hanson, G. N. Geochemical evolution of the suboceanic mantle. Journal of the Geological Society of London 134, 1–19 (1977).
Wood, D. A. Variably Veined Sub-Oceanic Upper Mantle - Genetic Significance for Mid-Ocean Ridge Basalts from Geochemical Evidence. Geology 7, 499–503, doi:Doi 10.1130/0091-7613(1979)7 < 499:Avvsum > 2.0.Co;2 (1979).
Sleep, N. H. Tapping of Magmas from Ubiquitous Mantle Heterogeneities - an Alternative to Mantle Plumes. Journal of Geophysical Research 89, 29–41, doi:DOI 10.1029/JB089iB12p10029 (1984).
Morgan, J. P. Thermodynamics of pressure release melting of a veined plum pudding mantle. Geochemistry Geophysics Geosystems 2 (2001).
Natland, J. H. Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone. Geological Society, London, Special Publications 42, 41–70, doi:doi.org/10.1144/GSL.SP.1989.042.01.05 (1989).
Pertermann, M. & Hirschmann, M. M. Partial melting experiments on a MORB-like pyroxenite between 2 and 3 GPa: Constraints on the presence of pyroxenite in basalt source regions from solidus location and melting rate. J Geophys Res-Sol Ea 108, doi:Artn 2125 10.1029/2000jb000118 (2003).
Pertermann, M. & Hirschmann, M. M. Anhydrous partial melting experiments on MORB-like eclogite: Phase relations, phase compositions and mineral-melt partitioning of major elements at 2–3 GPa. Journal of Petrology 44, 2173–2201, doi:10.1093/petrology/egg074 (2003).
Brueckner, H. K., Zindler, A., Seyler, M. & Bonatti, E. Zabargad and the Isotopic Evolution of the Sub-Red Sea Mantle and Crust. Tectonophysics 150, 163–176, doi:Doi 10.1016/0040-1951(88)90300-9 (1988).
Bonatti, E., Ottonello, G. & Hamlyn, P. R. Peridotites from the Island of Zabargad (St John), Red-Sea - Petrology and Geochemistry. J Geophys Res-Solid 91, 599–631, doi:DOI 10.1029/JB091iB01p00599 (1986).
Vannucci, R. et al. Rare earth and trace element geochemistry of clinopyroxenes from the Zabargad peridotite-pyroxenite association. Journal of Petrology Special Lherzolite Issue, 255–269 (1991).
Johnson, K. T. M., Dick, H. J. B. & Shimizu, N. Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research 95, 2661–2678 (1990).
Bryan, W. B. & Dick, H. J. B. Contrasted abyssal basalt liquidus trends: evidence for mantle major element heterogeneity. Earth and Planetary Science Letters 58, 15–26 (1982).
Regelous, M., Weinzierl, C. G. & Haase, K. M. Controls on melting at spreading ridges from correlated abyssal peridotite - mid-ocean ridge basalt compositions. Earth and Planetary Science Letters 449, 1–11, doi:10.1016/j.epsl.2016.05.017 (2016).
Kushiro, I. Compositions of magmas formed by partial zone melting of the earth's upper mantle. Journal of Geophysical Research 73, 619–634 (1968).
Pilet, S., Baker, M. B. & Stolper, E. M. Metasomatized lithosphere and the origin of alkaline lavas. Science 320, 916–919 (2008).
Yoder, H. S. & Tilley, C. E. Origin of Basalt Magmas - an Experimental Study of Natural and Synthetic Rock Systems. Journal of Petrology 3, 342-+, doi:DOI 10.1093/petrology/3.3.342 (1962).
Roeder, P. L. & Emslie, R. F. Olivine-Liquid Equilibrium. Contrib Mineral Petr 29, 275-&, doi:Doi 10.1007/Bf00371276 (1970).
Ulmer, P. The dependence of the Fe2+-Mg cation-partitioning between olivine and basaltic liquid on pressure, temperature and composition. Contrib Mineral Petr 101, 261–273, doi:10.1007/BF00375311 (1989).
Sun, S.-S. & McDonough, W. F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications 42, 313–345, doi:10.1144/gsl.sp.1989.042.01.19 (1989).
Mcdonough, W. F. & Sun, S. S. The Composition of the Earth. Chem Geol 120, 223–253, doi:Doi 10.1016/0009-2541(94)00140-4 (1995).
Newman, S. & Lowenstern, J. B. VolatileCalc: a silicate melt–H2O–CO2 solution model written in Visual Basic for excel. Comput Geosci-Uk 28, 597–604, doi:https://doi.org/10.1016/S0098-3004(01)00081-4 (2002).
Cartigny, P., Pineau, F., Aubaud, C. & Javoy, M. Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H2O/Ce, δ D, CO2/Nb) in the North Atlantic mantle (14° N and 34° N). Earth Planet. Sci. Lett. 265, 672–685, https://doi.org/10.1016/j.epsl.2007.11.011 (2008).
Le Voyer, M., Kelley, K. A., Cottrell, E. & Hauri, E. H. Heterogeneity in mantle carbon content from CO2-undersaturated basalts. Nat Commun 8, 14062, doi:10.1038/ncomms14062 (2017).
Bennett, E. N., Jenner, F. E., Millet, M. A., Cashman K. V. & Lissenberg, C. J. Deep roots for mid-ocean-ridge volcanoes revealed by plagioclase-hosted melt inclusions. Nature 572, 235–239, https://doi.org/10.1038/s41586-019-1448-0 (2019).

There is NO Competing Interest.

Download PDF

Version 1

posted

You are reading this latest preprint version

Explosive Alkaline Volcanism on the SW Indian Ridge

Status:

Version 1

Abstract

Figures

Full Text

Discussion

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1