Linking planetary–scale spatio-temporal trends in magma compositions and volcanic resurfacing on Mars

In the continental crust, the probability of dike propagation out of magma chambers is governed by thermal, rheological, and pressure conditions of magma chamber‐wall rock systems. Incremental injection of melt into an average‐size, laccolith‐shaped, midcrustal magma chamber produces a volume of mobile magma at the bottom of the chamber that has the potential to escape as dikes through the upper, immobile portion of the chamber and the roof. Here we numerically model the conditions needed for dike propagation out of a magma chamber during continuous and episodic injections of melt into the chamber. The roles of magma buoyancy and overpressure from melt injections in generating dikes are explored within 1.78 × 104 to 1.78 × 108 Pa·s range of magma viscosities (μmag), 10 to 40 GPa range of elastic moduli (E) of the immobile top portion of the magma chamber, and 10 and 20 kyr durations of chamber growth. During episodic, high‐flux melt injections (tens of km3/yr), magma overpressure can reach >100 MPa and initiate dike propagation even when μmag and E are near the high ends of the examined ranges. The probability of generating dikes diminishes when the injection flux is lower. Continuous low‐flux injections favor magma accumulation because injection overpressure never exceeds 20 MPa. During either continuous or episodic growth of magma chamber, there is never a sufficient amount of mobile magma in the chamber for dikes to be induced by magma buoyancy alone.

Spinel-peridotite xenoliths entrained by Plio-Pleistocene alkaline basic lavas from Sardinia (Italy) indicate a complex petrological history of the uppermost lithospheric mantle. They mostly show protogranular textures and are characterised by a four-phase equilibrated assemblage, ranging in composition from lherzolites (up to 18% of Cpx) to harzburgites, suggesting that the Sardinian subcontinental mantle underwent partial melting episodes with extraction of basic magmas. Trace element analyses (LAM-ICP-MS) and Sr- Nd isotope data, carried out on clinopyroxene (Cpx) separates, indicate a multistage history of depletion and enrichment processes. Clinopyroxene from Cpx-rich lherzolites are characterised by a LREE depletion, with low 87Sr/86Sr (0.70262-0.70391) and high 143Nd/144Nd (0.51323 - 0.51286) values, while clinopyroxene from less fertile lherzolites and harzburgites show LREE enrichments, higher 87Sr/86Sr (0.70410-0.70461) and lower 143Nd/144Nd (0.51288-0.51251). Nd model ages (relative to CHUR) of the most LREE-depleted samples, with 87Sr/86Sr<0.703, suggest that partial melting events occurred during Pre-Palaeozoic times. Modelling of the HREE distribution in clinopyroxene indicates that the Cpx-rich lherzolites could be interpreted as a residue after low (< 5%) melting degrees of an inferred fertile source, while higher melting degrees (up to 20-25%) are necessary to fit the Cpx composition of the most refractory harzburgites. Subsequent metasomatic processes are testified by the isotopic/ LREE enrichments, mainly recorded in the Cpx-poor peridotites. This fact implies that the most refractory domains of the mantle are more easily percolated by fluids, while Cpx-rich domains are less permeable to metasomatic agents, as indicated by experiments on melt connectivity in peridotite materials. Geochemical modelling suggests that the above mentioned enriched compositions can be obtained by metasomatising previously depleted mantle peridotite with a small amount (< 3%) of a strongly alkaline silicate melt. Neither the inferred metasomatic agents nor the Plio- Pleistocene Sardinian lavas show the HIMU geochemical imprint which, in addition to enriched mantle EM components, is recognised in Cenozoic anorogenic magmas throughout Central Europe (Wilson and Downes, 1991). The available data therefore indicate that the lithospheric mantle beneath Sardinia is heterogeneously enriched mainly by EM components, which reflect the complex multistage evolution occurring over the last 500 Ma. Similar geochemical features are observed in other samples of the European lithospheric mantle, such as the peridotite xenoliths entrained in alkaline lavas from the Massif Central (Zangana et al., 1997), and Tallante (Southern Spain; unpublished data). Analogous metasomatic enrichments can also be recognised in the most residual peridotites from the Pyrenean and Lanzo massifs (Bodinier et al., 1991; Downes et al., 1991). This suggests that the observed geochemical features were probably acquired during pre-Middle Mesozoic times, due to the repeated percolation of uprising EM metasomatic fluids in the European lithosphere. It should be emphasised that this metasomatic signature has not generally been observed for the lithospheric mantle of the African plate, where Cenozoic anorogenic magmas and associated mantle xenoliths are characterised by a prevalent HIMU metasomatic component (Beccaluva et al., 1998 and reference therein).

Melting of the primitive martian mantle at 0.5–2.2 GPa and the origin of basalts and alkaline rocks on Mars

Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle–ductile transition around a magma chamber. Yet, typical storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures $$\lesssim$$ 1.5 kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures $$>rsim$$ 2.5 kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only within a relatively narrow range of pressures around 2 ± 0.5 kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption. Volatile exsolution and crustal viscosity dictate that the optimum pressure for the growth of an eruptible magma reservoir is 2 kbar in all tectonic settings and crustal compositions, according to thermomechanical modelling.

This thesis consists of three separate parts, each addressing a specific aspect of the surface morphology of Mars. In Part 1, a comparison of large volcanic features is made using spacecraft imaging data. The Elysium volcanic province on Mars and the Tibesti volcanic province in Chad, Africa are studied using Mariner 9, Earth Resources Technology Satellite and Apollo photography. Elysium Mons on Mars and Emi Koussi on Earth show remarkable similarities in summit caldera and flank morphologies. Each has a large central caldera approximately 12 km in diameter and from 500 to 1000 m deep which contain numerous craters and large, irregular pits. Channel-like features which head at the calderas and taper downslope show evidence of collapse and possible lava erosion. Elysium Mons rises some 14 ? 1.5 km above its base and the summit is about 17 km above the 6.1 mbar mean martian pressure surface. Cratering data indicate most of the apparent cratering is endogenic in origin. The subdued, hummocky terrain on the flanks is distinctly different from the slopes of the younger Tharsis Ridge volcanoes, showing little if any sign of recent material flow. The lack of aqueous erosional forms on Elysium Mons argues strongly against recent ([approx.]10[superscript 5] to 10[superscript 6] year) pluvial episodes. The forms and associations of features throughout the Elysium region suggest central volcanism started earlier in Elysium than in Tharsis, and that the source of the Elysium volcanics is chemically evolved, with evidence of silicic magma. Finally, the data are consistent with the view that the martian crust has been stable and essentially motionless for an extended period of martian geologic time. Part 2 attempts to determine the age relationship between the large, sinuous channels on Mars and the terrains in which the channels are found. Crater counts, plain and mantle superposition, and geographic and geologic associations suggest that the channels are extremely old, are spatially and temporarily related to the ancient cratered terrain and are genetically related to the processes of fretting and chaos formation. There appears to be no evidence for recent channel activity. Part 3 presents the results of an investigation of the morphologic characteristics of the plains which separate the craters in the heavily cratered regions of Mars. These intercrater plains appear to be composed of stratified consolidated and unconsolidated materials, probably loose debris blankets and volcanic flows. The topmost layer of the plains unit varies from location to location. An older, cratered surface may be partly exposed where the kilometers-thick plains unit is locally incised and eroded. The association of chaotic terrain, fretted terrain and major channels with the plains suggests that the volatile(s) presumed to be necessary to produce these erosional landforms may have been present among the plains materials. It is speculated that the unconsolidated material is impact-generated debris and eolian deposits, suggesting an early atmosphere conducive to material transport and possibly flowing liquids.

The development of discrete volcanic centers reflects a focusing of magma ascending from the source region to the surface. We suggest that this organization occurs via mechanical interactions between magma chambers, volcanic edifices, and dikes and that the stresses generated by these features may localize crustal magma transport before the first eruption occurs. We develop a model for the focusing or “lensing” of rising dikes by magma chambers beneath a free surface, and we show that chambers strongly modulate dike focusing by volcanic edifices. We find that the combined mechanical effects of chambers, edifice loading, and dike propagation are strongly coupled. Chambers deeper than ∼20 km below the surface with magmatic overpressure in the range of 20–100 MPa should dominate dike focusing, while more shallow systems are affected by both edifice and chamber focusing.

Osmium isotope compositions of detrital Os-rich alloys from the Rhine River provide evidence for a global late Mesoproterozoic mantle depletion event

The best‐studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from 1975–1984. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the “breakout” pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this “stopping” pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.

Sources of water for the outflow channels on Mars: Implications of the Late Noachian “icy highlands” model for melting and groundwater recharge on the Tharsis rise

Calcium isotope constraints on OIB and MORB petrogenesis: The importance of melt mixing

Understanding mantle melting above subduction zones requires an evaluation of the behaviour of elements for which the mantle contribution greatly exceeds any subduction contribution. In this paper we present a compilation of partition coefficients for a suite of these elements (Nb, Zr, Y, Yb, Ca, Al, Ga, V, Sc, Fe, Mn, Co, Cr, Mg, Ni) for temperatures of 1200–1300°C, oxygen fugacities of QFM ± 1 and sub-alkaline compositions. These coefficients yield good-fit mantle depletion trends for abyssal, orogenic and trench-wall peridotites. Modelling of pooled melts from mantle melting columns, presented as FMM (fertile MORB mantle) normalized patterns, give signatures of the composition and degree of melting of the mantle wedge that are generally independent of the subduction component. In particular, patterns formed from melting of fertile mantle exhibit normalized element abundances in the order VHI &gt; HI &gt; MI (VHI = very highly incompatible, HI = highly incompatible and MI = moderately incompatible) at low degrees of melting, becoming VHI = HI = MI at high degrees of melting. With derivation from progressively depleted sources, the patterns for moderate degrees of melting change to VHI &lt; HI = MI at moderate degrees of depletion and VHI &lt; HI &lt; MI at high degrees of depletion. The details, but not the principles, can be varied by changing the shape of the melting column, the porosity of the mantle during melting, the potential temperature of the mantle, and the temperature and depth of initiation of melting. Bivariate plots of elements of contrasting compatibilities (Cr-Yb, Sc-Yb, Nb-Yb) can be contoured according to the degree of depletion or enrichment of the mantle and the degree of melting, with selected plots also emphasizing the role of garnet (Ti-Yb) and oxygen fugacity (V-Yb). Evaluation of data from present-day volcanic arcs suggests that: (1) intra-oceanic arcs with associated active backarc basins are derived principally from fertile MORB mantle that has lost up to about 3% melt in a previous melting event; (2) this depletion takes place in spinel lherzolite facies, supporting models that relate it to backarc basin melting events; (3) oceanic arcs with no associated backarc basins are derived principally from fertile MORB mantle, though enriched sources can be important locally; (4) intra-continental arcs are commonly derived from enriched mantle, probably because of the involvement of sub-continental lithosphere; (5) degrees of melting are probably high (in the order of 25–30%) in intra-oceanic arcs on thin crust, decreasing to 150r less in areas of thicker lithosphere; (6) some 10% melting can be explained by volatile addition to the mantle, the remainder by decompression.

Young volcanic centres of the Bransfield Strait and James Ross Island occur along back-arc extensional structures parallel to the South Shetland island arc. Back-arc extension was caused by slab rollback at the South Shetland Trench during the past 4 myr. The variability of lava compositions along the Bransfield Strait results from varying degrees of mantle depletion and input of a slab component. The mantle underneath the Bransfield Strait is heterogeneous on a scale of approximately tens of kilometres with portions in the mantle wedge not affected by slab fluids. Lavas from James Ross Island east of the Antarctic Peninsula differ in composition from those of the Bransfield Strait in that they are alkaline without evidence for a component from a subducted slab. Alkaline lavas from the volcanic centres east of the Antarctic Peninsula imply variably low degrees of partial melting in the presence of residual garnet, suggesting variable thinning of the lithosphere by extension. Magmas in the Bransfield Strait form by relatively high degrees of melting in the shallow mantle, whereas the magmas some 150 km further east form by low degrees of melting deeper in the mantle, reflecting the diversity of mantle geodynamic processes related to subduction along the South Shetland Trench.

Petrology of the plio-quaternary volcanism of the South-Central and Meridional Andes

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Geochemical studies of mantle-derived magmas erupted at the Earth surface have shown that one of the main factors controlling their chemistry is the degree of partial melting. However, geologists rarely have direct access to pristine mantle melts, and laboratory experimental partial melting of peridotite essentially documents the chemical composition of large melt fractions. Therefore, several aspects of melting models remain hypothetical. In particular, the major element composition of the lowest melt fractions from upper mantle peridotite is still a matter of debate. Here, the approach was to identi~ and examine directly near-solidus mantle melts preserved as quenched glass inclusions in minerals in upper mantle xenoliths. Contrary to interstitial glass samples found in xenoliths which are open systems and, consequently, are prone to chemical modification during ascent of the host xenolith, melt/fluid inclusions inside minerals behave as closed and isochoric systems. Previous studies of glass inclusions in xenoliths have shown that high-silica glasses are a common feature of spinel lherzolite minerals (Schiano and Clocchiatti, 1994), and a general link between these silica-rich inclusions and the character of near-solidus partial melts of peridotite has been suggested. The new contribution of this study is the documentation of a series of glass inclusions in mantle minerals which, after homogenization by heating, show a continuous suite of chemical compositions clearly distinct from that of the host lavas (Schiano et al., 1998). The compositions range from silicic, with nepheline-olivine normative, 64wt.% SiO2 and 11 wt.% alkali oxides, to almost basaltic, with quartz normative, 50 wt.% SiO2 and 1 -2 wt.% alkali oxides. An experimental study of the inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at mantle temperature and pressure (T 1230~ and P ~1.0 GPa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr203 and poorer in SiO2, K20, NazO, A1203 and CI with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts. Furthermore, the results extend previous studies arguing in favour of silica-rich melts at low degree of melting, in establishing that near-solidus primary melts of peridotite could have SiO2 contents > 60 wt.% for MgO contents < 1 wt.%. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.

Research Article| October 01, 1992 Mantle decarbonation and Archean high-Mg magmas Garth R. Edwards Garth R. Edwards 1Faculty of Science, Athabasca University, Athabasca, Alberta T0G 2R0, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Garth R. Edwards 1Faculty of Science, Athabasca University, Athabasca, Alberta T0G 2R0, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (10): 899–902. https://doi.org/10.1130/0091-7613(1992)020<0899:MDAAHM>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Garth R. Edwards; Mantle decarbonation and Archean high-Mg magmas. Geology 1992;; 20 (10): 899–902. doi: https://doi.org/10.1130/0091-7613(1992)020<0899:MDAAHM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Magnesium-rich mane to ultramafic extrusions were most common in the Archean and pose interesting petrological problems. The high Mg content of komatiites (>18 wt%, for example, is usually interpreted as indicating an origin at higher temperatures than exist in mantle melting zones in the modern Earth. Current contrasting models for the origin of komatiites in the mantle require either high degrees of melting or lower degrees of melting at great depth. A potential complementary mechanism for Mg enrichment in magmas involves the melting of magnesite-bearing garnet Iherxolite. In this model, the ascending primary mafic or ultramafic magma is enriched in MgO by the loss of some off the CO2 to the adjacent mantle at pressures of ∼2.2 GPa, where the magma becomes saturated with CO2. To generate komatiite in this way from a picritelike parent, for example, requires that the primary magma lose some of its major and trace element components to the adjacent mantle concurrently with the CO2. Production of magnesian magmas by magnesite breakdown may not have required the heat or depth of those produced by other means; this mechanism may help to explain some apparently low Archean geothermal gradients, as well as the contemporaneity of Archean diamonds and komatites. The mantle magnesite could have formed by direct reaction of primordial CO2 or CO with hot, protomantle material during Earth's accretionary period. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.