Martin Menzies

Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China

Abstract In eastern China Palaeozoic kimberlites and Cenozoic basalts have been erupted through the same Archaean crust, thus providing deep probes of the cratonic lower lithosphere over a period of 400 Ma. While Palaeozoic diamondiferous kimberlites point to the existence of thick, refractory lower lithosphere in the east, Cenozoic basalt-borne xenoliths reveal the presence of hot, thin, less refractory lower lithosphere. Remnants of the Archaean lithosphere may have survived as harzburgites which are chemically similar to those from the Kaapvaal craton but very different from recently accreted lherzolites. In the absence of convincing evidence for supra-subduction or intraplate processes it is believed that the dramatic change of lithosphere architecture in the Phanerozoic was caused by indentor tectonics resulting from the collision of India and Eurasia. Passive reactivation and remobilization of the Archaean lower lithosphere, in particular metasome horizons, contributed to Cenozoic magmatism aligned along major lithospheric faults.

Geologic, geochemical, and geophysical consequences of plume involvement in the Emeishan flood-basalt province

Prevolcanic kilometer-scale lithospheric doming in the Emeishan large igneous province, southwest China, allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography, geology, geochemistry, and geophysics of the region. Systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments, the extent of erosion, thickness, and chemistry of volcanic rocks, and the crust-mantle structure. These features, which are best explained by a mantle plume, may be used to track older plume sites in the geologic record.

A brief Oligocene period of flood volcanism in Yemen: implications for the duration and rate of continental flood volcanism at the Afro-Arabian triple junction

40Ar39Ar dating of mineral separates and whole-rock (WR) samples has established that basaltic continental flood volcanism (CFV) began between 30.9 and 29.2 Ma in northwestern and southwestern Yemen, respectively. Rhyolitic volcanism commenced at 29.0–29.3 Ma throughout Yemen. Lower basaltic lavas were erupted every 10–100 kyr, whereas upper bimodal volcanic units were erupted every 100–500 kyr, which reflects generation of rhyolitic magmas from basalts that resided for longer periods in lithospheric magma chambers than during the early phase of exclusively mafic magmatism. The youngest dated flood volcanic units were erupted between 26.9 and 26.5 Ma throughout Yemen. The duration of preserved CFV defined by 40Ar39Ar dating (4.4 myr) contrasts with the wide range of WR KAr dates previously obtained in Yemen (> 50 myr). 40Ar39Ar step-heating studies of WR samples has shown that this discrepancy is due to the disturbed Ar systematics of volcanic samples. Most samples have experienced post-crystallization loss of radiogenic Ar and/or contain excess Ar, with only ca. 25% of the WR KAr dates within 1–2 myr of true crystallization ages. WR KAr data can be screened for reliability using the radiogenic Ar yield and 40K36Ar ratio, which reflect the Ar retentivity of the sample, the likelihood that alteration has disturbed a samples Ar systematics, and the susceptibility of the sample to a finite amount of Ar loss or the presence of a finite amount of excess Ar. Examination of existing WR KAr data in the Ethiopian part of this flood volcanic province, using these parameters, suggests that much of these data are also misleading. Two phases of flood volcanism are inferred in Ethiopia and Eritrea at 38-30 Ma and ca. 20 Ma. The older phase is equivalent to that in Yemen, and is consistent with the progression in basal volcanic ages obtained in Yemen moving from north to south. The younger phase is related to the onset of upper crustal extension and incipient Red Sea-Gulf of Aden rifting. The sequence of events — surface uplift (?), flood magmatism and subsequent upper crustal extension — in Yemen is consistent with the involvement of a mantle plume at the Afro-Arabian tripe junction. However, the overall eruption rate for this flood volcanic province is only 0.03 km3/yr, much slower than that postulated for other plume-related provinces such as the Deccan or Siberian Traps, but perhaps comparable to the Parana-Etendeka province, which also contains significant amounts of rhyolitic volcanic products like those of Yemen-Ethiopia. The highly variable eruption rates in individual provinces must reflect the very different character of individual plumes, or the control of lithospheric structure and plate tectonic stresses on the surface manifestations of plumes. The long duration of CFV and large amounts of rhyolitic volcanism at the Afro-Arabian triple junction may be attributed to the relatively slow separation of the African and Arabian plates compared with, for example, the rifting of India and the Deccan Traps.

Nd and Sr isotope geochemistry of hydrous mantle nodules and their host alkali basalts: implications for local heterogeneities in metasomatically veined mantle

Nd and Sr isotopic data on pargasite Iherzolite inclusions, kaersutite megacrysts and their host alkali basalts are presented here to clarify some questions regarding isotopic equilibration during mantle metasomatism and the role of metasomatism in basalt genesis. Five alkali basalts from Nunivak Island within the Aleutian back-arc basin, have87Sr/86Sr ratios of 0.70251–0.70330 and143Nd/144Nd ratios of 0.51289–0.51304. On a Nd versus Sr isotope composition diagram the basalts overlap the fields of MORB and ocean island basalts. Pargasites and mica separated from hydrous nodules found in these basalts have a range in87Sr/86Sr of 0.70256–0.70337 but identical143Nd/144Nd ratios of 0.51302. The metasomatic fluid represented by the pargasite is in isotopic equilibrium, both for Sr and Nd, with the dry mantle as represented by diopside. Eight alkali basalts from the Ataq diatreme, South Yemen, have87Sr/86Sr range of 0.70335–0.70426 and143Nd/144Nd range of 0.51252–0.51305. On a Nd versus Sr isotope composition diagram the basalts from Ataq plot in two distinct fields, (1) within the field of ocean island basalts, and (2) within the range of continental rift basalts but to the left of the Nd-Sr correlation line, somewhat similar to the Skye and Oslo rift basalts. Diopside and pargasite separated from three nodules at Ataq have a more complex history than those at Nunivak. Two nodules contain pargasite and diopside with identical87Sr/86Sr ratios but different143Nd/144Nd ratios. A third nodule contains diopside with a143Nd/144Nd ratio similar to that of other diopsides. The Nunivak basalts are derived from a source with a time-integrated light-REE depletion, in contrast to the light-REE-enriched nature of the basanites. This is best explained by a recent metasomatic event in the source region which increased the LIL element content of the peridotite thus accommodating higher degrees of melting. The Ataq volcanic rocks seem to tap different sources characterized by both light-REE enrichment and depletion, in contrast to the uniform source of the Nunivak basanites. Production of the Ataq basanites is believed to involve anataxis of metasomatically veined continental mantle where local mantle heterogeneities survived the melting event.

Determination of partition coefficients between apatite, clinopyroxene, amphibole, and melt in natural spinel lherzolites from Yemen: Implications for wet melting of the lithospheric mantle

Plio-Quaternary basaltic volcanic fields along the south coast of Yemen contain mantle xenoliths of anhydrous and metasomatized amphibole-(± apatite) bearing spinel lherzolites. Partial melting of clinopyroxene and/or amphibole in a closed system produced silicate glass from which new clinopyroxene, olivine, and spinel have crystallized. Partition coefficients between mineral phases in the matrix and particularly those (apatite, clinopyroxene, amphibole) formed during the metasomatic event, and between glass and the new clinopyroxene crystallized from it have been calculated by analyzing trace element composition with an ion microprobe. Partition coefficients between apatite and clinopyroxene or amphibole are large (e.g., ≫ 1) for the REEs, Ba, Sr, Y, and Hf and confirm that apatite, when present in the mantle, is an important repository for these elements. On the other hand, partitioning between amphibole and clinopyroxene is very close to unity for most of the analyzed trace elements, except for Ba, Nb, Zr, and Hf. Trace element ratios such as Zr/Nb, Ba/Zr, or Ba/Nb can thus be used in basaltic rocks to assess the presence of amphibole in their mantle sources. Partition coefficients between clinopyroxene and melt are within the range of published values for most of the analyzed elements, with some low values for Zr (e.g., 0.030–0.353) and near unity for the HREE. Using partition coefficients between amphibole and clinopyroxene and Depx/melt from the literature, we calculate the partitioning of trace elements between amphibole and a basaltic melt. The Damp/melt obtained are lower than most of the published values, but are in good agreement with recent values measured in experiments at 1.5 GPa and 1100°C. The partition coefficient for Nb between amphibole and melt is always low (<0.20), which indicates that residual amphibole during partial melting in hydrated mantle (amphibole-bearing peridotite) cannot account for Nb depletion in arc magmas.

Matching conjugate volcanic rifted margins: 40Ar/39Ar chrono-stratigraphy of pre- and syn-rift bimodal flood volcanism in Ethiopia and Yemen

Abstract 40 Ar/ 39 Ar dating of mineral separates and whole-rock samples of rhyolitic ignimbrites and basaltic lavas from the pre- and syn-rift flood volcanic units of northern Ethiopia provides a temporal link between the Ethiopian and Yemen conjugate rifted volcanic margins. Sixteen new 40 Ar/ 39 Ar dates confirm that basaltic flood volcanism in Ethiopia was contemporaneous with flood volcanism on the conjugate margin in Yemen. The new data also establish that flood volcanism initiated prior to 30.9 Ma in Ethiopia and may predate initiation of similar magmatic activity in Yemen by ∼0.2–2.0 Myr. Rhyolitic volcanism in Ethiopia commenced at 30.2 Ma, contemporaneous with the first rhyolitic ignimbrite unit in Yemen at ∼30 Ma. Accurate and precise 40 Ar/ 39 Ar dates on initial rhyolitic ignimbrite eruptions suggest that silicic flood volcanism in Afro-Arabia post-dates the Oligocene Oi2 global cooling event, ruling out a causative link between these explosive silicic eruptions (with individual volumes ≥200 km 3 ) and climatic cooling which produced the first major expansion of the Antarctic ice sheets. Ethiopian volcanism shows a progressive and systematic younging from north to south along the escarpment and parallel to the rifted margin, from pre-rift flood volcanics in the north to syn-rift northern Main Ethiopian Rift volcanism in the south. A dramatic decrease in volcanic activity in Ethiopia between 25 and 20 Ma correlates with a prominent break-up unconformity in Yemen (26–19 Ma), both of which mark the transition from pre- to syn-rift volcanism (∼25–26 Ma) triggered by the separation of Africa and Arabia. The architecture of the Ethiopian margin is characterized by accumulation and preservation of syn-rift volcanism, while the Yemen margin was shaped by denudational unloading and magmatic starvation as the Arabian plate rifted away from the Afar plume. A second magmatic hiatus and angular unconformity in the northern Main Ethiopian Rift is evident at 10.6–3.2 Ma, and is also observed throughout the Arabian plate in Jordanian, Saudi Arabian and Yemeni intraplate volcanic fields and is possibly linked to tectonic re-organization and initiation of sea floor spreading in the Gulf of Aden and the Red Sea at 10 and 5 Ma, respectively.

Strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the Snake River Plain: evidence for Archean basement

Abstract Xenoliths of intermediate to felsic granulites found in evolved lavas from the Snake River Plain have been analyzed for Nd, Pb, and Sr isotopic compositions and related trace element contents. Overall, they exhibit wide ranges in present-day values of 87 /Sr 86 Sr (0.70235–0.83011), 143 /Nd 144 Nd (0.51023–0.51148) and 206 /Pb 204 Pb (13.43–24.65). The Rb Sr, U Pb, Th Pb, and Sm Nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas. However, Pb Pb and Sm Nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2.8 Ga. Nd model ages are significantly older (ca. 3.1–3.4 Ga) for many of the xenoliths. Precambrian metasediments exposed along the southern margin of the Snake River Plain have Sm Nd systematics similar to those of the xenoliths. These results suggest that at least two significant thermal events occurred during Archean evolution of the crust in this region: (a) early (ca. 3.1–3.4 Ga) additions of mantle-derived magmas to the crust, and (b) regional metamorphism at 2.8 Ga accompanied at least locally by magmatism. Concordance between the Sm Nd and Pb Pb ages suggests that these isotopic systems are little affected by high-grade metamorphism in this case. The distribution of xenoliths in Snake River Plain lavas supports the presence of Archean crust beneath much of southern Idaho, although such rocks rarely are exposed at the surface. Thus, Nd and Pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking.

Characteristics of volcanic rifted margins

Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

Volcanic ash layers illuminate the resilience of Neanderthals and early modern humans to natural hazards

Marked changes in human dispersal and development during the Middle to Upper Paleolithic transition have been attributed to massive volcanic eruption and/or severe climatic deterioration. We test this concept using records of volcanic ash layers of the Campanian Ignimbrite eruption dated to ca. 40,000 y ago (40 ka B.P.). The distribution of the Campanian Ignimbrite has been enhanced by the discovery of cryptotephra deposits (volcanic ash layers that are not visible to the naked eye) in archaeological cave sequences. They enable us to synchronize archaeological and paleoclimatic records through the period of transition from Neanderthal to the earliest anatomically modern human populations in Europe. Our results confirm that the combined effects of a major volcanic eruption and severe climatic cooling failed to have lasting impacts on Neanderthals or early modern humans in Europe. We infer that modern humans proved a greater competitive threat to indigenous populations than natural disasters.

SrNdPb isotopic and trace element evidence for crustal contamination of plume-derived flood basalts: Oligocene flood volcanism in western Yemen

Abstract Oligocene flood basalts from western Yemen have a relatively limited range in initial isotopic composition compared with other continental flood basalts: 87 Sr 86 Sr = 0.70365−0.70555 ; 143 Nd 144 Nd = 0.5129−0.51248 (ϵNd = +6.0 to −2.4) ; 206 pb 204 Pb = 17.9−19.3 . Most compositions lie outside the isotopic ranges of temporally and spatially appropriate mantle source compositions observed in this area, i.e., Red Sea/Gulf of Aden MORB mantle, the Afar plume, and Pan-African lithospheric mantle Correlations between indices of fractionation, silica, and isotope ratios suggest that crustal contamination has substantially modified the primary isotopic and incompatible trace element characteristics of the flood basalts. However, significant scatter in these correlations was produced by: (a) the heterogeneous isotopic composition of Pan-African crust; (b) the difference in susceptibility of magmas to contamination as a result of variable incompatible trace element contents in primary melts produced by differing degrees of partial melting; (c) the presence or absence of plagioclase as a fractionating phase generating complex contamination trajectories for Sr; (d) sampling over a wide area not representing a single coherent magmatic system; and (e) variation in contamination mechanisms from assimilation associated with fractionation (AFC) to assimilation by hot mafic magmas with little concomitant fractionation. The presence of plagioclase as a fractionating phase in some suites that were undergoing AFC requires assimilation to have taken place within the crust and, coupled with the limited LREE-enrichment accompanying isotopic variations, excludes the possibility that an AFC-type process took place during magma transfer through the lithospheric mantle. Isotopic compositions of some of the inferred crustal assimilants are similar to those postulated by other workers for an enriched lithospheric mantle source of many flood basalts in southwestern Yemen, Ethiopia, and Djibouti. The western Yemen flood basalts contain 0–30% crust which largely swamps their primary lead isotopic signature, but the primary SrNd isotopic signature is close to that of the least contaminated and isotopically most depleted flood basalts. LREE HFSE and LILE HFSE ratios also correlate with isotopic data as a result of crustal contamination. However, Nb La and K Nb ratios of >1.1 and 87 Sr 86 Sr ∼ 0.7036; 143 Nd 144 Nd ∼ 0.51292 ; 206 Pb 204 Pb ∼ 18.4−19.0 . This, coupled with low LILE HFSE ratios, suggest the source has characteristics akin to the Afar plume. A mantle source isotopically more depleted than Bulk Earth, but not as depleted as MORB, coupled with LILE depletion, also characterises other examples of plume-derived flood volcanism. This mantle reservoir is responsible for the second largest outbursts of volcanism on Earth and has radiogenic isotopic characteristics akin to PREMA mantle, but the incompatible trace element signature of HIMU mantle.