Igneous Rock Associations 8. Arc Magmatism II: Geochemical and Isotopic Characteristics
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Bibliographic record
Abstract
Geochemical and isotopic data provide insights into the origin and evolution of magmatism found at destructive plate margins. Tholeiitic magmas are dominant in the early stages of oceanic island-arc genesis and calc-alkalic magmas are most common in mature oceanic arcs and in continental arcs where they may range from basalt to rhyolite in composition, including voluminous intermediate (andesitic) rocks. Experiments suggest that calcalkalic mafic magmas are formed by melting of a hydrated mantle wedge and undergo low pressure fractional crystallization under near-H2O saturated conditions. Intermediate to felsic magmas are derived in a wide variety of ways, including the fractionation of a more mafic parent, mixing between mafic and felsic magmas (a process supported, in many cases, by field and textural evidence), crustal contamination, or partial melting of the crust. All these processes appear to take place, to some degree, in arc systems, although in any given arc system, one mechanism may predominate. Arc-related calc-alkalic and tholeiitic basalts typically show moderate degrees of light rare-earth-element (LREE) enrichment, and flat heavy rare-earth-element (HREE) profiles, indicating an origin in a shallow (spinel lherzolite) mantle. More evolved magmas exhibit Eu anomalies, consistent with low pressure plagioclase fractionation. Compared to within-plate settings, tholeiitic and calc-alkalic arc magmas have lower abundances in high-field-strength (HFS) elements, possibly because these elements are bound during the accessory phases in the mantle wedge, and are stable during partial melting. Compared to arc tholeiites, calc-alkalic magmas have higher abundances of incompatible large ion lithophile (LIL) elements reflecting enrichment in the mantle wedge source. This characteristic depletion in HFS, and enrichment in LIL, elements, in arc magmas is the basis for a variety of discrimination diagrams. These diagrams constrain processes operating in modern and ancient arc systems and include chondrite-normalized, MORB-normalized and mantle-normalized spidergrams, which are characterized by jagged patterns of trace-element abundances (in contrast to the relatively smooth patterns of within-plate suites). Some arc suites have depleted initial 143Nd/144Nd and lower initial 87Sr/86Sr than the bulk earth, and are similar to MORB. Other suites have enriched isotopic patterns consistent with the influence of subducted oceanic sediments on the composition of the magma. Samarium-Nd and Rb-Sr isotopic studies can be used to distinguish between felsic magmas derived from fractional crystallization of a more mafic parent (which would have similar values) and those derived from the melting of ancient crust. SOMMAIRE Les donnees geochimiques et isotopiques fournissent des indications quant a l'origine et a l'evolution du magmatisme des marges de subduction des plaques tectoniques. Les magmas sont principalement tholeiitiques dans les premieres phases de formation des arcs insulaires oceaniques, alors qu'ils sont principalement calco-alcalins pendant les phases terminales des arcs insulaires oceaniques ainsi que dans les arcs insulaires continentaux, ou leur composition peut aller du basalte a la rhyolite, dont des volumes considerables de roches de composition inter-mediaire (andesitique). Des experiences permettent de penser que les magmas mafiques calco-alcalins sont formes par la fusion d'un biseau mantelique hydrate qui subit une cristallisation fractionnee a basse pression en des conditions de quasi-saturation en H2O. Les magmas de composition intermediaire a felsique resultent de mecanismes tres varies, dont le fractionnement d'une roche mere plus mafique, le melange de magmas felsiques et mafiques (mecanismes mis en evidence par des donnees de terrain et l'analyse texturale), la contamination crustale, ou la fusion partielle de la croute. Tous ces mecanismes semblent se produire, au moins partiellement, au sein d'arcs insulaires, mais l'un d'eux peut constituer le mecanisme predominant de quelque systeme d'arcs insulaires particulier. L'enrichissement modere typique des basaltes calco-alcalins et tholeiitiques d'arcs insulaires en elements legers des terres (LREE) rares ainsi que le profil plat de leur contenu en elements lourds des terres rares (HREE) sont des indicateurs d'une origine mantelique peu profonde (iherzolithe a spinelle). Les magmas plus evolues affichent des anomalies en Eu, ce qui concorde avec un fraction-nement a basse temperature des plagioclases. Compares a ceux des contextes intra-plaques, les magmas tholeiitiques et calco-alcalins d'arcs insulaires affichent des contenus moindres en elements a grande intensite de champ, peut-etre parce que ces elements sont lies pendant les phases accessoires dans le biseau mantelique, et sont stables pendant la phase de fusion partielle. Compares aux tholeiites d'arcs insulaires, les magmas calco-alcalins ont des contenus plus eleves en elements lithophiles a grands champs ioniques incompatibles, ce qui est le reflet d'un enrichissement au sein du biseau mantelique source. Cet appauvrissement caracteristique en elements a grande intensite de champ (HFS) et cet enrichissement en elements lithophiles a grands champs ioniques (LIL) des magmas d'arcs insulaires forment la base d'une variete de diagrammes de discrimination. Ces diagrammes permettent de preciser les processus en jeu des systemes d'arcs insulaires modernes et anciens et incluent des diagrammes radiaux normalises pour les chondrites, pour les basaltes de dorsales oceaniques (MORB) et pour le manteau, lesquels son caracterises par des profils anguleux irreguliers des courbes de contenus en elements traces (en contraste avec les profils relativement reguliers des suites intra-cratoniques). Certaines suites d'arcs insulaires montrent des ratios initiaux 143Nd/144Nd appauvris et 87Sr/86Sr inferieurs a celui de la valeur planetaire actuelle, et qui sont semblables a celui des basaltes de dorsales oceaniques. D'autres suites ont des profils isotopiques enrichis, ce qui correspond a une influence de sediments oceaniques subductes sur la composition du magma. Les etudes samariumneodymium et rubidium-strontium peuvent etre utilisees pour differencier entre les magmas felsiques issus d'une cristallisation fractionnee d'une roche mere plus mafique (qui montrerait des valeurs similaires) et ceux provenant de la fusion d'une croute ancienne.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it